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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