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Dear Clemente,
How high is your MS source pressure? Increasing it will also increase your signal.
Cheers,
Amandine
________________________________
From: Stable Isotope Geochemistry [[log in to unmask]] on behalf of Clemente Recio [[log in to unmask]]
Sent: 03 March 2015 01:39
To: [log in to unmask]
Subject: Re: [ISOGEOCHEM] N analysis by Isoprime GC-C-IRMS

Dear Wolfram,

Many thanks for your reply.

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

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 AA)

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.

Cheers,

Clemente



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,

Wolfram



-----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.

Thanks,

Clemente


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*******************************************
Dr. Clemente Recio
Laboratorio de Isótopos Estables
Edificio Multiusos I+D+i
Univ. de Salamanca
C/ Espejo, 2
E-37007 SALAMANCA
              SPAIN
Phone: (+34) 923 29 45 00, Ext. 1540 (Automatic Switchboard)
E-mail: [log in to unmask]
Web page: http://www.usal.es/isotopos
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