Chromatography Forum

I'm almost about to scream "PROBLEM RESOLVED" (almost!)

After trying flow rate adjustments which made no difference (thanks for the tip anyway!), I stood around looking at the two instruments and tried to figure out what was the easiest piece to pull and swap. The electron multiplier is pretty easy to remove, so I shut down both instruments, pulled both EMs, and put the newer one in the older instrument.

Until now the older GCMS has given consistently non-linear curves, with reponse increasing by a factor of ~3 when concentration doubles. After swapping the EM's I just got this below from the older GCMS. These are reinjections from Friday's solutions (http://i43.tinypic.com/ridjx3.jpg), so it's not great, but it is straight!

I want to switch back to the old EM and rerun again to confirm, as well as go back to some of the other assays I've been trying to work on, before I rule this issue as completely resolved.

Yes both instruments had the same stationary phase and inlet. I've tried clipping columns, replacing liners, seals etc etc. I'm almost to the point of separating the two GCs from their respective MSs and switching them over to demonstrate the problem is with the MS and not the GC side

That might be a better idea than it sounds at first.

[/quote]I understand that if you take certain points a few may look linear. Bear in mind these are not the only curves/analytes I've run.[/quote]

But they are the only curves that you have shown us - are we telepathic

[/quote]I've ruled out the derivitization procedure (I think) by preparing a calibration curve that is diluted then derivitized, and another which is a stock derivitized, then diluted.[/quote]

Which it would have been good to have known sooner.

[/quote]This is appearing more and more with different analytes, although it only seems to affect the analytes I actually NEED to work....!!! It's affected the ZDDP assay, an EDTA assay (methylation), a FAME assay and most recently is now appearing with 2-bisethylhexyl-phosphate. All of these are non-linear on the first GCMS and linear on the second. But OFN and Pentadecane is linear on the first GCMS..!!![/quote]

All the non-linear calibrations are derivatized, which often means that there is a more gunk going into the injection when compared to straight reference substances just dissolved in clean solvent. Have a look at the TIC (and please post it so that we can all see), there may be things in the background that are interfering. Also you need to run a blank - i.e. everything, including the derivatizing reagent but with no analyte. Please post that as well, TIC and SIM.

[/quote]I prepared a calibration curve of BEHP in duplicate from 0.1 to 5 µg/mL in DCM. This is prepared from a point dilution of a 10µg/mL working stock solution in DCM, i.e. dilute 0.5mL of the stock with 0.5mL of DCM to get the 5µg/mL, dilute 0.2mL of the stock with 0.8mL DCM to get the 2µg/mL etc etc. One curve was run on the first, another on the second. [/quote]

So these were derivatized after dilution ?

This is plainly a complicated problem, which is why it is absolutely necessary to design the trouble shooting in a way that clearly eliminates certain possibilities. At this stage there are still three main contenders: adsorption on active sites in the bad system, a concentration dependent derivatization problem (including interfering impurities), something wrong with the bad MS itself.

Peter

It would be a good idea to run the EM from the old instrument in the new instrument. This way you can look to see if the problem follows the part. There are times that simply removing a part and replacing it back in the instrument will seem to fix a problem.

Thanks for all the comments. I'm still testing to confirm the detector is the issue. I am also trying the "bad" detector in the good GCMS to confirm the part is the problem.

I've ruled out the derivitization procedure (I think) by preparing a calibration curve that is diluted then derivitized, and another which is a stock derivitized, then diluted.

Which it would have been good to have known sooner.

See page 1, post#8:

Also, just to confirm the prep method (derivitization) wasn't the cause, I prepared a high concentration standard and diluted that down sequentially.

All the non-linear calibrations are derivatized, which often means that there is a more gunk going into the injection when compared to straight reference substances just dissolved in clean solvent. Have a look at the TIC (and please post it so that we can all see), there may be things in the background that are interfering. Also you need to run a blank - i.e. everything, including the derivatizing reagent but with no analyte. Please post that as well, TIC and SIM.

Except the BEHP (which is a typo and should actually be TBEP, tris-2-butylethyl-phosphate, not BEHP), this is not derivitized. Until I saw it with this analyte I was also thinking it was a derivitization/methylation issue. There is nothing interfering in the TIC's, and we run a BLK (which is a standard zero containing all prep reagents but no analyte) with all analyses. Also, and not yet mentioned, after the first non-linear ZDDP curves, I ran a method for organotins (derivitization with tetraethylborate) which worked fine.

I prepared a calibration curve of BEHP in duplicate from 0.1 to 5 µg/mL in DCM. This is prepared from a point dilution of a 10µg/mL working stock solution in DCM, i.e. dilute 0.5mL of the stock with 0.5mL of DCM to get the 5µg/mL, dilute 0.2mL of the stock with 0.8mL DCM to get the 2µg/mL etc etc. One curve was run on the first, another on the second.

So these were derivatized after dilution ?

Nope, just point dilutions of a 10ppm stock.

This is plainly a complicated problem, which is why it is absolutely necessary to design the trouble shooting in a way that clearly eliminates certain possibilities. At this stage there are still three main contenders: (1)adsorption on active sites in the bad system, (2)a concentration dependent derivatization problem (including interfering impurities), (3)something wrong with the bad MS itself.

(1)I'm thinking not because I've changed liners, columns, seals etc etc over and over and that made no difference. And I've seen no non-linear curve on the good GCMS.
(2)I don't think so since I confirmed derivitization wasn't the cause by diluting down a 10ppm derivitized stock with hexane and with derivitized blank and then running. Both curves matched and both were non-linear.
(3)the detector is definitely looking like the problem!

As a final comment, I've managed to reproduce the non-linearity on the good GCMS with the good detector by cranking the EM voltage all the way down below normal operating level. When I crank it back up to operating level and reinject the samples, it's linear. To me this suggests that at normal operating level the bad detector is performing to the same level as a good detector at low voltage.

Thanks for ALL the tips guys!

ps....

Replacement EM tube from instrument manufacturer: $3650
Replacement EM tube "prepared according to the original specs" direct from OEM source: $585
priceless....

Congradulations - and now can you come work on my instruments? I've had two go off line today!!!!