Page 62 - Modern Analytical Chemistry

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Chapter 3 The Language of Analytical Chemistry 45

interferences. Samples in which the analyte is present at a trace or ultratrace con-
centration usually must be analyzed by a concentration method. If the quantity of
sample is limited, then the method must not require large amounts of sample.
Determining the concentration of lead in drinking water requires a method
that can detect lead at the parts per billion concentrations. Selectivity is also im-
portant because other metal ions are present at significantly higher concentrations.
Graphite furnace atomic absorption spectroscopy is a commonly used method for
determining lead levels in drinking water because it meets these specifications. The
same method is also used in determining lead levels in blood, where its ability to
detect low concentrations of lead using a few microliters of sample are important
considerations.

3 E Developing the Procedure

After selecting a method, it is necessary to develop a procedure that will accomplish
the goals of the analysis. In developing the procedure, attention is given to compen-
sating for interferences, selecting and calibrating equipment, standardizing the
method, acquiring a representative sample, and validating the method.

3 E.1 Compensating for Interferences

The accuracy of a method depends on its selectivity for the analyte. Even the best
methods, however, may not be free from interferents that contribute to the mea-
sured signal. Potential interferents may be present in the sample itself or the
reagents used during the analysis. In this section we will briefly look at how to mini-
mize these two sources of interference.
In the absence of an interferent, the total signal measured during an analy-
sis, S meas , is a sum of the signal due to the analyte, and the signal due to the rea-
gents, S reag

S meas = S A + S reag = kn A + S reag (total analysis method) 3.9
(concentration method) 3.10
S meas = S A + S reag = kC A + S reag
Without an independent determination of S reag , equation 3.9 or 3.10 cannot be
solved for the moles or concentration of analyte. The contribution of S reag is deter-
mined by measuring the signal for a reagent or method blank that does not contain method blank
the sample. Consider, for example, a procedure in which a 0.1-g sample is dissolved A sample that contains all components
of the matrix except the analyte.
in 100 mL of solvent. After dissolving the sample, several reagents are added, and
the signal is measured. The reagent blank is prepared by omitting the sample and
adding the reagents to 100 mL of solvent. When the sample is a liquid, or is in solu-
tion, an equivalent volume of an inert solvent is substituted for the sample. Once
S reag is known, it is easy to correct S meas for the reagent’s contribution to the overall
signal.
Compensating for an interference in the sample’s matrix is more difficult. If the
identity and concentration of the interferent are known, then it can be added to the
reagent blank. In most analyses, however, the identity or concentration of matrix
interferents is not known, and their contribution to S meas is not included in S reag. In-
stead, the signal from the interferent is included as an additional term

S meas = k An A + k In I + S reag (total analysis method) 3.11

S meas = k A C A + k I C I + S reag (concentration method) 3.12