User`s guide

Method Development
222
temperatures, up to 300°C and more, for efficient thermal
desorption.
1
As a general policy it is a good idea to use similar sorbents in a tube
so that conditioning the combined packings is facilitated. For
example, if you use both Tenax (polymer, max temp 350°C) and
Carbotrap (carbon, max temp 400°C) in the same tube, you can only
recondition the tube to the lower temperature. If both packings were
carbon you could obviously condition the tube to 400°C without ill
effects.
Determination of Adsorbent Strength
A more accurate approach to tube packing selection is to assess the
adsorbent strength by determining the retention volume for
individual analytes. This may be measured as follows:
A chromatographic column is packed with a known weight of the
adsorbent to be evaluated. It is convenient to use a column 1 metre
long or greater in order to separate the test analyte and any solvent in
which it may be introduced. The column is operated at a range of
temperatures between 150°C and 250°C. The retention volume of
the analyte at each temperature is recorded together with the column
flow rate. As the mass of packing is known, these retention times
and volumes may be converted to specific retention volumes in
liters/gram at each temperature.
If the log of the specific retention volume is plotted against the
reciprocal of the absolute column temperature to give a linear
relationship, this line may be extrapolated to give the log of the
retention volume at 20°C. A graph of this form for heptane adsorbed
on Chromosorb 106 is shown in Figure 64. The retention volumes
for a number of compounds on a Tenax GR column have been
determined. Table 14 shows the extrapolated values for the retention
volumes at 20°C and gives a good illustration of the range of
compounds that may be quantitatively adsorbed on Tenax GR.
1. A. Braithwaite and F.J. Smith, Chromatographic Methods, Fifth
Edition, (Glasgow, 1996), p 225