CHROMATOGRAPHY - Introduction to Soil Chemistry - page 191

then a pure sample of that compound can be added (this is called “spiking”)
to the unknown, and a sample of this mixture analyzed. If only one larger peak
is obtained at the specific R f values of both the unknown and the known, they
are the same ( R t can be handled similarly). When mixtures of similar compo-
sition are to be analyzed repetitively by a chromatographic method, it is
common to prepare a list of the compounds that are expected to be present
along with either their R f or R t values. In this way these components can be
“identified.”

9.7. QUANTIFICATION

GC and HPLC allow for ready quantification of the components exiting the
column in that the area under the peak in the chromatograph is proportional
to the amount of component present. However, to make a quantitative analy-
sis, it is essential to have a calibration curve for each component of interest.
This means making solutions of differing concentration, injecting them, and
finding the relationship between peak area and amount of material present in
a manner similar to that described in Chapter 8, Sections 8.8 and 8.9, for col-
orimetric analysis. In many cases the software that control the chromatograph
can be set up to automatically do this analysis.
For TLC quantitative analysis is more difficult, although it can be accom-
plished in some instances. If such an analysis is undertaken, great care must
be taken to ensure that there is an excellent relationship between the spot
characteristic measured and the amount of material present.

9.8. CONCLUSION

Chromatography in its various forms is extremely important in the isolation
and identification of complex mixtures found in the environment and in soil.
It is applied to components that are isolated from soil by extraction. The
extraction medium is important in that it must be compatible with the chro-
matographic method and analyte detection method. Samples for gas chro-
matography must be volatile in that they must be in the gaseous state for
analysis. Samples for HPLC and TLC must be soluble to some minimal extent
in the elutant being used. Detection of components after separation is per-
formed by either optical, spectrophotometric, or electrical methods. In the case
of TLC plates, separated components may be found by spraying the plates with
a reagent that reacts with the component(s) present to produce a spot where
the component has eluted. By chromatographing known compounds, a list of
R f values can be prepared and used to “identify” the components in an
unknown mixture. In the case of GC and HPLC, the same thing can be accom-
plished by preparing a list of R t values for compounds and used to “identify”
components of an unknown mixture. Identification by R f and R t values is not