ENCYCLOPEDIA OF ENVIRONMENTAL SCIENCE AND ENGINEERING - INSTRUMENTATION: WATER AND WASTEWATER ANALYSIS ppt

Thus, a sample can contain components in a number of physical states: i.e.; dissolved, in ionic and molecular form; insoluble, as are bubbles of gas, and suspended and colloidal chemical

INTRODUCTION

In the observation of our pollution problems there seems

to be an attitude of separation on the part of the human

observer from the polluted lake or stream In reality water

is so pervasive in our life; it is such a large part of our

bodily mass and surrounds us in clouds, fog, rain, snow,

lakes, rivers, and oceans We seem to accept its presence

without much thought However, we all are part of the

eco-system and, therefore, pollution is an intimate condition

of our lives—not something unconnected to us Much of

the human population appears to have been separated from

their ecological heritage and membership Perhaps this

is the reason pollution is so endemic to our world; many

people had seen pollution as something displaced from

their intimate reality

In the last thirty years the threat and cause of damage

to ecological and human health from polluting surface and

ground water and acid rain and snow, as well as air pollution,

global warming, and the destruction of the ozone layer has

increasingly occupied our consciousness and our everyday

life The society from young school children to adults

read-ing newspapers and watchread-ing television are aware that we

are heirs to serious environmental problems Polls indicate

the great extent of this concern Recently the concerns of

various national governments have led to international

con-ferences dealing with the ozone problem and discussion of

global warming Perhaps the convergence of several

envi-ronmental conditions that threaten to change planet earth’s

ecological system have awakened the irresponsible amongst

the citizenry, government administrators, scientists and

engi-neers, and the industrial establishment to finally realize that

we are all part of the ecological system and have a vital

inter-est in the control of pollution

The Clean Water Acts of the U.S Congress and

envi-ronmental action of various States and similar actions in

Canada have resulted in some improvement in natural

water quality in North America The role of the Green

par-ties and the citizenry has had a similar effect in Western

European nations In Eastern Europe there is increasing

concern about pollution problems Much remains to be

done in the areas of irrigation, non-point source pollution,

acid rain and snow, the effect of air pollution on water

pollution, protection of ground water from hazardous

wastes, and the further reduction of pollution from

indus-trial sources Extensive human effort and resources have

been dedicated to detect and measure water pollutants and understand their effect on human populations and on the ecological system, as well as on the collection and rec-tification of wastewater in treatment facilities However, much more remains to be done

A realistic primer may help us to visualize the overall effects of water pollution Sitting by an ecologically healthy lake or stream, we observe a proliferation of life—plants and animals familiar and cherished by us Comparing that to our experience of being next to a polluted water body, we would notice different plants, not attractive to us and the presence

of foul offensive odors (However for a lake acidified by acid rain, very clear waters, devoid of life, are observed.) The system has changed from being aerobic (presence of dis-solved oxygen) to anaerobic (lack of dissolved oxygen) The water body has changed so that it is no longer attractive to

us nor can it serve as a water resource A lack of dissolved oxygen in the water has changed the living conditions so that anaerobic fauna and flora can reside there Two conditions can cause this situation: i.e., an excess of nutrients (such as nitrates or phosphates) serving to facilitate the growth of plants and an excess of biodegradable organic matter serv-ing as food for the microbial population These pollutants originate from human biological waste and human activities such as agriculture and industry

An excess of biodegradable organic matter leads to an accelerated growth of the microbial population Since they are aerobic and require dissolved oxygen in the water for respiration, a large population could deplete the dissolved oxygen supply leading to the asphixiation of fish, other ani-mals and insects and the death of plants Then anaerobic fauna and flora will flourish producing reduced gaseous substances, such as ammonia and hydrogen sulfide These gases are toxic and unpleasantly odiferous Although water can be reaerated by the air above its surface to provide a supply of dissolved oxygen, the process is very slow allow-ing for the conditions of oxygen depletion to exist for long periods of time

Another mechanism leading to the same result is caused

by an excess of nutrients The presence of excessive amounts

of nitrates and phosphates spur algae growth in the water body The upper layers of algae shield the lower layers from sun light This situation causes death of the lower layers

of algae adding large amounts of biodegradable organic matter to the water body and an explosion in microbiologi-cal growth Thus, through the action described above the

dissolved oxygen content is greatly reduced and

anaero-bic conditions develop Another category of pollution are

the toxic substances entering water bodies, such as some

synthetic organic materials and toxic metals and non-metals:

they cause the death of aquatic plants and animals disrupting

the water ecosystem Non-biodegradable substances may be

toxic, cause problems due to their physical nature, or detract

from the beauty of nature

From a consideration of the foregoing descriptions of the

mechanisms of pollution effects, a number of parameters for

the determination and control of water pollution can be listed

For example degradable organic matter, non-biodegradable

substances, dissolved oxygen, nitrates and phosphates, and

toxic metals, non-metals and organic matter are classes of

substances requiring methods of analysis

However the previous and rather bare outline of the

pol-lution scenario does not expose the complex problems in

describing the ecological mechanisms affected by pollution

and their attendant solutions The definition of a problem is

necessary if one is to prescribe a solution The more

com-plete the definition, the more precise and comprehensive the

proposed interpretation can be Unfortunately, we do not

have the luxury of unlimited time to adequately define the

various environmental problems; we must institute actions

using the knowledge at hand and update and improve our

interim solutions as we approach a more complete definition

of each of the problems Indeed, the answers to the problems

of water pollution and abatement have been undertaken in

this vein

The large question, what do we measure, brings us to the

complexity of the issue, since what we measure is connected

to why we measure a particular property or component The

attempt to answer these questions cannot be undertaken in

this relatively short article, however, a very limited response

will be given to these questions

This article will describe the operation and use of

chemical instrumentation both in the laboratory and in

monitoring instrumental systems, for data collection

nec-essary for refining the definition of the environmental

water problems, monitoring of processes to treat

waste-waters and drinking water, and the ecological monitoring

of natural waters

WATER AND WASTEWATER ANALYSIS

In the last fifty years the advances in electronics have made

possible the development of the sophisticated

instrumenta-tion and computer systems which serve very well the

pur-poses described herein Development of chemical sensors

and their combination with instrumentation has resulted in

the laboratory and monitoring chemical measurement

instru-ments so commonly found in laboratories, environmental

monitoring systems and manufacturing plants In addition

the interfacing of these instruments and computer systems

results in effective and creative data handling, computation,

and prediction

A general consideration of the analysis of water and wastewater samples brings forth several factors to consider What characteristics need to be monitored and for what reasons? How do we obtain a representative sample of the source to be analyzed and how do we preserve its integrity until an analysis is completed? What constitutes our present methodology and with what biological, chemi-cal, physical and instrumental means do we carry out these measurements? However, a primary consideration in answering these questions relates to the nature of water and wastewater samples

The Nature of Water-Related Samples and Sampling Considerations

Nature of Water-Related Samples The category of water

and wastewater samples can include water samples, sludges, benthic muds, plant matter and so forth Samples may be taken from a number of systems: for example, natural water bodies, process streams from wastewater treatment and manufacturing plants, benthic environments, marshes, etc Different procedures for sampling can be required for each variety of sample based on their unique chemical, physical, and/or biological nature

Water is alluded to as the universal solvent for good reasons; it is the best solvent humans experience In addi-tion to dissolved substances water can also transport insol-uble, suspended, and/or colloidal matter Thus, a sample can contain components in a number of physical states: i.e.; dissolved, in ionic and molecular form; insoluble, as are bubbles of gas, and suspended and colloidal chemical substances; and biological organisms in a variety of sizes The determination of the identity and concentration of unique chemical and biological components is important The presence of these components give to the water sample biological, chemical, and physical characteristics—such as physiological qualities, acidity, alkalinity, color, opacity and

so forth

Sludges and mud samples are heterogeneous mixtures containing water with dissolved matter and the sampling procedure must not change the composition of the mix-ture Plant matter, having its own unique characteris-tics, requires the proper procedures for sampling. 1 Many samples display time-based changes once taken from the source for a host of reasons Changes are evident over various time scales For example suspended matter settles during a time period as determined by particle size giving

a change in opacity and/or color, chemical reactions may occur amongst components, gases and volatile substances may diffuse to the surface of the sample and evaporate, gases or volatile substances in the air space above the sample may condense and dissolve at the sample interface, etc Substances in benthic samples can experience air oxi-dation and plant matter can lose moisture and so forth All

of these changes present a deviation of the sample’s position and characteristics from the source The serious-ness of the changes depend on the purpose of the analysis

