Hi Bill,
Long time no see ... I hope your doing well.
I don't have any answers to your questions ... but I do have lots of
questions about achieving/maintaining good linearity on a GCC setup
and there must be a lot of similarities to a TC-GC.
For openers, my experience is limited to our GCC-III system -
Delta+XL. I have no experience with TC/GC, but wonder if the problems
you are experiencing are simply larger manifestations of the same
fractionation problems that affect 13C - GCC? Chris Rhodes gave a
talk about GCC - IRMS at the 2005 CF-IRMS workshop during which he
made the comment that a GC is "essentially one big fractionation
device" (or some words to that effect) which I found both disturbing
and greatly soothing ...
Paul Eby also gave a great talk at the workshop about GCC CH4
standards and in it made the point that avoiding manual syringe
injections is critical. He recommended using sample loops (see also
his posting to ISOGEOCHEM #009903).
From your posting, I assume you are doing manual injections?
Do you have a dual GCC and TC-GC system? How is the linearity in GCC mode?
My experience is based on manual injections using syringes varying
from 10 ul to 250 ul in size. I don't see how to EASILY avoid using
different sized syringes (and inlet splits) as the concentrations of
gases we receive vary so widely and sometimes we have very little gas
to work with. I measure linearity daily, and typically get 0.15
permil/[V] (or worse ... yikes) over 0.5 to 5.0 [V]. Trying a sample
loop is on my "list of things to do" but I haven't figured out what
modification are needed to the GC inlet (do you plug the purge vent?
If so, where? Must the sample loop be heated? What is a small volume
sample loop going to do to the EPC of the GC electronics?) Any
comments from the list are welcome ...
Something else which might give you a clue is a discussion on
ISOGEOCHEM about HD and "time shifts" (ISOGEOCHEM #11774).
Sorry I can't be of more help, but I'm interested in hearing any
responses from the list on how to improve (and maintain) good GC -
IRMS linearity ...
Cheers,
Steve
At 11:27 AM 11/21/2005, you wrote:
>Good Morning/ afternoon everyone!.
>
>We have been experiencing a problem with our Delta+XL Trace TC/GC system
>running (at the moment) Methane for Hydrogen Isotopes.
>
>While reproducibility is good as are Stability and H3 values
>(6.5ish) we have a linearity
>drift of 10+ per mil between high and low injection sizes - I am not
>worried too much
>about this as it seems steady and reproducible so can be corrected
>for if need be.
>
>However, increasing the GC temp (55C to 100C) will shift the values
>some 15-20 per
>mil (more positive). It also changes (cleaves) the 3/2 ratio trace
>somewhat). You can
>see the potential problem when we want to look at any other
>hydrocarbon fractions that
>would involve temp ramping.
>
>Since we are still new to the system, we are a little lost as to
>"WHAT IS NORMAL"?.
>
>We have spent the last weeks doing the more obvious things, changing
>septa syringes, liner cleaning and leak checks. We have also replaced
>guard column and swapped the column (CarbonPlot) as well but the troubles
>seems to remain with the same temp effected, floating values.
>
>While doing are swapping of bits listed about we sprung a major leak
>related to the
>furnace tube and/or connections. We took this opportunity to put in
>a new Aluminium
>Oxide and remove potential issues with it. We are still seeing this
>shift - back to square
>one - which has lead me to call out for assistance.
>
>Tuning mass 18 in the cup 29 gives a water peak of 540mV (higher
>then the <300mV
>listed but not a panic situation) and mass 30 in the 30 cup at 145mV
>(which is oddly
>higher then the 30-50mV listed). These could indicate nafion
>troubles and the NO
>mass30 may indicate reduction over (Cu) death. This is bypassing the GC flow
>(backflush open) and through the GC Combustion lll unit.
>
>1) If we are not using the Red reactor (@ room temp) can is still
>poison the makeup He
>stream?
>
>2) How much shifting would you expect (if any) in hydrogen values with GC
>temp manipulation?
>
>3) Can you get really bad linearity even though the H3 and On-Off
>Stability looks good?
>
>Sorry for the long ramble but I am hoping to provide you all with as
>much info as
>possible.
>
>Hope you're all having a better time then we are heheh.
>
>Cheers,
>
>
>Bill Mark
>Isotope Technician
>Environmental Isotope Lab
>University of Waterloo
>Phone (519)-885-1211 Ext 6747
>FAX (519)-746-0183
>http://sciborg.uwaterloo.ca/research_groups/eilab
Stephen Taylor
Lab Manager, Isotope Science Lab,
University of Calgary, Physics & Astronomy,
2500 University Dr. N.W.
Calgary, Alberta,
Canada, T2N 1N4
(p) 403 220 8268
(f) 403 220 7773
[log in to unmask]
www.phas.ucalgary.ca/isl
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