Agriculture Reference
In-Depth Information
CHAPTER
9
CHROMATOGRAPHY
Chromatography and its derivatives, such as solid-phase extraction, discussed
in Chapter 7, is a powerful, essential tool for the analysis of soils. Of the
many forms of chromatography that have been developed, gas and high-
performance liquid chromatography are the most commonly used, particularly
in the analysis of soil extracts. Chromatographic methods, as are spectroscopic
methods, are almost exclusively referred to by their acronyms: CG—gas
chromatography, LC—liquid chromatography, HPLC—high-performance
liquid chromatography,
1
TLC—thin-layer chromatography, CE—capillary
electrophoresis. Also, because there are subdivisions to each type of
chromatography, the acronyms are often lengthened, such as GLC—gas-liquid
chromatography (liquid because there is a liquid coating the stationary phase).
Chromatographic methods are also often used as so-called hyphenated
methods, where their output is used as the input for an identification method
such as mass spectroscopy. These “hyphenated” methods are also most often
referred to by their acronym, for example, GC/MS—gas chromatography/mass
spectroscopy and HPLC/MS—high-performance liquid chromatography/mass
spectroscopy. Note that although UV-Vis is hyphenated, it is not a hyphen-
ated method in that it does not consist of two different methods of analysis.
9.1.
FUNDAMENTALS OF CHROMATOGRAPHY
All chromatographic methods function on the same principle, which is the par-
titioning of components of a mixture between two phases: (1) a stationary
phase, which may be a solid, liquid, or gel, and (2) a mobile phase, which may
be gas, liquid, solution, or a varying mixture of solvents. When a mixture is
introduced to a chromatographic system, its components are alternately
adsorbed and deadsorbed, that is, partitioned between, the stationary and
mobile phases. Partitioning is caused by different polarities of the stationary
and mobile phases and the compounds being separated. Compounds in the
mixture have different affinities for the phases and will move at different rates
in the chromatographic system and thus be separated.
1
High-performance liquid chromatography is also called
high-precision
and
high-pressure liquid
chromatography
.
