Large Volume Injection (LVI)

[WK]

Dear All,
I have a Varian 1079 injector with Saturn GC-MS.
Yesterday I tried for first time L.V.I.
So I used default values:
Injector 70degC - ie below bpt ethanol (solvent)
1 min wait
200degC/min to 250degC
Split on 50:1 for 1 min
off for 2 mins (while injector ramps)
on at 3mins
Column 50degC for 3mins then 20degC/min to 250.

I have used 2ul only and used both normal and slower injection speeds.

Column 30m x 0.25 x 0.25 RTX5 which is in good nick.
Liner 2mm id without wool

Main problem is I get nice sample peak - but for larger components I get
broad identical ghost peaks straight after the main peak.

Hope you can help
Best Regards
WK

[rubyfan]

Are you using the standard 8200 injection mode? If so, make sure your rinse solvent is the same as the sample solvent (ethanol). Mixed solvent injections can make peaks split.

I've had really good luck doing large volume injections with the 1079 injector on our Varian. I routinely use 80uL injection volumes with no issues. I would guess than the standard 2mm unpacked liners are fine for injection volumes < 10uL, but I use a similar liner with silanized wool for injections > 10uL.

Good luck!! Continue to optimize your system, I think you'll like the results once you've figured it out.

[AICMM]

Wk,

It appears to me that you are at odds with yourself. You intend to put a large volume on cool, then you ramp split which means that analyte is splitting with solvent which defeats the whole purpose. I would suggest, if you can, inject, then open splitter for a minute or two (and if possible raise the inlet 10 or 15 C) to vent off the excess etoh, then close the split and crank the injector temp and refocus on cool column. I would also increase your column hold to 4 minutes so that all of the inlet fun is well over (and everything is solidly re-focused) before you begin you temperature ramp.

Best regards.

[WK]

Thanks for the replies:
Rubyfan - have you got some injection parameters that you can share with me? At least I could try to reproduce those and then work from there.
I have tried for 2 days with different split times,injector ramps & column times with no luck - my 1079 takes a while to cool from 250 to 70!
WK

[rubyfan]

WK --

Essentially, this is what I do. All of my solutions are in acetonitrile, so that is what I've optimized for. FYI, acetonitrile boiling point is 82C.

I program the 100uL syringe to uptake the sample at 10uL sec (I generally inject 80uL). The injection rate is 4uL/sec. The injector temperature is 78C for 1.3 minutes, then ramped at 200C/min to 250C. As for the injector split, it is turned on at 75:1 for the first 1.2 minutes, then the split vent closes and doesn't reopen until 3.0 minutes. At that time, I program the injector to open at a 200:1 split. Finally, the GC oven temperature is held at 78C for 4 minute, then I start my temperature ramp.

Hope this is helpful!! Also, I have a tank of CO2 gas connected to my 1079 injector, so it cools very quickly. You might look into purchasing a tank. If you push the GC setup button on the 3800, select option #2 (view heated zones), it should tell you whether your 1079 is configured to work with liquid CO2 or liquid nitrogen. Liquid nitrogen will cool better but you need special gas lines to set this up. I just use standard copper tubing with my tank of CO2 and it performs just fine. Good luck!! Let us know how you make out.

-Aaron

[WK]

Hi Rubyfan
Thanks for the info - I shall modify slightly for ethanol and try.
Then I shall try your conditions for acetonitrile.
I am looking to inject dilute tinctures you see and monoterpenes present have bpt about 150degC so with splitless the monoterpenes peak shapes are poor (later peak shapes are better).
The peak shapes appear much better by LVI except for these annoying wide twin peaks. (Kilimanjiro has twin peaks if you like Monty Python!).
I have both nitro and CO2 available in setup but no plumbing around the back of the GC.
Do your analytes have high bpt/MW?
WK

[rubyfan]

All of my large volume injections are generally for detection of trace level explosives. I guess the highest boiling ones are RDX/Tetryl.

What Saturn system do you have? We have an older Saturn 2000 but I still think it's about the best mass spec available. The sensitivity is phenomenal and the system requires little maintenance. I much prefer using this system for trace level detection rather than our newer Agilent 5975. I'd like to trade in the 5975 and get the Varian triple quad. If only management would let me!!

-Aaron

[WK]

Hi Rubyfan,
It was purchased around 1999 and it does not have MS/MS.
No CI either. Its really simple to pull apart, clean and put back together.
I have concluded that I just cannot trap my monoterpenes since they have boiling points lower than (150degC + initial column temperature).
Its not solvent polarity mis-match either.
I will have to go back to sample extraction and concentration again.
Thanks
WK