Dr. Meier-Augenstein,

I forgot to mention that we used to use guard column at the front end of GC column.  But it seems to us that this doesn't really protect the GC column, as we still see the similar chromatograph problem with such guard column. 

River

--- On Tue, 10/13/09, Wolfram Meier-Augenstein <[log in to unmask]> wrote:

> From: Wolfram Meier-Augenstein <[log in to unmask]>
> Subject: Re: [ISOGEOCHEM] GC-C-IRMS issue
> To: [log in to unmask]
> Received: Tuesday, October 13, 2009, 2:55 AM
> Dear River He,
> 
> 
> Do you run the steroids underiviatized or derivatized? If
> yes to the
> latter, what derivatisation do you use?
> 
> Also, do you use a retention gap (= 1 m deactived FS
> capillary between
> injector and column)?
> 
> How do you inject; split or splitless?
> 
> Lastly, have you checked your column/s after initial
> conditioning for
> active sites by running a Grob test? Similarly, do you
> check yor
> column/s one the problem starts to manifest itself with a
> Grob test run?
> An alkane test will tell you next to nothing if a column
> has developed
> active sites since active sites predominantly affect only
> polar
> compounds.
> 
> 
> 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 River He
> Sent: 13 October 2009 07:45
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] GC-C-IRMS issue
> 
> Dear All,
> 
> I am posing this note to ask any suggestion and advice
> about the
> problems we have been struggling with our GC-C-IRMS for
> quite a while.
> In fact, we are a little confused.  
> 
> In our lab, we have a Thermo GC-C-IRMS (Delta V Plus) for
> d13C analysis
> of steroids. Since we bought this system, we have begun to
> deal with the
> chromatograph problem; the tailing of sample peaks. 
> Normally after
> putting on a new GC column, the first few batches are fine,
> especially
> the first one or two. BUT very quickly, the chromatograph
> of samples
> deteriorates; the tailing gradually shows up and gets worse
> and worse at
> very fast rate, and after few weeks, the tailing is so bad
> that we
> cannot analyze any sample without changing a new column. 
> 
> System ontamination by sample was easily considered a
> primary cause, as
> once a while we do see "dirty" samples with high CO2
> background. Cutting
> column, cleaning injector and changing liner or septa
> barely work for
> us(well only worked for once or twice probably).  In
> fact, we do
> routinely cut column, change septa, liner and condition
> column. Then, we
> also thought the column may decay due to leaking or
> moisture from the
> samples. Keeping this in mind, in the last few months, we
> had closely
> monitored these two factors and it seems to us that they
> are not the
> reasons either. The Ar background of our system at straight
> mode is
> alway at the level of 80 mV with oven temperature at 60 C
> and its
> reading only increases 20 to 30 mV at the temperature of
> 310 C.
>  
> Another thing we observed also kind of confuses us more.
> Although the
> chromatographs of our steroids samples are very poor with
> huge tailing,
> but we can get pretty decent peaks when we run mixture of
> alkanes. This
> tells us that the column seems okay.  I am not sure
> whether the
> oxidation tube is a problem.  The current one is
> relatively new and we
> just changed it in August.  Also, we regenerate
> oxidation tube for at
> least one hour between batches and do this at least
> overnight during
> weekend.  
> 
> We will greatly appreciate if anyone can give us suggestion
> and advice
> to cope with our problems.  BTW, the GC colum
> wecurrently use is
> DB-17MS. 
> 
> THanks in advnace,
> 
> River He
> SMRTL, USA
> 
> 
>      
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