strange 28/32 problem on GC-MSD

[mty]

Hi All,

We are using a 6890/5973 in EI mode. We have a PTV installed, and when it works, it is great and when it doesn't, it is awful. Anyway, we had service yesterday on the PTV (using way too much liquid nitrogen--turned out to be a loose nut somewhere in between the GC and MS). My tech is trying to tune the instrument, and we are seeing 32 and 28, and they pulse every 20 scans or so. They are in the 4:1 ratio I'd expect for an air leak. m/z 32 pulses to about 35-40% of the 69 from PFTBA, and then goes right back down--the pulse lasts about 2-3 scans. I've done troubleshooting on air leaks before, but I have never seen one pulse like this. Does anyone have ideas where to look? This happened after service, so I'm inclined to check things myself at the moment.

Thanks in advance!
Michele

[LCbob]

Could be something to do with the ptv, check what it is doing while you are scanning the air/water, remember there will be delay before the air/water increases. Initially I would remove column from the ptv, plug it with a septa and recheck.

[AICMM]

mty,

I'm curious. Does the problem happen with the inlet set to some high temp like 150 or with the inlet at low temp like 35?

Best regards,

AICMM

[Bigbear]

Is your method splitless? We have found to get the best air/water reading the Agilent GC inlets need to be in split mode. We do this for tuning as well.

[JI2002]

I saw the same problem before. Using the grease from Agilent (part number in the 5973 manual) on the O-ring of MS chamber did the trick. Out of cutiosity, PTV cryo is on the GC, are you sure the nut between GC and MS has anything to do with Liquid N2?

[MX304]

I saw the same problem before. Using the grease from Agilent (part number in the 5973 manual) on the O-ring of MS chamber did the trick. Out of cutiosity, PTV cryo is on the GC, are you sure the nut between GC and MS has anything to do with Liquid N2?

I addition to the MS chamber O-ring, it's a good idea to replace the o-ring that seals the knob that you unscrew to vent the chamber. It's the smaller of the two liner o-ring sizes, so you probably have some lying around. Another trick on the MS door O-ring is to use the oil that goes in a diffusion pump instead of the grease.