GC/Ms Peak area drop

[chronomag]

Peter, if that is the case, then there isn't a mass calibration problem. The other two compounds, azobenzene and octadecane in the Agilent mix, did not have peak area drop over time.

So what would be the point of running in full scan, or in doing an exact mass test?

Now that we have established that the test mix tractable components are not affected in SIM (which we did not know before) there would be no point in running full scan or doing an exact mass test - a classic case of the more you tell us and the sooner you tell us, the more we can help and the quicker we can hep.

Merry Christmas !

Peter

Good to know. For what it's worth (nothing?) the Agilent engineers did understand fully that the tractable compounds have good %rsd (1-3 %) over 24 hrs in SIM.

So I have saved the time of setting up and running a pointless "Exact Mass" test.
I'll ask them if they have any other interesting ideas.

[James_Ball]

Probably not an exact mass or mass drift problem unless you are looking at a very narrow mass range. Typical SIM application look at unit wide masses such that 210 would be 210 +/- 0.5amu.

What type of samples are you running?

If the samples have a lot of heavy hydrocarbons in them, like oil, sludge, or plant matter, they you will need to clean the inlet and trim the column quite often.

Do you use glass wool in the inlet liner? PAHs work much better if you are using glass wool to get consistent vaporization since they have such a high boiling point.

I recently had a column damaged because a helium cylinder got turned off over a weekend. The PAHs, especially the late eluting ones began to tail very badly and were not consistent at all. Replacing the column brought everything back to normal.

[chronomag]

After a lengthy delay, Agilent has been out a couple of times to work on the machine.
Most noteworthy, the transfer line nut was bent, towards the back of the oven, so from the front it was difficult to notice.
The bend caused the threads to eventually strip, and the column was being damaged with each tightening.
Also, replaced were the filaments and the column (to an ultra-inert 5MS)

The NPD checkout standard was recommended for checkout after the repair, and the results were better than before. The RSD for malathion could be better, but at least there is no constant signal drop.

7 min 8 min 9.5 min
inj azobenz. octadec. malathion

1 101387 1077258 37729
2 98879 1042300 33621
3 102888 1084180 30056
4 101119 1103115 31459
5 108281 1092417 30866
6 103463 1107151 31121
7 102018 1094915 31308
avg 102576 1085905 32309
%CV 2.8 2.0 8.1

Now to check the RSD for our PAH standard.

I have yet to attempt the exact mass test, that will be next.
Just by random chance, there exists the possiility that malathion (or any of our PAHs) are off in mass by more than the azobenzene and octadecane.

[chronomag]

Probably not an exact mass or mass drift problem unless you are looking at a very narrow mass range. Typical SIM application look at unit wide masses such that 210 would be 210 +/- 0.5amu.

What type of samples are you running?

24HR air filter samples, extracted, filtered, and concentrated. They are pretty well cleaned up before injection. The PAH levels are in the low ppm range for the dirtiest samples.

If the samples have a lot of heavy hydrocarbons in them, like oil, sludge, or plant matter, they you will need to clean the inlet and trim the column quite often.

The samples don't have heavy compounds, oil, or wax.

Do you use glass wool in the inlet liner? PAHs work much better if you are using glass wool to get consistent vaporization since they have such a high boiling point.

We normally use deactivated glass wool in the inlet.

I recently had a column damaged because a helium cylinder got turned off over a weekend. The PAHs, especially the late eluting ones began to tail very badly and were not consistent at all. Replacing the column brought everything back to normal.

It is possible that the previous column was damaged by heat at some point, or by the bent transfer line nut and stripped threads.

[chronomag]

The PAH repeat injections still show some downward drift, although not as bad as before.
This is with a new deactivated liner, new septa, new conditioned column, multiple solvent injections prior, clean source, tuned etc.

[James_Ball]

Have you tried running the PAH samples using Hydrogen carrier gas? Over time it will keep the source, inlet and column cleaner. I have been running it for some pesticide work and it works great, after the initial high background you get when first switching.

[chronomag]

Have you tried running the PAH samples using Hydrogen carrier gas? Over time it will keep the source, inlet and column cleaner. I have been running it for some pesticide work and it works great, after the initial high background you get when first switching.

I'm not sure I would enjoy having H2 as a carrier gas...intersting though.
There is now a pricey source option available from agilent that is self-cleaning, I think it uses H2.

Have you run malathion before and seen any downward drift with peak areas ?

[chronomag]

Does anyone running PAH regularly on a GC/MS have historical data that tracks changes in area counts over time?

We usually don't run repeat injections of the same concentration std during our regular routine.
Looking back at last years data, I do see that the area counts for the same concentrations vary quite a bit from week to week.

Looking at just the 50 ng/ml standard, the area shifts +/- 50% for some of the 17 PAHs, less for others.