com-and its use There must be no perturbation of the sample

composition or physical characteristics which will nullify

or seriously distort the analysis results and be detrimental

to the purposes of the analysis

Preservation is a means of preventing decomposition

and change of various biological or chemical components in

the sample Standard Methods provides a treatment for the

preservation of specific components in a variety of samples

where preservation is possible (see Table 1). 2

Sampling Considerations Two types of changes must be

considered in order to define and carry out a proper

sam-pling program, i.e., time-based changes in the source to be

sampled and in the sample taken from the source

Time-based changes in the source may be short or long

term The short-term changes taking place are indicative of

biological, chemical, and/or physical interactions in the time

span assigned to repetitive sampling Long-term changes

in the sources are related to ecological trends It is

impor-tant to be aware of each of these types of changes so that

sample storage changes are not mistaken for sample source

changes

In order to obtain a legitimate sample of water or

waste-water for analysis one must understand the nature the sample

and its time-based changes Once a sample taken from the

source it may begin to change for many reasons as discussed

in the previous section, IIA, 1 The sample must be

represen-tative of the sample source, that is it must have the same

bio-logical, chemical, and physical characteristics of the source

at the time the sample is taken Therefore the sampling

pro-cedure must not cause a change in the sample relative to the

source if there is to be an accurate correspondence between

the sample analysis and the nature of the sample source at

the time the sample was taken

To prevent confusing the two variables, ecological changes

in the source and interactions in the sample that can occur in

the same time span in a series of samples, preservation of

the samples is undertaken to deter changes after sampling

However, preliminary sampling and testing may be necessary

to indicate the type of time-based changes occurring Then a

reliable program can be established with some certainty

Three kinds of sampling schemes are undertaken in

consideration of the sample source characteristics—grab,

composite and integrated samples

A grab sample is one taken at a given time and place

If the source doesn’t change greatly during a long passage

of time or within a large distance in all directions from

the sampling point, the grab sample is useful The results

from grab samples are said to represent the source for the

given values of distance from sampling location and time

However a time and place series of grab samples is needed

to establish the constancy of analytical values for different

times and distances from the original grab sampling point

With that information the sampling frequencies and times

for a sampling program can be established The sampling of

solids, such as, benthic muds and sludges, requires great care

in order to obtain truly representative samples

A series of samples collected and blended to give a time-averaged sample is known as a composite or time-composite sample In another procedure the volume of each sample of the series collected is proportional to flow

of the sample source, namely the water body or waste stream The samples of various volumes are composited

to provide the final time-composite sample This type of sample gives an average value over a time period and saves analysis time and cost Sampling frequencies and the total time span of the series depends on the source Composite sampling may be used for process streams to determine the effect of unit processes or to monitor a plant outfall for daily or shift changes However, biological, chemical, and physical parameters that changes on storage during the sampling time period can’t be reliably determined in time-composited samples and another sampling protocol is needed (see Table 1)

At times, simultaneous samples are needed from various locations within a given source, such as a river or lake Grab samples are then composited, usually based on volumes pro-portional to flow, and are called integrated samples These samples are used to determine average composition or total loading of the source which varies in composition in its breadth and depth The sampling program for such sources is complex and requires careful consideration for each unique source

Water and Wastewater Parameters

A large number of water quality parameters are utilized in the characterization, management and processing of water and wastewater Table 2 lists a number of these param-eters separated into three categories—physical, chemical and biological It is obvious that only some parameters are considered to be pollution factors because they indicate con-ditions of water during processing or in the natural state The STORET system of the USEPA lists more than four hundred parameters separated into six major groups and is used for the analysis, collection, processing and reporting of data. 3 Table 3 gives a sampling of groups of parameters in this system Not all of these parameters are used frequently, since many are rather unique to particular waste effluents In actuality, a very small number are used in the analysis of a particular sample

Water quality parameters may be divided into two groups, specific and non-specific water quality parameters Specific parameters refer to chemical entities of all types, e.g., ions, elements, compounds, complexes, etc For example, in Table 2 some specific parameters are ammonia, all metals listed, dissolved oxygen, nitrates, sulfates, and so forth Non-specific parameters are included in three categories and some examples are as follows: chemical (hardness, alkalinity, acid-ity, BOD [biochemical oxygen demand], TOC [total organic carbon], COD [chemical oxygen demand], chlorine demand), physical (salinity, density, electrical conductance, filterable residue), and physiological (taste, odor, color, turbidity, sus-pended matter) Many of these non-specific parameters are

TABLE 1 Summary of special sampling or handling requirements a,*

Minimum Sample

Size mL

Sample

Maximum Storage Recommended/ Regulatory

Carbon, organic,

Total

add H3PO4 or H2SO4 to pH
2 7 d/28 d

chlorination

Organic,

acid/L if residual chlorine present

7 d/7 d until extraction; 40 d after extraction

(continued)

very important in water and wastewater characterization and

instruments are available to measure specific and non-specific

parameters

Methodology

The large variety of tests carried out on water and

waste-water samples and sources have been codified and are

included in the laboratory reference in the United States

entitled “Standard Methods for the Examination of Water

and Wastewater” and is commonly referred to as Standard

Methods This compendia of methods is regularly updated

At the present time the 19th edition published in 1995 is in

use 2 and a supplement was issued in 1996 Supplements are

used to update methods on an ongoing basis in order not to

unduly prolong the publication of the new edition However

not more than one supplement appears to have been published

for each edition

Three professional organizations jointly write and edit this manual—the American Public Health Association, the American Water Works Association and the Water Environment Federation (formerly the water pollution Control Federation) It is published by the American Public Health Association Over five hundred profession-als belonging to these organizations and others participate

in Standard Methods It was first published in 1905 and an interesting history of its genesis is given in the preface to the 19th edition. 2

At one time methods were segregated between water and wastewater test methods, however, since the 14th edition in

1976, that division ceased In the 19th edition, methods are classified in ten groups: Introduction, Physical Aggregate Properties, Metals, Inorganic Nonmetallic Constituents, Aggregate Organic Constituents, Individual Organic Constituents, Radioactivity, Toxicity, Microbiological Examination, and Biological Examination

TABLE 1 Summary of special sampling or handling requirements a,* (continued)

Minimum Sample

Size mL

Sample

Maximum Storage Recommended/

Regulatory #

Purgeables* by

purge and trap

1000 mg ascorbic acid/L if residual chlorine present

7 d/14 d

immediately; refrigerate

48 h/N.S.

§ Refrigerate  storage at 4C, in the dark

# Environmental Protection Agency, Rules and Regulations 40 CFR Parts 100–149, July 1, 1992 See this citation for possible differences regarding container and preservation requirements N.S  not stated in cited reference; stat  no storage allowed; analyze immediately

a If sample is chlorinated, see text for pretreatment

Twenty-five years ago the dearth of instrumentation was

used in Standard Methods. 4 However, in the present edition

the following instrumentation is employed in the

method-ologies: molecular spectroscopy (visible, uv, ir), atomic

spectroscopy (absorption, flame, ICP), chromatography

(gas, ion, liquid), mass spectrometry (GC/MS, gas

chroma-tography/mass spectrometry), electro-analytical techniques

(polarography, potentiometric and amperometric titrations,

selective ion electrodes), radio-activity counters (gas filled

and semiconductor detectors and scintillation counters), and

automated continuous-flow methods

Also included in Standard Methods are aspects such as safety, sampling, mathematical treatment of results, reagents, apparatus etc It is fortunate, indeed, that such a compre-hensive work is available and that it is regularly revised Enlightened editorial leadership and the many members of the Standard Methods committees in the last twenty years can be credited for the steady increase in the inclusion of instrumentation in Standard Methods In the increasing com-plexity of environmental and ecological problems and guid-ance of Standard Methods is a valuable and practical support

in obtaining necessary analytical data

The American Society for the Testing of Materials, ASTM, is an important compendium for the analysis of raw and finished material products A large section is devoted

to the analysis of water and wastewater in the context of processing and usage. 5

The EPA has published instrumental methods for the analysis of priority pollutants and other substances controlled

instru-In addition to the requirements of the process industries, the needs of the water and wastewater area have spawned the development of some specialized laboratory, monitor-ing and process control instrumentation Some examples are the total carbon and organic carbon analyzers, biologi-cal oxygen analyzers and the residual chlorine analyzer Monitoring and data acquisition systems, in conjunction with this instrumentation, are increasingly used in waste-water management and plant process control Certainly a number of physical parameters such as temperature, flow rate, pressure and liquid level have been measured instru-mentally in the process industries, including wastewater treatment plants, predating the development of this wide variety of analytical instrumentation. 7

Instruments utilized in the measurement of parameters important to wastewater analysis, treatment and manage-ment can be divided into two categories based on application Monitoring of water bodies and waste treatment processes require monitoring instruments which are characterized by ruggedness and capability of unattended operation and data storage and/or transmission A second category, laboratory instruments, in many instances, may be more sophisticated, sensitive to the surrounding environment and also have data storage and transmission capabilities Each type has its spe-cific utility in the scheme of analysis and data acquisition for wastewater characterization and processing In many instances monitoring instruments are laboratory devices which were ruggedized and prepaared for field use Thus the variables measured and the principles of operation are the same in many cases Some examples of variables mea-sured by laboratory and monitoring instrumentation are pH, conductivity, DO (dissolved oxygen), specific cations and

TABLE 2 Some water quality parameters a

Dissolved oxygen Hardness Heavy metals:

Chromium Copper Iron Lead Manganese Mercury Magnesium Nitrate, nitrite Organic compounds:

Detergents Herbicides Pesticides Phenol Oils and greases Oxidation–reduction potential pH

