Hello everyone,

 

I have a Thermo TC/EA, and have been running solid organic samples using the standard packing configuration, and with a GC column (0.6 m 5A mol sieve) temperature of 80 deg C, a reactor temperature of 1450 deg C, and a flow rate ~ 100 mL/min.  

 

I have been trying to run O and H isotopes on hair samples, but I have been having a curious problem with the O isotopes.  In my runs (sequences), I will start with a set of reference materials (RM's), then have a set of samples (run in triplicate), then another set of RM's followed by another set of triplicate samples, then another set of RM's followed by another set of triplicate samples, and ending with a set of RM's.  What I have been seeing is that the RM's for O which don't contain any nitrogen (or a small amount of N) run great, and I get reproducible results throughout the whole run.  However, with my RM that contains an appreciable amount of N (i.e. caffeine), AND with my hair samples, which also contains a lot of N, the reproducibility on any given triplicate is fine, BUT the isotope value of the same sample will vary, depending on its position in the run (i.e. the O isotope value from the first set of triplicate samples is different at the beginning of the run to those collected in the middle, and also the end of the run).  I have checked my chromatography, and I am getting isotopic baseline separation between the N peak and the CO peak.

 

I was wondering if anyone else has run into these problems, and if anyone has any suggestions as to why this is happening and how to fix it?

 

Thanks very much for any suggestions.

 

Michelle

 
Michelle Chartrand, PhD
Postdoctoral Researcher
G.G. Hatch Isotope Laboratory 
Marion Hall, Room 101
Earth Sciences Department, University of Ottawa
140 Louis Pasteur 
Ottawa, Ontario, Canada K1N 6N5
Phone (office): 613-562-5800 ext 6966
Fax: 613-562-5192
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