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tures will result in higher bleed levels. To minimize
column bleed, proper conditioning procedures
must be followed when installing a new column.
New columns generally are pre-conditioned by the
manufacturer and should not require a significant
amount of conditioning unless they are installed in
highly sensitive detectors.
Conditioning columns at high temperatures actually
can damage the column if the carrier gas contains
trace amounts of oxygen or there are leaks in the
carrier gas lines. These can oxidize the stationary
phase and cause column bleed. To demonstrate
this, we introduced increasing volumes of room air
onto an Rtx® -5 capillary column at 360°C; notice
the elevated baseline after every injection of
air(Figure 4). After exposure to oxygen, several
hours of continued conditioning with clean gas
were needed to return the baseline to its original
level. If trace amounts of oxygen are introduced
continuously onto the column through carrier gas
impurities or leaks, the baseline may never return
to its original level. Therefore, it is imperative to
use high-quality oxygen and moisture traps on all
carrier gas lines and to thoroughly check for leaks
using an electronic leak detector before condition-
ing the column.
Because oxidation of the phase is related to condi-
tioning temperatures, keep them as low as possible.
Figure 4
Trace amounts of oxygen in the carrier gas will cause an elevated baseline.
GC_EX00389
0 min. 50 100 150 200 250 300 350
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Condition columns at 20°C above the final tempera-
ture of the analysis or at the maximum operating
temperature of the column, whichever is lower. For
low-polarity phases such as Rtx ®-1 (100%
dimethylpolysiloxane) and Rtx® -5 (5%
diphenyl/95% dimethyl polysiloxane) columns, oxy-
gen will not begin to damage the phase until above
270°C. Therefore, longer conditioning times at
lower temperatures may be more effective in reduc-
ing column bleed without risking damage from oxy-
gen in the carrier gas.
Does sample contamination contribute to
baseline rise?
High molecular weight contaminants in your sam-
ples can cause the baseline to rise during tempera-
ture programming. Conditioning the column usually
can remove these contaminants, but prolonged
bake-out at high temperatures increases the risk of
phase oxidation. Solvent rinsing is an alternative
that can remove high molecular weight contami-
nants without the need for high-temperature condi-
tioning.
Summary
Controlling baseline rise is an important factor in
achieving accurate quantitative and qualitative chro-
matographic analyses. The analytical column is not
the only cause of this problem. The injector and
detector also can contribute to baseline rise.
Proper conditioning can reduce column bleed but,
first, care must be taken to eliminate trace amounts
of oxygen and leaks in the carrier gas line. Sample
contamination in the analytical column should be
reduced or eliminated by solvent rinsing. In addi-
tion, regular maintenance for the injector and
detector are necessary for accurate analyses.
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250
200
150
100
50
picoamps
0.5µL-4.0µL on-column injections
of air; Oven temp.: isothermal to
360°C