GC/MS quality gase

[d***@hotmail.com]

Hi,

I'm currently restoring a Finnigan MAT GCQ with Finnigan GC back to
service. I'm using 99.999% helium bought from Praxaix as carrier gas
for the GC and as damping/vent gas in the MS. The air/water mass spec
looks sort of OK. The peak for H2O+, N2+ and O2+ are all well over
20%
of the H3O+ peak, but I'm concerned about a high ion (TIC) count in
the air/water spectrum. Praxair says their 99.999% helium contains
less than 2 ppm of water but I think there should be well under 1 ppm
water for sensitivity. I'm sure the helium we have is more than
adequate as damping/vent gas in the MS. My question is what are the
best suppliers of GC/MS quality hydrogen as carrier gas and MS
quality
methane as CI gas? Price is a consideration.


My second question is related to the use of taps. We've priced out a
triple purpose water/oxygen/hydrocarbon which is less expensive than
the price differential between 99.999% helium and the best research
quality helium from Praxair. However, traps can create clutter and
other problems. What is the current wisdom concerning the use of
traps? Can we get GC/MS quality hydrogen and methane and avoid the
use
of traps and do so economically over time? The triple purpose trap is
rated for two years use.


d.

[Richard]

Post by d***@hotmail.com
Hi,
I'm currently restoring a Finnigan MAT GCQ with Finnigan GC back to
service. I'm using 99.999% helium bought from Praxaix as carrier gas
for the GC and as damping/vent gas in the MS. The air/water mass spec
looks sort of OK. The peak for H2O+, N2+ and O2+ are all well over
20%
of the H3O+ peak, but I'm concerned about a high ion (TIC) count in
the air/water spectrum. Praxair says their 99.999% helium contains
less than 2 ppm of water but I think there should be well under 1 ppm
water for sensitivity. I'm sure the helium we have is more than
adequate as damping/vent gas in the MS. My question is what are the
best suppliers of GC/MS quality hydrogen as carrier gas and MS
quality
methane as CI gas? Price is a consideration.
My second question is related to the use of taps. We've priced out a
triple purpose water/oxygen/hydrocarbon which is less expensive than
the price differential between 99.999% helium and the best research
quality helium from Praxair. However, traps can create clutter and
other problems. What is the current wisdom concerning the use of
traps? Can we get GC/MS quality hydrogen and methane and avoid the
use
of traps and do so economically over time? The triple purpose trap is
rated for two years use.
d.

That is starting to sound like a leak in your MS. Have you tried a
different gas cylinder to see if the air/water goes away? If not its
your vacuum system. As I said before, UHP gases are all we ever use
in my lab (seven GC/MS). You have to distinguish between water and O2/
N2 in your system. Water can take a while to pump out, especially if
a vacuum system has been sitting vented for a long time. We expect N2
and O2 to be a very low percent of the m/z 69 peak during tuning.

The trouble with using traps and lower quality gas is the trap gets
used up at unpredictable rates. Its not possible to give a trap
rating of x years use. Depends on how much water/organics are being
absorbed from the gas and one bad cylinder can waste a new trap. What
happens when a trap gets "full" is known as breakthrough- the stuff
its trying to filter out suddenly starts coming through the trap and
into your MS. Then there are is the work wasted while you figure out
where the background and ghost peaks are coming from.

You still don't say where your are. We have had very good luck with
Airgas and Matheson cylinders. However any company can have a bad
cylinder for some reason.

Richard

[Jim]

I agree with Richard about the possibility of a leak in the vacuum
system. Have you checked? In our lab, we use "dust off" to check for
leaks and look up the CAS # to see what the spectrum looks like for
"dust off" and look for the 3 highest M/z peaks while spraying around
the transfer line, gaskets, ion gagae tube, etc in the vacuum system.

If you use a trap, use an indicating trap on the end of your trap system
to show when its used up. We always use traps on our systems.

Jim

[Ronald Thompson]

***@hotmail.com wrote:


I currently use Airgas's BIP grade helium in my GC/MS. The cylinder has
a built in filter. When we made the switch a few years ago we saw a
decrease in air/water/oxygen compared to UHP grade.

Ron