D. Harvey - Modern Analytical Chemistry (794078), страница 18
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The CCV and the CCB are analyzed before the first and after the last sample, and after every set of ten samples.Whenever the CCV or the CCB is unacceptable, the results for the most recent setof ten samples are discarded, the system is standardized, and the samples are reanalyzed.
By following this protocol, every result is bound by successful checks on thestandardization. Although not shown in Figure 3.7, the CLP also contains detailedinstructions regarding the analysis of duplicate or split samples and the use of spiketesting for accuracy.3G The Importance of Analytical MethodologyThe importance of analytical methodology is evident when examining the resultsof environmental monitoring programs. The purpose of a monitoring programis to determine the present status of an environmental system and to assess longterm trends in the quality of the system. These are broad and poorly definedgoals. In many cases, such studies are initiated with little thought to the questions the data will be used to answer.
This is not surprising since it can be hardto formulate questions in the absence of initial information about the system.Without careful planning, however, a poor experimental design may result indata that has little value.1400-CH03 9/8/99 3:51 PM Page 49Chapter 3 The Language of Analytical Chemistry49StartStandardizationICV, ICBOK?NoIdentifyandcorrectproblemYesCCV, CCBOK?NoYesRun 10samplesDiscard resultsfor last set ofsamplesCCV, CCBOK?NoYesYesMoresamples?NoEndFigure 3.7Schematic diagram of a portion of the Contract Laboratory Program protocol for the analysis of trace metals by graphite furnace atomicspectrophotometry, as specified by the Environmental Protection Agency.
(Abbreviations: ICV = initial calibration verification; ICB = initial calibration blank,CCV = continuing calibration verification, CCB = continuing calibration blank)1400-CH03 9/8/99 3:51 PM Page 5050Modern Analytical ChemistryThese concerns are illustrated by the Chesapeake Bay monitoring program.This research program, designed to study nutrients and toxic pollutants in theChesapeake Bay, was initiated in 1984 as a cooperative venture between the federal government, the state governments of Maryland, Virginia, and Pennsylvania,and the District of Columbia.
A 1989 review of some of the problems with thisprogram highlights the difficulties common to many monitoring programs.11At the beginning of the Chesapeake Bay monitoring program, little attentionwas given to the proper choice of analytical methods, in large part because theintended uses of the monitoring data were not specified. The analytical methodsinitially chosen were those standard methods already approved by the EPA.
Inmany cases these methods proved to be of little value for this monitoring project.Most of the EPA-approved methods were designed to detect pollutants at theirlegally mandated maximum allowed concentrations. The concentrations of thesecontaminants in natural waters, however, are often well below the detection limitof the EPA methods. For example, the EPA-approved standard method for phosphate had a detection limit of 7.5 ppb.
Since actual phosphate concentrations inChesapeake Bay usually were below the EPA detection limit, the EPA methodprovided no useful information. On the other hand, a nonapproved variant ofthe EPA method commonly used in chemical oceanography had a detection limitof 0.06 ppb. In other cases, such as the elemental analysis for particulate forms ofcarbon, nitrogen, and phosphorus, EPA-approved procedures provided poorerreproducibility than nonapproved methods.3H KEY TERMSaccuracy (p. 38)analysis (p. 36)analytes (p. 36)calibration (p.
47)calibration curve (p. 47)concentration techniques (p. 38)detection limit (p. 39)determination (p. 36)matrix (p. 36)measurement (p. 36)method (p. 36)method blank (p. 45)precision (p. 39)procedure (p. 36)protocol (p. 37)quality assurance and qualitycontrol (p. 48)robust (p. 42)rugged (p. 42)selectivity (p. 40)selectivity coefficient (p. 40)sensitivity (p. 39)signal (p. 37)standardization (p.
