Packed column injectors consist of carrier gas inlet, a septum, septum purge, injector insert, heater block, and column connection; but the heart of this technological feat is another set of gas lines out of the injector-another path that the vaporized sample can take. This is called the split line or vent.
The manufacturers of these systems design them so that the carrier gas flow onto the column is constant-to maintain the chromatographic requirements of the column and yield reproducible retention times for analytes. At the same time, the amount of gas that goes out the split vent controls the amount of sample that enters the column.
If the split vent is closed, via a computer controlled split valve, then all of the sample introduced into the injector goes on the column.
If the split vent is open then most of the vaporized sample is thrown away to waste via the split vent and only a small portion of the sample is introduced to the column.
And finally, a very neat aspect of this is that the amount of gas exiting the split vent can be varied while keeping the flow onto the column constant.
This means that the AMOUNT of the split (called the split ratio) can be varied. A common split ratio is 50 to 1. That is, for every 50 units of gaseous sample that are thrown away to waste, 1 unit goes on the column.
The analyst keeps careful control of the split ratio so that results from the chromatography can still be quantified.
Chromatographic peaks that show up as, say 2.5 ng of compound X really represent 2.5 x 50 = 125 ng of analyte X in the original sample.
Also notice that this mass (125 nanograms) would have overloaded the column if all of it ended up on the capillary column. A split injection, and no sample dilution required
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