peak height variability

[PhilNZ]

I'm having problems with the peak area/height variability of my standards. They won't stay within the expected day-to-day variability. One week they are at a certain height, and the next week they have dropped to about a third of their original size. The next day they increased slightly again, or do whatever else they want to do.

I use an internal standard, and the peak height/area of that stays pretty much the same throughout the weeks and doesn't follow the pattern of increase/decrease of the other peaks as you would expect if something was up with the instrumentation. It would suggest something is up with the standard solution rather than the GC-MS. We use a QP2010 Plus from Shimadzu.

I keep the standards in a -20C but allow them to come up to RT for a few hours. I've even let them warm up over night, and it doesn't seem like the compounds are stuck on the glass surface of the vial.

I've also made up a new standard from scratch and compared that to the old standard solution, but the two came back as the same when I analyzed them side by side.

We use 100 ul BSTFA/TMSI 98:2 for derivatization (dry), then add 1mL isooctane before injection, and we tune the instrument every day.

Has anyone else ever had this issue?

Cheers

[Don_Hilton]

Without a bit more detail I can speculate on a number of possibilities. I have seen variation in response of silylated compunds. It can be reaction conditions, the condition of reagents, time and temperature of the reaction, dirt in the inlet. Column age and condition.

A bit more detail of the types of copunds might help. Also, how do areas look? Do the peaks tail more when they tend to be shorter?

[PhilNZ]

I wasn't sure what sort of details you need to make an assessment, so just let me know what you need to know and I'll try and answer as best as I can.

We're looking at a range of endocrine disrupting compounds (EDCs), so they all have at least one -OH group that is silanized. The list is quite varied, we have antimicrobials, detergents, hormones, etc.

The peak shapes are always nice and symmetrical. It's just the size that changes unpredictably. We used to have a problem with the derivatization conditions, but we've standardized it now. We divide out the BSTFA into smaller vials to reduce air exposure, and we change the inlet regularly. Column is about 6 months old.

[Don_Hilton]

I assume that you are describing changes in peak height for a standard mixture of a specific conentration. Let's start with the preparation of the standard sample from the stock solutions you keep to the solution for injection.

Can you describe the steps and anything that might change from one run to the next. Concentrations of anlaytes, compunds present that might derivatize but are not of interst, and BSTFA quantity may be important.