GC-MS TIC problem

[Marvaz]

Hello everyone,

We are working in a 7890A GC connected to 5975C MS and we are analyzing PCB's with a DB-5 column in ground water and VOC's with an HP-VOC column. Before the sample collection we have standardized both of the methods and made calibration curves with calibration standards and we were waiting to collect the samples and when the samples were collected we analyzed first VOC's because they are more volatile than PCB's.
When we installed the DB-5 column an electricity brake down happened for hole night and both of the filaments burned. We changed one of them (the second ordered, didn't come yet) and when we tried to run a standard, the chromatogram was far from what we expected. With the same oven program, the same compounds and concentration, same column, everything same, the retention times has changed, some compounds are co-eluting and some others disappeared, and the size of the peeks is getting smaller as the session in moving.
Has anyone any idea about what might be the cause? is the filament change? is the fact that we haven't installed both of filaments? maybe something else happened in the instrument?
I am posting you photos of TIC before and after the filament change to understand better...

before


and after


Hope someone will have the answer...
Thanks in advance..!!!

[CE Instruments]

Doubt it is the filaments that have caused the change. Are you sure you ae running exactly the same program. Looks to me like you may be either running constant pressure mode instead of constant flow or that you have reached the maximum pressure that you can but this is not enough to maintain flow. Check method and gas supply.

[Marvaz]

Thanks for the reply...

I checked and the method is exactly the same and I am running constant flow mode not constant pressure.

Can wrong installation of the column be the cause ?

[Johnny Rod]

How is the vacuum compred to before? Coudl you have a small leak?

[triplequad]

Did you run an ATune after the filament change...?

[Peter Apps]

You have symmetrical broadening only of late eluting peaks, and no tailing. This is extremely unlikely to be due to anything downstream of the column.

Am I right in understanding that you did not run any of the PCBs after you ran the VOcs and before the power cut ? If this is the case then you do not know whether the peak shapes were good before the power cut ?

The most likely culprit is dirt in the inlet - quite possibly a fragment of septum - from the analyses of the VOCs, or the column being wrongly installed in the inlet. Poor installation at the MS end is possible, but I would expect a marked change in peak size.

Peter

[Marvaz]

The vacuum is the same as it was before the column change and when I run ATune there is no indication that there is a leak, it is normal...

Peter you understand right, I didn't run any PCB's before the power cut so I don't know what was going on with the peeks, and I believe, as you said, that poor installation is the problem, so I de-install and re-install the column and now I wait the system to stabilize....

I hope I will not have to ask you again...

Thanks for the help...

Marios

[Peter Apps]

Did you notice anything wrong with how the column was installed ?

Please let us know if the re-installation was successful - it is always useful to have feedback.

Peter

[James_Ball]

Seems there is a large increase in the column bleed at the end of the second run compared to the first. Maybe some air is getting into the column. If there is a leak that opens when the oven temperature increases it could be causing a leak that allows air in and lowers the overall pressure so that the peaks are eluting late the farther you get into the run. This would also be causing the carrier to be a mixture of air and helium near the end of the run which would cause the broad peaks along with the higher column bleed.

Do you see m/z 40 or 44 in the raised portion of baseline near the end of the run?

Also you said you only installed one filament, did you leave the other broken filament in there or remove it? If you removed it then there is no longer a positive charged target for the electrons to be drawn to on the opposite side of the ionization chamber.

[Marvaz]

I did re-install the column but nothing changed...

and finally the solution was that somehow the thermal aux 2 was unchecked and turned off (maybe from the electricity power cut?). When I checked it and run a sequence everything was right again....

thank you all for your concern and your help...!!!

Marios

[Peter Apps]

Hi Marios

Thanks for the feedback. What is thermal aux 2 ? - the transfer line ?

Peter

[Marvaz]

Hi Marios

Thanks for the feedback. What is thermal aux 2 ? - the transfer line ?

Peter

Hi Peter,

I don't know exactly cause I am new to the gc/ms world but i believe that it is something that provides heat in the transfer line...

if I am wrong and someone knows for sure please correct me...

[James_Ball]

Yes, Thermal Aux #2 is the transfer line or MS Interface which ever way you say it.

[Yama001]

Keep an eye on that Aux setting - it has been a while for us, but we used to see that routinely turning off on us. No idea why it was happening, or why it stopped, but we got in the habit of verifying it was still checked off in the method.