NItrogen ramping in my base line

[chemomet78]

Hi,
I work with GC-MS. In last few days, I got I very high nitrogen ramp after about 75 sec of the run, which continues till about 275 sec then the baseline decline to the normal state. This appears with splitless mode, however, in split mode it's OK. I changed the moisture, hydrocarbon, oxygen and the MS filter. In addition, I changed the gas cylinder, and switched from hydrogen to helium. I changed the inlet septum and the liner as well, but still have the same problem. Before all of this, I checked for leaks. there are no leaks.

any help and suggestion is highly appreciated.
Thanks

[willnatalie]

Are you manually injecting or is it automated?
Also Are you just seeing Air and nothing else?
In short, if an auto sampler is present and when injection occurs you see only N2, its possible the needle a syringe could be bad. The sample is not being picked up because air leaks into the syringe.

[chemomet78]

Thanks for your reply Willnatalie ....
I inject manually, but this nitrogen ramp appears even without injection i.e: in column blank runs.

[Peter Apps]

Do you see oxygen increasing along with the nitrogen ? If you do you have air getting in. If not this is probably not nitrogen at all, but a siloxane from the column or septum. There are some earlier posts on the forum about this. What is it that you changed from hydrogen to helium ? - were you using hydrogen as carrier gas ?

Peter

[Don_Hilton]

Are you using splitless with a purge off time about 75 sec? When you run splitless with a purge off, the split vent is opened, typically to a very low flow. Air will diffuse back in through the split vent.

To get rid if the background air, you need to increase the purge flow through the inlet. (You will reduce, but not eliminate the effect.)

[chemomet78]

Hi,
Thank you for your response ...
Yes, I was using Hydrogen as a carrier gas. Then, when I got this problem, I switched into Helium to see if there is any difference or not. I don't have any oxygen appearing in the system.
The purge off time that I used was 180 sec not 75. I tried today to decrease it to 60 sec, the ramp ended at almost 160 sec instead of 280 sec. Decreasing the purge off time to 30 sec, the ramp started at 100 sec and ended at 130 sec. Decreasing the purge off time into 20 sec, the ramp started at 100 sec and ended at 120 sec. The time of this ramp is the same time that the vent valve is closed. This only is clear at m/z 44, but in the TIC the base line looks fine.

Thanks

[Don_Hilton]

My abilty to type sometime gets me. I had intended to type 275 rather than 75. - and had ignored the dead time of the column.

It looks like the ramp is startign about 70 seonds and ending about 90 seconds after the purge valve opens (purge off time)? If I've got this right, does increasing the purge flow keep the air background down?

[chemomet78]

I think the ramp starts directly after the dead time of the column, after 100 sec and ends just by opening the purge valve i.e: the ramp is only there during the period of the purge off.
I changed the inlet seal, as I thought might cause leak, but nothing improved.

[philippemottay]

It seems to me there is a leak in splitless mode. From your description I take it it is an agilent. You can try to monitor the 18/28 peaks with the purge valve on and off. If I am right it will the 28 will go up when the valve is off (allow for the dead time).
I believe changing the gold seal at the bottom of the injector will fix it (until next time). It is something I have often seen but was not able to fix for a long time.

Philippe

[Ron]

What model of GCMS do you have?

[chemomet78]

Thank you philippemottay for your reply,
I thought like you, and yesterday I already changed the gold seal. No improvement.
The GC model is Agilant 6890 with TOF MS.

[Don_Hilton]

I also run a 6890 with a TOFMS. Mine is a LECO GCxGC system. I typically have column flow rates of about 0.6 to 1 mL/min - which leaves a very low head pressure. I find that I have to take the purge flow up to about 50 mL/min for these column condtions to avoid air in the background.

[chemomet78]

I run the same system LECO GCxGC-TOF. I already tried to increase the purge flow from 40 to 50 ml/min and higher, I used 80 ml/min and even 200 ml/min, but nothing changed. In the split mode everything is OK.

[Don_Hilton]

From my reccolection of the plumbing of the 6890, the inlet purging in splitless mode should be the same a physical situation as the inlet running in split mode. And, in split mode you seem to be able to avoid the air background... And I assume with the same column flow. There seems to be somethign funny about the inlet switching. And I'm stuck beyond this point. (I was able to end my search for a solution to problem with similar symptoms on my instrument a few weeks ago by kikcing up the purge flow...)

[Ron]

The 6890 is very different from the 5890 in many respects, and one of those is the way the instrument does split mode injections. In the 5890 you always had the same total flow in split or splitless mode as long as you didn't change the split ratio, and back pressure control was always used to control the column flow. In the 6890 the GC goes into forward pressure control mode and the split flow is turned off until the purge valve is turned on. During the splitless time the only flows are the column flow and the septum flow, and it is possible for air to backstream into the system.

I don't know what the current firmware does, but the early instruments would go back into splitless mode at the end of the run, and if the instrument sat overnight a significant amount of air could enter the system. I lost at least one column this way.

I hope that this issue has been resolved, but any time you have a long splitless time, especially with a low head pressure, there is a real possibility of air entering the system even if the system is in split mode until the injection is made.