I've been having a few problems with our EA for the last few weeks. Here
are the details:
It's a Carlo Erba NC 2500 connected up to a Micromass Optima. The Optima is
working fine for dual inlet work, but the EA has not been used for about 18
months.
Linearity tests for N2 and CO2 reference gases give a range of 0.015-0.020
permil over the full range of refgas sizes, i.e. there is nothing wrong
with the mass spec. A run of fifteen solid organic standards of widely
differing sizes (equivalent to 0.05-0.75mg N) gives a scatter of d15N
values with a standard deviation of 0.14 permil - in other words nitrogen
works OK.
However, the situation for carbon is problematic. A similar run of
standards, (attempted organic standards as well as graphite) gives a range
of d13C values, with the largest samples being 1.8 permil heavier than the
smallest samples.
I assumed that the problem was probably related to poor combustion - there
is a good flash combustion - sample drop time is 21 seconds, not dissimilar
from the recommended values. He pressure/flow is 150 kPa,  88 ml/min. O2
pressure/flow is 60 kPa / 20 ml/min - I have had the O2 up to 150 kPa / 55
ml/min with no effect on the d13C/sample size variation. I've also had the
GC oven up to 110°C for four hours to rid the column of any crud. The water
trap and furnace tubes have new chemicals and there are no major leaks.
Also there is a good (R2 = 0.999) linear relationship between the
integrated area of mass 44 and the mass of the sample.
I've also fiddled with the way the peaks are integrated, with no success.
Any ideas as to what could cause the d13C/samplesize variation, and what I
can do about it? As I say, it is evidently an EA problem, not the mass spec.

cheers,

Jason


*********************************
Dr. Jason Newton
University of California Santa Cruz
Department of Earth Sciences
Santa Cruz
CA 95064
USA

email: [log in to unmask]
Tel: +1  (831) 459 5751
Fax: +1  (831) 459 3074