Phosphates Potassium Sodium Sulfate Total organic carbon (TOC)

a Reprinted from Ref (4), p 1438 by courtesy of Marcel Dekker, Inc

anions, and a variety of specific and nonspecific parameters

by automated analyzers However, the instrumental

appear-ance and some unique functions related to unattended

oper-ations may differ

Monitoring and data acquisition systems are also

con-sidered in this article In the control of wastewater treatment

systems and plants, data may be obtained exclusively from

monitoring instrumental systems or a combination

includ-ing data from laboratory instruments via a laboratory data

system and/or from data entered through terminals

Analytical instrumentation can be classified according

to principles based on various physical phenomena These

general categories are spectroscopy, electrochemistry,

radio-chemistry, chromatography, and automated chemical

analy-sis The instrumentation described in this article is organized

according to these categories

INSTRUMENTATION

Structure of Instruments

An instrument is a device that detects a physical property

or chemical entity through the conversion of a physical or

chemical analytical signal to an energy signal, usually

elec-trical, with subsequent readout of the energy signal

Three main parts comprise an instrument: that is a

chemi-cal or physichemi-cal sensor, signal conditioning circuits, and

read-out devices The sensor develops a signal, usually electrical,

in response to a sample property and the signal conditioning

circuit modifies the signal in order to allow convenient

read-out display of the signal Finally, a readread-out device displays

the signal, representative of the sample, in terms of a reading

on an analogue or digital meter, a recorder chart, an

oscil-loscopic trace, etc Figure 1 delineates the three major parts

and functions of an instrument, sample properties and) to be measured, and instrumental criteria

Sensors A sensor, the primary contact of the instrument with the sample, is a device that converts the input energy derived from a sample property to an output signal, usually electrical in nature The relationship between the input energy

(measurand), Q 1 , and the output energy, Q 0 , is expressed in the form:

Q 0  f ( Q 1 ) (1) and is known as the transfer function The sensitivity is given

in the equation

S  dQ 0 / dQ 1 (2) When the transfer function is linear, the sensitivity is constant throughout the sensor’s range However, the sensitivity (gain

or attenuation factor) is dependent on the value of the ferential fraction in equation 2 The sensor threshold is the smallest magnitude of input energy necessary to obtain a measurable change in the output

Readout signals may be digital, D (discrete), or analog,

A (continuous), in form and are a function of the nature of the

input signal and the sensor and the design of the signal

condi-tioning circuits These signals are interconvertible using A / D

or D / A devices Fast reacting sensors and circuits, however,

are utilized for producing digital signals, where, formerly, analog signals were obtained

Two varieties of sensors, chemical and physical, are in use on various instruments The physical sensor allows the conversion of physical energy from one to another One example is a photocell that converts an impinging light beam

TABLE 3 Number of STORET listings for water analysis

parameters in group

Organic materials

to an electrical signal and is used in spectrometers A second

example is a piezoelectric crystal-based sensor that converts

a mechanical force to an electrical charge translatable to a

potential The piezoelectric effect is reversible; an electric

charge will cause a mechanical dislocation in the crystal

Another example of a physical sensor is a platinum

resis-tance thermometer where the resisresis-tance of a platinum wire is

altered by a change in temperature

Chemical sensors are devices that allow the analyte or

target material through one of its specific chemical

param-eters to ultimately generate an energy signal, usually

electri-cal, in a transducer through the agency of a selective chemical

or physical chemical reaction A transducer is a material

structure inside of which or on whose surface the specific

chemical or physical chemical reaction takes place leading

to the generation of the energy signal Thus, there are two

parts to the chemical sensor, the interface zone or area where

the selective reaction takes place and the usually non-specific

transducer. 8 Figure 2 illustrates, functionally, the parts of a

chemical sensor

An example of a chemical sensor is a potentiometric

electrode Here the selective chemical reaction, the redox

reaction of the analyte, is in equilibrium at the electrode

sur-face imposing a potential that is proportional to the

loga-rithm of the concentration of the analyte as described by the

Nernst equation For example, a copper electrode in a

solu-tion of copper ions will take on a potential in response to the

concentration of copper ions The logarithm of the copper

ion concentration is proportional to the electrode potential Another illustration of a chemical sensor is an amperometric electrode, where a current arises due to the redox reaction of the analyte when the electrode is at the appropriate poten-tial The concentration of the analyte is proportional to the magnitude of the current A platinum electrode maintained

at the redox potential for the silver/silver ion redox system will detect the concentration of silver ions A membrane electrode is another type of chemical sensor The fluoride electrode consists of a lanthanum fluoride (LaF 2 ), thin, crys-tal membrane On the outside surface, the sample side of the membrane, the fluoride ions, F, from the sample are attracted electrostatically to the lanthanum ion, La 3, at the surface of the membrane to form a complex The complexed entities do not penetrate very deeply into the surface The amount of F complexed is a direct function of its activity

(see Section III,B,2, a ) and represents a selective physical

chemical reaction A membrane potential arises because the opposite side of the membrane is exposed to a standard activity of F giving a net difference in potential between the two sides The membrane potential is the non-specific electrical signal of the sensor

Signal-Conditioning Circuits These circuits modify the signal produced by the sensor so as to provide an accurate representation of the sensor signal with optimal electrical characteristics to drive the readout device In Figure 1 a number of signal conditioning modes are given and can be

Thermocouple Katharometer Thermister Bolometer

Electrical Electrode pair Electrodes, AC system Membrane electrode Mechanical

Balance force transducer Force transducer Hydrometer Viscosity pipet Nuclear

Ionization tubes Scintillation counters Photographic plates Cloud chamber Semiconductor detectors

Amplification Arithmetic operation Chopping

Comparison to reference Digitization

Rectification Stabilization

Analog Meter Oscilloscope Recorder Digital Nixie display Point plotter Printer Tape, paper, or magnetic

Criteria

Band width Noise figure Sensitivity Signal-to-noise Time constant Error Hysteresis Nonlinearity Scale Zero displacement

Instrument

FIGURE 1 Diagram of instrumental functions Reprinted from Ref (4), p 1442 by courtesy of Marcel Dekker, Inc.

placed in four categories—modification of sensor output,

amplification, mathematical operation, and signal

modifica-tion for readout

The electrical components used in these circuits are of

two types, active and passive elements Active elements,

such as solid state devices add energy to a circuit; whereas

passive elements, such as resistors, capacitors, inductors,

diodes add no energy Both elements are combined to form

active and passive circuits Active circuits change signals in

a complex way Passive elements are used in active circuits

to provide necessary conditions for the proper functioning

of active circuits Some active devices are ionization

cham-bers, vacuum phototubes, operational amplifiers and gas

dis-charge tubes

Readout Devices The sensor signal modified by the

conditioning circuits is ultimately converted into a visual

form by the readout device or output transducer The

read-out signal may be analog or digital requiring a compatible

readout device Analog readout devices comprise

record-ers, metrecord-ers, oscilloscopes, photographic plates and

integra-tors; printers, computers and digital meters with optical

displays provide digital readouts A digital computer may

be interfaced to an instrument, in order to compute values

from a digital output signal and produce a hard (printed)

copy of the data using a printer Analog output signals may

be digitized in order to utilize a computer The advantages

of digital outputs are the statistical benefit derived from

counting and analog outputs are advantageous in feedback

control systems

Analog Devices The automatic recording potentiometer

or potentiometric recorder has been, over the years, the

most frequently used readout device providing a

continu-ous trace on a chart of an analog signal Its operation is

based on a low power servomechanism utilizing a feedback

system The instrumental signal to be measured is

com-pared to a standard reference signal The amplified,

differ-ence or error signal activates the pen-drive motor moving

the pen on the chart to a position representing the

magni-tude of the analog signal The control of the pen, based on

the error signal, denotes the feedback system and the total

system is referred to as a servomechanism. 9,10 Two types of

recorders, the Y -time or X – Y, allow the recording of a signal,

Y, as a function of time or of two signals representing the ordered (data) pair, x, y, respectively In the Y -time device,

a constant-speed motor moves the chart in the x direction while the servomechanism deals with the y signal The X – Y recorder has two servo- systems, one for each signal, x and y

However, recorders may be limited by the rate that the data flows from the instrument Some recorders can adequately respond to signals during fast scans For example fast scans

in cyclic voltammetry of about 1 volt/sec can be transcribed using a recorder, however, at faster rates an oscilloscope is necessary

Almost any instrument can utilize a potentiometric

recorder A Y -time analog recorder is commonly used to trace gas and liquid chromatograms; the abscissa, X axis,

is for retention volume or time and the ordinate is for the detector response

The oscilloscope is a measuring device with complicated circuitry that allows accurate display and measurement of non-sinusoidal or complex waveforms The oscilloscope’s basic part is the cathode ray tube, CRT A CRT is a vacuum tube containing an electron gun pointing to a fluorescent screen at the tube’s end The electron gun provides a beam whose movement is controlled by two sets of deflector plates perpendicular to each other The plates receive the signals representing the waveforms These analog signals are dis-

played on a fluorescent screen as Y -time or X-Y curves The

display is photographed to provide a hard copy of the analog data The oscilloscope can display data that is generated at high rates, since there are no mechanical movements used

in manipulating the electron beam Where very fast events must be recorded, an oscilloscope is an effective readout device. 11 (See the previous paragraph on the potentiometric recorder.) Oscilloscopes have facilities to store, compare, and manipulate signals

Analog meters are based on the D’Arsonval meter ment The electrical current signal passing through a moving coil, to which is fixed a pointer, induces a magnetic field in the coil A static magnetic field from a permanent horseshoe magnet surrounds the coil The interaction between the two fields causes the movement of the coil: the degree of move-ment is determined by the magnitude of the signal current Analog meters require the analyst to interpret or read the output signal value by the position of the indicator needle

move-or pointer using a calibrated scale mounted on the meter

A resistance placed in series with the meter movement allows

FIGURE 2 Chemical sensor.

