In my lab we don't use the reverse flow system but I'm not entirely sure as to why this should cause what old sdinosaurs like used to call the Mexican hat peak shape.
Double peaks are typically caused by a dead volume in the system; on a TC/EA this could be inside the reactor or in the GC column (or both). An overload (too large an injection volume) can also contribute to ghosting.
To avoid / minimise the risk of double peaks we pack our glassy carbon tube differently for liquids as compared to the standard packing for solids. We don't use a crucible (obviously) and we pack the tube with glassy carbon chips just shy of the brim. On top of the glassy carbon tube we use a stainless steel insert (instead of the standard graphit insert). We use 1.2uL syringe to inject typically 150-180nL of water. The syringe needle is longer than normal to ensure the tip of the needle protrudes from the insert into the small void on top of the glassy carbon chips in the glassy carbon tube.
So, if you are satisfied your GC column packing hasn't collapsed thus creating a dead volume in the column, play around with the packing of the glassy carbon tube, the length of the syringe needle and the injection volume; I should also mention we sieve the glassy carbon chips so the chips we end up using to pack the tube are more or less the same overall shape and size. Using the irregulary shaped chips as they come shipped can cause void volumes within the packing.
All the best,
"Facts do not cease to exist because they are ignored."
Aldous Huxley, "Proper Studies", 1927
Professor W Meier-Augenstein, PhD, CChem, FRSC
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From: Stable Isotope Geochemistry [[log in to unmask]] on behalf of Robert van Geldern [[log in to unmask]]
Sent: 25 March 2013 10:20
To: [log in to unmask]
Subject: [ISOGEOCHEM] TC-EA water injection with bottom feed
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:
*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.
Dr. Robert van Geldern
GeoZentrum Nordbayern / Applied Geosciences
University of Erlangen-Nuremberg
91054 Erlangen, Germany
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