Chromatography Forum

I am currently trying to validate a method for 71 pesticides using a new Agilent 7000 GC/MS triple quad/GC:7890A.

I am using a system with a multi-mode inlet and blackflush. We are using a cold splitless injection.

My problem is more than half of the calibration curves are quadratic. This would be fine, but those compounds with quadratic curves are showing inconsistent response througout the length of an average run. I can't seem to get a calibration check standard to pass within 20%.

I tried running a sequence with 5 repetitions of: standard, followed by 3 samples. For the quadratic curve compounds, the response for the standard started poor, got better, but then got poor again. I think this is normal, but not to the extent that the injections varied. Some injections of the same standard were even 40% different for those trouble compounds. For the linear ones, the % difference was within 20% without fail.

Has anyone had issues with this and identified the root of the problem? If not, any recommendations on how to remedy this? When Agilent ran some demo standards for us before we bought the instrument, they had the same problem with quadratic curves but told us it was something we could fix in our method. I'm starting to wonder if that's really true.

Thanks so much!
-Chrissy

This one might be worth a discussion with the sales rep. When I did apps work (different company). I don't believe that I ever told the customer that they could do somethign that I was not able to do with their samples.

Chrissy
Could you please write on what is common to the problematic pesticides?
Are those the more polar ones or late to elute pesticides.
Do you see with the problematic compounds some peak tailing and lower TIC intensity?
Possible reasons could be active sites in the backflush device or too low ion source temperature but I am not sure about it.
If you wish to comunicate off-line please write me and possibly send a file of these pesticides and list which are the good and bad and maybe I could come up with a hint to the root of the problem
Best wishes
Amirav
amirav@tau.ac.il

Are these OC/OP pesticides by any chance? Whilst I'm an LCMS guy, I know the GCQQQ method developer I worked alongside had the exact same issues with his Agilent 7000 GCMS instrument. If I remember correctly it was caused by a number of problems including active sites. I believe he reduced the problem by adding triolein ( a triglyceride)to his injections as well as revising his choice of internal standard.

Hope that helps,

Aviator