CHEMICAL SENSOR

INTERFACE ZONE TARGET

the measurement of voltage Resistance may also be

mea-sured with the meter One weakness of this device is its low

internal resistance causing loading errors by high impedance

signals. 11

A meter is used in the analysis of single samples or

samples analyzed, serially, at a slow rate on a spectroscopic

instrument at one frequency or wavelength Meters also

are employed to indicate proper adjustment of potentials,

currents, temperatures, etc for various instruments

An electronic voltmeter, EVM, is more sensitive and

accurate than the D’Arsonval-based meter previously

described, particularly for signals with high impedance

The internal resistance is 10 Mohms (megaohms, 10 6 ohms)

or more for d.c (direct current) signals and 1 Mohm for

a.c (alternating current) signals The circuits use solid state

devices compared to the earlier device, a VTVM (vacuum

tube voltmeter) Current and resistance is measurable with

the EVM Its application parallels those for the

D’Arsonval-based meter. 11

A photographic plate or film may be used to collect data

in the time domain where all the data are displayed

simul-taneously, that is a spectrum in emission spectroscopy The

radiation in the dispersion pattern of the sample reflected or

transmitted from the prism or grating impinges on the

pho-tographic plate

Electronic integrators determine the area under a curve

and are superior in precision to the ball and disk

integra-tor and the several hand methods widely utilized They

may be based on operational amplifier or transistor

cir-cuitry Some potentiometric recorders have a second pen

controlled by an integrator and the density of the pen’s

excursions determine the area under the curve This last

type is not as convenient as the electronic integrators that

can correct for baseline changes Chromatographic peak

areas for GC and HPLC (high performance liquid

chroma-tography), anodic stripping analysis peaks, spectroscopic

curves, etc are integrated as a means of quantitation and

analysis of an analyte

Analog computers are available but are not used now to

any great extent

Digital Devices The digital computer or microprocessor

interfaced to the instrument brings a broad capability to the

display and processing of instrumental data Data reception

and storage is convenient when real time computation and

display are not required Mathematical calculations,

includ-ing the areas under curves, graphic and tabular displays,

correlation with previously collected data, and many other

operations can be carried out at one’s convenience Real

time processing can be accomplished on a time-sharing

basis or with a dedicated computer The visual display is at

a video monitor and a printer provides a hard (printed) copy

of the raw and calculated data, graphs, and other

informa-tion Computer devices include microprocessors and micro-,

mini-, and mainframe computers The instrument must be

carefully interfaced to the computer and this task requires

much electronic skill Instruments providing spectral

read-outs, the need for number crunching and repetitive analyses

can benefit greatly from a computer interface Some ments that utilize Fourier transform analysis require a com-puter capability and many instrumental techniques have been revolutionalized by computer use The use of the computer 12

instru-in the reduction of noise instru-in instru-instrumental signals by ensemble and boxcar averaging has greatly improved the quality of instrumental data. 12

Digital meters measure analog signals and provide

a digital readout A/D conversion of the analog input is accomplished electronically The digital data is displayed

as numeric images using solid state devices such as LEDs, light emitting diodes, and LCDs, liquid crystal displays, and lamps such as, NIXIE, neon, and incandescent bulbs The LED is the more convenient device because its seven seg-ment readout display uses lower currents and voltages than the lamp displays The LED’s red image, due to the semi-conductor gallium arsenide doped with phosphorus, may

be increased in intensity by using more semiconductor in the LED The image color of LEDs may be fabricated to be green or yellow, also. 11,13 LCDs operate by means of polar-izing light They use reflected light for viewing, a seven-segment and dot matrix readout display, an a.c voltage, consume very little power and are more fragile than LEDs. 14 The LCDs and LEDs are the newest and most convenient display devices

Digital meters can be used in place of the analog variety The former are more accurate and easier to read

Instrumental Parameters and Definitions Instrumental

characteristics of operation and data treatment and statistics are defined by a number of parameters A definition of each term is as follows:

• The range of frequencies (information) in the signal is called the bandwidth During amplifica-tion, some amplifiers cannot respond to the range

of frequencies in the signal producing an amplified signal with a narrower bandwidth

• The baseline is the signal obtained when no sample is being examined and reflects the noise inherent in the instrument

• Calibration is the process relating instrument response to quantity of analyte In general a series of standard solutions or quantities of ana-lyte are analyzed on the instrument taking reagent blanks into account and using a similar matrix

as the sample under consideration The response data are plotted to provide a calibration curve where error bars indicate the precision of the method. 15 Other calibration procedures such as the methods of standard additions 16 and of internal standards 17 have advantages in specific situations The former is helpful in ameliorating interferences from the sample matrix and the latter in correcting for changes in instrument response particularly in

quantity-GC, and ir (infrared) and emission spectroscopy. 18

• The gain refers to the ability to amplify a signal and is the ratio of the output to input signal The

gain may refer to voltage, current or power

ampli-fication and its input and output impedances

• Noise refers to random signals, usually

continu-ous, that restricts the lower detection limit and

accuracy of the signal Noise arises from

elec-tronic components and environmental sources and

cannot, at times, be completely eliminated

• The ratio of the amplitude of the signal to that of

the noise is called the signal-to-noise, S/N, ratio

This ratio gives the ability to distinguish between

signals and noise, that is a measurement of the

quality of an instrument One cannot usually

dis-tinguish the signal from the noise when the ratio

is less than about 2 or 3

• Resolution or resolving power is the

capabil-ity of displaying two signals differing slightly

in value The resolving power, R, of a

mono-chrometer concerns absorption of emission

spec-tral signals,

R  λ / d λ (3)

is the wavelength under consideration and d λ is the

differ-ence of wavelength between the two signals In mass

spec-trometry resolution refers to the separation of two mass

peaks Ms and Ms  dMs, where dMs is the difference in

masses so that

R  Ms / dMs (4) For resolution for chromatographic methods see Part Two

Section III,B,4, a

• Response time refers to the time needed for a pen

of a potentiometric recorder to travel the total

ver-tical distance on the Y axis

• Sensitivity, S, describes the ratio of the change in

the response or output signal, dI 0 of the instrument

to a small change in the concentration or amount

of the analyte, dC The ratio is given as follows:

S  dI 0 / dC (5)

• Linear dynamic range, LDR, describes the

mathematical relationship between amount or

concentration of the analyte and the response of

the instrument An increase in the analyte

quan-tity results in a linear increase in response The

size of the range of quantities accommodated

by the instrument response is the key factor for

this parameter For example in voltammetry

the LDR is 108 to 103 M (molar), five orders

of magnitude, in (ultraviolet) uv–visible

spec-trophotometry, about 10 to 100, and for a GC

with a FID (flame ionization detector) the LDR

extends from 101 to 10 7 ng (nanograms, 109

grams) or eight orders of magnitude Obviously

the sensitivity remains constant in contrast to a non-linear dynamic relationship

• The reagent blank or blank in a spectroscopic determination is the signal obtained by the solution

of the reagents without any analyte The sample matrix is important to include, if known, in the blank In many instances the effect of the matrix is determined indirectly

• Accuracy defines, mathematically, the absolute

error, e a , inherent in the method when comparing

the analytical result, x i , with the true value, x t , of the analyte content of the sample

e a  ( x i  x t) (6) Preparing a standard sample containing an accurately known concentration of the analyte is required This is not a simple task, because homogeneity of any mixture is difficult to obtain and ascertain

• The precision of a method is concerned with the repeatability of the analytical results for a number

of analyses on the same sample There are several ways of expressing precision; standard deviation

is a very effective and meaningful measure The

standard deviation, sd, for small sets of data is

1

1 21

∑

⎡

⎣

Here x i is the experimental value, x a , the average of the

exper-imental values, and No, the number of values The standard

deviation is a measure of the average uncertainty of all the

measurements in the data set, x i , that is x 1 , , x N . 18,19

Types of Instruments

Analytical instruments can be classified according to egories based on various physical phenomena The general categories used in this article are spectroscopy, electrochem-ical analysis, radiochemical analysis, chromatography, and automated analysis Table 4 illustrates these categories

Spectroscopy

Introduction Spectroscopic instruments include optical and

other types of instruments The optical instruments analyze electromagnetic radiation, emr, while other spectroscopic instruments deal with sound, mixtures of ions, electrons, and other forms of energy Other optical methods utilize instru-ments that make refractometric and polarimetric measure-ments Refractometric measurements will be discussed in the section on liquid chromatography

Spectroscopy, classically, is that area of science where the electromagnetic radiation, emr, emitted from or absorbed

by a substance is resolved into its component wavelengths

indicating its intensity and presented as a spectrum In

this category absorption, emission, and photoluminescence

(fluorescence and phosphorescence) spectroscopy using

x-ray, ultraviolet–visible (uv–vis), and ir radiation, and the

measurement of turbidity, or suspended matter by

neph-elometry and turbidimetry, are included However, today in

a broader sense, spectroscopy includes the following:

reso-lution of electrons of many energies by uv and x-ray

photo-electron, Auger etc spectroscopy; sound waves by acoustic

spectroscopy; ions by mass number by mass spectroscopy;

and absorption of radiowaves by atoms and electrons exposed

to a magnetic field in nuclear magnetic resonance and

electron spin resonance spectroscopy The phenomena of

absorption, emission, photoluminescence (fluorescence and

phosphorescence), and scattering are the bases of scopic instruments

b Spectroscopic instruments Spectroscopic instrumentation is differentiated with respect to the wavelength range of the instrument, that is x-ray, uv, visible, and ir and type of instrument, i.e absorp-tion, emission, photoluminescence (fluorescence and phos-phorescence), and turbidity The energy sources, sample cells, wavelength selection devices (gratings, prisms, filters, crystals) and sensors may differ for these various instru-ments These parts are listed in Figure 3 for the wavelength regions of from 100 to 40,000 nm (nanometer, 109 meters) X-ray and non-optical spectroscopic instruments are not included

TABLE 4 Bases for instrumental methods

e, ions, or electric field Ion formation/seperation in electric or

magnetic field

Mass spectroscopy Electricity (arc, spark), heat (flame,

plasma)

ESCA)

spectroscopy

voltammetry

differential temperature vs increasing temperature heat flow to sample vs increasing temperature Temperature vs volume of reagent

Thermogravimetric analysis Differential thermal analysis Differential scanning calorimetry.

