|The liquid sample can be seen
in this photo of a 10-microliter GC syringe between approximately the 2
and 3 microliter marks. The narrow steel plunger is to the right and not
The photo shows the result of loading first air, then sample, and finally more air. This assures that there is no sample in the needle during insertion into the hot injector (which would volatilize and lead to pre-injection of part of the sample, resulting in doubled peaks) and that there is a backing of air to blow all of the sample into the injector quickly.
|This shows a loaded syringe being
positioned by the back injector of the dual injector HP 6890 gas chromatograph.
For clarity in the photo the student is posing with one hand, he will use both hands while inserting the syringe - one holding the narrow metal plunger (which would blow out due to carrier gas pressure upon piercing the rubber septum) and the other grasping the glass barrel.
The syringe needle is fully inserted prior to injection.
Determine as best you can how much run time to allow - or how many peaks to expect. Thoughtless use of the instrument wastes time: stopping the run after 15 minutes when the last compound came off in 2 minutes is an obvious waste, but stopping too early can be just as bad. If a run is stopped early a compound (or more) is still on the column and will come out later, spoiling the next run and both runs must then be repeated. Ask for advice if you're not sure, but be aware this is not an instrument which will stop itself, beep and spit the data out.
Take care not to bend the syringe plunger when you insert the sample.
Rinse the syringe 5 times between injections with the anhydrous ethyl ether provided.
Tell the instructor if there seem to be problems - steadily increasing retention times can indicate that the septum needs to be replaced, for example.
©2001,2002 Daniel A. Straus
|A look inside the GC oven. Notice the coils.|
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