Problems with Internal Standard

[diana23]

Hi!

I have an issue with my Quattromicro GC. I use and internal standard and I use equal amounts of IS in Calibration curve and in samples, but when analyzing samples, IS in samples is always higher than in calibration curve (recoveries of 200%) and If I inject again a calibration point, the response is much higher than the reponse I obtained in the calibration curve. It looks like the response is increasing after each injection.

what can be the problem?

[Steve Reimer]

Either the detector is getting more sensitive as you go or you're are adding more analyte to the GC column.
Does the response increase when you analyze a series of standards or does it only happen when you run samples?
Are the samples in the same solvent as the standards? Or is there some leftover polar solvent present in small quantities?
Does the response drop when the instrument sits idle?
Is this a split or splitless injection?
Is this purge and trap?

[diana23]

Hi!

No, the response doesn't increase when I inject a series of stardards, only if I inject samples.
Calibration standards and samples are dissolved in the same solvent hexane, but to prepare the standards I use an extract of different matrices (same type as my samples) dissolved in hexane because of the matrix effect.
No, the response doesn't drop when the instrument sits idle.
It is a splitless injection
No, it is not purge and trap.

thanks for your help

Diana

[mckrause]

You are probably seeing the post-injection effects of matrix in the injector port. We see this a lot in pesticide analysis of food extracts, where we'll get a vastly different response between check standards injected before and after the sample. We've seen increases in the analyte response of upwards of 300%, depending on the complexity and type of food that was extracted. I wouldn't have expected it for hexane but our experience certainly mirrors yours.

BTW, we've never found a work around other than go back and clean up the sample some more.

[diana23]

Hi!

we are also analyzing food but I obtain good results of IS for these samples when I inject those in a similiar instrument (also Quattromicro, same GC but different autosampler). For example in that instrument I obtain 100% for IS and in the bad one I obtain 200%.