Page 43 - Tandem Techniques

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the silica with the trifunctional reagent and then with water, in a similar manner to the synthesis of the
oligomeric phase. However, with the trifunctional silane the polymer is produced as opposed to an
oligomeric chain.

All three types of phases behave differently, particularly when in contact with solvent mixtures of very
high water content [17-19] but have no specific influence on any associated tandem instrument. Reverse
phases are used in over 75% of all LC analyses. However, these stationary phases are almost always
employed with mobile phases consisting of aqueous solvent mixtures. Such solvents can pose certain
difficulties when used with tandem instruments such as the IR spectrometer or the NMR spectrometer.
These difficulties and their solution will be discussed under the appropriate spectroscopic technique.

The characteristics of the LC system that are important for tandem operation are, again, the solute
concentration leaving the column and the volume flow rate. The concentration of solute in the eluting
solvent from a LC column can range from about 5 x 10- g/ml to 1 x 10 g/ml. The eluent
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concentrations that can be used with LC detectors is seen to be several orders greater than that used
with GC detectors. The actual concentration will again depend both on the mass of sample injected and
the dimensions of the column. The column flow rate will range from about 5 µl/min., for a microbore
column about 0.5 mm I.D., to about 2 ml/min., for a standard packed column about 4.6 mm I.D. Again
it is seen that any interface used for a particular tandem instrument will need to be carefully designed to
accommodate both the elution characteristics of the LC column and the inlet requirements enjoined by
the tandem instrument.

Thin Layer Chromatography

Thin layer chromatography (TLC) is basically a form of LSC, but with two distinct differences. The
separation is achieved on a lamina sheet of stationary phase, as opposed to a cylindrical column, and the
mobile phase is not pumped through the system, but allowed to percolate through the stationary phase
layer by surface tension forces. The stationary phase,