Hi Jason--
In lieu of a proper manual, some tips:
1. Got Goo? I phosphates have been run in your system, the
Conflo probably is coated on the inside with an orange
goo/film/dust.
2. Clean open split (after closing off to mass spec ;) Especially needed if capillaries coated in goo. Carefully pull out capillaries and wipe them down, wipe down rim of open split. Clip the ends of the capillary, and re-insert to appropriate level. If it does not look too dirty, you may get away with just adjusting capillary levels, though you may have to clip the line going to the mass spec. IF you noticed that the sample line is in too far, it might be blocked and just pulling that back out will make a world of difference.
3. Benzoic acid tests. Runs well, amt% wise, under almost
any conditions, but it does sublimate--so avoid drying it, esp.
vacuum drying. That also means your sample bottle may have been
homogeneous at one point, but may not be that way now.
4. Phosphates don't mix with sulfates. If you run silver phosphate after barites, you CO signal disappears, but benzoic acid and the sulfates themselves run fine. So don't mix an match. Cleaning and repacking reactor fixes problem.
5. D/H and hetero atoms--not the best combination. (look up D/H on caffeine)
6a. Backgrounds. it may take a day or so at temp for a
newly packed reactor's backgrounds to settle down. The <200
(m/z 28) at 1325 oC after 3 days should happen sooner. The
background is due to glassy C reacting to stuff in alumina tube at
temp, to permeability of alumina tube at temp, perhaps leaks in
the alumina tube, and other leaks. IF there's "air" trapped in a
capsule that slowly leaks out--there's your virtual leak. Ar
levels, m/z 40, should be below 100 mV on middle cup, but do an Ar
leak check if level above 60 ish mV. Check at 1325 and compare to
value at working temp--it shouldn't change much, unless you have a
leak/crack opening up at temp. As long as Ar levels are low, you
do not have to worry about air leaks--and getting NOx forming in
the source.
6b. backgrounds. Depends on temp you like to run at. We
run at 1450 and see high backgrounds for CO (m/z 28). Waiting a
day (or a bit more) at temp to let levels stabilize is not
uncommon. We recently got decent results with backgrounds at ca.
500 mV on m/z 28. If it's stable and Ar levels are low, try
running. Note, I've also seen backgound jump as a storm blows
thru and the humidity jumps up.
7a. Autosampler. Zero-blank? Instead of purging for 5 min,
try pressurize-release (and I presume your samples are properly
encapsulated), with opening and closing of valve to reactor to
flush that out after the first few pressurize and release cycles.
Test flushing/seals by looking at baseline levels with and with
out isolation valve on autosampler open/closed--using regular He
feed only (aka purge valve closed). Background levels should only
shift by a few mV if there's not a problem with the autosampler.
If there is a problem, just close isolation valve and
re-flush/vent the autosampler. If it still persists after a few
re-flushings, look for other leaks.
7b. Autosampler. seals. Large o-ring and middle vent leaks should be picked up via Ar leak check, unless leak is large--whereby it could be picked up w/ a He leak detector. Rarely the seal on the tube going to the isolation valve leaks, and that is hard to detect via Ar leak checking.
8. But is it really packed properly? Look down the view port (with shade 3 glasses or perhaps sunglasses--too bright otherwise), and bottom of crucible should be glowing just as bright are rest of crucible. A shade cooler could be ok, especially if glassy C chips underneath can be seen glowing (if you can see down side of crucible). Put a graphite funnel on top if using a square crucible--make a short one out of graphite tubing or rod--nothing fancy, just a beveled tube to keep sample from falling outside crucible. Top or funnel might start to be outside hot zone--that's ok.
9. Be patient. We had better runs if we wait a hour or so after loading samples before starting run. Perhaps helps with air in capsules or adsorbed water.
10. Conditioning set. Sometimes the first set of standards appears a bit anomalous. We'll start with a junk sample, benzoic, the set of 4 for house standard A, 4 of B, then 4 again of A.
11. Standards--lots of standards--house or commercial. A standard deviation of +/- 0.2 means you'll see 0.4 per mil spreads (yes 18O). We like to run them in groups of four to make sure we seeing trends and not noise. That means use the 100 position carousel.
12. D/H on hydrous minerals...yeah, that's actually tricky too....
Well, that's enough of that--the Friday softball and beer is about to start...
take care,
gerry
[log in to unmask]"> Hi All,
I need some TCEA help. Our TCEA is pretty new but very sporadically used and I have yet to develop good practices. Each time I come back to it I seem to have to start from scratch! I need some advice on basics such as what backgrounds are acceptable, what precisions are acceptable, what tailing is normal, etc. I need info on running organics for dD and d18O. Does someone have lab methods / standard operating procedures that they can share?
I realize that is beyond a normal Isogeochem request so I am willing to pay for advice (maybe hire someone as a consultant?).
Thanks on this Friday afternoon (after a week of trying to figure it out and a pile of samples waiting!).Jason
Jason Curtis, Ph.D.
Stable Isotope Mass Spec Lab manager
Senior Associate-In Geochemistry
Department of Geological Sciences
University of Florida
Gainesville, FL 32611
+1-352-275-8642