D. Harvey - Modern Analytical Chemistry (794078), страница 17
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In this section we will briefly look at how to minimize these two sources of interference.In the absence of an interferent, the total signal measured during an analysis, Smeas, is a sum of the signal due to the analyte, and the signal due to the reagents, SreagSmeas = SA + Sreag = knA + Sreag(total analysis method)Smeas = SA + Sreag = kCA + Sreag(concentration method)3.93.10Without an independent determination of Sreag, equation 3.9 or 3.10 cannot besolved for the moles or concentration of analyte. The contribution of Sreag is determined by measuring the signal for a reagent or method blank that does not containthe sample.
Consider, for example, a procedure in which a 0.1-g sample is dissolvedin 100 mL of solvent. After dissolving the sample, several reagents are added, andthe signal is measured. The reagent blank is prepared by omitting the sample andadding the reagents to 100 mL of solvent. When the sample is a liquid, or is in solution, an equivalent volume of an inert solvent is substituted for the sample. OnceSreag is known, it is easy to correct Smeas for the reagent’s contribution to the overallsignal.Compensating for an interference in the sample’s matrix is more difficult. If theidentity and concentration of the interferent are known, then it can be added to thereagent blank.
In most analyses, however, the identity or concentration of matrixinterferents is not known, and their contribution to Smeas is not included in Sreag. Instead, the signal from the interferent is included as an additional termSmeas = kAnA + kInI + Sreag(total analysis method)3.11Smeas = kACA + kICI + Sreag(concentration method)3.12method blankA sample that contains all componentsof the matrix except the analyte.451400-CH03 9/8/99 3:51 PM Page 4646Modern Analytical ChemistrySolving either equation 3.11 or 3.12 for the amount of analyte can be accomplishedby separating the analyte and interferent before the analysis, thus eliminating theterm for the interferent. Methods for effecting this separation are discussed inChapter 7.Alternatively, equations 3.11 or 3.12 can be solved for the amounts of both theanalyte and the interferent.
To do so, however, we must obtain two independentvalues for Smeas. Using a concentration method as an example, gives two equationsSmeas,1 = kA,1CA + kI,1CI + Sreag,1Smeas,2 = kA,2CA + kI,2CI + Sreag,2that can be solved simultaneously for CA and CI. This treatment is general. Thecomposition of a solution with a total of n analytes and interferents can be determined by measuring n independent signals, and solving n independent simultaneous equations of the general form of equation 3.11 or 3.12.EXAMPLE 3.3A sample was analyzed for the concentration of two analytes, A and B, undertwo sets of conditions. Under condition 1, the calibration sensitivities arekA,1 = 76 ppm–1kB,1 = 186 ppm–1kA,2 = 33 ppm–1kB,2 = 243 ppm–1and for condition 2The signals under the two sets of conditions areSmeas,1 = 33.4Smeas,2 = 29.7Determine the concentration of A and B.
You may assume that Sreag is zerounder both conditions.SOLUTIONUsing equation 3.12, we write the following simultaneous equations33.4 = (76 ppm–1)CA + (186 ppm–1)CB29.7 = (33 ppm–1)CA + (243 ppm–1)CBMultiplying the first equation by the ratio 33/76 gives the two equations as14.5 = (33 ppm–1)CA + (80.8 ppm–1)CB29.7 = (33 ppm–1)CA + (243 ppm–1)CBSubtracting the first equation from the second gives15.2 = (162.2 ppm–1)CBSolving for CB gives the concentration of B as 0.094 ppm. Substituting thisconcentration back into either of the two original equations gives theconcentration of A, CA, as 0.21 ppm.1400-CH03 9/8/99 3:51 PM Page 47Chapter 3 The Language of Analytical Chemistry473E.2 Calibration and StandardizationAnalytical chemists make a distinction between calibration and standardization.9Calibration ensures that the equipment or instrument used to measure the signal isoperating correctly by using a standard known to produce an exact signal.
