5973 contamination

[cjm]

Hi all,

I'm running a 5973 with a 6890 for air sample analysis. I'm trying to locate the source of some seemingly common contaminants, but I'm struggling to isolate the problem. The system has a concentrator using nitrogen cryo, and I have since removed this from the system, so I just have a 60m Rxi-1 installed directly to the inlet and the MSD.

Originally the issue was high 28 and 32 which I assumed was just an air leak, however when changing parameters, oven, flow, split, splitless, not only do the raw abundances change, but the ratios change as well, and it no longer appears to be air. 28:32 ratio varies between 1.4 and 2.6, and is never consistent. When temps and flows are stable, I can get from <10% 28:69 to over 100%.

Also very strange, is when increasing oven temp to 300 (below the max isothermal of 330) and scanning with PFTBA open, I get 207 to become base, with 281 right behind. 207 jumps around as well, from nearly 200% of 69, to <20%. I see negligible 149 suggesting rubber/plastic isn't the issue.

I've already replaced the nearly two year old column with a fresh one, and still no change. I've capped the MSD and proven it's working ok, with negligible 207, 281, 28, 32.

System does not yet have gas traps installed, however the gas source splits between two systems, and the other system shows no issues.

I'm questioning the source of the ions 207 and 281 in particular. I'm about ready to strip out all the helium lines thinking there's something sitting in the tubing contaminating the system. Any ideas would be appreciated.

[James_Ball]

207 and 281 are normally column bleed, and if there has been that much air flowing through the column when it is at elevated temperatures they will be quite high since the stationary phase has been oxidized.

Since the air is low with the MS inlet capped, the leak must be in the inlet somewhere. It it is the normal 6890 and probably more than 10 years old since it has a 5973, I would begin to suspect there is a crack in the gas lines where they attach to the inlet top. Lifting and lowering these over the years will cause the lines to crack from flexing them over and over as inlet maintenance is done. Especially if they are entering from the left side of the inlet with the lines tucked away nicely under the plastic cover over the inlet plumbing. I reroute mine so the lines come in from the right hand side and when doing maintenance they will lift up through the slot in the cover without needing to bend them to clear the inlet.

If the leak is not there, then it could also be the EPC gas valves, those can also go bad and cause air leaks.

The fact that another instrument on a shared gas line is ok says it is between where the gas line connects to the instrument and the head of the column, so at least it narrows down where to look. I chased a leak like this for months one and that is when I found it in the lines to the inlet top. Very tiny cracks and could only find it with a good leak detector.

[cjm]

Thank you for the response. Because this system is used for "air" analyses, the inlet is rarely touched. Column/Inlet maintenance is performed maybe once over the lifetime of the column, which could exceed two years. I'll try using the leak detector around the EPC as I couldn't find any leaks at the inlet. Hopefully it will reveal itself.

[cjm]

Update: I installed a triple gas trap at the instrument and re-plumbed gas lines. Nitrogen still way elevated, O2 now down to almost zero, exactly what I'd expect to see with a functional trap. So now I'm waiting on a grade 6 helium tank that probably cost a million dollars to prove it's a contaminated tank. Then it's back to the 207/281 ion contamination issue. Hopefully the high air was responsible for the elevated levels of 207/281. Will update next week.

[James_Ball]

If the other system on the shared gas line is also a GCMS and did not show the contamination then it is more likely a leak between where that line splits off from the main line to the affected instrument.

The trap removing oxygen does indicate that it is coming in prior to the instrument. I always like to use one of the small indicating oxygen traps right at the instrument connection to let me know if there is a leak in the incoming gas line or bad tank. If it begins to change color quickly I know there is a problem, and it will also catch the tiny amount of oxygen that gets in with each tank change.

Also Grade 6 is a little overkill, a 4.5 or 5 should be clean enough. But any grade can have a contaminated tank once in a while if they are not checking them well at the supplier.

[cjm]

I've searched at every connection possible for a leak and cannot find one. Our tank (grade 5 purity by the way) regulator is set to 85 psi delivery pressure, which is more than the instrument needs, and even a small leak should be detectable. Also, the instrument with trouble is a 5973, the neighboring instrument is a 5975, both have diffusion pumps. Would you expect air to behave differently?

Also, we have had trouble with a lot of the helium tanks and routinely have to monitor new tanks for spikes in air or nitrogen. Not sure why this is becoming an issue more often.

[James_Ball]

I've searched at every connection possible for a leak and cannot find one. Our tank (grade 5 purity by the way) regulator is set to 85 psi delivery pressure, which is more than the instrument needs, and even a small leak should be detectable. Also, the instrument with trouble is a 5973, the neighboring instrument is a 5975, both have diffusion pumps. Would you expect air to behave differently?

Also, we have had trouble with a lot of the helium tanks and routinely have to monitor new tanks for spikes in air or nitrogen. Not sure why this is becoming an issue more often.

I am pretty sure the 5973 and 5975 use the same oil diffusion pump and are almost identical except for the electronics.

I have had a few leaks that were difficult to find, one was in the interface at the MS. If you take them apart you find it is two concentric stainless steel tubes with a weld joining them together which is hidden inside the heating assembly, I actually had one instrument develop a crack in that weld and was a pain to find until I sprayed Dustoff into the insulation around the heating element. There is also the seal between the interface and the vacuum manifold that can leak. Those both can be intermittent and difficult to find. Another had a crack just past the threads where the column nut attaches, it was more obvious since the tip was bent upwards slightly where someone had been over tightening the nut.

