I just wanted to thank everyone who responded to my plea for help.   
The problem was apparently due to a leaky split/splitless valve. We  
are now getting correct isotopic values for our standards throughout  
the chromatogram and are happily running once again.

All the best,
Sue

On May 10, 2006, at 12:00 PM, Susan Ziegler wrote:

> Yes I did replace all the o-rings possible.  The 2 just under the  
> valve and the three sealing the connections to the gas lines for  
> both the purge and the s/sl.  That didn't seem to do the trick in  
> this case unfortunately.
>
> On May 10, 2006, at 11:46 AM, Brian Jones wrote:
>
>> Hi Susan,
>> Have you tried replacing the rubber o-ring just under the valve.  
>> These rings
>> are fairly cheap and I have found in the past that they tend to be  
>> the
>> weakest link in the valve system.
>>
>> Brian Jones
>> Stable Isotope Ecology Lab
>> Texas A&M University
>> College Station, Tx. 77843-3146
>> 979-845-8916
>>
>> -----Original Message-----
>> From: Stable Isotope Geochemistry [mailto:[log in to unmask]]  
>> On Behalf
>> Of Susan Ziegler
>> Sent: Wednesday, May 10, 2006 11:26 AM
>> To: [log in to unmask]
>> Subject: Re: [ISOGEOCHEM] GC-C-IRMS issues (split/splitless valve  
>> leak)
>>
>> Thanks to everyone for their suggestions on our fractionation issue.
>> It was much appreciated.
>>
>> It turns out that I have a leaking split/splitless on/off valve in
>> the injector system. I have removed the valve in question and purged
>> it with compressed air, taken it apart and cleaned it.  After that
>> the leak was diminished but subsequently returned within 6 hours. In
>> the past toggling the valve between split and splitless would fix the
>> leak found in splitless mode and to prevent such leaking toggling
>> every few days did the trick.  So this valve leaves much to be  
>> desired!
>>
>> I am using an HP6890 (about 6 years old) and have just found out that
>> I cannot purchase a new s/sl onoff valve through Agilent Technologies
>> (the company that now makes the HP instruments) and instead have to
>> purchase an entire manifold that the valve is mounted onto at a cost
>> of $2300 USD.  Does anyone know where I might be able to find a  
>> new s/
>> sl on/off valve (PN G1544-60710) for the 6890 split/splitless
>> injector?  This is apparently an inherent problem with these
>> injectors so I would really like to find out if there is an
>> alternative to buying the very expensive manifold every time the
>> valve fails.
>>
>> Thanks again for the input.
>>
>> All the best,
>> Sue
>>
>>
>> Susan Ziegler
>> Associate Professor
>> Department of Biological Sciences
>> University of Arkansas
>> Fayetteville, AR  72701
>> 479.575.6944 (lab 6342)
>> 479.575.4010 (fax)
>> http://www.uark.edu/~susanz/
>
> Susan Ziegler
> Associate Professor
> Department of Biological Sciences
> University of Arkansas
> Fayetteville, AR  72701
> 479.575.6944 (lab 6342)
> 479.575.4010 (fax)
> http://www.uark.edu/~susanz/

Susan Ziegler
Associate Professor
Department of Biological Sciences
University of Arkansas
Fayetteville, AR  72701
479.575.6944 (lab 6342)
479.575.4010 (fax)
http://www.uark.edu/~susanz/