47)technique (p. 36)total analysis techniques (p. 38)validation (p. 47)3I SUMMARYEvery discipline has its own terminology. Your success in studyinganalytical chemistry will improve if you master the language usedby analytical chemists. Be sure that you understand the differencebetween an analyte and its matrix, a technique and a method, aprocedure and a protocol, and a total analysis technique and aconcentration technique.An analytical method is selected on the basis of criteria such asaccuracy, precision, sensitivity, selectivity, robustness, ruggedness,the amount of available sample, the amount of analyte in the sam-ple, time, cost, and the availability of equipment.
These criteria arenot mutually independent, and it often is necessary to find an acceptable balance among them.In developing a procedure or protocol, consideration is givento compensating for interferences, calibrating equipment andstandardizing the method, obtaining an appropriate sample, andvalidating the analysis. Poorly designed procedures and protocolsproduce results that are insufficient to meet the needs of theanalysis.1400-CH03 9/8/99 3:51 PM Page 51Chapter 3 The Language of Analytical Chemistry513J PROBLEMS1.
When working with a solid sample, it often is necessary tobring the analyte into solution by dissolving the sample in asuitable solvent. Any solid impurities that remain areremoved by filtration before continuing with the analysis.In a typical total analysis method, the procedure mightreadAfter dissolving the sample in a beaker, remove anysolid impurities by passing the solution containingthe analyte through filter paper, collecting thesolution in a clean Erlenmeyer flask. Rinse the beakerwith several small portions of solvent, passing theserinsings through the filter paper, and collecting themin the same Erlenmeyer flask. Finally, rinse the filterpaper with several portions of solvent, collecting therinsings in the same Erlenmeyer flask.For a typical concentration method, however, the proceduremight stateAfter dissolving the sample in a beaker, removeany solid impurities by filtering a portion of thesolution containing the analyte.
Collect and discardthe first several milliliters of solution before collectinga sample of approximately 5 mL for further analysis.Explain why these two procedures are different.2. A certain concentration method works best when the analyte’sconcentration is approximately 10 ppb.a. If the sampling volume for the method is 0.5 mL, aboutwhat mass of analyte is being measured?b.
If the analyte is present at 10% w/v, how would youprepare the sample for analysis?c. Repeat for the case in which the analyte is present at10% w/w.d. Based on your results, comment on the suitability of thismethod for the analysis of a major analyte.3. An analyst needs to evaluate the potential effect of aninterferent, I, on the quantitative analysis for an analyte, A. Shebegins by measuring the signal for a sample in which theinterferent is absent and the analyte is present with aconcentration of 15 ppm, obtaining an average signal of 23.3(arbitrary units). When analyzing a sample in which theanalyte is absent and the interferent is present with aconcentration of 25 ppm, she obtains an average signal of 13.7.a.
What is the analyte’s sensitivity?b. What is the interferent’s sensitivity?c. What is the value of the selectivity coefficient?d. Is the method more selective for the analyte or theinterferent?e. What is the maximum concentration of interferent relativeto that of the analyte (i.e., [interferent]/[analyte]), if theerror in the analysis is to be less than 1%?4. A sample was analyzed to determine the concentration of ananalyte. Under the conditions of the analysis, the sensitivity is17.2 ppm–1.
What is the analyte’s concentration if Smeas is 35.2and Sreag is 0.6?5. A method for the analysis of Ca2+ in water suffers from aninterference in the presence of Zn2+. When the concentrationof Ca2+ is 50 times greater than that of Zn2+, an analysis forCa2+ gives a relative error of –2.0%. What is the value of theselectivity coefficient for this method?6. The quantitative analysis for reduced glutathione in blood iscomplicated by the presence of many potential interferents.In one study, when analyzing a solution of 10-ppbglutathione and 1.5-ppb ascorbic acid, the signal was 5.43times greater than that obtained for the analysis of 10-ppbglutathione.12 What is the selectivity coefficient for thisanalysis? The same study found that when analyzing asolution of 350-ppb methionine and 10-ppb glutathione thesignal was 0.906 times less than that obtained for the analysisof 10 ppb-glutathione.
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