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Split Mode
The split injection mode is preferred for the analysis of relatively
concentrated samples. The sample is split in the injector with a
representative portion entering the column. The split injection mode provides
the shortest sampling time which leads to sharp chromatographic peaks.
Use the 4.0 mm open insert, or the packed 4.0 mm ID insert when operating
in the split mode.
In the split injection mode, the sample volume is typically 2
µ
L or less. Early
eluting compounds usually appear as very sharp peaks. In some cases, the
peak width is less than one second. Thus, it is important that you inject the
sample as quickly as possible. If you are using the CP-8400 AutoSampler
use the Standard Split/Splitless mode of injection. If you are using a non-
Varian AutoSampler or you are using the User Defined Mode of operation on
either the CP-8400 or the 8200 AutoSamplers, if the sample injection time
(the time between the insertion and removal of the syringe needle from the
injector) exceeds the peak width, peaks can broaden, tail or chromatographic
performance will be degraded. With broader, later eluting peaks, it is less
important that you inject the sample quickly. The split ratio (fraction of
sample that enters the column) is the ratio of the flow of carrier gas out the
split vent to the flow through the column.
Splitless
Mode
The 4 mm insert packed with glass wool is typically used for isothermal
splitless injection. The small capillary section of the glass insert fits snugly
around the syringe needle and restricts backflash of the sample vapor
during injection. In the splitless injection mode, the sample enters the
column during a variable sampling time at the beginning of the analysis.
This period is typically 30-90 seconds during which there is no flow from the
injector to the split vent. After the sampling time, the injector is vented to
remove any residual solvent and sample out of the injector.
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