Page 64 - Modern Analytical Chemistry

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Chapter 3 The Language of Analytical Chemistry 47
3 E.2 Calibration and Standardization

Analytical chemists make a distinction between calibration and standardization. 9
Calibration ensures that the equipment or instrument used to measure the signal is calibration
operating correctly by using a standard known to produce an exact signal. Balances, The process of ensuring that the signal
measured by a piece of equipment or an
for example, are calibrated using a standard weight whose mass can be traced to the
instrument is correct.
internationally accepted platinum–iridium prototype kilogram.
Standardization is the process of experimentally determining the relation-
standardization
ship between the signal and the amount of analyte (the value of k in equations The process of establishing the
3.1 and 3.2). For a total analysis method, standardization is usually defined by relationship between the amount of
the stoichiometry of the chemical reactions responsible for the signal. For a con- analtye and a method’s signal.
centration method, however, the relationship between the signal and the ana-
lyte’s concentration is a theoretical function that cannot be calculated without
experimental measurements. To standardize a method, the value of k is deter-
mined by measuring the signal for one or more standards, each containing a
known concentration of analyte. When several standards with different concen-
trations of analyte are used, the result is best viewed visually by plotting S meas
versus the concentration of analyte in the standards. Such a plot is known as a
calibration curve. A more detailed discussion of calibration and standardization calibration curve
is found in Chapter 5. The result of a standardization showing
graphically how a method’s signal
changes with respect to the amount of
analyte.
3 3 Sampling
E.
Selecting an appropriate method helps ensure that an analysis is accurate. It does
not guarantee, however, that the result of the analysis will be sufficient to solve the
problem under investigation or that a proposed answer will be correct. These latter
concerns are addressed by carefully collecting the samples to be analyzed.
A proper sampling strategy ensures that samples are representative of the mate-
rial from which they are taken. Biased or nonrepresentative sampling and contami-
nation of samples during or after their collection are two sources of sampling error
that can lead to significant errors. It is important to realize that sampling errors are
completely independent of analysis errors. As a result, sampling errors cannot be
corrected by evaluating a reagent blank. A more detailed discussion of sampling is
found in Chapter 7.

3 4 Validation
E.
Before a procedure can provide useful analytical information, it is necessary to
demonstrate that it is capable of providing acceptable results. Validation is an eval- validation
uation of whether the precision and accuracy obtained by following the procedure The process of verifying that a procedure
yields acceptable results.
are appropriate for the problem. In addition, validation ensures that the written
procedure has sufficient detail so that different analysts or laboratories following the
same procedure obtain comparable results. Ideally, validation uses a standard sam-
ple whose composition closely matches the samples for which the procedure was
developed. The comparison of replicate analyses can be used to evaluate the proce-
dure’s precision and accuracy. Intralaboratory and interlaboratory differences in the
procedure also can be evaluated. In the absence of appropriate standards, accuracy
can be evaluated by comparing results obtained with a new method to those ob-
tained using a method of known accuracy. Chapter 14 provides a more detailed dis-
cussion of validation techniques.