Though I haven't gone down this particular route (yet), here is my
tuppence worth based on deliberations for a similar project.
In 1M methane molecules at natural abundance, one would expect to find
3,999,360 H and 600 D. Adding 25ppm CD4 would add indeed only 100 D to
this pool of hydrogen.
For the natural abundance pool, it's a safe assumption to make that all
(well, almost all) D will be present as CDH3 (600 molecules) next to
999,400 molecules of CH4. To this you are adding 25 molecules of CD4
giving you a total of 700 D in the presence of 3,999,360 H.
That said, methane is usually depleted in D, so assuming 500 D for 10e6
methane would correspond to 3,999,500 H thus giving a d2H value of
-166.69. Adding 100 D would increase the d2H value to -0.03, i.e. very
close to VSMOW and IAEA-OH1 (d2H -3.9).
Conversion of methane into H2 will be nigh impossible by wet-chemistry
means such as Zn, U or Mn (these have been developed for the reduction
of water), so that leaves thermal reduction (aka pyrolysis). Thermolysis
of methane into carbon and hydrogen and recombination to H2 will result
in almost all D ending up as HD (just as well since our instruments
don't measure m/z 4) if CH4, CDH3 and CD4 enter the pyrolysis reactor
As calculated above, the resulting d2H value is still natural enough as
not to require any changes to your Delta_plus_XL (i.e. adjustment of cup
As for chromatography, on "normal" GC phases where one can "predict"
what separation to expect between undeuterated and deuterated
homologues, methane is not retained at all and comes off the column at a
time defined by its dead volume and one's carrier gas flow.
As to how well it would be possible to separate CD4 from CH4 on
PLOT-columns, I'm afraid I haven't got the foggiest since I haven't
tried that yet myself. However, considering the shift from -166 to
almost 0 o/oo between natural and spiked methane there would be no need
If this would be my project, I would be tempted to have a go how
reliable/reproducible analysing the methane (spike vs. natural) works
on-line (GC/TC-IRMS) and not even worry about chromatographic separation
(quite the opposite actually - within reason) by injecting pure methane
(or methane in a matrix of whatever other gases you'd expect to be
present in your ultimate samples) onto a SilicaPLOT or CarboPLOT-P7
column to separate CH4 from other permanent gases (and HVOCs).
Hope this helps.
> -----Original Message-----
> From: Stable Isotope Geochemistry
> [mailto:[log in to unmask]] On Behalf Of Janet Hope
> Sent: 02 December 2004 21:21
> To: [log in to unmask]
> Subject: Deuterium enrichment experiment
> Sorry, forgot the heading
> Dear listmembers,
> I have been asked for help on a forthcoming project and am
> looking for advice from anyone who may have gone down this track:
> We are thinking of using CD4 as an isotopic tracer for
> methane migration in the subsurface. We expect to use methane
> doped with CD4 in a dilution ratio of 25 ppm of CD4 in CH4
> (deltaD CH4 of -200 per mil vs VSMOW). The methane will be
> converted to hydrogen offline and the options are to analyse
> the hydrogen under continuous flow on a Delta Plus XL or dual
> inlet (external lab).
> Is it na´ve to assume that in 1000000 CH4 molecules there are
> approx 500 D atoms and approx 4000000 H atoms to give a
> natural abundance of -200permil for CH4 and for addition of
> CD4 to 25ppm we would only be adding another 100 D atoms.
> Will the conversion of methane to hydrogen result in a random
> scrambling of H and D resulting in a delta D of approx
> -40permil (600D to 4000000 H atoms) or will CD4
> preferentially form D2. Do different methods Zn, U, high T
> pyrolysis result in different isotopic 'scrambling'.
> For the hydrogen produced is there a problem using the Delta
> Plus XL which is setup for natural isotopic abundance.
> What problems (detector overload/memory effects/damage
> electronics etc) would we expect if the analysis was done on
> methane under GC-TC-IRMS with 1) no effective GC separation
> or 2) chromatographic separation of CD4 and CH4 (monitoring
> m/z 3 and 2).
> Janet Hope
> Isotope and Organic Geochemistry
> Petroleum and Marine Division
> Geoscience Australia
> ABN: 80 091 799 039
> Tel: (02) 6249 9487 / 9594
> Fax: (02) 6249 9961
> email: [log in to unmask]
> web site: http://www.ga.gov.au