Chromatography Forum

I have been running the same method to test for terpene concentrations in a direct injection for months, and have always gotten stable results. However, just recently the results appear to have greatly increased, so that my former standards are now reading off the charts compared to the calibration I initially made. I'm using a Shimadzu QP-2010

A standard of 25 PPM now has TIC areas in the millions, while before it topped out at the low 100k range.

Curiously other methods do not seem affected though I need to do more thorough inquiry. The SIM method I commonly use for other analytes is not showing anything like this.

A split ratio of 250 gives results similar to what 25 used to give. I cleaned the split vent but that wasn't it.


Method is the same as before when I was getting stable results:

Hello

If you're using the same method and you haven't changed any parameters, I'd check:

-Flow on split vent line (perhaps real flow is different than displayed one)
-you mentioned that "A split ratio of 250 gives results similar to what 25 used to give". If there is something wrong with GC/MS I wouldn't expect results exactly 10 times higher or lower (errors are much more scattered) so make sure there is nothing wrong with sample prep - dilutions, pipettes etc

Regards

Tomasz Kubowicz

I'm just running the standard solutions that have been in the freezer the whole time, so there's no sample prep to speak of. The results aren't exactly 10x, that was just my approximation. I'll check the flow like you are saying.

EDIT: An independent flow meter confirmed the flows are what they should be.