Dear All,

Here at Reading we operate a Sira Series II mass spectrometer
for 2-H, 13-C and 18-O analysis.  Since new in 1989 we have
had problems analysing Deuterium (2-H): SLAP always gave an
isotopically heavy result around -390 per mil compared with
the expected -428.  There was some discussion on this topic
in Isogeochem a few years ago, when other Sira II users reported
similar problems which apparently remained unresolved.

Recently, while troubleshooting a faulty mass 2/3 beam display,
we changed the original OPA-104 chip in socket IC-2 on the low
mass head amplifier for an equivalent replacement OPA-128 chip
obtained from Pro-Vac Services here in the UK.  Since then, we
have had excellent SLAP data around the expected -428 value!
VSMOW continues to analyse near to zero so that we no longer
have to apply a significant "stretch" correction for all our
Deuterium data.

Other Sira II users experiencing similar problems with their
SLAP analyses may wish to try this simple modification and see
if it works for them!

Michael Isaacs,
Dept. PRIS,
University of Reading,
Reading RG6 6AB.

Tel: +44(0)118-9875123x7974
Fax: +44(0)118-9310279
Email: [log in to unmask]