Random Oxygen (m/z 32) peak in EPA 8270 runs on a HP5973

[Darber99]

I'm having oxygen peaks occur at random retention times throughout some instrument blank runs (usually only one peak per run but can be rather large) after cleaning my source. PFTBA scan shows proper vacuum with no leaks (N2 (>4%) and O2 (~1%)at resting state and tunes properly. I've heard of the diffusion pump "burping", but do not know what could cause this. Any help would be appreciated. Much thanks.

Darber

[Don_Hilton]

I've known seals in a mass spec to "burp." The duration is typically short and you see m/z 29 and 40 with 32. Are these spikes narrower than your chromatographic peaks and do you see the other ions associated with air with these spikes?

[Darber99]

Yes, mass 40 (~40%-45% of 32) and 29 (~10%) are present in the peak. Carbon dioxide (m/z 44) is also present at about 30% of Oxygen. We only scan down to m/z 29 to avoid dealing with Nitrogen. Which seals on the MS would be causing this?

[Don_Hilton]

I don't remember the '73 very well. But I think I remember an o-ring type of seal that seals the instrument on the portion that gives you access to the ion source. (This is a problem that occurs with 0-rings in other types of mass specs as well.) Clean the rubber seal carefully. A bit of dust lying across the seal can cause these burps. And, inspect the seals. If you have had a particle trapped in the seal, it can cause a deformation, which will allow for leakage - like burps.

I would suggest step 1 to be vent the instrument, check and clean seals ( methanol on a chemwipe will do nicely) and pump it back down.

I'll leave my favorite step 2 for later. there is a technique I learned to use on a '72, but might make a mess of the seals on the '73.

Perhaps someone from Agelent will jump in and give a better step 2.

One other thign you can do is get a can of dust-off or something similar and spray along the seals in the mass spec. If the spectrum of the burps changes to look like the spectrum of the gas in the dust-off can, you are probably close to the leak. (But this works only if the burping is sufficiently frequent to work this technique in a reasonable time frame.)

[aldehyde]

There is a seal around the analyzer door and another o ring around the diff pump. Did you change diff fluid or something recently? If it hasn't been messed with in awhile I would doubt that it would spontaneously start having trouble.

What maintenance have you done recently?

You can try getting a can of compressed duster, check the CAS number on the back to find out whats in it (usually a halogenated ethane) and then go in manual tune and monitor that ion in profile mode. Spray the compressed gas around the seals of the vent valve, analyzer door, MSD interface and see if you get a big spike.

Another option is to stick a chunk of dry ice in a plastic solvent bottle and spray CO2 around those same trouble spots while monitoring ion 44.

Its interesting that you're not getting a constant leak, does it happen only when things are heated? Maybe your interface ferrule is cracked and you only get a leak when the oven heats up.

You may want to block the interface with a new nut and ferrule, vent the system and just stick the column in like normal and trim it a few inches from the interface and block it with a septa (or if you have a no-hole ferrule use that.) The system will pump down much faster than with an actual column installed and it will allow you to 'half split.'

Don't forget you could have a crack somewhere around the inlet, blocking off the MSD and leak goes away = problem is in the GC side.

I had a leak once that only presented itself with hydrogen carrier but if I switched to nitrogen it was fine, and only for low split ratios. Once I monkeyed around with the inlet enough trying to figure it out the leak got bigger and was happened regardless of what gas I used, but small cracks can be particularly hard to pinpoint when they only occur under certain conditions like high T.

[Bigbear]

If you are running in splitless mode, some air can enter through the injection port

[AICMM]

Why are you scanning that low? If you really need an ion that low somewhere in your chromatogram, can you set it up to scan that low for just that peak?

Best regards,

AICMM

[mckrause]

AICMM is correct - no need to scan that low in 8270. Start your scanning at 35 - you'll save yourself a lot of headaches.

You have classic symptoms of a cracked ferrule. I'd replace the interface ferrule - if you are serious about running 8270 then move to the SGE aluminum ferrules - incomparable!

Don mentions wiping down the o-rings with methanol. I'd go farther - give them a good dosing of methanol. It swells the o-ring slightly and you'll get a better seal. Also, make sure you wipe all contact surfaces carefully - the '73 is really leaky around the o-ring.

What kind of septa are you using? if your syringe is coring and you're accumulating in the injector then you could be experiencing different problems.