N2 for GC-MS "Rest" Gas

Discussions about GC-MS, LC-MS, LC-FTIR, and other "coupled" analytical techniques.

18 posts Page 2 of 2
James_Ball wrote:

Set it to He and column flow of 1ml/min and split ratio to 100:1 and it will flush out in just a few minutes. If you put it into manual tune and scan until the N2 drops to almost nothing you know the flush is complete.

Good suggestion-thanks!

I'm fortunate in that the "gas hog" 5890 is further down-stream on the plumbing than the 7820, so even without changing anything it seems to help with purging the lines.

Also, of some interest to me is that just playing around before leaving yesterday, I told the 5890 that it was running N2. With a 30mx.25mm column at 50ºC, it's actually not capable of maintaining a .500 ml/min flow of N2(it can do He). ~.54mL/min was as low as I could get it. The EPC module on the 5890 is definitely a lot less refined than it is on newer instruments...
I started noticing issues with my 5975 on Tuesday(first time I'd tried to use it since doing this) and after exchanging a few emails with Brenda today I'm posting this as a caution and also walking away with my tail tucked between my legs...

When I did the switch over last week, since I was opening the gas line anyway, I figured it was a good time to shut everything down and also clean the sources in both MSs. The 5975 initially came back online fine with the valve installed but only flowing helium.

As I said, a few days ago, I started noticing issues-specifically the vac pressures were a lot higher than I expect(low 10^-4 torr) for this instrument and the tuning looked AWFUL. Although I could get it to autotune, I still had a lot of noise on the 502 peak and the EM voltage jumped up over 400V from the last good tune(up to the high 1800s from the mid 1400s). It would also peg the 28 signal at what I assume is the instrument max. I initially thought a HUGE vac leak, and a pump down with a blank ferrule seemed to confirm that it was in the column fitting. With the blank ferrule, I was getting ~1.5x10^-5(diffusion pump) and it tuned at a reasonable EM voltage. The only problem was that I couldn't FIND the problem-I went through I don't know how many new ferrules(.4mm ID for .25mm ID column, 15% graphite/vespal) and of course tightened them after temperature cycling.

Finally, I emailed Brenda today, who confirmed that I'd done most of the troubleshooting she would have tried and asked for the tune reports. I sent her the "bad" one, one without a column, and one from a month ago that showed what I'd consider "normal." I also sent her screen caps of the 28 m/z peak at different flow rates.

She looked it all over and said it looked like I was getting a whole lot of N2 into the system, and asked if I'd changed my carrier filter without purging it as new ones come purged with N2.

Going on that, I first shut off the N2 cylinder completely and bled the line to make sure my valve wasn't faulty and allowing N2 and He to mix(although that didn't seem likely, since the 5971 was fine). That didn't change anything, so I bypassed the carrier filter completely, and everything returned to normal in probably less than 5 minutes.

I'm GUESSING that I'd just managed to get too much N2 in the carrier filter, and it wasn't purging out at the flow rates going in to the GC.

So, based on that, it looks like I need to relocate the filter to a location between the He cylinder and the switching valve, not right before the instrument. I know that right before the instrument is most effective, but honestly I use high grade gases and consequently my carrier filters last ages anyway since about all they catch is the occasional air bubble from switching cylinders.
I try to only put the small indicating oxygen filters right before the instrument. I put the larger filters farther back up the line, usually before it splits out to multiple instruments. The small indicating oxygen trap will catch the little bit of air that might get in the line and will change color if you have a large leak, and they are easier to flush out.

On my instrument with the switching valve, I put the valve right next to the inlet connector, with the filters before the valve. I use one valve per instrument so one can be running helium while the other is in rest mode.
The past is there to guide us into the future, not to dwell in.
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