GC/MS Large volume hexane injection overwhelm vacuum?

[MSCHemist]

Hi all I attended a Pittcon presentation on Large Volume GC injection. It got me playing with some numbers. I currently inject 2ul of hexanes in one of my trace analyses pulsed splitless on a 3316 liner. Playing with flowcalc if I set my Quickswap preassure (which will be the column outlet pressure) to 11 psi, and the injector to 180 deg, and the pressure pulse to 30 psi [30m .25.25 db-1) I can inject 8ul of hexanes and still be at 80% liner volume.

My concern would be the MS vacuum. With the current 2ul it goes pretty high into the E-4 torrs [standard turbo usually operates at 3-4E-5 torr]. Would that much hexanes damage the system?

[thohry]

Regarding the analyzer volume and the capacity of the pump, I don't think that's a problem for the system.
Anyway, backflush can be a problem I think.

[James_Ball]

As long as you can delay the filament on until after the hexane is removed from the analyzer you should be ok. I would think most vacuum systems can handle it, but turning on the filament during that high analyzer pressure time will certainly shorten the lifetime of the filaments.

[MSCHemist]

Thanks yea I don't turn the filament on until 6 minutes and may analytes don't come out until 10min. The hexanes are usually out by 4min. Backflash should not be an issue with the low inlet temp and high inlet pressure.

[MSCHemist]

I did the experiment today. I was only able to inject 5ul which was as much as my 7683b would allow with a 10ul syringe and saw nearly a 3 fold increase in area count. If I want to go to 8ul I will need a 25ul syringe.

The total vacuum gets as high as 1.5E-3 torr while the hexanes is coming through though.

[James_Ball]

Something that sounds counter intuitive but we have found to work is using split injections to actually gain area counts versus splitless injections. Using a 1ul injection we gained sensitivity using a 5:1 split versus the sensitivity we had doing a splitless injection, this on both our GCMS and GC analysis. Try the 5ul injection with a 5:1 split and see what happens. It would be interesting to see what the results are with larger injection volumes.

[MSCHemist]

I may give it a try but I am looking at my analytes in the low ppb range. Specifically I was looking at phenylboronic acid derivitized 3-monochloropropane diol and was able to clearly quantitate it down to 5ppb.

Also Agilent recommends against very low splits as the EPC has trouble reprucibly creating the conditions. They claim low reproducibility of injections.