D. Harvey - Modern Analytical Chemistry (794078), страница 14
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The filter is placed in a dish containing a nutrient broth and incubated. At the end of the incubation period the number of coliform bacterialcolonies in the dish is measured by counting (Figure 3.1). Thus, municipal waterdepartments analyze samples of water to determine the concentration of coliformbacteria by measuring the number of bacterial colonies that form during a specified period of incubation.3B Techniques, Methods, Procedures, and ProtocolstechniqueA chemical or physical principle that canbe used to analyze a sample.methodA means for analyzing a sample for aspecific analyte in a specific matrix.procedureWritten directions outlining how toanalyze a sample.Suppose you are asked to develop a way to determine the concentration of lead indrinking water. How would you approach this problem? To answer this question ithelps to distinguish among four levels of analytical methodology: techniques, methods, procedures, and protocols.1A technique is any chemical or physical principle that can be used to study ananalyte.
Many techniques have been used to determine lead levels.2 For example, ingraphite furnace atomic absorption spectroscopy lead is atomized, and the ability ofthe free atoms to absorb light is measured; thus, both a chemical principle (atomization) and a physical principle (absorption of light) are used in this technique.Chapters 8–13 of this text cover techniques commonly used to analyze samples.A method is the application of a technique for the determination of a specificanalyte in a specific matrix.
As shown in Figure 3.2, the graphite furnace atomic absorption spectroscopic method for determining lead levels in water is different fromthat for the determination of lead in soil or blood. Choosing a method for determining lead in water depends on how the information is to be used and the established design criteria (Figure 3.3). For some analytical problems the best methodmight use graphite furnace atomic absorption spectroscopy, whereas other problems might be more easily solved by using another technique, such as anodic stripping voltammetry or potentiometry with a lead ion-selective electrode.A procedure is a set of written directions detailing how to apply a method to aparticular sample, including information on proper sampling, handling of interferents, and validating results. A method does not necessarily lead to a single procedure, as different analysts or agencies will adapt the method to their specific needs.As shown in Figure 3.2, the American Public Health Agency and the American Society for Testing Materials publish separate procedures for the determination of leadlevels in water.1400-CH03 9/8/99 3:51 PM Page 37Chapter 3 The Language of Analytical ChemistryGraphite furnaceatomic absorption spectroscopyTechniquesPb inWaterMethodsProceduresProtocolsAPHAPb inSoilPb inBloodASTMEPAFigure 3.1Figure 3.2Membrane filter showing colonies of coliformbacteria.
The number of colonies are counted andreported as colonies/100 mL of sample.Chart showing hierarchical relationship among a technique, methods using that technique,and procedures and protocols for one method. (Abbreviations: APHA = American PublicHealth Association, ASTM = American Society for Testing Materials, EPA = EnvironmentalProtection Agency)PourRite™ is a trademark of Hach Company/photocourtesy of Hach Company.Finally, a protocol is a set of stringent written guidelines detailing the procedure that must be followed if the agency specifying the protocol is to accept the results of the analysis. Protocols are commonly encountered when analytical chemistry is used to support or define public policy.
For purposes of determining leadlevels in water under the Safe Drinking Water Act, labs follow a protocol specifiedby the Environmental Protection Agency.There is an obvious order to these four facets of analytical methodology. Ideally, a protocol uses a previously validated procedure. Before developing and validating a procedure, a method of analysis must be selected. This requires, in turn, aninitial screening of available techniques to determine those that have the potentialfor monitoring the analyte.
We begin by considering a useful way to classify analytical techniques.3C Classifying Analytical Techniques1. Identify the problemDetermine type of informationneeded (qualitative,quantitative, or characterization)Identify context of the problem2. Design the experimentalprocedureEstablish design criteria(accuracy, precision, scale ofoperation, sensitivity,selectivity, cost, speed)Identify interferentsAnalyzing a sample generates a chemical or physical signal whose magnitude is proportional to the amount of analyte in the sample.
The signal may be anything wecan measure; common examples are mass, volume, and absorbance. For our purposes it is convenient to divide analytical techniques into two general classes basedon whether this signal is proportional to an absolute amount of analyte or a relativeamount of analyte.Consider two graduated cylinders, each containing 0.01 M Cu(NO3)2 (Figure 3.4).
