Loss of sensitivity when switching to H2 as carrier gas

[anamariboyes]

I switched from He to H2 as carrier gas on my GCMS.
I have a Thermo Fisher ISQ LT.

I mainly do SPME taste and odor analysis, and ever since I switched to H2, I cannot get a decent calibration or repeatability of standards. I tried running reps of known standards and have aprox. 40% loss of concentration .
I have tried standards at the high end of the curve and low end, both resulting in poor repeatability.

Repeller voltage is 13.8

Is loss of sensitivity normal when using H2 as carrier?

Standards are all new.

Any insight out there?

Anamari

[Steve Reimer]

There is a loss of sensitivity with hydrogen. How much will depend on your system. I assume that you have looked at the Thermo presentation on hydrogen in mass spec. Have you contacted someone at Thermo?

[anamariboyes]

Yes, I have contacted Thermo. They said I shouldn't see much of a loss with H2 but at such low concentrations they couldn't guarantee it.
I am trying to modify my method to see if minor tweaks can help with the issue. Thanks for the reply.

Anamari

[rb6banjo]

I watched an Agilent webinar on this topic and they indicated that you would likely see a loss in sensitivity. For me, that's very bad as I'm almost always looking for a "micro-sniff" of some off-flavor/off-aroma compound. So, I won't switch unless absolutely forced to do so.

[James_Ball]

You may need to reduce your carrier flow rate to regain your sensitivity. H2 doesn't pump out as well as He does so you will have less vacuum at the same flow rate. Most want H2 so they can run the faster analysis, but with MS it doesn't work that way. Even at the same ml/min flow rate you will see a slightly faster run time, but the MS will not let you take advantage of the very high flows you can get with H2 in regular GC systems.

Also make sure you retune with the H2 carrier, the setting will possibly be different, I know it was different when I tried H2 on my Agilent systems.

[anamariboyes]

Thanks everyone.
RB6banjo, I too look for micro sniff stuff so it is affecting my sensitivity at such low concentrations, and unfortunately, I was kindly "obliged" to switch over. The cost of He I guess was becoming too high. Now I have to learn to work with the change...oh well.

James_Ball, thanks for the advice to reduce my carrier gas. I did actually reduce from 1.2ml/min to 1.0 ml/min. I may have to try maybe slightly lower rate to see if this enhances the sensitivity any more.

I did a re-tune once I changed out my ion source with the appropriate one for H2. I also played around with oven ramping temps since my compounds are showing up a bit earlier and I did not want to "over bake".

I may be faced with having to draw a new minimum detection limit if I cannot drop down to the 2 ppt that I once had with He...the good ole days.
Thanks much, all the support is wonderful.
Anamari

[mckrause]

Remember that flow rates for H2 and He are radically different to get the same linear velocity!! Plus, the pumping issue is much more pronounced if you are using a turbo rather than a diffusion stack - a dif actually pumps H2 slightly more efficiently than He.

While you can use higher linear velocities with H2 and not suffer a significant loss in separation, I highly recommend running the H2 at the LOWEST linear velocity that will afford you separation for MS. Lots and lots and lots of bad things happen in the MS with H2; if you minimize your flow rates you minimize the other effects as well.