Sensitivity loss on GCxGC-TOFMS

[kishorebits]

We bought a new (6 months back) LECO/Pegasus GCxGC-TOFMS with a cryogenic free modulator. Initially the sensitivity of the detector looked promising. However, I observed there is a sudden loss in the sensitivity (more than 10 times). I took following actions to get back the sensitivity. However, I did not get any success.

Tuning: Performed tuning several times and check whether there was any change in the detector gain.
Checks of leak: I did PFTBA tune and leak checks and confirmed that there are no leaks in the system. Additionally, I did external leak check and confirmed that there are no leaks.
Filament: I suspected that filament might have blown off. Therefore, I used another filament to do the analysis. However, there was no improvement in the sensitivity.
Change of liner and septum: I changed the liner and septum just to rule out this possibility.
Change of helium gas: Helium gas cylinder was running low. Therefore, I changed it to a new one.

Can anyone suggest what could be a done to get back the detector response.
Regards,

[Peter Apps]

If it is only 6 months old the instrument should still be under some kind of guarantee (I got twelve months on new Varians) so your best move will be to contact the supplier and make it their problem.

Peter

[kishorebits]

Thanks for the quick reply. Its still under warranty. The problem is we donot have a local support in Singapore. Usually there is usually a time gap of 1 month when ever we call for support. Therefore, I am trying to resolve the issue on my own before the LECO engineer is here to solve the problem. Moreover, I am puzzled about sudden drop sensitivity. Can suggest me if I can try something mean while?

[Peter Apps]

IMHO letting customers wait a month for technical support on instruments that are still under warranty should be a good way to go out of business. If I were you I would be working my way up the spply hierarchy until I got someone who makes things happen

In the meatime please post full details of what it is that you are analysing, and all the instrument set up and settings, both GC and MS.

Peter

[AICMM]

kishorebits,

I fully agree with Peter.

1) Log your service call first and then continue with the troubleshooting. 2) Tell them you will bad mouth them endlessly if it takes them a month. 3) From your post I assume that the MS passed its' tune with the same basic sensitivity that it had when it was installed? If so, I would start by looking at the GC side but without more information about the GC parameters it is difficult to know where to begin.

Best regards.

[Don_Hilton]

I have a LECO Pegasus and keep it running well - let me see if I can help out here. First two questions: What version of ChromaTOF are you using? and if 3.32 or later you should be able to check with the diagnostics window to see what detector voltage it takes to get 100,000 counts on m/z of the PFTBA signal. (This will check the sensitivity of the mas spectromter itself without any problems in the MS affecting the reult.)

If you can get 100,000 counts with a voltage between 1400 and 1600 V the MS is totally fine - and the problem is on the GC end. Between 1600 V and 1800 V - as new as the instrument is, we may have to look more.

You say that you did an external leak check - but I'm not sure how you did it. Did you use a leak sniffer? The pressfits used to join the GCxGC columns can be absolutely wonderful when they work - and a total pain when they don't. If helium is bleeding out through the pressfit, your sample is also. (And depending on flow conditions you can still pass a leak test.) But, first things first. Let me know the sensitivity you are seeing in the mass spec by itself and we'll follow if from there.

(edited 10:36 am Atlanta, GA, USA time - adding in a bit more troubleshooting information)

Don

[kishorebits]

Thanks for all the replies with suggestions. I found that there is a leak at the Press fit connection. I some how missed this area when I did a external leak check before. After Don Hiltons suggestion (highlighting Press fit ) and others suggestions (that problem could be related to GC) I have done external leak check again. To my surprise, I found the there is a leak at press fit connection. I am trying to replace this press fit and check whether I can get back the sensitivity. I will update about the progress soon.

[Peter Apps]

Thanks for the feedback. with a clean cut on the columns that you are trying to join the pressfit should work fine.

Peter

[Don_Hilton]

If you cut the ends of the columns, use a new press fit. Sometimes a bit of the polyimide from the column will stick to the inside of the pressfit. And if there is a piece of polyimide stuck to the pressfit, the newly cut column end will not seat properly (it will still leak - or will pull apart easily).

Wipe the ends of the column with methanol (or actonitrile has been suggested) before putting the pressfit together. This supposedly either lubricates or softens the polyimide. It does seem to make the pressfit hold together better.

If a pressfit comes apart, as it did for you, sometimes just pushing it back together will work.

I've been developing methods for GCxGC applications for over five years now - and the press fit is the first place to look for sensitivity issues in GCxGC. You will find that there are some columns that will seal nicely in a press fit and stay in place for a long time - others will be a total pain. I have become convinced that it is the polyimide that is the problem - and you just have to live with it - unless you can find the same stationary phase and dimensions avaialble from some other column vendor. I've used. You can get polyimide to "glue" the colulmn into the pressfit, which I have done ocasionally.

[kishorebits]

After fixing the leak. I could get back the sensitivity. I had a tough time with the press-tight fit connector. I changed press fit connector twice but still found a leak at higher temperature. Finally I switched to a low dead volume union instead of press fit connector. After one cycle of heating and cooling, I found a leak with the union too (probably because of ferrules). But after tightening it one more time, the leak is fixed. Things are working fine now. I got the sensitivity back . Thanks again every one. Special thanks to Don Hilton.

[Peter Apps]

That's good, thanks for the feedback.

Peter