Problems with internal standards

[mrpj]

Hi,

I got a problem with the ratio between internal standard 1 and 2 on just one of the GC-MS machines.¨

To approve the results I need the internal standard 1 to be 2-3 times bigger response than standard 2.
Internal standard 1 is composed of 1-valine ethyl ester and 2 is composed of trioctylamine.

Some info:
GC 6890
MSD 5973
14,5 m x 250 µm x 0,25µm HP-5MS column
The method ramps from 75 to 300 C degrees with a total run time about 15 min and screens after drugs.

[chromatonewbie]

Hi there,

a few things I would start off with:

1. is the inlet liner dirty and in need of replacement?
2. has the inlet temperature changed?
3. has the split ratio changed?
4. how many sample pumps before injection?
5. are the scan/sim amu range/ions the same?
6. how do the autotune peaks compare with the other instruments?
7. has the purge time changed?

(I'm assuming you have used the same solution on all of the instruments)

cheers
Liz

[mrpj]

Thanks, I'll check point nr. 3.-7.

[chromatonewbie]

One other thing I thought of - your run time is quite short - if you run a solvent blank after it, what is the % carryover of compounds from your previous run of reference standard? Carryover/contamination can also affect ratios.