Diagnostic:
Heating and temperature control issues
Retention time
and/or selectivity changes which show a diurnal pattern (e.g.,
decreasing gradually during the day and then increasing again at
night), especially when accompanied by baseline drift, are often the
result of laboratory temperature changes, especially for methods run at
"ambient" temperature. Try temperature-controlling the column; it the
results improve, then you had a temperature problem.
Even
temperature-controlled systems can have their problems. If you are
seeing diurnal variations in temperature-controlled systems, consider
placing a temperature probe in contact with the column and monitoring
the column temperature.
Instrument-to-instrument
variations in retention or selectivity are often the result of
differences in heat transfer in different column compartment designs.
In general, "block" designs (in which the column is held in direct
contact with a temperature-controlled metal block) will provide
slightly higher "on-column" temperatures that will "air bath" designs
which depend on air circulation, even if both systems are functioning
properly and maintaining the same environmental temperature (metal
conducts heat better than air!). As with the diurnal variations noted
above, if you suspect temperature problems, you should monitor the
actual column temperature. An even better approach is to mount a
temperature probe in a tee fitting just downstream from the column and
monitor the actual mobile phase temperature.
The higher the
operating temperature, the greater the liklihood of problems cause by
inadequate preheating (i.e., having the incoming mobile phase
significantly cooler than the column inlet). Under these circumstances,
both longitudinal (down the length of the column) and axial (across the
column) temperature gradients may form, with the result that sample
molecules move at different speeds in different parts of the column.
Depending on the details of the gradient and migration velocities, the
problem can show up as tailing, shoulders, flat-top peaks, or split
peaks. Try doubling the length of preheater tubing.
Ref: LC-GC
16
(12) 1080 (1998)
Ref: LC-GC
17(11)
1006 (1999)
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LC Resources, Inc. all rights reserved