GC-MS 6890/5973 carrier

[Archimonde1308]

Given how expensive helium is, is nitrogen a good carrier for the 5973 MSD if the compounds I am scanning are all having molecular weights above 70 amu? The column is an HP-5 non polar column, 30m x .32um *.25um

[MSCHemist]

From what I've heard N2 is crappy for MS ionization. It is like having basketballs bouncing arround in the source. N2 is too large a molecule.

[mckrause]

You have several problems with N2 - the most severe is the Van Deemter plot is steep. You have to run at optimum velocity or really lose plates count in a hurry. Unfortunately, since the MS is a vacuum source, the head pressure for optimum nitrogen flow is a negative pressure. You can fix this to some extent by installing a restrictor between the column and the MS, but it's not ideal.

You do lose some ionization efficiency, but if you're running a turbo you'll make some of it back up in better vacuum (turbos are a lot more efficient for nitrogen than for helium). MSChemist is correct in that nitrogen has a huge electron capture cross-section relative to helium, but if you have filament control you can increase the filament current and gain back some of the ionization.

Chromatography is going to be SLOW compared to helium - probably take about 50% more time for the equivalent chromatography, and you literally can't do it without electronic pressure control. You may get some tuning issues - we've only tried it for BFB, so I don't know what happens if you try to tune to DFTPP criteria.

[James_Ball]

You have several problems with N2 - the most severe is the Van Deemter plot is steep. You have to run at optimum velocity or really lose plates count in a hurry. Unfortunately, since the MS is a vacuum source, the head pressure for optimum nitrogen flow is a negative pressure. You can fix this to some extent by installing a restrictor between the column and the MS, but it's not ideal.

You do lose some ionization efficiency, but if you're running a turbo you'll make some of it back up in better vacuum (turbos are a lot more efficient for nitrogen than for helium). MSChemist is correct in that nitrogen has a huge electron capture cross-section relative to helium, but if you have filament control you can increase the filament current and gain back some of the ionization.

Chromatography is going to be SLOW compared to helium - probably take about 50% more time for the equivalent chromatography, and you literally can't do it without electronic pressure control. You may get some tuning issues - we've only tried it for BFB, so I don't know what happens if you try to tune to DFTPP criteria.

May want to try H2 carrier, if you are not doing EPA specific tune work.

Right now I am experimenting with changing one of my 5973s over to H2 but am having a little trouble with the m/z 95-96 ratio in BFB. Pass is 5-9% and when I started out it was 100% 95 and 80% 96, got it down to 16% 96 and today I replaced my small diameter draw out plate with the large diameter one suggested by Agilent. I hope that gets me the other 8-10% reduction I need.