Chemical Pollutants in Air, Water, Soil, and Solid Wastes Phần pps

PRECISION AND ACCURACY OF ANALYSISQUALITY ASSURANCE AND QUALITY CONTR OL IN ENVIRONMENTAL ANALYSIS Quality assurance and quality control programs mandate that every labora-tory follow a

Handbook of Environmental

Analysis

Water, Soil, and Solid Wastes

Pradyot Patnaik, Ph.D.

LEWIS PUBLISHERS Boca Raton New York London Tokyo

ANALYTICAL TECHNIQUES

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© 1997 by CRC Press LLC

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PRECISION AND ACCURACY OF ANALYSIS

QUALITY ASSURANCE AND QUALITY CONTR OL

IN ENVIRONMENTAL ANALYSIS

Quality assurance and quality control programs mandate that every labora-tory follow a set of well-defined guidelines so as to achieve analytical results at

a high degree of accuracy The term quality assurance refers to a set of principles that are defined, documented, and strictly observed such that the accuracy of results of analysis may be stated with a high level of confidence and legally defensible The quality assurance plan includes documentation of sampling events, receipt of samples in the laboratory, and their relinquish to respective individuals to perform analysis; all of which are recorded on chain-of-custody forms with dates and times, as well as the names and signatures of individuals responsible to perform the tasks The plan, in a broader sense of the term, also includes quality control

The laboratory quality control program has several components: documen-tation of standard operating procedures for all analytical methods, periodic deter-mination of method detection levels for the analytes, preparation of standard calibration curves and daily check of calibration standards, analysis of reagent blank, instrument performance check, determination of precision and accuracy

of analysis, and preparation of control charts Determination of precision and accuracy of analysis and method detection limits are described under separate subheadings in the following sections The other components of the quality control plan are briefly discussed below

The preparation of a standard calibration curve is required for many colori-metric and gas chromatography analyses A fresh calibration check standard at any selected concentration should be prepared daily and analyzed prior to sample analysis If the response for the check standard falls outside of ± 15% standard deviation for the same concentration in the standard calibration curve, then a new calibration curve should be prepared

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AN ALYSIS OF ORGANIC POLLUTANTS

Gas chromatography (GC) is the most common analytical technique for the quantitative determination of organic pollutants in aqueous and nonaqueous

sam-ples In environmental analysis, a very low detection limit is required to determine the pollutants at trace levels Such low detection can be achieved by sample concentration followed by cleanup of the extract to remove interfering substances Sample extractions and cleanup procedures are described in detail in Chapter 5

of Part 1 of this text

Aqueous samples containing volatile organics can be directly analyzed by

GC (without any separate sample extraction steps) interfaced with a purge and trap setup The analytes in the sample are concentrated by the purge and trap technique, (as discussed in the following section), prior to their analysis by GC

or GC/MS The volatile organics in soils, sediments, and solid wastes may be analyzed in a similar way by subjecting an aqueous extract of the sample to purge and trap concentration Alternatively, the analytes may be thermally desorbed from the solid matrices and transported onto the GC column by a carrier gas

At the outset, one must understand certain principles of GC to assess if it is

a proper analytical tool for the purpose If so, how to achieve the best separation and identification of component mixtures in the sample with reasonable precision, accuracy, and speed? And what kind of detector and column should be selected for the purpose? It is, therefore, important to examine the type of compounds that are to be analyzed and certain physical and chemical properties of these compounds Information regarding the structure and the functional groups, ele-mental composition, the polarity in the molecule, its molecular weight, boiling point, and thermal stability are very helpful for achieving the best analysis After

we know these properties, it is very simple to perform the GC analysis of component mixtures To achieve this, just use an appropriate column and a proper detector Properties of columns and detectors are highlighted below in the fol-lowing sections

Efficiency of the chromatographic system can be determined from the number

of theoretical plates per meter Although this term primarily describes the property and resolution efficiency of a column, other extra column variables, such as the

