Chromatographic Methods for Environmental Analysis - Fundamentals of Environmental Sampling and Analysis - page 279

SW-848 methods have also been published for the analysis of Cl ,F ,Br ,NO 3 ,
NO 2 ,PO 3

4 ,SO 2 4 (method 9056), Cl (method 9057), and Cr 2 O 2 7 (method 7199).
Whereas IC is the method of choice for anions, its applications to cations
(metals) analysis are limited in the environmental field. This is because sensitive and
powerful alternative instrumental techniques are readily available, such as those
introduced in Chapter 9 (atomic spectroscopic techniques). Cationic IC has achieved
only certain importance in the analysis of alkali metals (Li þ ,Na þ ,K þ ), alkaline
earth metals (Mg 2þ ,Ca 2þ ,Sr 2þ ,Ba 2þ ), and in the determination of NH 4 -N in
drinking water.
Recent advances in IC include the coupling of IC with ICP or ICP-MS that can
analyze various species of metals including Hg, Se, and As. The speciations of these
three elements are particularly important because of the vast difference in their
toxicity and bioavailability in the ecosystem (refer to Sec. 7.2.3). Although less
environmentally important, there are many other species that can be measured by IC,
including low molecular weight amines, quaternary ammonium compounds,
oxyhalides, weak organic acids, silicates, aliphatic and aromatic sulfonic acids,
carbohydrates, and amino acids.

10.5 PRACTICAL TIPS TO CHROMATOGRAPHIC
METHODS

10.5.1 What Can and Cannot be Done with GC
and HPLC

At this point, a beginning chromatographer perhaps wants to know what types of
chemicals can or cannot be analyzed by a specific chromatographic technique
described above. To some extent, the two major chromatographic techniques GC
and HPLC are complementary. All gases and volatile compounds can be analyzed
by GC, but not by HPLC. Nonvolatile organic compounds and thermally (50-300 C)
unstable compounds cannot be analyzed with GC unless their structures are changed
through derivatization (see Section 7.5). These nonvolatile and thermally unstable
compounds, however, can be measured by HPLC. For some semivolatile compounds
of environmental interest (e.g., PAHs, nitroaromatic compounds, and explosives),
both GC and HPLC can be used. HPLC is preferred in cases when direct analysis of
aqueous sample is needed to avoid time-consuming extraction procedure. In other
cases, GC is the instrument of choice because a variety of more sensitive detectors
are available.
Because volatility is related to boiling point and molecular size, we can extend
the above generalizations to some specific compounds that are of common concern.
Smaller nonionic organics tend to be more volatile, hence organic compounds
(containing up to 25 carbons) can likely be analyzed by GC. For the same reason,
HPLC has the advantage of handling compounds with larger molecular weight
(MW 1000), such as those of biomolecules. Fortunately, most chemicals of

environmental concerns are the small molecules havingMWof less than 500. Besides

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