Air - Water Spectrum MS

[lusi]

I need some advice on the following:
We have a Thermo DSQII MS and when we put it on the air-water spectrum was fine. The MZ 28 was smaller than the 18. This kept until the afternoon at least. Next morning when we checked agin the MZ 18 and 28 were almost the same height.

I thought it may have been a leak somewher even though the leak detectors and the automatic leak detector in the GC did not indicate such.

I changed the septum, tightened the nuts etc and then the spectrum was good again. Next morning the same thing happened the MZ28 and 18 were the same height.

Anyway, tried for two more days and the same things keep repeating. It is fine all day but in the mornings the MZ28 come back up.

Any ideas?

Thanks

[Mikaelalfredsson]

Interesting to know would be what your maintenance tune says about the leak/air percentage.

normally I would guess on a loose/damaged Transferline ferrule (not the column but the bigger behind).

If you fix the nut closest to the MS you can carefully retighten this one.

Can happen if you run high column temperatures. for the column I can recommend SilTite ferrules and nuts, they work really good.

could also be normal, again check what your tune report says (don´t forget to tick leack check in tune options).

M

[lusi]

Dear M.
Thanks for the input.
The system was down for a while. Actually it has been more down than up. A series of boards had to be changed and this took a while. So I really do not have a maintenance information about the air-water percentage.
Besides all I have been trained to do was check the spectrum visually. Not even sure if there is another way to do it, the manual does not indicate so.
I did try to do the mass calibration once and got a message indicating it was unable to calibrate the detector gain..
I did use the leak detector - hand held one, and I even used methanol (just in case) but they didnt indicate any leaks.
We are going to change out the gas - its about time. But I dont know if that would affect anything...

[Mikaelalfredsson]

Hi lusi,

In the DSQ Autotune, you can choose doing either a full automatic tune or just a maintenance tune.

the later is something that is recommended that you do more frequently, to compensate for dirt build ups.

Anyway if you choose the Maintenance tune and in that window tick also the air leak check. you will have
the air leak in the report.

if you have too much of a leak the tune will fail.

Since you wouldnt want to run with too much oxygen around, this is a good check to do at least after every column change so you know your system is tight.

you say you get the "unable to calibrate detector gain" my first guess is the Ion volume is not completly alligned. so please check that. should be easy to see on top of the volume itself if you have an ion burn that is not centred over the inlet hole.


Do your DSQ have the air lock so you can remove the ion volume without stopping?, then you can just insert the tool and give it a little push to let it seat better..

also after each cleaning you should make sure that when you hold the source block in your hand and insert the volume, it should be easy to insert but seat firmly, so you can lift the source with it.

M

[lusi]

Thanks M. Changed the gas. Put on the instrument - air-water looked good. Ran the maintenance tune. Leak check passed, 13.2% of reference. I honestly dont know what that means. Will try the full tune later.

The ion volume has not been touched since it was installed about 4 years ago. Added to that most of the time the instrument was done. Since the boards kept going and replacement takes months!!!.

Honestly I dont know much about the instrument since it was down so often and the training we received was the basic training in the book.

I am not sure how exactly to check the ion volume physically. I dont recall any of the manuals indicating, but then I never did check of that.

I doubt that the DSQ has an air lock but I will ask the suppliers.

[aldehyde]

After doing maintenance on the system it is common for air and water to stick around for several hours, best to tune the next day. Once it has equilibrated for enough time your O2/N2/water should all really be <5%. Older systems may have slightly elevated values, 13% is high.

The percent is because the abundance of the O2/N2/water is reported as a ratio of the most abundant peak measured (with Agilent instruments the tune uses an internal calibrant perfluorotributylamine PFTBA and the 'base peak' is 69.)

[lusi]

Thanks Aldehyde...I did tune a while after and it was at 7%.

So maybe it was too soon.

Also I am noticing the Mz28 is constantly rising. Why is that so?

[aldehyde]

What is it at? 28 (N2) is usually in a 3:1 ratio with 32 (O2). If you have an abnormally high N2 it is possible you have an air leak before your gas filter, which would scrub out O2 but not N2.

[Peter Apps]

I have seen 28 as a peak from column bleed (based on all other possible causes having been eliminated) - there is some previous discussion in the forum on exactly this issue.

Peter

[lusi]

Aldehyde, wont the automatic leak test on the GC pick up a leak? The leak test passed. Also the filter looks pretty good still.

I also leak check physically. All the connections, except at the septum (which I changed a few days ago). Plus the column evaluation was really good.

The MS autotune leak test passed and its about 5.1% of the reference gas?

Peter, I did a column evaluation and it was pretty good.

I just dont get why the Mz28 holds all day and overnight it rises. Especially over the week-ends. Am I missing something.

Could the calibration gas used during the autotune cause the subsequent air-water spectrum to be off for a while?

[Peter Apps]

[quote="lusi"]
Peter, I did a column evaluation and it was pretty good.

I just dont get why the Mz28 holds all day and overnight it rises. Especially over the week-ends. Am I missing something.

quote]
Hi Lusi

The column does not have to be bleeding excessively to generate enough m/z 28 to make the software think that you have a leak. The overnight and weekend build up is a clue that the column is involved - what temperature is the oven when the sytem is in standby ?.

Peter

[lusi]

Thanks Peter.

I use like 60 or 40. I dont know what is the bets temperature to be honest.

The column can be up to 340 I think.

[Peter Apps]

Hi Lusi

As an experiment - keep the column at 180C overnight and then check for "air" (m/z 28).

Peter

[lusi]

Thanks Peter.

What do you think the prob is?

I was going to shut down and check the connection at the liner and restart.

[Peter Apps]

I think that the problem is that siloxane phases generate a 28 m/z fragment that makes it look as if there is an air leak when in fact there is not one - have a look at the forum archives.

Peter