Problems with quantitation of Adipate/Phthalate by GC/MS

[manuelf]

I'm using a HP GC 6890 and 5973 MSD and I would like to analyze Di(2-Ethylhexyl)adipate and Di(2-Ethylhexyl)phthalate by EPA method 525.2 with the temperatures ant Temp programs suggested there.
I prepared calibration solutions in Acetone within the range of 1-10ug/ml and injected 1ul with an autosampler. The resulting calibration curve is not linear and I have increasing response factors with the increasing amount of analytes and %RSD above 30.
I tried this on 2 different instruments with different autosampler (CTC PAL and Agilents own) also different septum types but always same findings.
Does anybody ever encounter a similar problem before? Thank you for your response.
manuelf@web.de

[chhubert]

There are two possible sources:-
1) the MSD: check your MSD's ion source. Dirty ion source may lead to narrowing linear range.
2) the GC column/inlet: dirty column/inlet is also another culprit of trapping analyte, especially when it is at low concentration. The % of absorption for analyte at low concentration is more prominent than that at high concentration.

[Ron]

Agilent came out with the inert source option because there were issues with linearity of polar compounds with the original source. Since you have the same problem on two different instruments with the same compounds it is more than likely an ion source issue. I don't know it the inert source is a solution to the problem for these compounds or not, I have heard from people using the inert source that there may be issues with this source also.

[ingochrist]

Did you try to confirm that the RSD with both those units is fine? I would recommend using somewith realy simple such as FID Mix or BTEX standard with a high split. That might help you determine the source of your problem. I can offer you additional advice for your CTC PAL if you think the autosampler is the source of your problems.

Best Regards,
Ingo Christ