Thank you every one for valuable responses.  It was great to see so much
discussion of H/D mass-spec systematics.  I have three points to make in
this message

1. Specific thanks to those who offered to loan me a magnet

2. Comments on Willi Brand's points about the Zn method

3. A challenge to VG to respond (to their colleague) about the relative
accuracy of Finnigan and VG mass-specs
                    - - - - - - - - - - - - - -
1. Christian France-Lanord of CRPG (Vandoevres les Nancy, France) and
David Bourne (Pro-Vac Services, Crewe, UK) both very kindly offered the
loan of a magnet.  Christian, you will be relieved to hear that I will
borrow from David, since it makes the transport easier (see copy of
response below), but thanks very much for your rapid and positive reply
anyway.
                    - - - - - - - - - - - - - -
2. I have to disagree with Willi about the Zn method (well I would be bound
to!).  To try to separate the problems of accuracy of the preparation
method and of the mass-spectrometry, I think that it would be hard to get
reproducibility of 0.1 to 0.2 per mil if the method were not giving 100%
yields.  The reason for wanting to borrow the magnet is because I have
faith in the old VG flight tube geometry and general measurement hardware -
otherwise how could we have got numbers between -426 and -427 so
consistently? (see the un-massaged data in the original paper).

The method still gives highly reproducible results, except that we now use
BDH Reagent grade Zn shot rather than Analar. (Sorry, John I never bought
any of your Zn to compare with the original method).  Below are the last
four analyses we made on SLAP, prepared by Mike Isaacs and measured on our
VG SIRA series 2 and on someone else's VG Optima.  (These were the data
that forced me to stop measuring H/D on the SIRA, depite the excellent
precision).

SIRA    -332.0 & -332.1

OPTIMA  -444.9 & -444.4

The data are typical of the within-batch reproducibility for the SIRA and I
am confident that if run on a 602 or other old geometry m.s. they would be
-427

(In contrast, carbonates run on the same SIRA series 2 seem to give results
that are both precise and accurate)
                    - - - - - - - - - - - - - -
3. These data deserve a response from  VG/Fisons/???  If anyone from the
former VG factory reads this please could you reply because the argument
has been a bit one-sided up till now.  Echoing what Torsten wrote, it would
be valuable to hear from VG/Fisons m.s. users about what unstretched
SMOW/SLAP numbers they obtain.

Thanks everyone

Max
________________________________________________________________________________
>Dear  Prof  Coleman,
>
>I came across your request for the loan of a 602 HD magnet on the Isogeochem
>>network.
>We have several of the 602 type HD magnets and would be quite happy  to loan
>you one for your proposed comparison.
>Please could you let us know  how the magnet carriage is mounted or would you
>also need a carriage?.
>On early models of the SIRA 12 this carriage was mounted vertically against the
>source housing making magnet adjustment rather awkward, the later had the
>carriage moving horizontal on top of the collector housing.
>
>If you are intrested then please let me know the appropriate details so that I
>can arrange for the necssary parts to be ready for your collection.
>
>Yours sincerely,
>
>David Bourne
>
>Pro-Vac Services>

To David Bourne
Pro-Vac services, Crewe, Cheshire

Dear David

Thank you for your most rapid and helpful response.

I would like to take up your offer.  Our SIRA 12 has its HD analyser
siamesed off the C02 one and the orientation of the magnet carrier is
horizontal.  To be more explicit the source is horizontal - so the ion beam
starts off shooting out in a horizontal direction.  The analyser tube bends
downwards so that the collector hangs vertically.  I hope that this makes
it clear but with this software I cannot include a sketch.

I will try to arrrange to pick up the magnet when I go to ICI, a visit
which is being arranged (and re-arranged) at present.

Again many thanks

Max Coleman

_______________________________________________________________________
Max Coleman, Professor of Sedimentology
Postgraduate Research Institute for Sedimentology
The University of Reading
PO Box 227, Whiteknights
Reading RG6 6AB, UK

Phone    +44 1734 316627
Fax      +44 1734 310279
e-mail   [log in to unmask]