Balances,for example, are calibrated using a standard weight whose mass can be traced to theinternationally accepted platinum–iridium prototype kilogram.Standardization is the process of experimentally determining the relationship between the signal and the amount of analyte (the value of k in equations3.1 and 3.2). For a total analysis method, standardization is usually defined bythe stoichiometry of the chemical reactions responsible for the signal. For a concentration method, however, the relationship between the signal and the analyte’s concentration is a theoretical function that cannot be calculated withoutexperimental measurements. To standardize a method, the value of k is determined by measuring the signal for one or more standards, each containing aknown concentration of analyte. When several standards with different concentrations of analyte are used, the result is best viewed visually by plotting Smeasversus the concentration of analyte in the standards.
Such a plot is known as acalibration curve. A more detailed discussion of calibration and standardizationis found in Chapter 5.3E.3 SamplingcalibrationThe process of ensuring that the signalmeasured by a piece of equipment or aninstrument is correct.standardizationThe process of establishing therelationship between the amount ofanaltye and a method’s signal.calibration curveThe result of a standardization showinggraphically how a method’s signalchanges with respect to the amount ofanalyte.Selecting an appropriate method helps ensure that an analysis is accurate.
It doesnot guarantee, however, that the result of the analysis will be sufficient to solve theproblem under investigation or that a proposed answer will be correct. These latterconcerns are addressed by carefully collecting the samples to be analyzed.A proper sampling strategy ensures that samples are representative of the material from which they are taken. Biased or nonrepresentative sampling and contamination of samples during or after their collection are two sources of sampling errorthat can lead to significant errors. It is important to realize that sampling errors arecompletely independent of analysis errors.
As a result, sampling errors cannot becorrected by evaluating a reagent blank. A more detailed discussion of sampling isfound in Chapter 7.3E.4 ValidationBefore a procedure can provide useful analytical information, it is necessary todemonstrate that it is capable of providing acceptable results. Validation is an evaluation of whether the precision and accuracy obtained by following the procedureare appropriate for the problem. In addition, validation ensures that the writtenprocedure has sufficient detail so that different analysts or laboratories following thesame procedure obtain comparable results.
Ideally, validation uses a standard sample whose composition closely matches the samples for which the procedure wasdeveloped. The comparison of replicate analyses can be used to evaluate the procedure’s precision and accuracy. Intralaboratory and interlaboratory differences in theprocedure also can be evaluated. In the absence of appropriate standards, accuracycan be evaluated by comparing results obtained with a new method to those obtained using a method of known accuracy.
Chapter 14 provides a more detailed discussion of validation techniques.validationThe process of verifying that a procedureyields acceptable results.1400-CH03 9/8/99 3:51 PM Page 4848Modern Analytical Chemistry3F Protocolsquality assurance and quality controlThose steps taken to ensure that thework conducted in an analytical lab iscapable of producing acceptable results;also known as QA/QC.Earlier we noted that a protocol is a set of stringent written guidelines, specifying anexact procedure that must be followed if results are to be accepted by the agencyspecifying the protocol. Besides all the considerations taken into account when designing the procedure, a protocol also contains very explicit instructions regardinginternal and external quality assurance and quality control (QA/QC) procedures.10Internal QA/QC includes steps taken to ensure that the analytical work in a givenlaboratory is both accurate and precise.
External QA/QC usually involves a processin which the laboratory is certified by an external agency.As an example, we will briefly outline some of the requirements in the Environmental Protection Agency’s Contract Laboratory Program (CLP) protocol forthe analysis of trace metals in aqueous samples by graphite furnace atomic absorption spectrophotometry. The CLP protocol (Figure 3.7) calls for daily standardization with a reagent blank and three standards, one of which is at the laboratory’s contract required detection limit. The resulting calibration curve is thenverified by analyzing initial calibration verification (ICV) and initial calibrationblank (ICB) samples.
The reported concentration of the ICV sample must fallwithin ±10% of the expected concentration. If the concentration falls outside thislimit, the analysis must be stopped and the problem identified and corrected before continuing.After a successful analysis of the ICV and ICB samples, standardization is reverified by analyzing a continuing calibration verification (CCV) sample and a continuing calibration blank (CCB). Results for the CCV also must be within ±10% of theexpected concentration. Again, if the concentration of the CCV falls outside the established limits, the analysis must be stopped, the problem identified and corrected,and the system standardized as described earlier.
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