Dear James,

A couple of references that may help you regarding O2 and mol. sieve are:

Cliff and Thiemens, Anal. Chem., 1994, 66,2791-2793

and

Thiemens and Meagher, Anal. Chem., 1984, 56, 201-203 

The use of molecular sieve requires quantitative removal from the sieve.  
If you are transferring directly from the sieve to the MS then I also 
suggest using liquid He to transfer.  A Toepler Pump may also work if it 
suits your system constraints.

Good luck.

Cheers,

Steven Cliff
Dept. of Chemistry
UCSD