Jonanthan - its most likely the H2 gas you are using for your reference -
many, if not most, North American suppliers of H2 compressed research gases
have dD values lower than -1000 permil, making the peak very hard to find.
You may need to find a different supplier or get someone to send you an
ampoule of H2 gas with "normal" dD values. Len
Leonard I. Wassenaar, Ph.D.
National Water Research Institute
11 Innovation Blvd.
tel: (306) 975-5747
fax: (306) 975-5143
From: Jonathan Horton [mailto:[log in to unmask]]
Sent: Monday, January 22, 2001 11:18 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] HD problems on Finnegan delta plus
I am using a new delta plus and am struggling to find hydrogen on it
(dual-inlet mode). I fill the bellows, and have looked for a peak with
BDAC value around 1000; I have tried retuning the source, but to no avail.
The instrument has been used with the precon for N2O, where it worked just
fine. Specifically what I've done is:
1) close SGE and look at background on cups 3,2,and 1 using B-scan (a
little water, N2, nothing else) (I can't figure out how to do a B-SCAN
using cups 7 and 8, which would be more useful in this case...)
2) put Oztech standard H2 into inlet A1 and into bellows (pressure
increases in bellows to >100) (I've tried 2 different standard bottles to
rule out bad gas)
3) leave source tuning as was for CO2; adjust BDAC from 10 to 3000 in
4) I find a very large peak (>10V) in cup 7 (mass 2) at around BDAC 2500;
small peak (0.2 V) in cup 7 at BDAC 1300; but at no point during the scan
is there ANY change in the voltage on cup 8 (mass 3) - it stays constant
5) changed source tuning settings to match values given for old (returned)
delta plus for HD mode (I don't have a record of source tunings used for
HD during install); repeat step 3, result same as item 4
6) when I get a peak (the large one at BDAC 2500 and the small one at BDAC
1300), I've looked at the peak shape - cup 7 has a peak shape, cup 8 is
always a flat line.
7) I've done a baseline check to see if the amplifiers are working: the
baselines (source off) for cups 7 and 8 are around 185, voltage readings
are -10 and -8 (respectively). With the source on, baselines are 210 and
185, voltage readings +8 and -8. So, turning the source on does not change
the baseline in cup 8, but does slightly change it in cup 7. I hope this
doesn't mean there's anything wrong with the amplifiers on cup 8... ???
I'm assuming that I still don't have the right combination of BDAC, HVDAC
and source settings (X1, extraction, etc.) to find the right signal. What
procedure should I use to try to find the correct settings? I assume there
are standard starting points, and that one simply tweaks the knobs in a
particular order to find the right settings. Or, maybe not! Is there a
calibration procedure for HD? In short, what do you do with a new
instrument to find the HD peak?
Thank you in advance for your help,
Department of Biology
Virginia Polytechnic Institute
and State University
Blacksburg, VA 24060-0406
phone: (540) 231-8713
fax: (540) 231-9307
email: [log in to unmask]