D. Harvey - Modern Analytical Chemistry (794078), страница 16
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Problems with selectivity become even greater when the analyte is present at a very low concentration.63D.5 Robustness and RuggednessrobustA method that can be applied to analytesin a wide variety of matrices isconsidered robust.ruggedA method that is insensitive to changesin experimental conditions is consideredrugged.For a method to be useful it must provide reliable results. Unfortunately, methodsare subject to a variety of chemical and physical interferences that contribute uncertainty to the analysis. When a method is relatively free from chemical interferences,it can be applied to the determination of analytes in a wide variety of sample matrices. Such methods are considered robust.Random variations in experimental conditions also introduce uncertainty. If amethod’s sensitivity is highly dependent on experimental conditions, such as temperature, acidity, or reaction time, then slight changes in those conditions may leadto significantly different results.
A rugged method is relatively insensitive to changesin experimental conditions.3D.6 Scale of OperationAnother way to narrow the choice of methods is to consider the scale on which theanalysis must be conducted. Three limitations of particular importance are theamount of sample available for the analysis, the concentration of analyte in thesample, and the absolute amount of analyte needed to obtain a measurable signal.The first and second limitations define the scale of operations shown in Figure 3.6;the last limitation positions a method within the scale of operations.7The scale of operations in Figure 3.6 shows the analyte’s concentration inweight percent on the y-axis and the sample’s size on the x-axis.
For convenience,we divide analytes into major (>1% w/w), minor (0.01% w/w – 1% w/w), trace(10–7% w/w – 0.01% w/w) and ultratrace (<10–7% w/w) components, and wedivide samples into macro (>0.1 g), meso (10 mg – 100 mg), micro (0.1 mg –10 mg) and ultramicro (<0.1 mg) sample sizes. Note that both the x-axis and they-axis use a logarithmic scale. The analyte’s concentration and the amount of1400-CH03 9/8/99 3:51 PM Page 43Chapter 3 The Language of Analytical Chemistry4310–10%Ultratrace10–9%10–8%10–7%ppb–log(% analyte as %w/w)10–6%10–5%Trace10–4%ppm10–3%1 g sample, 1% analyte10–2%Minor0.1 g sample, 10% analyte0.1%0.01 g sample, 100% analyte1%ggm1m010100%0.1 0.01100 10µg1µg10µg010 gm110%10Major1MacroMesoMicro0.1 0.01100 1010.1 0.01100 101gmgµgngUltramicro–log(Weight of sample)sample used provide a characteristic description for an analysis.
For example,samples in a macro–major analysis weigh more than 0.1 g and contain more than1% analyte.Diagonal lines connecting the two axes show combinations of sample size andconcentration of analyte containing the same absolute amount of analyte.
As shownin Figure 3.6, for example, a 1-g sample containing 1% analyte has the sameamount of analyte (0.010 g) as a 100-mg sample containing 10% analyte or a 10-mgsample containing 100% analyte.Since total analysis methods respond to the absolute amount of analyte in asample, the diagonal lines provide an easy way to define their limitations. Consider,for example, a hypothetical total analysis method for which the minimum detectable signal requires 100 mg of analyte. Using Figure 3.6, the diagonal line representing 100 mg suggests that this method is best suited for macro samples andmajor analytes.
Applying the method to a minor analyte with a concentration of0.1% w/w requires a sample of at least 100 g. Working with a sample of this size israrely practical, however, due to the complications of carrying such a large amountof material through the analysis. Alternatively, the minimum amount of requiredanalyte can be decreased by improving the limitations associated with measuringthe signal.
For example, if the signal is a measurement of mass, a decrease inthe minimum amount of analyte can be accomplished by switching from a conventional analytical balance, which weighs samples to ±0.1 mg, to a semimicro(±0.01 mg) or microbalance (±0.001 mg).Figure 3.6Scale of operation for analytical methods.Adapted from references 7a and 7b.1400-CH03 9/8/99 3:51 PM Page 4444Modern Analytical ChemistryConcentration methods frequently have both lower and upper limits for theamount of analyte that can be determined. The lower limit is dictated by the smallest concentration of analyte producing a useful signal and typically is in the partsper million or parts per billion concentration range. Upper concentration limitsexist when the sensitivity of the analysis decreases at higher concentrations.An upper concentration level is important because it determines how a sample with a high concentration of analyte must be treated before the analysis.
