Hi Bjoern,
If you didn't already do so I would bake the GC column (with the helium flow
on but not into the mass spec) at 125'C overnight and see if that helps.
The columns don't last forever so if this doesn't work it might be time to
invest in a replacement. Depending on your budget there is a new generation
of CN columns available with much lower back pressure and better baseline
separation.
Cheers

Nick..


Nick Collins
President, MS Solutions Inc.

MS Solutions Inc.
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-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On Behalf
Of Bjoern Wissel
Sent: Thursday, February 14, 2008 10:14 AM
To: [log in to unmask]
Subject: [ISOGEOCHEM] EA problem with prolonged peak retention times

Hi there,

I have been running basic C+N stable isotope analyses on a Costech EA 
connected to a Finnigan Delta plus for a number of years without problems.
Recently, I observed an elevated N2 baseline (30 to 40 mV instead of 15 to
20 
mV) in combination with prolonged peak retention times (N2 peak starts at
180 
sec instead of the normal 100 sec) and peak durations are longer as well.

Since then, I checked for leaks (pressure and Ar), changed seals, water
trap, 
combustion / reduction columns, He tank, GC column, and connection to the 
ConFlo but nothing changed the elevated baseline and late peaks. The flow 
rate is constant at 120 measured at the outlet of the EA and directly at the

GC column.

I am wondering if anyone came across this or a similar problem, or does 
anyone have any ideas what could cause this phenomenon. I would greatly 
appreciate any comments and suggestions.

Thanks, Bjoern