My research facilities only limited to an IRMS and GC-MS without coupling. I am having trouble to use these instruments in finding the isotope ratios and concentration of halocarbons in the solid and aqueous samples.
What kind of stable isotope ratios do you want to measure? 13C, 2H, 37Cl, 81Br? And which specific compounds are you looking for?
Having measured 13C on PCBs already, I would say that the abundances of chlorinated hydrocarbons are so low that alternatives like using LC separations which I have done for sugars and amino acids would be precluded if you want isotope compositions on those halogenated compounds extracted from sediments or phyytoplankton.
if you want isotope compositions on those halogenated compounds il's necessary to extracte from sediments or phyytoplankton. What kind of stable isotope ratios do you want to measure? 13C only per exemple ..!!
Stephen, thank you very much for your useful information. That means there is no other possible alternative already? :(
Yes, extraction would be a first step, but then to separate the components by any method that would be sufficient for IRMS (minimum, depending on the system) requires perhaps uM of compound. GC/IRMS you can get away with nM of compound.
An alternative would be to find someone to collaborate with who already owns a GC/IRMS.
As far as I know there is only GC-C-IRMS available to measure delta 13C values of chloro and bromomethane.
I think is only GC-C-IRMS available to measure delta 13C values of chloro and bromomethane.
Agreed! I was thinking of longer chain compounds than gasous species. If your sample was big enough..KG? Has anyone reported bromomethane in phytoplankton?
Yes, Dr. Stephen, the work done by Scarratt and Moore, 1996. Production of methyl chloride and methyl bromide in laboratory cultures of marine phytoplankton. Marine Chemistry. 54, 263.
Thank you very much to those who contributed here, I appreciate your advice and discussion. I was hoping that if there is an algorithm or conversion factor available to predict/estimate the said isotopic ratios from the IRMS analyses.
Dear Dr. Stephen, another question. Is it feasible if we concentrate the samples, followed by LC separation? Anyone? Please advice.
As already mentioned by previous respondents, for your chosen application there is no substitute for actual measurement by GC/C-IRMS. LC is not a suitable method to analyse or "concentrate" VOCs and HVOCs such as methyl chloride. Ideally, you would need to a hybrid system with a conventional MS coupled to the same GC so you get unambiguous compound identification (by MS) and 13C isotopic composition from the same GC peak. Concentrating trace VOCs from gaseous samples (be it air or a gas used to purge a liquid / suspension) requires cryo-trapping. To obtain enough material for an off-line IRMS analysis you would have to purge and trap / concentrate large volumes (>5 L). Even for GC/C-IRMS some cryo-focusing may be required but at least here sample volumes will be much smaller and hence more manageable. For a method to analyse halogenated VOCs at ppb and below for 13C have a look at M.E. Archbold et al., Rapid Commum Mass Spectrom., 2005, vol 19, pp 337-342.
You may try MS for measuring 13C ratio.But it is rather tough for sample preparation than GC-IRMS. The best choice up to date is GC-IRMS.
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