Page 45 - Tandem Techniques

Page 27

spectra are usually obtained by the somewhat tedious 'spot' recovery process.

An excellent treatment of TLC separation techniques and the spectroscopic examination of TLC plates
has been given by Poole [22] and is strongly recommended to those readers interested in TLC or
tandem techniques employing TLC as the primary separation process.

The Function of the Chromatographic Column

During the development of a chromatogram, two processes are active in the column that proceed
concurrently and more or less independently of each other. These processes are common to both GC
and LC columns and have two-dimensional equivalents in TLC.

First, the individual solutes contained in the sample move through the column at different rates, due to
their different distribution coefficients with respect to the stationary phase. This results in the individual
solutes moving away from each other as they move along the column and are thus separated.

Second the individual solute bands begin to spread, due to the various dispersion processes that occur. It
follows that the column must be designed to contain this dispersion so that each solute peak can be
eluted discretely. The spreading of the peak, both in the column and in any interface between the
column and a second instrument, is extremely important in tandem systems. This is so because just as
the peak dispersion must be contained in the column so that the peaks do not merge, so must the
dispersion in the interface between the two instruments be constrained, so that the integrity of the
separation is maintained in the tandem instrument. Furthermore, the dispersion in the column controls
the total volume of the eluted peak.

Thus the sensing cell in the associated instrument will have a maximum permissible volume to ensure
that, at any time, only one solute peak can be present in the cell and be monitored. More than one
sample in the sensing