Cylinder 1 contains 10 mL, or 0.0001 mol, of Cu2+; cylinder 2 contains20 mL, or 0.0002 mol, of Cu2+. If a technique responds to the absolute amount ofanalyte in the sample, then the signal due to the analyte, SA, can be expressed asSA = knA373.1where nA is the moles or grams of analyte in the sample, and k is a proportionalityconstant. Since cylinder 2 contains twice as many moles of Cu2+ as cylinder 1, analyzing the contents of cylinder 2 gives a signal that is twice that of cylinder 1.Select methodEstablish validation criteriaEstablish sampling strategyFigure 3.3Subsection of the analytical approach toproblem solving (see Figure 1.3), ofrelevance to the selection of a method andthe design of an analytical procedure.protocolA set of written guidelines for analyzinga sample specified by an agency.signalAn experimental measurement that isproportional to the amount of analyte (S).1400-CH03 9/8/99 3:51 PM Page 3838Modern Analytical ChemistryA second class of analytical techniques are those that respond to the relativeamount of analyte; thusSA = kCAtotal analysis techniquesA technique in which the signal isproportional to the absolute amount ofanalyte; also called “classical” techniques.concentration techniquesA technique in which the signal isproportional to the analyte’sconcentration; also called “instrumental”techniques.3.2where CA is the concentration of analyte in the sample.
Since the solutions in bothcylinders have the same concentration of Cu2+, their analysis yields identical signals.Techniques responding to the absolute amount of analyte are called totalanalysis techniques. Historically, most early analytical methods used total analysistechniques, hence they are often referred to as “classical” techniques.
Mass, volume,and charge are the most common signals for total analysis techniques, and the corresponding techniques are gravimetry (Chapter 8), titrimetry (Chapter 9), andcoulometry (Chapter 11). With a few exceptions, the signal in a total analysis technique results from one or more chemical reactions involving the analyte. These reactions may involve any combination of precipitation, acid–base, complexation, orredox chemistry.
The stoichiometry of each reaction, however, must be known tosolve equation 3.1 for the moles of analyte.Techniques, such as spectroscopy (Chapter 10), potentiometry (Chapter 11),and voltammetry (Chapter 11), in which the signal is proportional to the relativeamount of analyte in a sample are called concentration techniques. Since mostconcentration techniques rely on measuring an optical or electrical signal, they alsoare known as “instrumental” techniques. For a concentration technique, the relationship between the signal and the analyte is a theoretical function that depends onexperimental conditions and the instrumentation used to measure the signal. Forthis reason the value of k in equation 3.2 must be determined experimentally.3D Selecting an Analytical Method(a)(b)Figure 3.4Graduated cylinders containing 0.01 MCu(NO3)2.
(a) Cylinder 1 contains 10 mL, or0.0001 mol, of Cu2+. (b) Cylinder 2 contains20 mL, or 0.0002 mol, of Cu2+.© David Harvey/Marilyn Culler, photographer.accuracyA measure of the agreement between anexperimental result and its expectedvalue.A method is the application of a technique to a specific analyte in a specific matrix.Methods for determining the concentration of lead in drinking water can be developed using any of the techniques mentioned in the previous section.
Insoluble leadsalts such as PbSO4 and PbCrO4 can form the basis for a gravimetric method. Leadforms several soluble complexes that can be used in a complexation titrimetricmethod or, if the complexes are highly absorbing, in a spectrophotometricmethod. Lead in the gaseous free-atom state can be measured by an atomic absorption spectroscopic method. Finally, the availability of multiple oxidation states(Pb, Pb2+, Pb4+) makes coulometric, potentiometric, and voltammetric methodsfeasible.The requirements of the analysis determine the best method.
In choosing amethod, consideration is given to some or all the following design criteria: accuracy,precision, sensitivity, selectivity, robustness, ruggedness, scale of operation, analysistime, availability of equipment, and cost. Each of these criteria is considered inmore detail in the following sections.3D.1 AccuracyAccuracy is a measure of how closely the result of an experiment agrees with the expected result.
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