Hi Paul,


I like the sound of your EA GC column backflush system but would have one 
question.

Would you think this will work with all stationary phases or is this 
approach 'limited' to say MolSieve 5A?

The reason why I'm asking is that I would be worried about 'compacting' the 
stationary phase or 'disrupting' the phase bed long term in such a way as 
to influence chromatographic performance.

I suppose the above would depend in some way on how the column was packed 
in the first place. If you make your own, you have of course much more 
control over that factor.


Best regards,

Wolfram



On Aug 2 2006, Paul Brooks wrote:

> Carolyn,
> 
> I had a similar problem some years ago with soil samples that had been 
> digested HF. I am not sure when, but sometime in the run we ended up with 
> a high standing mass 30 that was only cured by changing the combustion 
> and copper tube and baking out the GC column. So it could be that there 
> is some compound inherent in the samples that causes this problem. This 
> seems likely since you can run 10 samples and then the 30 peak starts, 
> some contaminant may be coming through the GC column.
> 
> I belive the low temperature (1080C) 18O pyrolysis systems for organics 
> suffer from a similar problem. With out low temperature 18O pyrolysis 
> system for cellulose I have built a back flush system for the GC column, 
> so that after every sample twice as much carrier gas is pushed back 
> through the GC column as went forward. Perhaps this type of approach 
> would help with your samples.
> 
> Hope this helps,
> 
> Paul Brooks.
> 
> 
> 
> At 09:08 AM 8/2/2006, you wrote:
> >Hi,
> >
> > We have been running dC and dN on our EA-IRMS (an NA1500 w/zero blank 
> > to a Delta plus XP) for about two months. The first month went great; 
> > this month has been nothing but headache.
> >
> > The first month's samples were dC and dN on whole rocks (HCl/HF 
> > cleaned) and bitumens and fractions thereof (sats, aros, etc). We took 
> > what dN results we could get, but there wasn't much N to be had in 
> > these. Some of the whole rock sample sizes went as high as 20mg. Key 
> > here though is that through all of these very large samples, the 
> > background mass 30 remained low, and the standard deviations for both 
> > the dC and the dN results were good.
> >
> > This month's samples are kerogens in whole rocks, also HCl/HF cleaned, 
> > also with low N concentrations, and many with some amount of pyrite. 
> > These samples tend to be smaller in size, in the <10 mg range, but as 
> > we run a carousel we see the mass 30 rise until halfway through the 
> > batch, the dN becomes nonsense. Worth noting is that the dC for all of 
> > these samples is good, both accurate and precise.
> >
> > Standards run in a batch without unknowns will stay good. Our blanks 
> > are truly blank. Initially I was sure that our Cu reactor was spent, 
> > but when I removed it, the Cu was only half used. After another 100 or 
> > 200 samples I changed both reactors again with no improvement in the 
> > conditions. I have also replaced the H2O trap, and baked out the packed 
> > column overnight. I have tried adjusting the O2 (timing and amount), 
> > but we were already seeing a good flash combustion.
> >  Except for the increasing mass 30 background, our peaks are really 
> > quite lovely (symmetrical and tight).
> >
> > Any ideas what might be causing the increasing mass 30 background? It 
> > invariably arrives after about 10 or so unknowns, making the remaining 
> > samples garbage. The high 30 background will go down after a few hours, 
> > but standards run at this point are not as good, and any unknowns will 
> > cause mass 30 to rise rapidly. The only recourse seems to be changing 
> > the reactors again, but the reagents are not used up. Is the pyrite 
> > poisoning our reactors?
> >
> >Any thoughts or advice?
> >Carolyn
> >_______________________________
> >Carolyn Colonero
> >Lab Manager
> >Massachusetts Institute of Technology
> >Summons? Geobiology Lab
> >E34-510
> >42-44 Carleton St.
> >Cambridge, MA 02142
> >Office: 617-253-7850
> >Lab: 617-324-4002
> 
> Center for Stable Isotope Biogeochemistry
> Valley Life Science Building Room 3060
> Integrative Biology - MC3140
> Berkeley CA-94720
> Phone: (510)-643-1748
> Fax: (510)-643-1749 
>