Enthalpimetric methods

The types of instruments can be characterized by some

simple diagrams regardless of wavelength range as given in

Figure 4 The basic difference between absorption and

emis-sion spectroscopy is the use of a transmitted emr energy

source in the former, while in the latter, the sample is

stimu-lated in a thermal or electrical energy source to emit radiation

Photoluminescence (fluorescence and phosphorescence),

stimulated by emr, is observed usually perpendicular to the

stimulating beam In nephelometry instrumentation similar

to photoluminescence is utilized; turbidimetry can employ

absorption instrumentation

A brief description of the basic parts of the instruments

using these phenomena follows

(1) Energy sources

As noted from the diagrams above, all but emission

instrumentation use energy sources that irradiate the sample

In Figure 3 the sources are indicated as a function of the wavelength range of their radiant emissions Various lamps, i.e., argon, xenon, H 2 (hydrogen) and D 2 (deuterium), and solid state radiators give continuous emissions, i.e., a range

of contiguous wavelengths and are used in molecular troscopic instrumentation Photoluminescence and nephelo-metric instruments use these sources

spec-For atomic absorption instruments hollow cathode lamps are utilized They are line (discontinuous) sources providing unique radiation with a narrow bandwidth char-acteristic of particular element An individual lamp is usu-ally employed for each element Some multielement lamps are available

X-ray sources include x-ray tubes or radioactive sources The x-ray tube consists of a tungsten cathode that emits elec-trons when heated The electrons accelerated by a large poten-tial strike the metal anode generating x-rays characteristic of

(a) Sources

Continuous Wavelength, nm

(b) Wavelength selectors

Continuous

Discontinuous

fluorite prism

(c) Materials for cells, windows, & lenses

(d) Transducers Photon detectors

Heat

Golay pneumatic cell Thermocouple (volts) or Bolometer (ohms) Photoconductor

Silicon diode

Tungsten lamp

Nernst glower (ZrO2 + Y2O3Nichrome wire (Ni + Cr) Globar (SiC) Hollow cathode

lamps

Photomultiplier Phototube

Photocell

TIBr - TII KBr

NaCl Silicate glass Corex glass Fused silica or quartz LiF filters Glass absorption Interference filters Interference wedgers Gratings with various number of lines/mm 50 lines/mm

KBr prism NaCl prism

Fused silica or quartz prism xanon lamp

3000 lines/mm

Glass prism

H2or O2 lamp

FIGURE 3 Components and materials for optical spectroscopic instruments (Courtesy

of Prof A R Armstrong, College of William and Mary.)

the particular metal target Metals such as chromium, copper,

iron, molybdenum, rhodium, silver, tungsten and others

com-prise the anode target A number of radioisotope sources emit

useful x-rays, e.g., iron-55 yields manganese K radiation,

cadmium-109 gives silver K radiation, and cobalt-57 provides

iron K radiation

(2) Sample interface

The sample is usually presented as a solution contained

in a cell made of material transparent to source radiation

for absorption and photoluminescence (see Figure 3) Solid

samples are also used Potassium bromide disks containing

homogeneously distributed powdered analyte are used in ir

absorption methods

However in atomic absorption spectroscopy the sample

is atomized in a flame, plasma or thermal heat source In

effect the sample container is that volume of flame, plasma

or heat source

Solutions, as well as solid samples in the form of pressed

disks, pieces of solids, or solid solutions in borax, are

con-veniently analyzed in an x-ray fluorescence instrument

Solutions of sufficient thickness are the best sample

prepara-tions because of their homogeneity; they may be contained

in mylar cells Obviously the solvent must not contain heavy

atoms that fluoresce Sample surfaces are directly exposed to

the x-ray beam (see Figure 5)

In emission instruments the solid sample is placed in an

energy source environment, e.g., an electrical arc or spark, a

flame, or plasma

(3) Wavelength selectors The wavelength selector allows isolation of a particu-lar wavelength segment of the source or transmitted beam

A monochrometer is a selector comprising a grating or a prism which disperses or separates the radiation continuously over a considerable wavelength region The effective band-width of the wavelength, isolated by slits placed before the sample, is quite narrow, 1 nm or less The grating operates on the principle of interference and the prism by dispersion Other wavelength selectors are interference and absorp-tion filters Their effective bandwidths are about 20 to 50 nm, respectively; they are not continuous An interference wedge

is continuous over a region with an effective bandwidth

of 20 nm

The dispersing device, a single crystal mounted on a rotating table or goniometer (see Figure 5a), is the wave-length selector used in x-ray spectrometers A specific wavelength and its second and third orders of reflection are diffracted at a given angle of the beam to a particular plane

of the crystal The angle of diffraction depends on the “d” or interplanar spacing of the crystal and the wavelength and is defined by Bragg’s law Some examples of diffracting crys-tals with their unique wavelength ranges are topaz—0.24 to 2.67 Å, sodium chloride—0.49 to 5.55 Å, and ammonium diphosphate—0.93 to 10.50 Å (An angstrom, Å, is 108cm.) Unlike a prism or a grating that disperses a total spec-trum in the spectral regions of the source of radiation, the x-ray monochrometer diffracts a unique wavelength and its orders of reflection depending on the angle of the beam to the crystal plane

to radiant energy in the visible (350 to 750 nm) region Light shining on a semiconductor coating, such as selenium or copper(I) oxide plated on an iron or copper electrode, gener-ates a current at the metal–semiconductor interface A second electrode, a transparent coating of gold or silver on the outer surface of the semiconductor, collects the electrons formed

by the action of radiant energy on the semiconductor The magnitude of the photocurrent is proportional to the number

of photons/sec impinging on the semiconductor This tor is insensitive to low light levels, slow in response, shows

detec-a tendency to suffer fdetec-atigue, detec-and hdetec-as detec-a high temperdetec-ature ficient However, photovoltaic cells are rugged, require no separate source of energy and are low in cost They are used

coef-in coef-inexpensive filter photometers

(ii) Vacuum photoemissive tubes 20

In a photoemissive detector two electrodes, a cathode with an electron emissive coating and an anode, are enclosed

in an evacuated tube When the saturation potential is applied

A Absorption a & Turbidity Energy

& Readout

Signal Processing

& Readout

Wavelength Selector

Wavelength Selector

Wavelength

Selector

Signal Processing

& Readout

Photoelectric Detector

Photoelectric Detector

Photoelectric Detector Sample

a uv-vis and AA (flame and electrothermal)

b arc, dc spark, inductively coupled & dc plasma, & flame

D Fluorescence c & Nephelometry

c uv, x-ray

FIGURE 4 Outline of spectroscopic instrumentation.

across the electrodes, the radiant energy or photons cause

emission of photoelectrons The photoelectrons are collected

at the anode giving rise to a photocurrent The photocurrent is

proportional to the power or radiant energy of the light beam

and is independent of the applied potential (see Figure 6)

The eleven chemical compositions of various photoemissive

cathode coatings determine the wavelength range and

sen-sitivity varying from the uv to the near ir spectral regions

The window in the tube must be transparent to wavelength

of interest The dark current is a small current flowing when

no light falls on the cathode and is due to thermal energy and

electron emission from potassium-40, 40 K, in the glass tube

It limits the sensitivity of the detector Although this detector

has about one tenth the sensitivity of the photovoltaic cell,

its signal may be amplified because of its large internal trical resistance compared to the photovoltaic detector The photoemissive detector is used for higher intensity radiation and lower wavelength scanning rates than used with other detectors

(iii) Photomultiplier tubes 20

A photomultiplier tube contains a photoemissive cathode followed by a sequential, electron multiplying assemblage

of about nine dynodes (electrodes) as illustrated in Figure 7 The voltage of each succeeded dynode increases by 75 to

100 volts Photoelectrons from the photoemissive cathode are accelerated by the voltage increase of the first dynode caus-ing the release of several electrons for each impinging pho-toelectron This multiplier effect continues as the electrons

Phase detector Balance

indicator

Calibrated attenuator

60-Hz power input Cooling water

30-Hz

unit

Synchronous motor

Chopper Cell Fluorescent screen

Light collector Dial

1800 rpm 75°

150°

0°

φ φ

FIGURE 5 (a) Geometry of a plane-crystal x-ray fluorescence spectrometer

Note that the angle of the detector with respect to the beam, 2θ, is twice that of

the detector to the crystal face, θ (Courtesy of Philips Electronic Instruments.)

