The "cleaning after running labeled" thread just above has some helpful
advice. Be very careful about cross contamination outside the EA. Run
labeled/unlabeled separately if possible (or unlabeled first in a
series). After running labeled, swap out the ash collector (or clean
out the ash), wipe down the carousel (and piston?--not played with that
design), and run a few house standards.
For very nice samples, we see less than .1% carryover if I recall
correctly--still too much for running natural abundance after labeled.
If you can, it's probably best to run less labeled before more labeled
samples. The amt % standards, for which we use or house isotope
standard, will be fine for amt %, but probably be a bit off for
isotopes--more of a measure of residual contamination of the system.
That's usually ok since you don't need the same precision with labeled
runs as with natural abundance.
Linda Godfrey wrote:
> Does any one have experience of successfully switching between
> isotopically enriched samples and natural abundance samples on a CF-IRMS.
> I think by double wrapping samples any possible surface contamination by
> the tin cups to the autosampler carousel/piston can be taken care of.
> There ought not be condensation of gases in the EA or capillary to the MS
> - is this correct or is it simply better to stay safe and not run enriched