Instrumental methods of analysis

5.3 Radiant energy and Electromagnetic Spectrum: The Radiant energy can be defined as the form of energy transmitted from one body to another in the form of radiation.. Absorption of ele

5.1 Introduction

An exciting and fascinating part of chemical analysis is the use of instrumentation, which interacts with all the areas of chemistry and with many other fields of pure and applied sciences The instrumental methods of analysis come under the branch of chemistry known as Analytical Chemistry Analytical chemistry may be defined as the science and art

of determining the composition of materials in terms of elements or compounds contained in them

In analytical instrumentation, the term analytical technique refers to a fundamental scientific phenomenon that has proved useful for providing information on the composition

of substances

The instrumentation techniques can be classified in three principal areas:

(a) Spectroscopy(b) Electrochemistry(c) Chromatography The analysis can be classified as:

(1) Qualitative analysis(2) Quantitative analysis

The qualitative analysis measures the property and merely indicates the presence

of analyte in matrix or which reveals the identity of the compounds in a sample The

quantitative analysis is a magnitude of measured property which is proportional to the

concentration of analyte in matrix or which indicates the amount of each substances present

in the sample

These analyses can be performed by two ways namely

(1) Classical Chemical Methods and

Instrumental Methods of Analysis

nuclear magnetic resonance, electron spin resonanceRadiation scattering Turbidity, Raman

Radiation refraction Refractometry, interferometry

Radiation diffraction X- ray, electron

Radiation rotation Polarimetry, circular dichroism

Electrical potential Potentiometry

Electrical charge Coulometry

Electric current Voltammetry – amperometry, polarography

Electrical resistance Conductometry

Mass-to-charge ratio Mass spectrometry

Rate of reaction Stopped flow, flow injection analysis

Thermal Thermal gravimetry, calorimetry

Radioactivity Activation, isotope dilution

5.2 The role of Analytical Instrumental Methods in the field of engineering:

Analysis of a chemical property of a compound of interest varies from the field for which the chemical compound actually finds its application For example in the field of engineering the analysis of hydrocarbons, nitrogen oxides and carbon monoxide present in automobile exhaust gases are measured to asses the effectiveness of smog- control devices Assessment of percentage composition of an inorganic metal in the steel industry is needed to achieve the desired strength, hardness, corrosion resistance and ductility

5.3 Radiant energy and Electromagnetic Spectrum:

The Radiant energy can be defined as the form of energy transmitted from one body

to another in the form of radiation Radiation involves electromagnetic waves of lower wave lengths to higher wave lengths such as γ-rays, X-rays, UV rays, visible spectrum, infra red rays, microwaves and radio waves The frequency and wavelength of electromagnetic radiation vary over many orders of magnitude For convenience; electromagnetic radiation

is divided into different regions based on the type of atomic or molecular transition that gives rise to the absorption or emission of photons The details of the electromagnetic radiations are given in the Fig 5.1

5.4 Spectroscopy

Spectroscopy mainly deals with the interaction of electromagnetic radiation with matter or any chemical substance When different regions of electromagnetic radiation interact with matter of chemical substance, they give rise to different kinds of spectroscopy

Absorption of electromagnetic radiation by the matter in the radio frequency region

can give rise to Nuclear Magnetic Resonance (NMR) or Electron Spin Resonance (ESR) spectroscopy based on the possibility of the resonance Absorption of

electromagnetic radiation by the matter in micro wave region, different rotational levels of molecules give rise to rotational spectroscopy Absorption of infra red radiation by the matter in the infra red region can produce molecular vibrations and hence it is known as

Vibrational spectroscopy Absorption of visible or ultra violet radiation by the matter in

the visible or ultra violet region can produce electronic transitions of atoms or molecules

and hence they are known as Electronic spectroscopy.

X-rays can be produced by the bombardment of metal targets with high speed electrons and the study of absorption, emission or scattering of X-rays by the matter can be

studied which is known as X-ray spectroscopy.

