Hi Dave,
Although not easily seeing, loops which conveys produced CO2 to the mass spect may be blockage. Phosphoric acid tends to crystallize on such part of tubing!
You can also check if the syringe is clean enough, I mean, needle actually drops the acid into vials.
Good luck

On Wed, Mar 18, 2015 at 7:07 AM, Dave Mattey <[log in to unmask]> wrote:
Dear all

I wonder if anyone can suggest reasons of the 'less obvious' variety why an Isocarb system is producing poor isotope data related to variable CO2 reaction yields? This is the system that drops acid into evacuated Wheaton vials and transfers the CO2 to the external cold finger.

The transducer readings of the CO2 yield are sporadically low, resulting in the sample being aborted (too small), or if it runs, the isotope compositions are anomalous.  It is an intermittent  problem, most yields are good, but some are poor. I’d like them all to be good!

Sample size is in the 80-120ug range
Vacuum seems good
Acid is good quality (new batch just made, problem continues)
Acid delivery is consistent (i.e. present in all vials)
There are no obvious blockages or leaks on the gas line from the needle to the cold finger, which is working normally

Has anyone experienced this and found a reason that is not so ‘obvious’ as no sample, no acid, vacuum leak etc?  Any ideas appreciated!

best wishes


Dave Mattey
Department of Earth Sciences,
Royal Holloway, University of London,
Egham Hill,
Egham, Surrey,
TW20 0EX

Marco A. Rubio R. 
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