Introduction to Soil Chemistry

thin-layer chromatography 187
They may also include a fluorescent indicator that aids in visualization once
the plate is developed.These adsorbants may also have hydrocarbons attached
to them such that reverse-phase TLC can be carried out.
9.4.4. Detection
Caution: Spraying thin layer plates with visualization reagents should
always be done in a functioning hood. These reagents may be caustic
and toxic, as is the case with sulfuric acid.
Thin-layer plates, once developed and dried, are sprayed with a visualization
reagent that allows the detection of the separated components. The visualization
reagent produces a colored spot where each component of the mixture is
on the plate. For example, a 0.1% solution of ninhydrin (1,2,3-indantrione
monohydrate) in acetone can be sprayed on a plate that has separated amino
acids on it. The amino acids will show up as blue to brown spots on the plate.
Sulfuric acid can be sprayed on a silica or alumina plate and heated to show
the position of organic compounds as charred spots (this reagent cannot be
used on cellulose plates since the whole plate will turn black). Reducing compounds
can be visualized by using a solution of ammonical silver nitrate, which
will produce black spots where the silver is reduced by the reducing compound.
This reagent is particularly sensitive for reducing sugars. There are a
whole host of different visualizing reagent that can be found in the book by
Hellmut et al. [8].
A method for detecting compounds that have UV absorbence is the use of
thin-layer plates that contain a fluorescence dye or indicator. Plates are developed
and then placed under an ultraviolet light. Compounds on the plate
may show up as bright or dark spots depending on their interaction with the
ultraviolet light and the fluorescence produced by the fluorescence indicator
[8,9].
9.4.5. Soil Thin Layer
A variation on thin-layer chromatography is the use of soil thin layers to investigate
the movement and degradation of organic compounds in soil. A soil
is sieved using a number 200 or smaller sieve, and this soil is used to produce
a suspension of soil in distilled water that is then spread on a glass sheet
to produce a soil thin layer. An organic component, often a herbicide, is
spotted on the dried sheet, which is developed using deionized water. The
movement and degradation of the organic compound under these conditions
can then be related to its expected movement and degradation in soil in the
field [10].