MS Detector v FID Detector

[mbdixon]

Please can some one suggest why this is happening:

I am doing migration analysis on Octadecyl 3-(3,5-di-tert-butyl-4-hydroxyphenyl)propionate in an isooctane matrix. I am injecting 1µl with a pulsed splitless injection onto a HP5 UI column 20 m x 180 μm x 0.18 μm with hydrogen as a carrier gas. The chromatography I am getting is great using an FID or MS detector, however, my calibration on the FID is linear whereas the MS is non-linear (quadratic). Why?

For the MS I run in SIM mode with 530m/z ion as my quantifying ion and 515m/z as my qualifier. In both FID and MS I use Bis(1-butylpentyl) adipate as an internal standard/ The exact same samples were used first for the MS run followed by the FID run.

[rb6banjo]

The linear dynamic range of the MS isn't as good as the flame.

When it comes to linearity over a very large concentration range, it is very difficult to beat a flame detector.

I see the same behavior you are seeing.

[mbdixon]

Thanks rb6banjo,

So assuming I can still see the lowest standard running it in a split mode should solve the problem?

[rb6banjo]

MS in my hands is generally quite linear on the low end of the concentration range. Since your problem is a "leachables" thing, you'll likely always be operating in that low end of the range. I'd expect that as long as you are able to measure the LOD and LOQ reliably, you're good to go.