Hello David,
If I may add my (late) part to the discussion, temperature stability for
hydrogen equilibration is important, much more than in the case of
CO2-H2O equiliobration. If you're interested have a look at "The
deuterium Isotope Separation Factor between Hydrogen and Liquid Water"
Rolston J.H. et al The journal of Physical Chemistry, Vol.80 No10 1976.
Finding an appropriate insulating material for the MultiPrep sample tray
has gone through several iteration as it had to be insulating of course,
safe up to 90oC as the same tray is used for carbonate reaction and it
had to be resistant to the needle so that it does not block it. The new
black insulating sheet which is now supplied with the MultiPrep sample
trays fulfills all those requirements and if it is not the one you have
you should get in touch with GV.
As far as the KelF discs are concerned, their main purpose was to
isolate CO2 from the rubber septa during the 5 hours of CO2-H2O
equilibration. If you try to keep CO2 in a vial with a rubber septum for
a few hours, it will diffuse throught the septum with the consequences
you can imagine on its isotopic composition. Kel F is a polymer which is
used in some of the valve seats on the dual inlet and which does not
affect CO2 or any other gas you can use in a dual inlet. It would be far
too hard to make a septum out of it which is why it was used as a disc
between the rubber septum and the CO2. As you rightly pointed out, if
you use the autofill procedure, your KelF disc is punctured by the
needle before the equilibration reaction and then becomes useless. That
is why when we used KelF discs for CO2-H2O equilibration, we used a
glove bag to fill the sample vial headspace with CO2, close them inside
the glove bag and put them in the sample tray un-punctured for
equilibration.
As you can imagine, this protocole does not apply to hydrogen!
But hydrogen does not show this diffusion problem with most standard
rubber septa.
Hydrogen equilibration does not require the KelF disc if you only run
hydrogen with your waters.
All that was before the Labco exetainers came to the isotopic world. The
septa in those exetainers holds both CO2 and H2 with no problem at all
and does not require the addition of Kel F. Be careful though, as there
are many different types of Labco exetainers, and different qualities of
exetainer caps. The one you want to order is the reference "778B with
MOSS caps". The larger diameter ones will not fit in the tray.
I must say at this moment with my MultiPrep I don't even use Kel F discs
at all even for CO2 from carnonates as the CO2 produced from the
phosphoric acid reaction is constantly extracted in the external cold
finger so it does not have time to diffuse through the rubber septa. I
haven't run very small samples yet, but certainly above 100 micrograms
KelF or no KelF does not make any difference.
As far as Hokko beads are concerned, I can only advice not to use the
beads as they are but stick them inside a "holder" which makes them easy
to handle. The "Hokko coils" supplied with the MultiPrep is a good
option. If you already have the beads, just buy small stainless steel
coils and do them yourself, it takes time to start with, but having to
put those electrostatic beads in small vials is sometimes challenging.
I would certainly not recommand to shorten the MultiPrep default pumping
times or transfer times. Anyway in Autorun mode, it would not save you
any time as usually sample n+1 is ready to enter the dual inlet before
sample n is analysed.
For equilbration, the default MassLynx parameters shoud be OK, 1atm for
the equilibration gas and 1.5 / 2 hrs equilibration should be OK.
Be careful to have something connected to FC even when running hydrogen
as it is using FC to flush the needle between samples, at least ours does.
One thing which may improve your D/H equilbration analyses is to use a
depleted H2 reference gas on your dual inlet. If you equilibrate natural
abundance waters, the H2 you will end up with after equilibration will
all be in the -700permil region. You can now buy depleted H2 gas
certified around -700permil. We have got some here purchased from Messer
Gresheim.
Sorry to have been so long. I hope this helps.
Best regards,
--
François FOUREL
UMR CNRS 5125 PEPS
Université Claude Bernard Lyon 1
2 rue Raphaël Dubois - Bât. Géode
F-69622 VILLEURBANNE Cedex - FRANCE
Tél : +33 (0) 4 72 44 62 42
FAX : +33 (0) 4 72 43 16 88
Email : [log in to unmask]
David Wheeler a écrit :
> Hello,
>
> Thanks to all who replied. I punched holes in the insulation and did a
> test run with the insulation and without the Kel-F. The results are
> better but still not as good as they should be.
>
> I think the next thing to work on is the timings. I didn't believe the
> timings in the manual (eg capillary transfer time = 15sec) and they were
> not what was set in the parameter files when I took over running the
> machine. Recently I've been wondering whether some of the pumping and
> transfer times are too long. The reason is that sometimes at the end of
> a run the hoko beads are up the side of the vial, which I interpret as
> being due to the water boiling (at 40 degrees) when the vial is
> evacuated for the hydrogen fill. So what sort of times do people allow
> for (1) the pumping out of a vial, (2) the hydrogen fill (I have the
> regulator set to 1 atm.), (3) the transfer of hydrogen to the water trap
> and (4) the drying time?
>
> I equilibrate at 40degrees for 1.5hrs. Is that correct?
>
> Lastly, I've had a look at the Labco site and can't see any vials which
> would fit my multiprep. Am I missing something?
>
> cheers,
> David
>
>
> ¤º°`°º¤ø¤º°`¤º°`°º¤ø¤º°`¤º°`°º¤ø¤º°`¤º°`°º¤ø¤º°`¤º°`°º¤ø¤º°`¤º°`°º¤ø
>
> David Wheeler
>
> Professional Officer
> Geochemistry Laboratory
> School of Earth and Environmental Sciences
> University of Wollongong,
> Wollongong, 2522, NSW
> Australia
>
> Phone 02 4221 4586 (office)
> 02 4226 5903 (lab)
> Fax 02 4221 4250
>
> E-mail [log in to unmask]
>
>
> 'Oh no, we won't give in,
> we'll go living in the past'
>
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>
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