Dear Wolfram,

Many thanks for your reply.

In our setup the order is GC --> Combustion 
interface --> Reduction furnace --> LN2 trap --> 

The length of 100 µm capillary inside the LN2 is 
about 20-25 cm; injection volume is 1µl of 
derivatized sample (460µgr to 1 mgr in 1 ml 
dichloromethane, depending on the nature of the 

Currently we are testing pure, reagent-grade AA 
(Glutamic acid, C5H9NO4 ; Alanine, C3H7NO2) 
hoping to create our own internal reference.

Chromatography, monitored via FID is fine. C isotope analysis is also fine.

But we do not get a significant signal for N, regardless of sample size.

All I can think is that CO2 is freezing inside 
the capillary to the extent of blocking it. The 
C/N ratio is large enough in the AA itself, and 
the derivatization used adds 5 additional Cs, so 
the amount of CO2 after combustion is high.

All suggestions are most welcome.



>Dear Clemente,
>Apologies for asking the obvious questions 
>first: where is your cold trap (at what point in 
>the interface) and what are the dimensions of 
>the trapping loop?
>Assuming everything is ok with the set-up of 
>your cold trap, are the AA samples you are 
>analyzing very abundant in one (or more) 
>particular amino acid(s); an amino acid you 
>might not be interested in?
>Example: collagen digest samples yield huge Gly 
>peaks.  When I was carrying out CSIA of bone 
>collagen digests we were mostly interested in 
>Pro and Hyp so I heart-cut the Gly peak from the 
>chromatogram so it would not enter the 
>combustion interface. This way we could run more 
>analyses before the cold trap became clogged.
>Best wishes,
>-----Original Message-----
>From: Stable Isotope Geochemistry 
>[mailto:[log in to unmask]] On Behalf Of 
>Clemente Recio
>Sent: 02 March 2015 10:57
>To: [log in to unmask]
>Subject: [ISOGEOCHEM] N analysis by Isoprime GC-C-IRMS
>Dear Colleagues,
>We are trying to do N in aminoacids by
>GC-combustion using an Isoprime MS coupled to an 
>Agilent GC, and are having difficulties trapping 
>the CO2 before the MS.
>I would very much appreciate hearing from any of 
>you with experience in similar work. It is 
>obvious that we are doing something wrong, and I 
>need help to identify our mistakes and sort 
>those out.
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Dr. Clemente Recio
Laboratorio de Isótopos Estables
Edificio Multiusos I+D+i
Univ. de Salamanca
C/ Espejo, 2
Phone: (+34) 923 29 45 00, Ext. 1540 (Automatic Switchboard)
E-mail: [log in to unmask]
Web page: