Page 61 - Modern Analytical Chemistry

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44 Modern Analytical Chemistry

Concentration methods frequently have both lower and upper limits for the
amount of analyte that can be determined. The lower limit is dictated by the small-
est concentration of analyte producing a useful signal and typically is in the parts
per million or parts per billion concentration range. Upper concentration limits
exist when the sensitivity of the analysis decreases at higher concentrations.
An upper concentration level is important because it determines how a sam-
ple with a high concentration of analyte must be treated before the analysis. Con-
sider, for example, a method with an upper concentration limit of 1 ppm (micro-
grams per milliliter). If the method requires a sample of 1 mL, then the upper
limit on the amount of analyte that can be handled is 1 mg. Using Figure 3.6, and
following the diagonal line for 1 mg of analyte, we find that the analysis of an ana-
lyte present at a concentration of 10% w/w requires a sample of only 10 mg! Ex-
tending such an analysis to a major analyte, therefore, requires the ability to ob-
tain and work with very small samples or the ability to dilute the original sample
accurately. Using this example, analyzing a sample for an analyte whose concen-
tration is 10% w/w requires a 10,000-fold dilution. Not surprisingly, concentra-
tion methods are most commonly used for minor, trace, and ultratrace analytes,
in macro and meso samples.

3 7 Equipment, Time, and Cost
D.
Finally, 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 relying
on instrumentation are equipment-intensive and may require significant operator
training. For example, the graphite furnace atomic absorption spectroscopic
method for determining lead levels in water requires a significant capital investment
in the instrument and an experienced operator to obtain reliable results. Other
methods, such as titrimetry, require only simple equipment and reagents and can be
learned quickly.
The time needed to complete an analysis for a single sample is often fairly simi-
lar from method to method. This is somewhat misleading, however, because much
of 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 be
analyzed per hour differs substantially from method to method. This is a significant
factor 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 of
necessary equipment and reagents, the cost of hiring analysts, and the number of
samples that can be processed per hour. In general, methods relying on instruments
cost more per sample than other methods.

3 D.8 Making the Final Choice
Unfortunately, the design criteria discussed earlier are not mutually independent. 8
Working with smaller amounts of analyte or sample, or improving selectivity, often
comes at the expense of precision. Attempts to minimize cost and analysis time may
decrease accuracy. Selecting a specific method requires a careful balance among
these design criteria. Usually, the most important design criterion is accuracy, and
the best method is that capable of producing the most accurate results. When the
need for results is urgent, as is often the case in clinical labs, analysis time may be-
come the critical factor.
The best method is often dictated by the sample’s properties. Analyzing a sam-
ple with a complex matrix may require a method with excellent selectivity to avoid