Margarete and all,

We typically run an internal standard (Sigma D-Glutamic acid, 9.5% N, 40.8% C) four times during a 40 slot NC determination for these types of samples. Average data for this standard from a recent run with weights ranging from 0.19 - 0.26mg (peak heights 3.5-5 nA N, 1.5-2nA C with diluter) show an sd of 0.35 per mil for 15N and 0.17 per mil for 13C (n=4). This is a typical result, sometimes we do better. When the system is having a good day, I have occasionally seen 15N and 13C precision below 0.1 per mil but this is exceptional.

Please contact me off list if you'd like further details.

Michael Isaacs

>>> Margarete Watzka <[log in to unmask]> 02/26/09 1:08 PM >>>
Hi Michael,
your last sentence about standards matching the size of the samples 
is an important point (as I know from own experience).
I would be interested what accuracy and precision for d15N (and d13C) 
you usually get at these small amounts of N (and C).

Best regards

Margarete





On 25 Feb 2009 at 12:40, Michael Isaacs wrote:

> Hi Dan,
> 
> It depends on the concentration of N and C in the material you want to
> analyse. Here at Trent we routinely run zooplankton samples with
> around 10% N and 40% C weighing 0.200mg on our IsoPrime / Eurovector
> EA and get good 15N and 13C data. If your amphipods have similar bulk
> compositions then I think its quite possible to analyse individual
> dried leg samples weighing 0.185mg to the required accuracy and
> precision. Just be careful to match your standards to your sample
> sizes.
> 
> Michael Isaacs
> -----------------
> Water Quality Centre
> Trent University
> 1600 Westbank Drive
> Peterborough
> Ontario
> Canada
> 
> Email: [log in to unmask]
> 
> >>> Daniel Nelson <[log in to unmask]> 02/25/09 12:09 PM >>>
> Hi
> I'm working with an endangered amphipod.  I would like to do some
> isotope analysis (C&N) to look at diet but I can't sacrafice wild
> individuals.  However, I do have a permit to remove walking legs for
> genetic analysis.  One large amphipod (dried) weighs approximately 4.0
> mg and one dried leg weighs approximately 0.185 mg.  What is the
> smallest sample weight that can be measured accurately?  Can I analyze
> individual legs, 3 or 4 legs, or do I need to do the whole individual?
>  I realize the whole individual would probably be best but again, I
> can't sacrafice individuals.  Thanks a lot.
> 
> Dan Nelson
> MS Student
> University of Idaho
> 

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