Different Quantification - Same Sample??

[mbdixon]

Hi Can anyone explain to me the cause of the following:

I am manually injecting the same sample into my 6890, under identical conditions, using a iso-thermal method with an internal standard to quantify at 10-200mg level deriviatised esters of olive oil. Yet with the same samples can be up to 10mg difference in quantification. The chromatography is good and I just cannot see what could be causing the difference.

Any thoughts?

[Johnny Rod]

Jjust to clarify, do you mean the differences between replicate injections from the same sample after derivatisation? Are there any trends or are the variations random? Any trends in the peak areas before you calculate the final mg content?

[mbdixon]

Yes, the same sample. Injected from the same vial. The differences seem to be pretty random and obviously with an internal standard I'm comparing the sample peak to the IS peak not taking absolute peak areas (or heights, I might see if it follows suit with using peak height when I have time....)

[Johnny Rod]

What's the internal standard? I would have a look at the raw peak areas and see if there are any trneds etc. going on there, there shouldn't be. FID I presume?

[Peter Apps]

This could be due to eratic inlet discrimination, especially if your internal standard and problem analyte have very different boiling points (or molecular weights). What are the instrument settings, what is your IS and which analytes give trouble ? Are there any analytes whose quantities ARE repeatable ?

Peter