Hi,
        I am getting a consistent 165 mV signal at mass 40 on our
EA/ConFlo/Mat252 system, using cup 4.  This suggests a leak in the
system and implies that the peak is in fact Argon 40.  However, there
are tests that I have performed that give rise to doubt that it is
Ar.  Other important points to consider:

1) background voltages on cups 2, 4, and 6 corresponding to masses
28, 29, and 30 are: 43, 32, and 150 mV.  ConFlo II specifications
dictate <50, <50, <200 mV.
2) background voltage on cup 4 for H2O at mass 18 is 1.2V with a
ConFlo II spec of <5V.
3) ConFlo II specifications for Ar in cup 4 should be <40 mV.
4) The only reason we know that mass 40 is high is that we desired to
lower the backgrounds on mass 28, 29, 30 in order to do smaller
sample sizes on the EA.  We decided to leak test the EA-ConFlo-Mat252
system using Ar gas and monitoring mass 40 on cup 4.  This lead to
the discovery that the background value on mass 40 is 4 times higher
than spec.
5) leak testing with Ar at all fittings between the EA and Mat252
(except top of open split in Conflo) shows no apparent leaks.
6) blowing the Argon into the top of the open split (with enough
flow/pressure to overcome He flow in bottom of tube) shows a rise in
signal on mass 40.
7) turning on He dilution in ConFlo control panel cuts mass 40 signal to 60 mV.
8) normal flow rate of He through EA is 120 ml/min at 150 psi.  We
looked at mass 40 signal as a function of flowrate without and with
dilution:
ml/min          without dilution                with dilution
120             156             53
80              228             56
140             142             61
8) If this was an atmospheric leak and if all of the mass 40 signal
was Ar, then a signal of 160 mV at mass 40 should generate a huge
signal at mass 29 corresponding to the atmospheric ratio of N2 to Ar.
9) thinking the mass 40 is Ar and may be contained in the UHP He tank
I created simple U-traps on the He feed line to the EA and the ConFlo
as well as the line out of the EA to the ConFlo and placed them in a
LN trap.  LN has a temp of -196 C, Ar has a boiling point of -186 C.
Ar gas should have been trapped in the LN bath, yet I detected no
drop in mass 40 signal.
10) swapping out UHP He bottles resulted in no significant difference
in mass 40 signal on cup 4.
11) closing SGE valve on MAT252 source block (capillary from ConFlo)
drops mass 40 signal  to 0.000 V
12) C3H4 has a mass of 40.  Can't think of a source of hydrocarbon on
the EA to ConFlo II system to produce C3H4.

Can anyone provide any suggestions?

TIA
Mike
--
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Michael M. Cheatham
312 Heroy Geology Laboratory                    Phone (315)-443-1261
Syracuse University                                     Fax   (315)-443-3363
Syracuse, NY 13244-1070

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http://www.geochemistry.syr.edu/cheatham/InstrPages.html

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