GC Headspace Smaller on 1st injection in a method

Discussions about GC and other "gas phase" separation techniques.

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Hi

We have a slightly odd effect when doing headspace injections. To explain, we first noticed this when manually injecting samples, due to our autosampler not working at the time.

The GC run time was set 20 minutes, in which we took about 4-5 injections from a single sample, looking for hydrogen, oxygen and nitrogen (takes about 4.5 mins for them to all come off). We take multiple injections per sample due to sample constraints during irradiations.

What we noticed is that our hydrogen peak is significntly smaller for the first injection, but the rest are then reasonably stable. Our autosampler has recently been fixed, so I tried taking multiple injections from some air samples, and the repeatability is excellent. The issue is the software only allows for 1 injection per method, and so we are concerned we are not getting a 'true' hydrogen reading. I checked this by then taking 3 manual injections in one method and the first gave a similar area to the autosampler method. The later two were both the same, however, again increased compared to the first sample. Has anyone come across something similar? Or have an idea what could be causing this?


Our set up is:
Agilent 7980B, w/ PAL RSI 85 autosampler
Argon carrier gas
30m 5x mol sieve column
100uL injections
Here is the image of the chromatagraph for the manual injections (peaks 2.807, 4.933 and 6.661 are air the othe 3 are nitrogen)

Image

This is the chromatagrah from the autosampler injection

Image
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