Hi Tao

I'm not exactly sure what you mean, but I think that the answer is 'No'.

The collectors (cups) are spaced so that 28, 29 and 30 or 44, 45 and 46 
or sulphur isotopes etc can go into them.  Although the physical 
separations of the beams for these isotopes are not the same, they are 
close enough that they CAN go into the same 3 cups.  It would be very 
difficult to get 29 and 40 to go in at the same time.  I think that you 
would have to peak jump between them and because of the small temporal 
instabilities your precision would probably not be good.  Having said 
that, repeated comparisons might provide an adequate result. 

Typically machines for these odd comparisons (such as for measuring 
variations in the proportions of major atmospheric gases) use a special 
collector layout, often with extra collectors, so that simultaneous 
measurement can be made.

I agree with Tom that a reference N might be a better bet. 

Hil

-- 
Hilary Stuart-Williams PhD
Research Officer - Stable Isotopes
Environmental Biology Group
Research School of Biological Sciences
The Australian National University
Canberra
ACT 0200 Australia

Tel 02 6125 2099
Fax 02 6125 4919
Mobile 0421 905 478