You may need to move your cups, but if you are going after natural 
abundances of 15N15N there is another, potentially more important 
issue.  The filament in your instrument contains a thin oxide coating 
that will emit CO from reaction with trace hydrocarbon contaminants 
and NO from reaction with N2 analyte.  These are low enough in 
abundance that you might not notice their contribution to 
measurements of singly-substituted molecules, but they can introduce 
large systematic errors in analyses of doubly-substiuted 
isotopologues (which typically make up only a few ppm of the analyte 
gas).  This issue can only be ignored if you are planning to analyze 
highly enriched gases.

John

>Hi All,
>             I am attempting to add m/z 30 to my nitrogen analysis. I 
>have managed to get the mass added in the ratio editor (thanks to a 
>previous posting by Gerald Olack in Nov 2004), however, I do not 
>know which cup to assign the mass. At present, my cup configuration 
>for nitrogen is set to cup 2 for m/z 28 and cup 5 for m/z 29. This 
>was determined by a Finnigan technician during install. This 
>instrument has 6 cups installed and I have tried placing m/z 30 in 
>cup 6 but I get no response in cup 6 when I check this with CO or N2 
>gas. My CO2 configuration uses cups 2 (m/z44), 4 (m/z45), and 6 
>(m/z46), shouldn't N2 be the same cups but different magnet step? I 
>have tried this and get no response on any cups. Anyone out there 
>with a MAT 252 and has managed to add m/z 30? Any help would be 
>greatly appreciated.
>
>Brian Jones
>Stable Isotope Ecology Lab
>Eller O&M Bldg. Rm 408
>Dept. of Oceanography
>Texas A&M University
>College Station, Tx. 77843-3146
>Lab 979-845-8916
>Cell 979-820-2745
>