Internal Standard Problem

[eccsmike]

We have purchased our first 6890/5975 GC/MS and would appreciate incite into a strange internal standard problem. The area counts of the internal standard are counts are increasing with increasing mass of target analytes on column. This occurs during calibration and is exhibited throughout the run. This has occurred as we make our first attempt at transferring one of our applications that we have been running successfully for 15 years on 5890/5972 systems. Our application is the analysis of soil and water sample extracts in 80/20 isooctane/acetone for ~60 neutral pesticides using EPA 8270D techniques. We run an 8 point initial calibration curve 10 to 2000 ng/mL using an RTX 200, 30m x 0.32u with .5 film, 2 ul injection. After injecting the standard curve, low level samples have lower IS area counts. We use 2 CCVs at 100 and 1000 ng. The area counts for the CCVs, 10 injections later, essentially match the area counts of like standards in the ICAL. We have tried different liners, a different column, a 1 ul injection, all to no avail. Agilent provided no ideas. The problem has persisted for several weeks without change.

Any thoughts would be appreciated.

Mike Linskens
ECCS
608-221-8700

[Steve Reimer]

With a 2 ul injection, depending on your inlet, you may have a expansion volume problem since you have actone in your sovent. If you are not getting all the water out of extracts you run the risk of overfilling your inlet and pushing some of your sample out the split vent or septum purge. Try reducing your injection volume and see if the problem lessens.
Acetone has a larger expansion volume than isooctane and water is greater still. Removing water from acetone is difficult, so much so that I would avoid using it for a final solvent.

[Don_Hilton]

I assume both instruments have the split/splitless inlet? Or does the newer instrument have the multimode inlet?

Splitless injection? If so, any effect of adjusting the purge off time?

[James_Ball]

I dealt with this same behavior doing Volatiles Purge and Trap analysis when I first started using the 5973MS. I had trouble with my internal standards rising as the calibration curve increased in concentration then after a few samples dropped to normal again. The only thing that solved the problem for us was switching to the ETP multipliers. Switched three volatiles system over and the problem went away, switched our semivolatile systems which run EPA8270 over and they had less problems also.