In my opinion it is less likely to be a block in the sample needle, as that
helium flow only feeds the column during the time that the Valco is making
an injection. Sample needle blockages (and we've had plenty) usually result
in a small first peak, followed by normal peaks with normal retention times
(in the case of partial blocks) or no peaks at all (complete block).
It could be something to do with the helium distributor (in front of the
rearmost Nafion dryer). This stainless steel line with brass Swagelok fittings
supplies Helium to all the bits of the Gas Bench which need it. If there is a
leak at one of the fittings, that would reduce the Helium flow to the column.
Similarly, the very fine capillary which feeds from this distributor to the
column via the Valco valve could be partially occluded, or the ferrules that
connect at the distributor or the Valco could also be partially blocking the
capillary (or leaking). Check these ferrules and fittings, and cut the capillary
slightly shorter at ecah end if in doubt about whether the ends are clean or
not (don't chop too much though, otherwise you'll get too much flow and
short retention times). Whatch out for al those other capillaries when doing
this though (I always seem to break at least one other one when working
inside the Gasbench!).
You can check the flow at the outlet of the column (needle valve shut, of
course). I can't remember exactly what it should be (0.7-1ml/min?) but it is
stated in the Gasbench manual. Don't bother checking the flow the other
side of the column (valco side) because (a) you'll have to take the column
temp down to ambient and take the helium pressure down very gradually
and (b) the flow rate recommended above is dependent on the colulm being
there, as it is the major flow restrictor.
Good luck!
Simon
Date sent: Tue, 23 Sep 2008 13:55:52 -0400
From: Penny Higgins <[log in to unmask]>
Subject: Sudden change in sample elution times - Gas
Bench
All,
I've been writing to this list a lot lately. The mass spectrometer is
finally behaving properly and I am finally trying to fix a problem
that appeared just as everything else went crazy.
When I run carbonate samples using our Gas Bench, typically it takes
about 300 seconds for the carbon dioxide sample peak to come out.
This has been the case for more than a year. Then, literally
overnight, this changed to about 400 seconds. Nothing on the Gasbench
was changed during the intervening night.
Since then, I've adjusted the GC temperature, which only changes the
elution times by a few 10's of seconds at most. I've also adjusted
the Helium carrier pressure, which makes the peaks come out at the
right time, but the flow is so rapid that my sample peaks diminish
far too quickly for an adequate analysis.
What am I missing?
Thanks,
~Penny
*******************************************************************
Dr. Pennilyn Higgins
Research Associate
"SIREAL"
Stable Isotope Ratios in the Environment Analytical Laboratory
Department of Earth and Environmental Sciences
University of Rochester
227 Hutchison Hall
Rochester, NY 14627
[log in to unmask]
Office: 209b Hutchison Hall Lab: 209 Hutchison Hall
Voice : (585) 275-0601 Outer lab: (585) 273-1405
FAX : (585) 244-5689 Inner lab: (585) 273-1397
http://www.earth.rochester.edu/SIREAL/index.html
*******************************************************************
--
Simon Eaton,
Senior Lecturer in Paediatric Surgery and Metabolic Biochemistry
Unit of Paediatric Surgery
Institute of Child Health
30 Guilford Street
London WC1N 1EH
Phone + 44 (0)20 7905 2158 (Office, Answer)
+ 44 (0)20 7905 2396 (Laboratory)
Fax + 44 (0)20 7404 6181
Mobile + 44 (0) 7762 180877
|
