As a check on instrument linearity for C and N analysis we typically
analyze methionine at weights varying from 0.1mg to 5mg.  This is in
addition to the several internal and normalization standards we also
analyze along with the samples. These linearity weights translate into a
peak amplitude ranging from 200mV to 13500mV for 15N with an overall
precision of around 0.3 per mil and a range of 200mV to 12500mV for 13C
with an overall precision less than 0.1 per mil (in some cases better than
0.05 per mil).  What we've noticed is that with 15N we get a slight
deterioration of results at this very low end of sample detection so we
try to keep our samples measured for 15N in a more precise range (peak
amplitudes between 500mV to 12500mV have a precision better than 0.2 per
mil and peak amplitudes between 1000mV and 12500mV have precision often
better than 0.15 per mil).  If samples cannot be measured within that
range due to lack of amount N or C then we need to make
researchers/clients aware that there is that added bit of error associated
with making measurements at that lower end of detection.  In addition we
try to match up our internal standard amplitudes with the samples being
analyzed by both matrix and weight.

Art


> 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
>>
>
> Department of Chemical Ecology and Ecosystem Research
> Faculty Center for Ecology
> University of Vienna
> Althanstr. 14
> 1090 Wien
> Austria
>
> Tel: ++43-1-4277-54258
> Fax: ++43-1-4277-9542
> email: [log in to unmask]
>


-- 
Art Kasson
Cornell Isotope Laboratory
Dept Ecology and Evolutionary Biology
E440 Corson Hall
Tower Road
Ithaca NY 14853 USA
Phone: 607.254.1382
Fax: 607.255.8088
COIL website
http://www.cobsil.com
Isogeochem website
http://www.isogeochem.com