Consider, for example, a method with an upper concentration limit of 1 ppm (micrograms per milliliter). If the method requires a sample of 1 mL, then the upperlimit on the amount of analyte that can be handled is 1 µg. Using Figure 3.6, andfollowing the diagonal line for 1 µg of analyte, we find that the analysis of an analyte present at a concentration of 10% w/w requires a sample of only 10 µg! Extending such an analysis to a major analyte, therefore, requires the ability to obtain and work with very small samples or the ability to dilute the original sampleaccurately.
Using this example, analyzing a sample for an analyte whose concentration is 10% w/w requires a 10,000-fold dilution. Not surprisingly, concentration methods are most commonly used for minor, trace, and ultratrace analytes,in macro and meso samples.3D.7 Equipment, Time, and CostFinally, analytical methods can be compared in terms of their need for equipment,the time required to complete an analysis, and the cost per sample. Methods relyingon instrumentation are equipment-intensive and may require significant operatortraining.
For example, the graphite furnace atomic absorption spectroscopicmethod for determining lead levels in water requires a significant capital investmentin the instrument and an experienced operator to obtain reliable results. Othermethods, such as titrimetry, require only simple equipment and reagents and can belearned quickly.The time needed to complete an analysis for a single sample is often fairly similar from method to method. This is somewhat misleading, however, because muchof this time is spent preparing the solutions and equipment needed for the analysis.Once the solutions and equipment are in place, the number of samples that can beanalyzed per hour differs substantially from method to method. This is a significantfactor in selecting a method for laboratories that handle a high volume of samples.The cost of an analysis is determined by many factors, including the cost ofnecessary equipment and reagents, the cost of hiring analysts, and the number ofsamples that can be processed per hour.
In general, methods relying on instrumentscost more per sample than other methods.3D.8 Making the Final ChoiceUnfortunately, the design criteria discussed earlier are not mutually independent.8Working with smaller amounts of analyte or sample, or improving selectivity, oftencomes at the expense of precision. Attempts to minimize cost and analysis time maydecrease accuracy. Selecting a specific method requires a careful balance amongthese design criteria. Usually, the most important design criterion is accuracy, andthe best method is that capable of producing the most accurate results. When theneed for results is urgent, as is often the case in clinical labs, analysis time may become the critical factor.The best method is often dictated by the sample’s properties.
Analyzing a sample with a complex matrix may require a method with excellent selectivity to avoid1400-CH03 9/8/99 3:51 PM Page 45Chapter 3 The Language of Analytical Chemistryinterferences. Samples in which the analyte is present at a trace or ultratrace concentration usually must be analyzed by a concentration method. If the quantity ofsample is limited, then the method must not require large amounts of sample.Determining the concentration of lead in drinking water requires a methodthat can detect lead at the parts per billion concentrations. Selectivity is also important because other metal ions are present at significantly higher concentrations.Graphite furnace atomic absorption spectroscopy is a commonly used method fordetermining lead levels in drinking water because it meets these specifications. Thesame method is also used in determining lead levels in blood, where its ability todetect low concentrations of lead using a few microliters of sample are importantconsiderations.3E Developing the ProcedureAfter selecting a method, it is necessary to develop a procedure that will accomplishthe goals of the analysis.
In developing the procedure, attention is given to compensating for interferences, selecting and calibrating equipment, standardizing themethod, acquiring a representative sample, and validating the method.3E.1 Compensating for InterferencesThe accuracy of a method depends on its selectivity for the analyte. Even the bestmethods, however, may not be free from interferents that contribute to the measured signal. Potential interferents may be present in the sample itself or thereagents used during the analysis.
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