I am using 4.5 grade Helium as carrier gas, it gives an intense 32 m/z peak for oxygen, I set the scan range 33-550 m/z to avoid peak at 32 m/z and increase S/N ratio ( i got the 2-3 fold of better sensitivity / less noise) is it chromatographically right? how it will affect the library search / else? what is the recommended range for scan mode?.
Dear Senthamizh
your carrier gas choice is wrong.
in case of normal GC for best result I will recommend grade 5.5 carrier gas which is about 99.9995% pure.
while for MS the recommendation is grade 6.0 which is 99.99999% pure.
Since GC is very sensitive technique therefore trace impurity will have significant impact on base line stability as well as S/N ratio also your detection limit goes down too.
Furthermore, this contamination will cause in the lowering the life of both column and detector.
as for the question about the scanning range, its depends upon your solvent in which you make sample as well the target molecular weight.
you can chose the range according to your sample or your target compounds.
Regards,
Dr Tariq
Dear Selvan,
Could try an oxygen filter in line, but pricey, and if too much oxygen in gas would wear out fast.
Regards,
Dr John
Hi Szenthamizh,
besides the receommendation you already got (4.5 grade He is really a bad choice): I hope you have tried a scan on mass 28 and/or 40. Just to make sure, that you don´t have a plain leak?
Regards
Stefan
PS Regarding the He: Just for fun, I would like to know if everybody here has currently a problem with He-Supply? Our supplier is heavily cutting down the delivery of He-tanks (we need ~ 15 50l/200bar tanks per month, last delivery was ~ 1 tank per week) due to a temporally shut down of two large He-production reservoirs. We are severly facing the problem and might be forced to shut down half a dozen of isotope mass-specs and a dozen GC´s. And, to anticipate the recommendation: No, due to ongoing projects we cant repace He with H2 at least for the GC´s. Wouldn´t work for mass-specs anyway...
Dear Stefan,
The world helium is running out!
Much was accumulated in old natural gas reservoirs in the USA during the cold war, from natural gas that contained a proportion as a result of billions of years of alpha decay.
These are probably running out.
Probably too many party balloons and squeaky voices at parties (followed by coma) in the west! (Sorry).
I have seen two contrasting situations:
One where my boss was draconian (and quite rightly so) where I had to make a 120 cubic metre tank last all year for three capillary GC's.
This could only be done if very careful leak checking was carrried out last thing on leaving. (By shutting the main valve and monitoring regulator pressure, it should not drop at all).
You can lose half a tank in a night if you do not do this eg from a perforated septum that has not been tightened.
The cylinder was in the lab virtually next to the GC's and linked by one piece of steel tubing.
Next was a defence research lab where helium was (for scaredy-puss health and safety reasons) located one hundred yards away in a concrete bunker (two 120 cubic metre cylinders on a manifold).
This leaked like a sieve and required two new cylinders every week.
I am surprised that the entire lab and area did not speak in falsetto voices!
(I remember having to sign for £20,000 worth in 1980!).
Experience with xenon is interesting (very rare and was £20,000 per cubic metre).
Used in hospitals as anaesthetic.
Here they recycle air from the room where it is used, and have a special rare gases recovery system.
I think I have seen Xenon used for SFC (once!) (MTV Cribs, again!).
It is possible that in the future Helium will have to be recycled and effluent from chromatographs recovered eg in as high a concentration as possible.
Hydrogen is viable (I am not sure about MS) especially with the new fast GC's (Ellutia UK see their site) where special capillary columns (Restek UK) are ohmically (electrically) heated at 1400 degrees per minute and there is technically no oven to explode.
Hope this helps!
Regards,
Dr John
PS: A jet separator before the MS using hydrogen would probably remove most of the hydrogen before entry into the MS (old time technology (Klondike Pete!)
PPS: Perhaps they should try prospecting for helium in granite or other ancient rock areas by drilling! Tiger economies could do this, excellent commercial prospects!
PPS: I tried helium once in the forensic lab in the good old days, where we held a Christmas party in the prep room next to the oxy-natural gas glass blowing torch (complete with oxygen cylinder) (and lit cigarettes), and spilling out into the lab where we extracted blood samples for drug analysis (R and D). (My boss had hep B from addict blood sample extraction in the days when it was not understood).
A good breath from a 120 cm tank resulted in a satisfying mezzo soprano, but paradoxically when I tried to sing the deep base parts of "old man river" my voice reacted in the opposite direction and the pitch went even higher! (Nearly blacked out!). Strange but no doubt explainable from basic physical and physiological principles!)
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