CP-3800 GC Operator’s Manual
71
1079 Modes of Injection
The 1079 injection mode is defined by the Split Vent program and by choice
of insert. The Split Vent is controller differently based on the pneumatics
installed, if EFC the split valve is controlled from the Injector section, or if
manual pneumatics the external events control the split valve. The following
are brief descriptions of the various modes of injection; more detailed
information on each mode is given later in this section.
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 3.4 mm open insert, the 3.4 mm ID fritted insert, or the packed 3.4
mm ID insert when operating in the split mode.
In the split injection mode, the sample volume is typically 1 µL 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 the sample injection time, (time between the
insertion and removal of the syringe needle from the injector) exceeds the
peak width, peaks can boraden, 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.
Isothermal
Splitless
Mode
The 3.4 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
clean any residual sample out of the injector.
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