My experiences, 50ug nitrogen probably was 3000 mV signal on cup 2 (mass28). It makes sense that the delta values are stable up than 35ug nitrogen (~2000mV). That is a stable linearity range, 0.03- 0.04 permil/ pervolt.
If the nitrogen content lower than the level, I will suggest that adjusting the maximum of the nitrogen reference input to round 2000 mV, and setting several reference events with different diluting times before EA start. (if you own a conflo IV)
Then you can build the calibration curve from those reference events on each of analysis.
It will be better, If you analyse the CN isotopes separately, you can run the reference events again after EA finished. You can use the closest reference intensity peaks (before and after EA event) to your sample to calibrate the background.
Dr. Nai-Jung, Wan 萬乃容
Agriculture, Food and Life
Supervisor SGS Taiwan Ltd.
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I have some animal tissue samples of small weight, ~0.08mg, ranging from 0.04 to 0.12mg I have measured composition (N & C) using IRMS. The average nitrogen content is ~ 8 ug.
I have found a very strong relationship between elemental nitrogen weight and δ15N values due to drift away from the correct values for those samples with low nitrogen content.
I have run a urea calibration across the range 0-70 ug Nitrogen and found that the delta value stabilises at approx. 35 ug, and prior to this the relationship is linear (0.127x + -0.034 =0). I only ran a single urea sample of each increment in the calibration.
All of my bat samples contain less than 20 ugN, and all fall within the linear region of the calibration curve.
Is anyone able to offer any advice as to the best way to correct my animal tissue samples to account for the drift away from true δ15N values?
Any advice is very much appreciated.
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