Dear Isogeochemers,

Several have asked how I actually ran 6ugms carbonate on my mass spec.
First, I am not implying that the precision of the measurement was anything
like running larger samples, in fact anything run with a major ion beam
less than 2e-9 is certainly suspect, IMHO, but, I have found that
occasionally when running smaller major beams (less than 2e-9) I have
generated numbers that are perfectly in line with those that were generated
with larger sample sizes and hence higher major beams (ie 4e-9 and
greater). For the most part we run 20 to 25 ugms CaCO3 minimum and on up to
50 to 75 ugms CaCO3 well within factory spec (+/- .05 or better for n=10
standards per run, and often it is better than this!), with the smallest
samples down at the lower limit of reasonable ion beam strengths and the
larger samples up in the 8e-9 to 1e-8 ranges (sometimes they are chopped).
In fact, because we have a common acid bath Autocarb device we do not like
to run much larger samples due to potential memory effects, time for
transfers, chopping, etc. For speed and precision we would rather go
directly into the inlet cold finger volume, balance some good strong beams
and run them. We have modified our reaction time by increasing it above
what the SIRA software allows by using the ^ engineer code. In the case
where we actually ran 6 ugms carbonate we were running a sample suite that
we new was small to begin with. We were happy to accept the fact that if
the samples were too small, then they were too small and the heck with it,
but we would try them anyway.
This is the methodology. Absolute vacuum integrity, low background and zero
cross seat leakage in all valves is a given. As are good zero enrichments
and standards run in comparable sizes. We obviously had to have a small
enough minimum gas volume in the ref bellows to balance the small samples
on the sample side so before the run I depleted the ref side of some of its
gas. I ran at 1000ma trap current (up from 800) but did not optimise all
the source parameters for the largest peaks (highest sensitivity). We
gained some sensitivity from the higher trap current but I still detuned
for stability with the electron volts and as an aside, I ran pretty
negative ion repellor as well. The key to the analysis however was the fact
that I had recently put in a new filament and all new source ceramics. It
is my experience that Vg filaments start out really great but through time
thier sensitivity degrades, my guess is that the thoria coating burns off
with time, or you just cannot help but add gradoo to the source even if you
are being extremely careful to only admit pure, clean and dry CO2 (Does it
pyrolyze and plate out on source surfaces as carbon film?) but I really do
not know what causes the problem, I entertain answers from the group. Over
time you still get good peak shapes and ratio flats but the sensitivity
starts to degrade within a month it then stabilizes (at some lower point)
for upto years provided you don't put any "crap" into the machine. My guess
is that if you wanted to run foram arms and legs you would have to
continually open up the machine and stick in a brand new filament as soon
as you saw that the sensitivity was starting to degrade after every couple
of runs. Obviously, something that most people would not want to do but
certainly within the means of those that are on the cutting edge of small
sample measurement capability. If any have further questions I will answer
as best I can.

Best Regards,
Cap