(b) Nondispersive x-ray absorptiometer (Courtesy of General Electric Co.)

contact the succeeding dynodes accelerated by ever higher voltages A cascade of a large number of electrons is collected

by the anode of the ninth dynode The final photocurrent can

be amplified, electronically, before readout The gain, G, can

be calculated as follows:

G  ( fs ) n (8)

where fs, the secondary emission factor for each stage, depends on the dynode emissive coating and n is the number of dynode stages Using values for fs of 3 to 10 for

older dynode emissive coatings and 50 for newer coatings

and n equal to 9 results in gains of about 10 4 , 10 9 and 10 15 ,respectively The response times can vary from 0.5 to 2 nsec (nanosec, 109 sec) The dark current can be decreased considerably by cooling the photomultiplier detector Since the dark current is a fairly constant value it may be sub-tracted or automatically nulled using a potentiometer The

FIGURE 6 Simple phototube circuit

(Reprinted from Ref (176), p 441

by permission of Prentice Hall, Inc.,

Englewood Cliffs, New Jersey.)

Incident radiation Grill

Shield

Tube envelope 1–9 = Dynode = electron multiplier0 = Opaque photocathode

10 = Anode 1

1

0 2

2 4

4 3

Focus ring

Semitransparent photocathode

Internal conductive coating

Incident radiation

Faceplate Focusing electrode

1–10 = Dynodes = Electron multiplier

11 = Anode

(b) (a)

FIGURE 7 Photomultiplier Design (a) The Circular-Cage Multiplier Structure in a Side-on Tube and (b) The Linear-Multiplier Structure in a Head-on Tube (Courtesy of the General Electric Company.)

photomultiplier tube is the most widely utilized detector in

optical spectroscopic instruments

(iv) Photodiodes

The photodiode, PD, is a small wafer of silicon dioxide

with a shallow layer of p and n material of the top and bottom

surfaces, respectively, to which are attached electrodes The

device is reverse biased When photons impinge on the

optically-active p diffusion region, electrons promoted to the

conduction band generate a photocurrent that is proportional

to the intensity of the optical light beam The PD detectors

are about ten times more sensitive than the vacuum

photo-emissive tube and are mostly responsive in the visible and

near ir regions Some tubes are sensitive to the uv region at

about 200 nm A lens is optically coupled to each small PD

wafer

Linear arrays or vidicon tubes of these multichannel

detectors allow nearly simultaneous detection of a spectrum

of wavelengths in instruments operated in the spatial mode

(see Section III,B,1, b,5 Instrumental ensembles), where

the detectors are swept electronically Optical multichannel

analyzers consisting of a monochromater, a multichannel

detector and a computer are used in flame emission and uv/

visible spectrophotometers. 21

(b) Infrared detectors 22,23

There are two categories of detectors used for the

spec-tral region above 1.2  m (micrometer, 106 m) namely, heat

and semiconductor detectors

(i) Thermocouples and thermopiles

A thermocouple is formed when two wires of a metal

are separately joined to the opposite ends of a wire of a

dissimilar metal If the two dissimilar metal junctions are maintained at different temperatures, a thermoelectric cur-rent will flow in the circuit Therefore, if one junction is maintained at a constant temperature, a thermoelectric cur-rent will be generated proportional to the temperature of the second junction The changes in the intensity of incident

ir radiation can be detected in ir spectrophotometers using this type of detector The sensitivity is 6 to 8 microvolts per microwatt and a temperature difference of 106C is detect-able (see Figure 8) A thermopile, consisting of a number

of series-connected thermocouples, may be miniaturized through thin film techniques to provide an effective ir detec-tor It has an 80 msec (millisec, 103 sec) response time with

a flat response below a frequency of 0.35 Hz (Hertz) (ii) Golay cell

The Golay cell is a pneumatic device similar to a gas thermometer The ir radiation shining on the blackened sur-face of a sealed cell containing xenon gas causes the gas to expand and distort a diaphragm, a part of the cell wall The moving diaphragm may be coupled to one plate of a capaci-tor transducing an ir intensity to a capacitance In another mechanism the beam of ir radiation is reflected from the mir-rored diaphragm surface to impinge on a photocell The area

of coverage of the beam on the photocell changes as a tion of the movement of the diaphragm The intensity of the

func-ir radiation affects the area of the beam that impinges on the photocell and ultimately the magnitude of the photocurrent The ir beam must be optically focused on the detector Its response time is 20 msec Sensitivity is about equal to that

of the thermocouple detector In the far ir it is an excellent detector (see Figure 3)

Toamplifier+

–

FIGURE 8 Thermocouple and preamplifier (Reprinted from Ref (180) With permission from

the Journal of Chemical Education.)

(iii) Pyroelectric detector

The pyroelectric detector is a thin wafer of material

such as, LiTaO 3 and LiNbO 3 , placed between two

elec-trodes to form a capacitor The detector material is a

non-centrosymmetrical crystal whose internal electric field is

changed as a function of temperature when it is below its

Curie temperature This detector is based on the change of

the capacitance of a substance with temperature and is

sen-sitive to the rate of change of the detector temperature The

changing radiation is modulated by chopping or pulsing

because the detector ignores steady unchanging radiation

Therefore this detector has a much faster response time than

those dependent on temperature directly Depending on

the circuit parameters, the response times may be 1 msec

or 10  sec and the responsivity (detector output/incident

radiation) is 100 or 1, respectively This detector has a large

ir range (see Figure 3)

(iv) Photoconductive and photovoltaic detectors

Photoconductive detectors are crystalline

semicon-ductor devices that experience an increase in conductivity

upon interaction with a photon The increase of

conduc-tivity is due to the freeing of bound electrons by energy

absorbed from the radiation A Wheatstone bridge is used

to measure the change in conductance (see following

Section III,B, 2,d ) The semiconductor materials include

the metallic selenides, stibnides or sulfides of cadmium,

gallium, indium or lead

Lead sulfide is commonly used as a detector in the near

infrared, 800 to 2000 nm, where it exhibits a flat response

The cell consists of a thin layer of the compound on a thin

sheet of quartz or glass kept under vacuum

Photovoltaic detectors have been discussed for uv/vis

spectroscopic instruments (see Section III,B,1, b.(4), (a), ( i )).

For ir applications the p -type indium antimonide detector,

cooled by liquid nitrogen, is available with a sensitivity limit

at 5.5  m However the two types of lead in telluride

detec-tors extend the ir range One detector, cooled with liquid

nitrogen, has a range of 5 to 13  m and a second one, cooled

with liquid helium, has a range of 6.6 to 18  m A minimum

response time of 20 nsec (nanosec 109 sec) is achieved

with these detectors

(c) X-ray detectors

Gas-filled and semiconductor detectors and signal

processors and readout used in the measurement of

radioactivity (see Sections III,B, 3, b,( 1 ),(a) and (b) and

(2)) are the applicable in x-ray spectroscopy

(5) Instrument ensembles

The design of an instrument depends on its use and

mon-etary considerations The several main modes of design are

designated temporal, spatial and multiplex In turn each of

these are of the dispersive or nondispersive type. 24

In the temporal category the instrument scans sequentially,

in time, the wavelength in order to determine the intensity

Dispersive systems employ monochrometers that are rotated

so as to position the selected wavelength on an aperature or

slit preceding the sample or detector Nondispersive systems utilize a series of absorption or interference filters that are interchangeable

Spatial systems display the total spectrum with taneous determination of the radiation intensities For a dis-persive instrument a monochrometer provides the dispersed radiation and a multichannel detector to detect their inten-sities Multichannel detectors utilized are a detector array (silicon diode array or vidicon tube), a number of individual detectors properly positioned, or a photographic plate In nondispersive systems the radiation beam is divided into a number of beams and each passes through a unique filter followed by a detector

Multiplex systems employ a single data channel where all the components of the signal are observed simultane-ously A Fourier transform is usually employed to resolve the complex signal into its components requiring the use of a computer There are distinct advantages to Fourier transform spectroscopy: namely, increased S/N ratio, increased energy throughput, large precision in wavelength measurement, and facility in its use However, thus far the instrument is costly

to acquire and maintain No further comment will be made

in this article about these instruments. 25,26 Dispersion instruments give more spectral detail because the wavelength selected has a narrower bandwidth wave-length spread However non-dispersive instruments are usu-ally cheaper, more rugged and have a higher signal to noise ratio Filter instruments are used frequently in monitoring equipment

(6) Absorption instrumentation

In the absorption process, radiation passes through the sample and a specific pattern of absorption of different wavelengths occurs leading to a spectrum for that sample For each wavelength the amount of light absorbed will differ and therefore the amount of transmitted light vary for each wavelength The intensity of transmitted light is inversely proportional to the concentration of sample and is measured

in absorbance, Ab, units The spectrum is the qualitative factor of identification while the intensity of the transmit-ted radiation is the quantitative measure Light is absorbed

in the uv and visible region by electrons in the atoms or molecules of a sample Some elements are identified by atomic absorption, AA, and some functional groups and species by uv/vis spectroscopy Absorption in the ri is due

to vibrational and rotational activity of atoms in lar groupings, such as functional groups, double, triple, and conjugated bonds, etc

