HI Rheane--

You might be having problems with the reactor holding onto its 
conditioning.  Though I prefer running a TC/EA at 1450, when I was doing 
GC-TC the temp was at 1400.  When you run above 1400, the permeability 
(be it micro-fissures or diffusion) of water/air thru the ceramic 
noticeably increases--and you'll see a rise in the baseline. For a GC-TC 
you can burn thru the carbon coating on the inside of the ceramic tubing 
and lose some of the reducing conditions.  What might be going on is 
that the first peak is, in effect, re-conditioning the reactor for the 
latter peaks.  You can try re-conditioning the reactor.  You may add a 
short alkane to the sample mix--one that elutes early and that you let 
thru the reactor (with sample dilution on) to condition the reactor.  
Eventually the reactor will die when it's leaking faster than it can be 
conditioned....

good luck,

gerry

On 2/24/2016 5:55 AM, Wolfram Meier-Augenstein (aps) wrote:
> Hi Rheane,
>
>
> >From what you describe it would seem your observations may be sample size related.
>
> To obtain reproducible d2H values on a GC-IRMS system peak heights should ideally be >3V.
>
> Another thing to look at would be your parameters for peak detection, integration and background correction to see if changes there might make a difference.
>
>
> Best,
>
> Wolfram
>
> ******************************************************
> Prof. Dr W Meier-Augenstein, CChem, FRSC
> Stable Isotope Forensics & Analytical Sciences
>
> Robert Gordon University
> School of Pharmacy and Life Sciences
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> ________________________________________
> From: Stable Isotope Geochemistry [[log in to unmask]] On Behalf Of Rheane DA Silva [[log in to unmask]]
> Sent: 24 February 2016 11:17
> To: [log in to unmask]
> Subject: [ISOGEOCHEM] need help to improve standard deviation for H2 stable isotope analysis.
>
> Hello Colleagues,
>
> I would really appreciate your advice on the issue of analyzing stable H2 isotopes.
> I work primarily on n-alkanes and hence as an instrument set up step I ran 10 replicate runs of a C10-C40 n-alkane mixture. variable concentrations led to variable standard deviation values.
>
> For example C-23, a 25ppm standard showed 20.8 standard deviation and a 12ppm standard gave a standard deviation value of 1.9.
> in the case of C-18, the 25ppm gave a value of 3.05 and the 12ppm showed 14.7. similar results were reflected in a 90ppm n-alkane mixture, where the higher n-alkanes gave the worst standard deviation values.
>
>
> the GC parameters are as follows, rate          Temp            hold time
>                                                  50                1.00
>                                     10           140               0.00
>                                     6.0          320               15.00
>   PTV injector via an autosampler at 50 oC going up-to 320 at 3 oC/sec and flow rate 1.22 ml/min.
>
> the GCC is set at 1450 oC
>
> IRMS: 3 reference pulses for 20 secs each and back-flush switching off at 430 secs and switching on at 3100secs.
> the standard deviation for std-on-off was 0.4 and H3 factor was found to be 5.948962.
>
> I would really be grateful for any suggestions that can lead up to improving and stabilizing the standard deviation for n-alkanes
>
>
>
> Rheane Da Silva
> Senior Research Fellow(VNJCT)
> Gas Hydrates Research Group
> CSIR - National Institute of Oceanography
> Goa-India
> 403004
>
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