you should be able to correct your sample with a Keeling-plot approach. Plot your urea delta values vs 1/N content, and you should have a linear relationship. Ideally, you repeat that for another substance with a strongly different N delta value, the intersection of the two lines will give you delta value and amount of N blank. In our experience, however, N delta values of N blank in EA measurements seem to be around -7 to -12 delta (diffusive fractionation?), if you cannot determine the N blank from two independent delta vs. 1/N content plot, you could use the intersection of you urea plot with -10 delta and see what this correction will do to your samples.
Von: Stable Isotope Geochemistry [mailto:[log in to unmask]] Im Auftrag von Victoria Kemp
Gesendet: Freitag, 24. Februar 2017 15:09
An: [log in to unmask]
Betreff: [ISOGEOCHEM] Urea calibration of low sample weight animal tissue
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 ä15N 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 ä15N values?
Any advice is very much appreciated.
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