Your situation appears different from what I experienced (I had a more
pronounced double H2 peak with a big valley in the middle). I think the
situation I saw can be a function of some H2 leakage around the two red
o-rings that seal the bottom fitting to the glassy tube. If some H2 can
escape that seal and take the longer path back up the ceramic tube with
the carrier helium, and then down through the top of the glassy tube
again instead of going straight out the bottom the first time, then you
would get two smaller peaks separated by a brief time interval. I can't
prove it, but I know I was seeing this with my bottom fitting after the
o-rings got some wear. The red o-rings that came with the IVA fitting
are rather fragile and wear quickly. I started wrapping a turn or two of
teflon tape over the o-rings before pushing the glassy tube over them,
and it solved the problem. The other advantage is, you never replace
o-rings again, just teflon tape when needed.
Hope this helps somebody.
On 3/25/2013 6:20 AM, Robert van Geldern wrote:
> Hi isotope folks,
> we run an Thermo TC/EA with a PAL autosampler for water injection.
> We recently modified it with a bottom feed adapter that reverses the helium flow which should reduce the memory effect of the system .
> A stainless steel insert is used on top of the glassy carbon reactor and the top fan is switched off. The top He capillary is shortened (2cm) and plugged with a 16" swagelock union with blind cap.
> Before the reversal of the helium flow we used a 0.5uL syringe that was recommended by Thermo but this delicate model tend to fail after a couple of injections (plunger blocks even with DI water). So we changed to a 10uL gastight SGE (#002987) syringe as recommend for the bottom feed design in  and set it to 700nL injection volume and about 80% dilution in the ConFlo IV.
> However, we always observe a "double" peak for hydrogen (or a peak with a shoulder).
> We tried all parameters (injection speed, post injection delay, hot needle injection, injection volume etc.) with not much success. All will influence the shape somehow but after a couple if injections we always back to the "shoulder peaks" for hydrogen.
> We tried again a 0.5uL syringe with 400nL injection volume and things looked better, but as most of the times the syringe failed after a couple of injections.
> Finally we used a standard 10uL non-gastight syringe (SGE #002980) from our Picarro and it seems that we got rid of the double peak.
> So it seems that our problem is related more to the syringe type and not so much to the other parameters as we kept them identical.
> You may see pictured here:
> My questions:
> *Has anyone used and tested different types of syringes and can recommend one?
> *Any other tips and tricks around the bottom feed design that tend to improve results in your system?
>  Gehre M, Geilmann H, Richter J, Werner RA, Brand WA. Continuous flow 2H/1H and 18O/ 16O analysis of water samples with dual inlet precision. Rapid Commun Mass Spec 2004;18:2650-60.
Jonathan D. Karr, PhD
Duke EnVironmental stable Isotope Lab (DEVIL)
French Family Science Center, Room 3322
Durham, NC 27708
for US mail include:
Campus Box 90338