Introduction to Soil Chemistry

186 chromatography
Retention times can be used for “identification” of eluting compounds in
the same way as with CG [4–6].
9.3.5. High-Performance Liquid Chromatography Applied to Soil
In soil analysis HPLC is used much like GC in that soil is extracted and the
extract, after suitable cleanup and concentration, is analyzed. One major difference
between them is that HPLC does not require that components be in
the gaseous phase. They must, however, be soluble in an elutant that is compatible
with the column and detector being used. Another difference is that
both a syringe and an injector are used to move the sample into the elutant
and onto the column. Detection is commonly by UV absorption, although both
refractive index and conductivity are also commonly used. Conductivity or
other electrical detection methods are used when analysis of ionic species in
soil is carried out [2,6,7].
9.4. THIN-LAYER CHROMATOGRAPHY
Thin-layer chromatography (TLC) is carried out on a thin layer of adsorbant
on a glass or plastic support (other supports have been used). It has sometimes
been referred to as planar chromatography since the separation occurs in a
plane. Paper chromatography, which is carried out using a piece of paper,
usually filter paper, is very similar to thin-layer chromatography and will not
be covered here.
9.4.1. Sample Application
In thin-layer chromatography a 10-mL or larger syringe is often used to place
sample spots on the thin layer before development. Alternatively, a glass capillary
tube or Pasteur pipette may be heated in a burner and pulled to obtain
a fine capillary suitable for spotting (see Figure 9.3).
9.4.2. Mobile Phases
Development of the thin-layer chromatogram is accomplished by placing a
small amount of elutant in the bottom of a suitable container then placing the
spotted thin layer in the container, sealing it, and allowing the elutant to ascend
the layer through capillary action.
9.4.3. Stationary Phases
A thin layer of adsorbant is applied to a support that may be a sheet of glass,
metal, or plastic (Figure 9.4, D). Adsorbants are typically alumina, silica gel,
or cellulose and may be mixed with gypsum to aid in adhering to the support.