Page 54 - Modern Analytical Chemistry

1400-CH03 9/8/99 3:51 PM Page 37

Chapter 3 The Language of Analytical Chemistry 37

Graphite furnace
Techniques
atomic absorption spectroscopy

Pb in Pb in Pb in
Methods
Water Soil Blood

Procedures APHA ASTM

Protocols EPA

Figure 3.1 Figure 3.2
Membrane filter showing colonies of coliform Chart showing hierarchical relationship among a technique, methods using that technique,
bacteria. The number of colonies are counted and and procedures and protocols for one method. (Abbreviations: APHA = American Public
reported as colonies/100 mL of sample. Health Association, ASTM = American Society for Testing Materials, EPA = Environmental
PourRite™ is a trademark of Hach Company/photo Protection Agency)
courtesy of Hach Company.

Finally, a protocol is a set of stringent written guidelines detailing the proce-
1. Identify the problem
dure that must be followed if the agency specifying the protocol is to accept the re-
sults of the analysis. Protocols are commonly encountered when analytical chem- Determine type of information
istry is used to support or define public policy. For purposes of determining lead needed (qualitative,
quantitative, or characterization)
levels in water under the Safe Drinking Water Act, labs follow a protocol specified
by the Environmental Protection Agency. Identify context of the problem
There is an obvious order to these four facets of analytical methodology. Ide-
ally, a protocol uses a previously validated procedure. Before developing and vali-
dating a procedure, a method of analysis must be selected. This requires, in turn, an
initial screening of available techniques to determine those that have the potential 2. Design the experimental
procedure
for monitoring the analyte. We begin by considering a useful way to classify analyti-
cal techniques. Establish design criteria
(accuracy, precision, scale of
operation, sensitivity,
selectivity, cost, speed)
3 C Classifying Analytical Techniques
Identify interferents
Analyzing a sample generates a chemical or physical signal whose magnitude is pro-
Select method
portional to the amount of analyte in the sample. The signal may be anything we
can measure; common examples are mass, volume, and absorbance. For our pur- Establish validation criteria
poses it is convenient to divide analytical techniques into two general classes based
Establish sampling strategy
on whether this signal is proportional to an absolute amount of analyte or a relative
amount of analyte.
Figure 3.3
Consider two graduated cylinders, each containing 0.01 M Cu(NO 3 ) 2 (Fig-
2+
ure 3.4). Cylinder 1 contains 10 mL, or 0.0001 mol, of Cu ; cylinder 2 contains Subsection of the analytical approach to
problem solving (see Figure 1.3), of
2+
20 mL, or 0.0002 mol, of Cu . If a technique responds to the absolute amount of relevance to the selection of a method and
analyte in the sample, then the signal due to the analyte, S A , can be expressed as the design of an analytical procedure.
3.1
S A = kn A
protocol
where n A is the moles or grams of analyte in the sample, and k is a proportionality A set of written guidelines for analyzing
constant. Since cylinder 2 contains twice as many moles of Cu 2+ as cylinder 1, an- a sample specified by an agency.
alyzing the contents of cylinder 2 gives a signal that is twice that of cylinder 1.
signal
An experimental measurement that is
proportional to the amount of analyte (S).