N2 for GC-MS "Rest" Gas

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

32 posts Page 2 of 3
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.
I just had my first Helium order of the year and the price is up to $621.50 a cylinder plus tax and freight (Chicago). So I am looking at plumbing nitrogen to my instruments for a rest gas.

Restek is warning about volatiles from the lubricant in the valve that JB linked. Is that an issue. I have 2 instrument 3/inlets and an aux epc for my quickswap.
MSCHemist wrote:
I just had my first Helium order of the year and the price is up to $621.50 a cylinder plus tax and freight (Chicago). So I am looking at plumbing nitrogen to my instruments for a rest gas.

Restek is warning about volatiles from the lubricant in the valve that JB linked. Is that an issue. I have 2 instrument 3/inlets and an aux epc for my quickswap.


I haven't seen any problems yet from the valves, but if you want to be more secure you can use two of the toggle valves, you just have to remember to close one when you open the other. The down side is if you leave them both open you get a mixture and if you close both you have no flow.

If you have Agilent 7890 and up GCs you can purchase an internal valve to electronically do the switching.

That looks like an outrageous price for Helium, unless maybe you are getting Grade 6 Research grade. We get Grade 4.5 (99.995%) which is good enough for what we do and I think we are about $300 for a (Airgas 300) cylinder.
The past is there to guide us into the future, not to dwell in.
Yea we order UHP I'll see if we can drop it down to 4.5.
The label on our 4.5 is UHP Helium. I think anything above 99.995% is considered UHP, 99.9999% is considered Research Grade I believe.
The past is there to guide us into the future, not to dwell in.
I contacted my vender Weldstar Purity plus and they told me dropping to a 4.7 would save only $50 then I contacted the lab for the parent company and they get Helium from US gas for $225/cyl. I'm not sure how there can be such a huge difference in price. Airgas was also charging us ~$500 when we switched from them about 5 years ago.
Some is volume discounts and also there is a difference if you pay cylinder rental or not. It is somewhat like buying a new car, you have to negotiate a price and some will end up with better pricing than others which seems strange for what you are buying.
The past is there to guide us into the future, not to dwell in.
I did as James suggested and installed the T valves so I could switch back and forth from He to H2 easily. H2 is considerably cheaper than He at ~$175 for a T size of UHP Hydrogen. Locally I have one vendor selling a T UHP He for $327 and the other selling the same size for ~$1100. I was using the latter vendor for most of the rest of my gases. You can guess which vendor I fell back on for He.

I am using dewar LN2 for purge gas and have used it for "Rest" gas. But I went with the hydrogen because I will eventually switch both my GC-MS to H2. Also, I am already using H2 as the carrier for my GC-FID. I've not seen any contamination from the T valves. But I did buy them brand new from the Swagelok dealer instead of half the cost on Amazon because of dubious provenance.

Note: My 5890/5972 can only flow at 0.6mL/min or higher when on hydrogen with a DB-VRX 20m 180um x 1um.
LALman wrote:
I did as James suggested and installed the T valves so I could switch back and forth from He to H2 easily. H2 is considerably cheaper than He at ~$175 for a T size of UHP Hydrogen. Locally I have one vendor selling a T UHP He for $327 and the other selling the same size for ~$1100. I was using the latter vendor for most of the rest of my gases. You can guess which vendor I fell back on for He.

I am using dewar LN2 for purge gas and have used it for "Rest" gas. But I went with the hydrogen because I will eventually switch both my GC-MS to H2. Also, I am already using H2 as the carrier for my GC-FID. I've not seen any contamination from the T valves. But I did buy them brand new from the Swagelok dealer instead of half the cost on Amazon because of dubious provenance.

Note: My 5890/5972 can only flow at 0.6mL/min or higher when on hydrogen with a DB-VRX 20m 180um x 1um.


When I was trying H2 for Volatiles I went with a longer column of 40m or 60m to increase the head pressure so I could achieve lower flows of H2. The run is a little longer but not terribly so and with a higher head pressure I had better control of the flow so the retention times were more reproducible. The same works for a semivolatile injection of Methylene Chloride, if you can run a longer column to increase the head pressure you reduce the backflash of the solvent for better peak shapes, and with H2 the run times are no longer than the shorter column using Helium.
The past is there to guide us into the future, not to dwell in.
Due partly to the situation in E. Europe the helium situation came to a head and it no longer was a price issue Airgas rationed us at about 30%. My new parent company went through about 20 cylinders last year so we are scrambling to do draconian conservation. We are getting nitrogen cylinders for all the makeups and we are getting the 3 way valves for the inlet and decided to go with hydrogen as a rest gas. That way if they forget to switch back and run it it shouldn't hurt anything and it will help clean the instruments.
We received a similar notice from Airgas, 30-50% reduction. We are already running in a high conservation mode so that much decrease is going to be difficult. Running 524.2 for drinking water volatiles requires helium for purging and to switch to nitrogen purge requires going to 524.4 which has all different QC requirements similar to 524.3 and would also require the time to get new certifications so at best if we switch to that it will be several months before it can be active.

Helium gets distilled from natural gas, and since the production of natural gas in the US has been hindered I imagine that is causing some of the "scarcity" of helium, even though world wide there is more than enough helium at current usage rates to last a few centuries at least.
The past is there to guide us into the future, not to dwell in.
MSCHemist wrote:
Due partly to the situation in E. Europe the helium situation came to a head and it no longer was a price issue Airgas rationed us at about 30%. My new parent company went through about 20 cylinders last year so we are scrambling to do draconian conservation. We are getting nitrogen cylinders for all the makeups and we are getting the 3 way valves for the inlet and decided to go with hydrogen as a rest gas. That way if they forget to switch back and run it it shouldn't hurt anything and it will help clean the instruments.


Could you provide the part number for the 3 way valve you are using? I've been searching for one to use for the same reason.
Restek resells it from Swagelok Restek part #23219 for 1/8" brass $116

It can also be ordered directly from Swagelok cheaper
B-41XS2 $62.10
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