5.5 Visible, UV and IR regions:

The visible light is a form of electromagnetic radiation which is in the region 380 nm-750

nm i.e 3800 A°- 7500 A°, The region of 3800 A° and less than that belongs to the violet region (U.V region), The wave length above the region 7600 A° constitute the infra – red region (IR- region).It is found that all kinds of electromagnetic radiation travel in the same speed i.e the velocity of the light The velocity is related to energy as

Ultra-E = hν = hc/ λ

where ‘E’ is the energy, λ is the wave length and ‘c’ is the velocity of light From the above equation we can infer that lower the energy ‘E’ greater will be the wavelength ‘λ’ The order

of energies of the electromagnetic radiations is given below

γ -rays > X-rays > U.V rays > visible light > Infra red rays > Microwaves > Radio waves

5.6 Interaction of Electromagnetic Radiation with Matter

Whenever electromagnetic radiation interacts with matter one of three things can happen

1 The electromagnetic radiation may undergo surface reflection

All electromagnetic reflections are governed by the same physical laws as reflections

of visible light Optics describes the general laws of reflection and may be applied to all

types of electromagnetic reflections ranging from radio waves to gamma rays

2 The electromagnetic radiation may be transmitted completely through the substance it encounters

If absolutely no energy is absorbed by the material, it is said to be transparent to the radiation The velocity of the radiation is usually slower in the transparent medium and as a

result the radiation usually undergoes refraction

Various materials are transparent at various wavelengths For example, lead glass

is transparent to visible light but not X-rays, whereas several thicknesses of black paper sheets are transparent to X-rays, but not visible light No known material is perfectly transparent

3 The electromagnetic radiation may be totally or partially absorbed by the substance In this process energy is transferred to the absorbing medium and this may cause significant changes to occur within the absorbing medium.

Because of the quantum nature of matter on atomic and molecular scales it has been discovered that energy can only be absorbed at the atomic or molecular level if the

energy of the incident radiation exceeds a specific threshold value Based on the reaction

of the compound to the radiant energy several instruments are designed to study their interaction and they can be classified as:

1 Absorption methods: Absorption spectroscopy

(a) UV spectrophotometer,

(b) IR spectrophotometer, (c) AAS

2 Emission methods: Flame photometry

3 Dispersion and scattering methods

5.6.1 Absorption method

Emission spectra

Ground State Excited state

Atom

EM Radiation

Atom emits particular wavelengths Absorption spectra

Ground State Excited state

Atom

EM Radiation

Atom absorbs particular wavelengths

This method deals with optical methods which are based on the response of a compound / element to radiant energy The response differs with the compounds i.e on exposure to radiant energy that they may absorb, emit or scatter radiation However all these interactions bring about changes in the electronic structure of the compound and the change can be subsequently evaluated Absorption spectrophotometry in the ultra violet and visible region is considered to be one of the oldest physical methods which are used for quantitative analysis and structural analysis It mainly deals with the interaction of radiation with matter

Principle

Absorption spectra arise from transition of an electron or electrons with in a molecule or

an ion from a lower to a higher electronic energy level and the emission spectra due to the reverse type of transition (Fig 5.2) For radiation to cause electronic excitation, it must be

in the respective region of the electro magnetic spectrum

5.6 Visible and Ultra-violet (UV) spectroscopy

When energy is absorbed by a molecule in the U.V region (100 nm-400 nm) or visible region (400 nm- 750 nm) it brings about some changes in the electronic energy of the molecule resulting on electronic transition of valence electrons When an electromagnetic radiation of UV region is made to pass through a compound having multiple bonds in its structure, it is observed that a part of the incident radiation is usually absorbed, and this results invariably in the transitions of valency electrons

5.7 ABSORABANCE, BEER- LAMBERT LAW

The intensity of absorption at maximum value (λ max) is related to the number of impringing photons being absorbed by the molecules Usually, only some of the photons are absorbed by the molecules The fractions of photons being absorbed at a given frequency depends on.

(a) The nature of the absorbing molecules;

(b) The concentration of the molecules The higher the concentration, the more molecules are present to absorb the photons;

(c) The length of the path of the radiation through the material The longer the path, the larger the number of molecules exposed and hence, greater the probability that a given photon will be absorbed

Absorbing molecules (pure liquid or solution)

Incident light, I 0 ℓ Transmitted light, I t

Light source

The absorption of light in the visible and near UV regions by a solution is governed by a photophysical law, known as the Lambert-Beer law

Lambert - Beer law:

When a beam of monochromatic light of intensity I is passed through a solution of concentration, C molar and thickness, dx, then intensity of transmitted light changes (due to absorption) by dI Then, probability of absorption of radiation is given by:

d I / I = - KC dx where K is the proportionality constant.