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© 1997 by CRC Press LLC

AN ALYSIS OF ORGANIC POLLUTANTS

BY GAS CHROMATOGRAPHY/

Gas chromatography/mass spectroscopy (GC/MS) is probably the best tech-nique to identify a wide array of unknown organic substances in sample matrices

It is also the most positive confirmatory test to determine the presence of pollut-ants in the sample Its application in environmental analysis has grown up enor-mously in the last decade

The method is based on the principle of chromatographic separation of components of a mixture on a GC column, followed by their identification from their mass spectra The compounds are separated on a suitable GC column, following which, the components eluted from the column are subjected to

elec-tron-impact or chemical ionization The fragmented and molecular ions are iden-tified from their characteristic mass spectra Thus, the substances present in the sample are determined from their characteristic primary and secondary ions and also from their retention times

For the analysis of organic pollutants in environmental samples, U.S EPA has classified them into two categories: volatile organics and semivolatile organics (Figure 1.4.1) The substances in the former category are those that are more or less volatile at ambient temperature and pressure This classification, however, is

based on the analytical technique used rather than the chemical structures of pollutants For example, chloroform and p-xylene are very different in their structures and chemical properties The only property that groups them together

is that both are volatile substances Although the volatile pollutants are expected

to have low boiling points, there are a few compounds such as ethylbenzene, xylenes, or dichlorobenzenes that have boiling points higher than that of water

V OLATILE ORGANICS BY PURGE AND TRAP METHOD

Two techniques may be applied to transfer the volatile analytes from the sample matrices onto the GC column: purge and trap technique and thermal desorption In the purge and trap method, an aliquot of aqueous sample (usually 5 mL for waste-waters and 25 mL for drinking waste-waters) is bubbled through by helium or nitrogen for 11 min The analytes are purged out from the sample and carried over with the purging gas onto a trap consisting of activated charcoal, tenax, and silica gel

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EXTRA CTION OF ORGANIC POLLUTANTS

AND SAMPLE CLEANUP

SAMPLE EXTRACTION

Organic pollutants in potable or nonpotable waters, soils, sediments, sludges, solid wastes, and other matrices must be brought into an appropriate organic solvent for their injection into the gas chromatography (GC) column Such

extrac-tion also enables the increase the concentraextrac-tion of analytes in samples by several order of magnitude for their detection at ppb or ppt level Depending upon the nature of sample matrices, various extraction techniques may be effectively applied for accurate and low level detection of organics These are outlined below

in the following schematic diagram

Figure 1.5.1 Schematic diagram of various extraction techniques.

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TITRIMETRIC ANALYSIS

Titration is one of the most commonly employed techniques in wet analysis Many routine analysis of wastewaters, potable waters, and aqueous extracts of sludges and soils can be effectively performed using various titrimetric techniques

In general, any titrimetric procedure involves slow addition of a solution of accurately known concentraion (a standard solution) to a solution of unknown concentration (sample to be analyzed) until the reaction between both the solutions

is complete In other words, the standard titrant is added slowly up to the point known as end point at which the solute analyte in the sample is completely

con-sumed by the solute in the standard solution The completion of the reaction is usually monitored by using an indictor, which causes a color change at the end point

Titrimetric methods generally employed in environmental analysis may be broadly classified into the following types:

2 General redox titration

3 Iodometric titration

4 Argentometric titration

5 Complexometric titration The above classification highlights the common analytical methods There

is, however, a great deal of overlapping as far as the chemistry of the process is involved For example, iodometric method involves an oxidation-reduction

reac-tion between thiosulfate anion and iodine It is, however, classified here under a separate heading because of its wide application in environmental analysis

Table 1.6.1 highlights some of the aggregate properties and parameters that can be determined by various titrimetric methods

ACID-BASE TITRATION

Acid-base titration involves a neutralization reaction between an acid and a soluble base The reactants may be a strong acid and a strong base, a strong acid

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© 1997 by CRC Press LLC