GC-MS loss of sensitivity via HS

[i44ran]

Hi,

I need some help....

I have tried to 2 series of THM standards via Headspace CTC autosampler on a GC-Ion Trap-MS. Vials are heated for 30 mins at 60 C.

I am beginning to find some problems. The sensitivity of the second series seems to be lower (and at times upto 50%) then the first series of standards. i.e. for 100 ug/l standard gave an area of 3000 (Arb units) the first time round, the second time it would give an area of ~ 1800-2200. They are prepared at the same time under the same conditions.

I have changed the syringe plunger (which has an effect), a new inlet liner and a new septum are placed and everything tightened.

I dont know what could be wrong... Any suggestions..

Thanks in advance..

i44ran.

[AICMM]

i44ran,

The lack of response tells me that the forum is looking for more information. Lots of people who have lots of experience with headspace but I think they need more data.

Best regards.

[masstocharge]

Hai i44ran,

More information about the instrumental parameters will give chance to discuss and solve the problem. can you give more instrumental details that includes MS ion source parameters.

[i44ran]

Hi, Sorry,

I am still learning this process of posting here, what kind of information would you require?

The ion source was at interface temperatures 280 ºC, source temperature 230 ºC, electron impact ionization mode (EI) 70 eV. The mass range was 40 – 350 a.m.u. in the full scan mode with a scan rate of 0.29 scans/s.

I was initially having acquisition errors, which was fixed by the engineer a few months back.

Thanks for you help...Aiccm and masstocharge.

144ran

[Peter Apps]

There are a number of possibilities - the most likely are that you lost volatiles into the headspace of the container that you made the standards up in (we need details of how you make up your standards) or that the caps on the HS vials are not tight enough. The MS may also be drifting (but probably not by enough to account for a 50 % difference).

Please post the headspace conditions, including the syringe volume and injection speed.

Peter

[i44ran]

Thx for help..

I dont think it could be the vials or its sealing or the method of prep, as i am running a whole series of calibration standards, and the whole set goes down (rather then a single vial) They are prepared at the same time and using the same method. This problem is relatively recent, and the way I used to do it has not changed from what was done previously without any issues.

I am using a 5ml water volume in a 10 ml vial. Heating for 30 mins at 60C. Injector speed 250ul/s. Syringe 1ml. 500ul head space injection.

I prepared 2 of each 100ug/l, 75, 50, 25, 10, 0. I run a standard set in the beginning and around 20 samples and one set in the end. The end one is usually much lower than the beginning one. E.g. i ran this just a week back - For a 100 ug/l standard, CHCl3 gave 4090 arb units for the first and 2286 Abt units for the second, via select ion integration.

Do you think it might be the problem with the MS itself.. like electron multiplier?

Thanks in advace

[AICMM]

i44ran,

Could still be caps. If you are using a non-teflon lined cap (ie silicone only) then THMs are very adept at slipping through. It's the basis for all the membrane extraction volatiles procedures such as MESI. This would account for all of the second standards being lower although they should not all be equally lower. The curve should have a different slope. So... prepare the first curve, then the samples, then prepare the second curve and run it.

Second, regarding the MS, tune and print. After running the back standards call up the same tune and check it out (don't re-tune!, just re-check the tune.)

Third thing to look at. If you have your headspace sampler plumbed to a split splitless injector put your THM standard (a microliter or two) in one of your vials, evaporate it completely and run it with a gas tight syringe directly in the split/splitless port. Then do the same at the end of the sequence and compare. This would isolate the headspace sampler and vials and only speak to GC/MS stability.

[Ron]

I would suggest adding an internal standard that is not a THM, and see if this shows the same behavior. This may help isolate the problem.

[masstocharge]

Dear i44ran,

If it is the case that whole ur HS-GC system is stable and repeatable, I may guess the problem. My thesis work is based on such problems. To give you an answer, i would like to know ...

1. MS instrument brand and model name.
2. Any Ion source cleaning was done before this problem?
3. Is you peak area drop is stronger for chlorinated compounds than others?
4. Voltage settings in Ion source. please include repeller and lense voltages from your tune file that is now and from the tune file that was used when there were no problems.

Good luck

[i44ran]

Hi

Sorry M/z, has been way whilst the instrument was moved. A new electron multiplier has been placed and the ion trap has been cleaned. But the instrument is now off-line with other problems.

In response to your questions

1. It is an Varian Ion Trap 2200 MS
2. I believe not cleaning was done to the traps until now
3. The instrument had not been used to analyse anything else apart from the THMs, hence don't really know.
4. The source was 70 eV. Sorry, i am trying to figure out this. Cant find this, maybe it was not saved in files.. Had a look, can only see axial modulation, RF voltage.. !

Thx for all your assistances..

i44ran

[JI2002]

scan rate of 0.29 scan/s sounds really slow....