Hi Claire,


Based on what you describe and what you told me off list, my suggestion
would be for a much simpler yet seemingly more radical approach.

All things being equal, retention times are a function of
solute/stationary phase interaction, temperature and carrier gas flow,
while carrier gas flow is ultimately determined by three things, the net
diameter of the narrowest connection, temperature and column head
pressure.

Your set-up is by and large typical for any TC/EA, your temperatures are
stable you say, which leaves column head pressure.

Off-list you mentioned that for a reading of 1 bar on the TC/EA manifold
gauge for carrier you measure an actual flow of 160 ml/min at the exit
of the GC column.

Clearly this eliminates pesky pieces of material acting as a
restriction.

Equally clearly this suggests something is seriously wrong with the
carrier gas supply through the TC/EA's manifold.  If your retention
times would not change, this may merely indicate a faulty pressure gauge
but since you observe fluctuating retention times and, hence fluctuating
flow rates I wonder if the pressure regulator in the TC/EA's manifold is
faulty (broken or "bent" diaphragm)?

My suggestion therefore would be to replace both gauge and regulator in
the manifold.  Conversely, you could try to bypass the carrier regulator
and adjust the He pressure of your delivery line down to approx. 1.5 bar
using the pressure regulator on your helium cylinder and then measure
the flow rate.

Incidentally, on our TC/EA a set pressure of 1.5 bar give or take at the
carrier gas gauge of the TC/EA's manifold corresponds to a flow of 90
ml/min plus or minus (today's reading: 88.2 ml/min).


Hope this help.


Best,

Wolfram


****************************
Dr W Meier-Augenstein, CChem, FRSC

Extension: 2624
E-mail: [log in to unmask]
URL: http://www.scri.ac.uk/staff/wolframmeieraugenstein

-----Original Message-----
From: Stable Isotope Geochemistry [mailto:[log in to unmask]] On
Behalf Of Lock Claire
Sent: 25 November 2008 11:39
To: [log in to unmask]
Subject: [ISOGEOCHEM] TC/EA retention time problems

 
Hello everyone,

I have a persistent but random problem with my TC/EA, but the problem is
sporadic and I haven't managed to track down the cause.

When measuring CO we observe changing retention times.  The variation
occurs between two things, firstly our "normal" run time, the N2 peak
elutes around 200 seconds and the CO peak elutes around 350 seconds,
secondly the "delayed" run or N2 elutes around 350 seconds and CO around
850 seconds.  Whichever retention time is observed is consistent for the
entire sample run.

We have observed this change with the same reactor tube, same gas
pressures, temperatures etc. The difference has occurred between loading
different samples i.e. the only thing that has been done is closing the
isolation valve re-loading the auto-sampler, purging it and re-opening
the isolation valve.

I have baked the GC, but this has not helped.
I have changed the reactor several times but the problem seems random
I have varied how I purge the samples and open the isolation valve with
purge left on and switched off, this hasn't affected it.
I have tried resetting the TC/EA
I have checked the ends of each line and they all seem fine.  I've
changed several ferrules which hasn't helped.

The argon background is around 5-8 mV. Stability and linearity of the CO
ref gas are good.  Mass 28, 29 and 30 backgrounds are stable.  

The heaters seem to be working ok, so I guess there must be something
wrong with the carrier flow, maybe a blockage which occassionally moves?
But I have been unable to locate it.

I am just about to change the reactor again and examine the one that's
in there to see if anything has become displaced or blocked, but if
anyone has any experience of similar problems, I'd be very grateful for
any insights.

Many thanks in advance for your help,

Claire  
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