SPME non-linear

[Monty_Python]

Can anyone suggest reasons why SPME shouldn't give a linear response? I've prepared samples that, when measured on a different GC-MS, show an almost perfect linear dose response, yet on the Agilent system they're all over the place.

The instrument has a new fibre (DVB/Carboxen/PDMS), new column, liner and septum (Jan this year), shows no issues with tuning.

Any suggestions?

[rb6banjo]

I see it too. Does your calibration data generally bend over toward the horizontal at larger concentrations? In general, I see better linearity with the 100% PDMS phase but also less sensitivity. You probably do this but I have to ask - do you add an internal standard? I have found that to generally improve linearity.

[Peter Apps]

Can anyone suggest reasons why SPME shouldn't give a linear response? I've prepared samples that, when measured on a different GC-MS, show an almost perfect linear dose response, yet on the Agilent system they're all over the place.

The instrument has a new fibre (DVB/Carboxen/PDMS), new column, liner and septum (Jan this year), shows no issues with tuning.

Any suggestions?

If "all over the place" means that points are scattered on both sides of the best line, then the root cause is poor repeteability. Why this should happen on an Agilent and not on another (nameless) intrument might have to do with inlet design, gas flows, temperature control, etc etc.

If you have a consistent trend away from linearity i.e. the points are consistently above or below the line at one end or the other the likely causes depend on what deviation you are seeing where. Points below the line are common with SPME simply because there is so little sorbent on the fibre, and it does not take much analyte to make sorbtion non-linear with concentration.

Peter

[Monty_Python]

Sorted it out - turned out the fibre was the issue. New fibre and all is well.

[Peter Apps]

Were you using a different fiber on the Agilent and the other anonymous instrument ?, or the same fiber on both ?

Peter