The spectroscopic curve or an instrument reading provides

an absorbance value for a chosen wavelength from which the concentration of the absorbing substance can be computed The absorbance value represents the degree of attenuation of the radiation of specific wavelength by absorbing substances

in the sample solution in the cell A constant, the absorptivity,

a, or molar absorptivity, e, can be calculated for a pure

sub-stance for a given wavelength and solvent The mathematical

relationship between the concentration of a substance, C, the

path length of the sample cell, b, and its absorbance units, Ab,

is expressed as follows:

molar absorptivity, e  Ab /( b, cm)( C, moles/L) (9)

absorptivity, a  Ab /( b, cm)( C, g/L). (10)

The absorptivity, a, can be expressed in a number of

con-centration terms (g/L, grams/Liter and mg/ml, milligrams/

milliliter) and path length terms (cm, centimeters and mm,

millimeters)

Equations 9 and 10 are a statement of Beer’s Law indicating

a linear relationship between the absorbance and the

con-centration for fixed conditions In analytical determinations

the concentration can be calculated using equations 9 or 10

Also used in analysis is a calibration curve, whose slope is

absorptivity It can be drawn using concentrations and

cor-responding absorbance values Beer’s Law prevails for many

substances, but there are deviations for some substances due

to chemical, instrumental, and physical phenomena

(a) Uv/visible instrumentation 27

A uv/visible instrument consists mainly of uv and

visi-ble energy sources (lamps), a wavelength selector (grating,

prism or filter), reference and/or sample cell, a detector (photodetector or photomultiplier) and a readout device (recorder, analog or digital meter, etc.) (see Figure 4A) Figures 9 and 10 illustrate the arrangement of these parts for photometers and spectrophotometers, respectively The distinction between the two types of instruments is that a photometer uses a filter and a spectro photometer a grat-ing or prism as a wavelength selector In Figure 9 the dif-ference between a single and double beam instrument is shown and the arrangement also refers to a spectrophotom-eter The various parts that transmit the light beam such as lens, cells, mirrors, transmission gratings and prisms must

be transparent to uv light and be fabricated of fused silica

or quartz Flint glass can be used in the visible region (b) Infrared instrumentation 28

Figure 11 is a schematic of a double beam ir tometer The parts and functions are similar to a un/visible instrument, however the arrangement differs—the light beam passes through the sample and then the wavelength selector

spectropho-in contradistspectropho-inction to the uv/visible spectropho-instrument (see Figures 4A, 9, and 10) Materials transparent to ir are the alkali metal chloride, bromide, and iodide salts

Filters Grating

Visible source

UV Source

Slits

Sample Compartment

Focal Point Detector

Tungsten lamp

Deuterium lamp

Concave grating Sector

mirror Sample

Reference Photomultiplier

Grid mirror

(a)

(b)

FIGURE 9 Single- and double-beam uv-visible spectrophotometers

(a) Beckman DU R Series 60, single-beam (Courtesy of Beckman Instruments, Inc., Fullerton, CA.) (b) Hitachi Model 100–60, double-beam (Courtesy of Hitachi Instruments, Inc., Danbury, CT.)

(c) Atomic absorption instrumentation 29

A schematic for a single beam atomic absorption

spectrophotometer is given in Figure 12 Both flame and

electrothermal atomizers may be utilized in this

instrumenta-tion (see Figure 4A) The hollow cathode lamp is the energy

source generating uv or vis radiation that passes through

the sample The flame or thermal area of the electrothermal

device acts as the sample cell where the sample solution that

has been nebulized (formed into a fine aerosol) is atomized

(Atomization is the formation of free atoms through thermal

energy in the flame or thermal area.) The flame is generated

from various fuel mixtures: acetylene and the oxidants air,

oxygen or nitrous oxide; hydrogen and the aforementioned

oxidants; and natural gas and air or oxygen Each of these

mixtures as well as the fuel/oxidant ratio determines the

flame temperature, a critical condition for atomization

(7) Emission instrumentation 30,31

Emission of emr by elements and some chemical

enti-ties, energized by flames, plasmas, and arc, is the basis of

emission spectroscopy The electrons are energized and

move to higher energy levels on absorption of the energy

On relaxation, the electron returns to a lower energy level

and the absorbed energy is emitted as radiation

Emission methods give rise to atomic spectra by a series

of atomization techniques: namely, flame, inductively pled argon plasma (ICP), electric arc and spark, and direct current argon plasma, DCP (see Figure 4B) An emission spectrophotometer capable of using plasma and arc and spark sources is illustrated in Figure 13

(8) Photoluminescence instrumentation The occurrence of fluorescence and phosphorescence (photoluminescence) refer to substances which on excitation

by radiation emit light on relaxation of the excited species

In resonance fluorescence the wavelengths of excitation and emission are the same However, in many cases the wave-length of emitted radiation is longer than that of the exciting radiation The difference in fluorescence and phosphores-cence is the time delay between excitation and emission The former is quite small (
106 sec), while the latter has a time delay of several seconds or longer Fluorescence can occur in

a number of organic and metal-organic complex molecules, and gases on excitation with uv light

(a) Uv/visible 32

A general schematic for this instrumentation is given in Figure 4C Fluorescence of molecular substances is measured

Field lens Entrance slit

Objective lens

Grating

Wavelength cam Light control

Exit slit

Occluder Sample

Sample Beam

Source

Comb M5

M6 M8

M7

M9

G2 G1 M12

M13

S2 S1 Filters

Thermocouple M14

C +

Sampling Area

in a fluorometer (see Figure 14) or a spectrofluorometer (see

Figure 15)

(b) Atomic 33,34

Atomic fluorescence is a fairly new instrumental

method that has been used for environmental samples The

instrumental arrangement is given in Figure 4C Because

of the dearth of commercial sources of the instrument and

no large benefits compared to other atomic spectroscopic instruments there is a small number of literature citations for its use

(c) X-ray 35 X-ray fluorescence spectroscopic instrumentation uti-lizes an x-ray source, i.e., x-ray tube or radioactive source,

to energize electrons of the inner orbitals of atoms which on

+–

Power supply

Motor Fuel Sample Oxygen

Flame

Rotating chopper

Hollow cathode tube

FIGURE 12 Components of an atomic absorption spectrophotometer The flame may be replaced by a furnace (Reprinted from Ref (176), p 464 by permission of Prentice Hall, Inc., Englewoods Cliffs, NJ.)

Measuring electronics

Microprocessor A/D

Quartz window Lens

Aperture Pivoted mirror

Mirror Moveable slit

Stepper motor

Concave diffraction grating

Mercury lamp

Source

Lens

Prealigned exit slit

Computer

FIGURE 13 A plasma multichannel spectrometer based upon Rowland circle optics (Courtesy of Baird Corp./

IMC Bedford, MA.)

Emission monochromator

Grating

Sample photomultiplier tube White

reflector

Reference photomultiplier tube

Grating

Absorbance compensating cell

Bean splitter

Sample compartment

Xenon lamp Excitation monochromator

FIGURE 15 A spectrofluorometer (Courtesy of SLM Aminco Instruments, Urbana, IL.)

REFERENCE LIGHT P

ATH

PHOTOMULTIPLIER

LIGHT INTERRUPTER

MOUNTING BLOCK LUCITE LIGHT PIPES

FORWARD LIGHT PATH

BLANK SHUTTER BLANK KNOB

DIFFUSE SCREEN

LIGHT SOURCE

relaxation emit fluorescence emission The wavelengths in the

fluorescence emission are unique for different elements This

information may be delineated by wavelength or energy

dis-persion instruments (see Figure 4C) Wavelength dispersion

is carried out with a crystal as a diffraction grating with

sub-sequent detection by a gasfilled detector (see Figure 5), while

energy dispersion may be accomplished by a lithium-drifted

silicon detector and energy-discriminative electronic circuits

(see Figure 16) Non-dispersive instruments use filters

Elements with atomic numbers greater than 7

(oxygen 8) fluoresce on irradiation with x-rays Useful

irradiating wavelengths extend from 0.5 to 2.5 Å Due to

the large absorption of wavelengths greater than 2.5 Å by

air and spectrometer windows, elements of atomic numbers

below 22 (titanium) cannot be detected Elements with lower

atomic numbers can be detected with a change of

atmo-sphere; down to aluminium (atomic no  13) in helium and

to boron (atomic no  5) in a vacuum

(9) Nephelometric & turbidimetric devices 36

The presence of turbidity, suspended matter and

col-loids, in liquid results in the scattering of a beam of incident

light passing through the liquid The scattering process is

elastic; the wavelength of incident and scattered light is the

same Particle shape and size distribution, size relative to the wavelength of the incident light, concentration of particles, and molecular absorption effect the angular distribution of scattered light intensity Since the scattering phenomenon is

so complicated analytical results are empirical depending on the use of standards However, differences in the design of instruments leads to different values for the same standard Turbidimetry refers to the measure of the decrease in the intensity of a beam of light undergoing scatter by suspended

or colloidal particles in a liquid If the transmittance is less than 90%, this method is effective A filter photometer illus-trated in Figure 9 is a suitable instrument

If the intensity of the scattered beam is measured at

an angle to the transmitted beam, then the phenomenon is known as nephelometry Right angle scatter is commonly used for a number of readings, although forward scatter

is more sensitive to large particles Stray light caused by scratches, dirt or condensation on cell walls leads to a positive error Figure 14, a simple fluorometer, measuring scattered light at 90 can be employed for nephelometric measure-ments A surface scatter instrument shown in Figure 17 is used to eliminate stray light No cell is employed since the flowing water sample surface is directly illuminated with the light beam Figure 18 illustrates a low range turbidimeter

Electron column

Si(Li) detector

Energy-to-Video

Keyboard

channel analyzer

Multi-Disk storage system

computer

Mini-FIGURE 16 Components of a typical energy-dispersive microanalysis system The Si(Li) detector is cooled in

a liquid nitrogen cryostat The charge pulse from the Si(Li) detector is converted in the preamp to a step on a

volt-age ramp The pulse processor converts the signal to a well-shaped voltvolt-age pulse with an amplitude proportional

to the energy of the x-ray (Courtesy of the Kevex Instruments, Inc.)

