GC-MS high base line

[Haohong]

Hi all,

This is my first post here. I'm using GC(7890B)-MS(5977A) for my research.

The following problem happened after I modified the path of a column (which was directly connected to MS) by connecting it to a column splitter (which was equipped in the GC chamber but wasn't used), and letting one of the outlet go to MS, and another outlet go to FID. I'm sorry I didn't vent the MS while I did the modification. I didn't change the GC configuration for the modification.

Then, when a sample was injected, both FID and MS can give sample peaks together, but a high and stable base line with abundance of 3500000 was observed in TIC (which used to be no more than 5000). MS fragments were from 28 (N2) and 32(O2) (peak intensity ratio of 4:1). The turbo pump seems working with 100% speed, but why 'Hi Vac' displays 'N/C', and I didn't hear any sound when it was initiated. I've tried the following actions but they didn't help: 1. tighten the column connections. 2. let the pump work for overnight. 3. vent and then pump down the MS and reboot it.

The AutoTune could pass, but the EM volt is as high as 2872. I found that there is no peak if I manually decrease the EM volt. Under such high EM volt, only the most abundant sample peaks as well as the high base line can be observed.

Does anyone could help me on these issues? Thanks!!

Best,
Haohong

[dblux_]

Before we start diagnosing your problem get back the old configuration of the column outlet directly connected to MS. Pump it down properly and report back how are things going on. You need to consider, depending on your column splitter, that vacuum may draw air or nitrogen from the FID to MS chamber.

[Haohong]

Before we start diagnosing your problem get back the old configuration of the column outlet directly connected to MS. Pump it down properly and report back how are things going on. You need to consider, depending on your column splitter, that vacuum may draw air or nitrogen from the FID to MS chamber.

Dear dblux,

Thanks for reply. According to your instruction, I vent, plugged the outlet to the FID and then pumped down. Unfortunately, the EM volt for AutoTune is still as high as 29XX, and the TIC revealed abundant N2 and O2.

Best,
Haohong

[dblux_]

You need to isolate the problem either to GC or to MS. Seems to me that you have a leak.
Plug the MS inlet with "no-hole" ferrule and then check if you have still high levels of nitrogen and oxygen.

[Peter Apps]

Before we start diagnosing your problem get back the old configuration of the column outlet directly connected to MS. Pump it down properly and report back how are things going on. You need to consider, depending on your column splitter, that vacuum may draw air or nitrogen from the FID to MS chamber.

Dear dblux,

Thanks for reply. According to your instruction, I vent, plugged the outlet to the FID and then pumped down. Unfortunately, the EM volt for AutoTune is still as high as 29XX, and the TIC revealed abundant N2 and O2.

Best,
Haohong

Details are important. If all you did was to plug the FID connection from the splitter, leaving the column connected to the MS via the splitter then you have not eliminated the splitter as a potential cause of the problem. As dblux said; connect the column direct to the MS.

[Haohong]

You need to isolate the problem either to GC or to MS. Seems to me that you have a leak.
Plug the MS inlet with "no-hole" ferrule and then check if you have still high levels of nitrogen and oxygen.

Dear dblux,

Thank you very much for help. I've fixed the problem in the following steps:
1. Make sure the MS part is working well: Plugged the inlet of MS, and AutoTune show much lower EM volt and the base line show not much N2 and O2 signal.
2. The kind Agilent engineer send me a manual for installation of 2-way splitter(with makeup gas) and an excel for splitter calculator, according to which I realised that the length, diameter of tubes for two detectors should be precisely calculated based on the column/detector flow rate, detector pressure, split ratio, etc.
3. I followed the instructions and it works.

Thank you again.

Best,
Haohong

[Haohong]

Before we start diagnosing your problem get back the old configuration of the column outlet directly connected to MS. Pump it down properly and report back how are things going on. You need to consider, depending on your column splitter, that vacuum may draw air or nitrogen from the FID to MS chamber.

Dear dblux,

Thanks for reply. According to your instruction, I vent, plugged the outlet to the FID and then pumped down. Unfortunately, the EM volt for AutoTune is still as high as 29XX, and the TIC revealed abundant N2 and O2.

Best,
Haohong

Details are important. If all you did was to plug the FID connection from the splitter, leaving the column connected to the MS via the splitter then you have not eliminated the splitter as a potential cause of the problem. As dblux said; connect the column direct to the MS.

Hi Peter,

Yes you are right. I found the MS part is working well only after I pulled the tubing from the MS inlet and plugged it. Finally I fixed the splitter problem by following step-by-step manual.

Many thanks!

Best,
Haohong