We have a CF-IRMS (EA-IRMS) set up in our laboratory. I usually get 25ugN-
250ugN linearity range for 15N and 600ugC-1000ugC linearity range for 13C.
We check linearity by running varied weights and see if we can get precise 
and accurate results based on the true value of the sample. As of now, this is 
the only way I know how to find linearity of IRMS and be sure that I can get 
precise and accurate results. 
Is there other way of finding linearity range of IRMS?
Please help us on what to do with samples with less than 25ugN and less than 
600ugC.
I dont know if we can still increase sensitivity of our IRMS.

Thanks
Ruben