Not seeing m/z above ~60 on Saturn 2000 MS.

[nickos012002]

Hi everyone

I have a Varian Saturn 2000 connected to a Varian 3800 GC.

The machine was running fine, showing a full range of m/z. I turned the machine off overnight and upon turning the machine on in the morning, the response was significantly lower and the MS was only detecting m/z lower than 100. An alkane reference sample containing alkanes up to C20 is still only showing peaks at ~55m/z.

The air is "OK" and the water is "high". This has always been like this - even when it was working and seeing high m/z's

The RF adjustment shows a straight line with a positive gradient showing all molecular weights from low to high.

The pump is pulling a good vacuum, the Vacuum current is ~180 and the ion gauge is reading 22uTor.

I have given both the column and the MS a bake out with no effect.

Any ideas people???

[JMB]

1. NEVER shut-down high vacuum equipment (ONLY for maintenance)

2. See 1 above

3. When you "turned the machine off overnight", did you vent or allow to come to atmospheric pressure ?

4. You should not have a "high" water signal.

5. I'm not familiar with your specific MS instrumentation, so cannot answer with detailed instructions. In general, continue the bake-out and pump-down. When stabilized, attempt to tune out to full mass range.

6. Does the Saturn have oil diffusion or rotary pumps ? You may have oil contamination on source/mass analyzer; if so, they will need cleaning.

[nickos012002]

Thanks for your response.

I guess i should clarify - i turned off the GC part of the instrument and the software. The MS part was left on overnight with the pump running. As soon as i turn the computer/software on, the MS side of the system runs well, indicating no leaks.

The saturn is running an oil diffusion pump.

If there is oil particulates in the trap, what would be indicative of this???

When you say "tune out to full mass range" Is that by adjusting the RF tuning? The RF tuning is showing a straight line with a positive gradient showing low to high molecular weights.

Sorry about all the questions - i'm a GC-MS newbie!

Cheers
Nick

[JMB]

Poor sensitivity is usually caused by,

1. dirty rods and/or ionization source
2. failing electron multiplier
3. faulty power supplies
4. faulty tuning
5. and for a GC/MS, dirty inlet/outlet of GC system. Clean/ replace glass liner; cut off the first inch (and maybe last inch) of capillary column. EDIT: make sure to get square, clean ends !!

As I said, I have no specific knowledge of the Saturn 2000. If the mass analyzer is a quadrupole, then the normal quad. bias against higher m/z values may have been exacerbated by dirty rods----hence suggestion to clean the rods.
Does the straight line, positive gradient RF tune indicate that the power supplies are OK out to high mass ?

Assume you are running in EI mode ???? Has the filament sagged and is not giving the full electron beam into the source ? Is the repeller dirty ?

For MS, cleanliness is next to godliness----so do the cleanings before suspecting faulty electronics.

Good Luck.

[James_Ball]

The 2000 would be the Varian Ion Trap I believe. We inherited one here in the lab from an acquisition and I am still trying to figure it out, totally different from a quad instrument.

Have you run through the full menu of auto tuning parameters yet?

You may want to search for tuning of ion traps to get more information. I wish I knew more about them to help.

[mckrause]

The 2000 is an ion trap, so different problems. I think you may be seeing an electronics problem, because you're seeing the opposite of what you normally would have if it were a tuning issues (ion traps detect high mass more readily than low mass, for whatever reason). However, I would certainly start with an autotune and make sure you can see all of the peaks in PFTBA.

[nickos012002]

Thanks for the replies folks. So it looks like an electronics issue. Is that easily fixed? I guess thats a "How long is a piece of string?" question!!!

The machine is currently operating as a GC-FID so i'll see how i go when i fire up the GC-MS again.

[MJD]

Hi

Did you manage to fix your issue?

You can check the hardware by performing a Diagnostic (from manual control click diagnostics)

If they all pass then probably you are losing the high mass ions because there are too many low mass ions (water!). It can take a while for the trap to get rid of all the humidity.

I wouldn't normally recommend turning off the GC and allowing the MS to remain on, I hope you cooled the transfer line, otherwise there is a risk of damaging that part of the analytical column and the stationary phase released will make the trap dirty.

Regards

Martin
http://www.atlas-labservices.com