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Hi Victoria,

The recent version of LIMS for light stable isotopes has an area based linearity correction available. It allows you to apply a linear, quadratic, logarithmic, or power fit to the data based on the standards available in the sequence. 

What I generally see is that below a signal of 5V/s, the bias in isotopic composition relative to area gets exponential. So over the whole range of my standards, a logarithmic fit seems to best describe the response we see. For me, the key is to bracket the range of response from unknowns with standards, and increase reps at the low range where things seem the least predictable. 

Since adding in corrections for linearity bias we have seen lower error in QC samples and have salvaged some samples that could only produce low yields. Still I would try to get known material in the range you are targeting and convince yourself any additional correction is really doing the best for the data. I reject any data that is outside of the range of acceptable standards in a sequence.

Regards,

Ben

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf Of Dyckmans, Jens
Sent: Friday, February 24, 2017 8:11 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] AW: Urea calibration of low sample weight animal tissue

Hi Victoria,

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.

Best,
Jens

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

Hi all, 

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 15N 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 15N values? 

Any advice is very much appreciated.

Many thanks, 


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