Jin,

I agree with Andrew Tait concerning your now dead ion gauge and that when you have
fixed the electronics then you can move on to finding your leak.

You should be able to find a leak that size easily with ethanol or acetone.  For
this you do not need to turn the source on but just watch the ion gauge reading.
Spray the solvent from a squeezy bottle slowly across each joint and gasket in the
high vacuum region.  When you find the leak you will see a large change in the ion
gauge reading.  If the reading goes up the solvent is being sucked in and if the
reading goes down the solvent has temporarily plugged the hole.

Do NOT use water to leak check.

In either case you will have to either tighten the joint or replace the seal.  You
did not say the age of the Optima but if it is an early one then the most likely
suspect are the viton seals for the source and collector housing lids.  Best to
replace these seals with new ones.

If you want to use a gas to leak check I must disagree with Andrew, don't use
nitrogen use argon.  As argon is only 1% of air the leaks are easier to see.  You
must then tune the mass spec to mass 40.  If  you have difficulty finding mass 40
let an aliquot of argon into the inlet system (if you have one).

This raises another question about your instrument.  Do you have a dual inlet as
well as continuous flow.  If you do have a dual inlet are all the valves leak
tight and is the waste turbo pump working correctly.

One final point.  For stable isotope instruments there is no need to ever degas an
ion gauge, you only risk causing damage (usually to the gauge filaments).  And you
should only degas an ion gauge with a vacuum better than 10-8 mbar.  If the gauge
is suspect, degassing will make it worse not better.

Cheers

Peter Stow

Jin Wang wrote:

> Dear list members:
> We have a Micromass Optima with Isochrome continueous flow interface here at
> Iowa State University. Recently we are having a vacuum problem. When CRT7
> valve is closed, the vacuum reading is only 3.6E-6 (normally it is
> about 1E-8 mbar). This happened gradually. If we reopened and then closed the
> valve, we can occasionally get a vacuum of 1E-7mbar. But now we can only get
> 3.6E-6mbar no matter the CRT7 valve is closed or not. We changed a new
> copper pad seal, a new knife edge and a new wavy washer, the vacuum is still
> 3.6E-6mbar. We degassed ion gauge, the reading is still same. When we tried to
> degas the ion gauge again, the computer gave us message, it reads "ion gauge
> 1 bad calibration data" and no vacuum reading in ion gauge display. I
> rebooted the computer but everytime when I try to turn the ion gauge on, I
> gets the same message. Any suggestions regarding to these two problems will
> be greatly appreciated.
> ---
>
> Yuexia Liang
> [log in to unmask]         U            1127 HNSB
> (515)294-9421(LAB)       S
> (515)294-3926(O)         A            Iowa State University



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