Perhaps I misunderstand your post, but from your description it sounds  
like your samples and standards might differ greatly in chemical  
composition (i.e., O2, H2O, Ar, etc.). If so, they would undergo  
significantly fractionations in the source, and might be subject to  
different sorts of isobaric interferences. Have you made measurements  
in which both sample and standard are purified N2?

John

On Jun 16, 2011, at 7:29 AM, Eleazar Padrón wrote:

> Dear colleagues,
>
> I am starting the analysis of 15N/14N ratio measurements on volcanic  
> gases by means of Thermo MAT 253. I would like to receive some  
> advices about the use of Dual Inlet system to make this type of  
> analysis. I have used atmospheric air as standard, but the results I  
> am obtaining are much heavier than expected. The sample is taken in  
> a evacuated bottle where a 5N NaOH solution retain the CO2 of the  
> volcanic gas.
> Can anybody help me on this?
>
> Thank you
>
> Eleazar
>
> __________________________________________________________________________________________________
>
> Eleazar Padrón González
> Instituto Tecnológico y de Energà as Renovables, S.A. (ITER, S.A.)
> Polà gono Industrial de Granadilla, s/n 38600 - Granadilla de Abona  
> Santa Cruz de Tenerife - España (Spain)
> TEL: + 34 922-747-700; FAX: +34 922-747-701
>
> __________________________________________________________________________________________________
>
> Antes de imprimir este e-mail piense bien si es necesario hacerlo.  
> El medioambiente es cosa de todos. Please consider your  
> environmental responsibility. Before printing this e-mail, ask  
> yourself whether you really need a hard copy.
>
>
> ADVERTENCIA:
> 	La información incluida en este e-mail es CONFIDENCIAL, siendo  
> para uso exclusivo del destinatario arriba mencionado. Si Usted lee  
> este mensaje y no es el destinatario indicado, le informamos que  
> está totalmente prohibida cualquier utilización, divulgación,  
> distribución y/o reproducciónn de esta comunicación sin  
> autorización expresa en virtud de la legislación vigente. Si ha  
> recibido este mensaje por error, le rogamos nos lo notifique  
> inmediatamente por esta misma và a a GRUPO ITER, S.A. y proceda a su  
> eliminación.
> DISCLAIMER: 	This message is intended exclusively for its addressee  
> and may contain information that is CONFIDENTIAL and protected by  
> professional privilege. If you are not the intended recipient you  
> are hereby notified that any dissemination, copy or disclosure of  
> this communication is strictly prohibited by law. If this message  
> has been received in error, please immediately notify us via e-mail  
> to GRUPO ITER, S.A. and delete it.