(The names turbidimeter and nephelometer appear to be used

interchangeably to describe a device measuring turbidity from

the intensity of scattered light.) The unit of measurement is

the NTU, nephelometric turbidity unit

Colored constituents in the sample can cause error by

absorbing light A correction can be made in a number of ways:

namely, by using a wavelength of light not absorbed by the

solution, by making an absorption reading of the clarified

solu-tion, or using an instrument that combines both readings. 37

(10) Other spectroscopic instruments

Mass spectrometry is treated in the gas chromatography/

mass spectrometry Part Two Section, 4,c, ( 2 ),( a ) NMR

(nuclear magnetic resonance) spectroscopy is not discussed

in this article The technique is, indeed, a most fruitful

means of identifying chemical entities and their structures

However, the use in water analysis is not a primary activity

In the identification for known natural and anthropogenic

materials its use would be invaluable No doubt the next

edi-tion of this article or an expansion of this article will contain

a section on NMR spectroscopy

(c) Applications of spectroscopic instruments

Standard Methods 2 includes a number of spectroscopic

methods using various instruments for the analysis of metals

(see Table 5) Colorimetric methods using uv/vis absorption

spectroscopy are available in Standard Methods for the

fol-lowing non-metals: bromide, fluoride, iodide, residual

chlo-rine, cyanide, ammonia, nitrate, nitrite, phosphate, sulfide

and sulfite The determination of turbidity in water by

neph-elometry and the analysis of sulfate ion by a turbidimetric

method appears in Standard Methods.2 See Part Two Section C for more applications

2 Electroanalytical instrumentation Electroanalytical chemistry encompasses a wide variety of analytical measurements and includes three different types of correlations The first type concerns the relationship between potential, current, conductance (or resistance), charge (or capac-itance) and the analyte For the second is the determination, during the titration, of the analyte and, ultimately, the endpoint

by electrochemical means The conversion of the analyte by an electric current to a convenient gravimetric or volumetric form

is the third type In this section a number of methodologies and their corresponding instruments will be discussed They include potentiometry, voltammetry, amperometry, coulometry, con-ductance measurements, and titrations using potentiometry and amperometry for endpoint detection

WEIR

SAMPLE OUT

LAMP

LENS

VENT WATER SURFACE

LENS LAMP

LIGHT BEAM

INST.

DRAIN

FLOW DIAGRAM FIGURE 17 Surface scatter turbidimeter (Used with

permission of Hach Co., Loveland, CO.)

Potentiometric Instruments An instrument consisting of

an electrochemical cell containing indicating and reference

electrodes and electronic means for measuring cell

poten-tial to within 0.001 volt (depending on the accuracy desired)

may be optimized to measure concentrations of various ions

and molecules The indicating electrode is the sensor which

gives the specific electrochemical detection of the analyte

in question, while, the function of the reference electrode

is to provide a stable reference potential for the indicating

electrode The potential of the indicating electrode changes

as a function of the concentration of the analyte according to

the Nernst equation,

E  E 0  (0.0591/n)Log α red / α ox (11)

The activities of the reduced and oxidized species are α red

and α ox , respectively, n is the number of electrons in the

redox reaction and E 0 is the standard reduction potential of the redox couple when the activity is one

For species, Sp, its activity, α Sp , is a variable related to its concentration in Moles/L The symbol, [Sp], represents the concentration of Sp in Moles/L The α Sp is defined by the equation,

The activity coefficient, f, varies inversely with the ionic strength of the solution (Ionic strength is a function that can be calculated from the concentration of ions in solution and their ionic charges.) Therefore, at high concentrations

of electrolytes, the ionic strengths are high, and the activity coefficients, small, less than one Activity and concentration are approximately equal when the ionic strength is low and f

is equal to values between 0.90 to 1

TABLE 5 Metals Analysis by Spectroscopy a

Co, Cu, Au, Ir, Fe, Pb

Cd, Cr, Co, Cu, Fe, Pb

Mn, Mo, Ni, Se, Ag, Sn

extraction e

or N2-hydrogen

Inductively Coupled Plasma (atomic emission)

Al, Sb, As, Ba, Be, B, Cd, Ca, Cr, Co, Cu, Fe,

Pb, Li, Mg, Mn, Ni, K, Se, Si, Ag, Na, Sr,

Tl, V, Zn Flame Emission

Li, K, Na, Sr Colorimetric/Spectrophotometric

Al, As, Be, B, Cd, Cr, Cu, Se, Si, Ag, V, Zn

a From Ref (2); b microquantities; c low conc.; d ammonium pyrrolidinedithiocarbamate; e methyl isobutyl ketone;

f 8-hydroxyquinoline.

We shall make a simplifying, practical, and slightly

erroneous substitution replacing activity with concentration

[Moles per liter] as follows:

E  E 0  (0.0591/n)Log[red]/[oxid] (13)

A convenient expression for the log of the concentration of

some species if the p function By definition it is

The most common p function is pH, defined by Log 1/[H]

as measured by a glass electrode (a membrane electrode)

However this function applies to other species, e.g., pOH,

pCa, pNO 3 etc The p functions can replace the log of the

reciprocal of the concentration terms

The specificity of the indicating electrode is determined

by the materials of construction and the structure There are

several types of indicating electrodes, metallic electrodes of

the first, second, and third kinds and membrane (selective)

electrodes including glass, solid-state, liquid membrane, gas

sensing, and enzyme and microbial electrodes The latter

two electrodes can be of the potentiometric and

amperomet-ric types For organizational convenience and simplicity all

these electrodes shall be discussed in this section entitled,

potentiometric instrumentation

(1) Metallic electrodes 12

An electrode of the first kind of a metal wire, rod or

plate in equilibrium with its metallic ion in solution For

example, a silver indicating electrode, a silver wire, in

con-tact with silver ions, Ag, gives the potential mathematically

described by the Nernst equation,

E  E 0  (0.0591/n)Log 1/[Ag] (15a)

e  E 0  (0.0591/n) pAg (15b)

Metallic electrodes of inert metals such as platinum, gold,

palladium, etc and non-metals such as carbon and boron

carbide may be used as indicating electrodes when the

redox couples are soluble species as Fe 2/Fe 3 However the

electrode response is not always reversible for some

com-binations of electrodes and redox couples leading to

non-reproducible potentials

An electrode of the second kind consists of a metal

elec-trode whose surface is coated with a slightly soluble salt of

that metal or a metal electrode in contact with a solution

con-taining a low concentration of a complex of that metal An

example of the former is a mercury electrode coated with

slightly soluble mercurous chloride making the electrode

potential sensitive to chloride ion concentration in the

solu-tion Most reference electrodes are of the former type Two

examples are the calomel (mercury/mercurous chloride) and

the silver/silver chloride electrodes Figure 19 is a schematic

of a calomel reference electrode. 38 A solution of known

con-centration of chloride ion bathes the electrode allowing the

potential to remain constant Potassium chloride tions at several different levels, 0.1 or 1.0 M or saturated are often used providing several different reference electrodes

concentra-As predicted by the equilibrium and Nernst equations for the calomel reference electrode (see equations 16a and 16b), different chloride ion concentrations will result in different potentials

(16a)

E  E 0  (0.0591/n)Log [Cl] 2 (16b) Since Hg 2 Cl 2 (s) and Hg(1) are a solid (s) and liquid (1), respectively, insoluble and not in ionic form in the electrode electrolyte (see Figure 19), they are not represented in the Nernst equation (2e represents the number of electrons taking part in the reaction.) Similar equations can be written for the silver/silver chloride reference electrode

A metal electrode made sensitive to a second metal is an electrode of the third kind The metal electrode is immersed

in the analyte solution containing a small concentration of the metal complex and of a similar complex of the second metal The electrode potential is dependent on the concen-tration of the ion of the second metal. 39

(a) Oxidation–reduction potential, ORP 40 The measurement of the ORP of a system can provide valuable information about its oxidative state It must be understood that the ORP value is the overall potential of the system being measured, not necessarily at equilibrium, and very seldom relates to one species With caution in interpreta-tion in mind, the ORP value can be used essentially in two area: biological and chemical inprocessing measurements and control

The ORP measurement is made in a cell using a polarizing indicating electrode, a reference electrode and a suitable potential readout device—an electronic voltmeter The indicating electrode, an electrode of the third kind, is usually a noble metal such as gold or platinum in the form

non-of a wire or button The redox system equilibrates cally with the indicating electrode and the reference elec-trode For the following system,

(17) one can write the reduction potential, E Mb  /Ma , using the Nernst equation, where b and a are the number of charges and b  a, and E 0 is the standard reduction potential of the system

EMb MaE0Mb Ma0.0591/ b( a)log aMa aMb (18)

If one assembles a cell to measure the ORP of this system using an indicating electrode and a standard hydrogen elec-trode, the cell potential value measured is the potential of the