On integrating the above expression, between limits I = I0 at x = 0 and I = I at x = ℓ, we get:

∫

I

dx KC I

dI

or Kcl I

Io

KCl I

Io

=

log 303 2

where ∈ = K/ 2.303 is called the molar absorptivity coefficient, and log I0 / I = A is called the absorbance.

which is Beer-Lambert’s Law, Thus’’ the absorbance (A) is directly proportional :

(i) to the molar concentration (C), as well as (ii) to the path length (ℓ)

substance is governed by Beer- Lamberts Law

5.9 Instrumentation-(Colorimetry)

There are five basic parts to a spectrophotometer The source provides radiation over

the wavelength range of interest White light from the source is passed through a

wavelength selector that provides a limited band of wavelengths The sample holder for analyte The radiation exiting the wavelength selector is focused on to a detector which

converts the radiation into electrical signals Finally the selected signal is amplified and

processed as either an analog or a digital signal (display) We will consider each of the

components separately

Fig.5.3 The general block diagram of a simple colorimeter

1 Light Source:

The source used in UV- spectroscopy should meet the following criteria

a) Beam produced should be in the detectable and measurable range

b) It should save as a continuous source of energy

Since incandescent tungsten filament lamp, is found to satisfy these needs , it is widely used The other source generally used is Tungsten filament incandescent lamp, hydrogen / deuterium discharge lamp and hydrogen gas lamps Tungsten filament incandescent lamps are used in the visible and adjacent parts of ultraviolet and infrared regions Hydrogen or deuterium lamps are used in the wavelength from 160 to 360nm Deuterium lamps provide maximum intensity

2 Monochromators

Filters and Monochromators filter the energy source in such a way that a limited portion is allowed to be incident in the sample Filters allow a wider bound of energy to pass through and they are used in filter photometers whereas, monochromators find their application in spectrophotometers

3 Sample holder:

The selection of material from which the cuvette is constructed is based on the selected range of measurement while its thickness depends on the read intensity of absorption Cuvetts with varied shapes are used (rectangular, cylindrical or cylindrical with flat ends) However, the main factor is that the windows of the Cuvetts should be normal to the beam direction Requirement of Cuvetts in terms of its make and thickness are as follows

UV region – quartz

Visible region – Glass absorption cells, silica cells and plastic containers

Cell thickness – 1, 2 and 5 cm

4 Photometer / Detector:

The mechanism behind the photoelectric devices is the conversion of radiant energy

to electrical signal

Basically 3 types of photometers are used:

a) Photovoltaic cells in which we detect the radiant energy by the current generation between the semiconductor and metal

b) Phototubes in which the energy absorption induces the solid surface to emit electrons and

c) Photoconductive cells in which the absorbed energy changes the electrical resistance

5 Signal Processing:

The electrical signal generated by the transducer is sent to a signal processor where

it is displayed in a more convenient form for the analyst Currently, most spectrometers come either with built-in processors or provision for interfacing to a personal computer

5.10.1 Single beam instruments and double beam instruments(UV-VIS)

The instruments currently used for UV/Vis absorption is the filter photometer which are shown in the following Fig 5.4 The filter is placed between the source and sample to

prevent the sample from decomposing when exposed to high energy radiation A filter photometer has a single optical path between the source and detector and is called a single –beam instrument Fig.5.4 shows the optical diagram of a single beam instrument Radiation from a source passes through the slit into the monochromator A reflection grating diffracts the radiation, and the selected wavelength band pass through the slit into the sample chamber A solid-state detector converts the intensity into a related electrical signal that is amplified on a digital read out

This type of instruments has the limitations with respect to the bandwidth which is relatively fairly large Hence this instrument is more appropriate for a quantitative analysis than for a qualitative analysis In addition the accuracy of a single beam spectrophotometer

is limited by the stability of its source and detector over time

5.10.2 Double beam Instruments:

Many modern photometers and spectrophotometers are based on a double-beam design; fig-b illustrates a double- beam in-time spectrophotometer in which two beams are formed

by a V shaped mirror called a beam splitter One beam passes through the reference solution to a photo detector and the second simultaneously passes through the sample to a second The outputs are amplified, and their ratio, or the log of their ratio, is obtained electronically or computed and displayed on the out put device

Double beam Instruments offer the advantage that they compensate for all but the most short-term fluctuations in the radiant output of the source They also compensate the wide variations of source intensity with wavelength Furthermore the double-beam design is well suited for continuous recording of absorption species.

or halogens The identification of the absorbing groups is done by comparing the spectrum

of an analyte with those of simple molecules, containing various chromophoric groups

b).Quantitative Analysis.

Absorption spectroscopy based on ultraviolet and visible radiation is one of the most useful tools available to the analyst for quantitative analysis The determination of an analyte’s concentration based on its absorption of UV or visible radiation is one of the most frequently encountered quantitative analytical methods

(i) Environmental Chemistry : To analyse metals in water and waste water

(ii) Clinical Chemistry : Determination of total serum protein, serum