lower GC results than before

[tiger]

Our GC was down and now reset up. We reedit the method and the chromatograph looks normal. However, after running the same sample, we found that the number now is 20% lower than before. I am sure not due to the sample and previous results are reliable since we did crosscheck. So i have no idea what's going on. I DID calibration after resetting up the method and all the coefficients are higher than 0.99. I really don't want to switch a new column before i can figure out it is due to software issue. Any thoughts are highly appreciated.

[aldehyde]

what are you trying to analyze. what kind of inlet, detector, what vendor is it?

[tiger]

we use Agilent 5890 SERIES II GC-FID cool on column and try to analyze biodiesel sample. The chromatography looks normal. quote="aldehyde"]what are you trying to analyze. what kind of inlet, detector, what vendor is it?[/quote]

[willnatalie]

I'm assuming you are using an auto injection system? Have you tried a different needle to see is the results are the same?

You might also speculate the jet, has it been cleaned recently?

Have you had to increase the H2 flow at all? Increasing the H2 (though I do not think the different would be that big) could decrease your analyte sensitivity.

The last thing I can think of; turn off flows for everything (H2, Air, makeup gas) except the flow though the column. Take a flow meter and measure the flow coming out of the top of the jet. Is the flow the same coming out of the jet as the supposed inlet flow? This would show the sensitivity loss is not due to a leak somewhere in the system.

good luck.