[LALman]

Other than a crack at the weldment, the worst leaking problem I've had with my 5973 is the other end of the weldment assembly; the little block with three o-rings that seats against the end of the EPC. For some reason this is the hardest place to seal for me.Just a little tweek of that block and I get air in my spectrum. I think the little o-rings are too flat, even when new.

[Arne]

Regarding inlet maintenance: Do change the septum if it is pierced for your analyses. That's where the air leak could be.

We have had great success with chasing leaks with "canned air" spray (R134a or similar Fluorocarbon) and lookign for spikes in the 69 m/z peak when spraying on different parts of the GC-MS assembly.

Regarding 207/281: It is normal for them to become much higher at temps > 300°C, especially if there is some air leaking in, as is the case for you.

[LALman]

Update: I installed a triple gas trap at the instrument and re-plumbed gas lines. Nitrogen still way elevated, O2 now down to almost zero, exactly what I'd expect to see with a functional trap. So now I'm waiting on a grade 6 helium tank that probably cost a million dollars to prove it's a contaminated tank. Then it's back to the 207/281 ion contamination issue. Hopefully the high air was responsible for the elevated levels of 207/281. Will update next week.

It cost me $235 to have Oneida Research Services analyze a UHP He 5.0 tank for Nitrogen, Oxygen, Argon, CO2, Moisture, Hydrogen, Methane, Ammonia, and Helium. Mine had 192 ppm N2, 18ppm Ar, and 3 ppm H2; making about 60,000 counts of background. They will send you the test cylinder if you need to do it.

[LALman]

If the other system on the shared gas line is also a GCMS and did not show the contamination then it is more likely a leak between where that line splits off from the main line to the affected instrument.

The trap removing oxygen does indicate that it is coming in prior to the instrument. I always like to use one of the small indicating oxygen traps right at the instrument connection to let me know if there is a leak in the incoming gas line or bad tank. If it begins to change color quickly I know there is a problem, and it will also catch the tiny amount of oxygen that gets in with each tank change.

Also Grade 6 is a little overkill, a 4.5 or 5 should be clean enough. But any grade can have a contaminated tank once in a while if they are not checking them well at the supplier.

I have a 207 peak all the time. But its not causing me any problems for 8260VOC. It accumulates overnight on my column and I think the source might be the bit of column in my inlet getting oxygen. I've found that for my 5973, its where the weldment attaches to the EPC that I have the most leaks. Just a little wiggle there and I get air leaks in my system. Leak detector won't help for that spot but you can spray difluoroethane "canned air" at that joint and looking for mass 51,

Maybe an inert section of column at the inlet and exit would help also. I say this because I have my largest 207 peak on the first run of the day and then its small. This suggests to me that its bleeding from the hot inlet section in the inlet.

By the way have any of you noticed a tetrahydrofuran peak coming from standby trap flow using dewar sourced nitrogen for purge gas?

[James_Ball]

If the other system on the shared gas line is also a GCMS and did not show the contamination then it is more likely a leak between where that line splits off from the main line to the affected instrument.

The trap removing oxygen does indicate that it is coming in prior to the instrument. I always like to use one of the small indicating oxygen traps right at the instrument connection to let me know if there is a leak in the incoming gas line or bad tank. If it begins to change color quickly I know there is a problem, and it will also catch the tiny amount of oxygen that gets in with each tank change.

Also Grade 6 is a little overkill, a 4.5 or 5 should be clean enough. But any grade can have a contaminated tank once in a while if they are not checking them well at the supplier.

I have a 207 peak all the time. But its not causing me any problems for 8260VOC. It accumulates overnight on my column and I think the source might be the bit of column in my inlet getting oxygen. I've found that for my 5973, its where the weldment attaches to the EPC that I have the most leaks. Just a little wiggle there and I get air leaks in my system. Leak detector won't help for that spot but you can spray difluoroethane "canned air" at that joint and looking for mass 51,

Maybe an inert section of column at the inlet and exit would help also. I say this because I have my largest 207 peak on the first run of the day and then its small. This suggests to me that its bleeding from the hot inlet section in the inlet.

By the way have any of you noticed a tetrahydrofuran peak coming from standby trap flow using dewar sourced nitrogen for purge gas?

Haven't used nitrogen for purge yet, but I have seen similar just from the helium carrier if sitting overnight, also a peak that elutes the same as 1,4-Dioxane, but not positive it is really that.

What temperature do you have the inlet and MS transfer line set to?

I normally keep my inlet about 180C, plenty high enough to not freeze out any analytes yet cool enough not to bleed off any of the column. Also they suggest the MS transfer line be kept about 30c lower than the column max or even less to prevent build up of column bleed on the source.

[LALman]

I run my inlet and transfer line both at 200C. My column is an Agilent DB-VRX 20m x 180um x 1 um with a max oven temp of 220C. My runs range from 40C up to 200C at 17 minutes. Then a quick ramp up to 230C with a 1.5min hold. The tetrahydrofuran peak is below MDL during routine runs. The first run of the day its ~150ppb. The line from my LN2 tank is 3/8" trace metal grade translucent PTFE tubing the rest is stainless or copper.

Trying to think of THF sources. The only THF I have in my lab are the cal standards (200 ppm in MeOH). Its a possibility that vinyl coating on bottles or bottle weights are degrading (that greasy goo that forms on any vinyl coated material) and diffusing into my teflon transfer line. Also, my lab drains are PVC so I suppose there could be offgassing. Although its been in place 20 years.

[James_Ball]

I wonder if THF is used when manufacturing the teflon tubing? If it was then that could be a source. Do you let the column rest at 35c over night or more like 150c? I keep mine at 150c on standby to help prevent things like that from collecting on the column.

Try putting a carbon trap just before the P&T on the nitrogen line and see if that helps.