Tim, Andy, and everyone else interested,

I sometimes get the impression that pyrolytic processes seem to be regarded as
less predictable reactions or black magic chemistry (alchemy ?). I hope that
this contribution adds some more clarity and helps lessen the confusion
regarding pyrolytic reactions in general and the setup of EA-Pyrolysis-IRMS
systems in particular.
Best regards

Roland


1. Background
Oxygen bearing traces in the carrier gas (H2O, O2, CO2 and perhaps CO) will be
converted to CO over hot carbon and will therefore contribute to the background
signal of a d18O measurement. Another source of such a background could be the
reaction between carbon and silica at elevated temperature (accelerated at
temperature >1000°C and/or in presence of hydrogen at elevated temp. At >1120°C
quartz is thermally unstable). The peak-background relation influences the
reproducibility and accuracy of your measurements. Influencing factors are
amount and isotopic composition of the background.

2. Oxygen isotope exchange reactions
Organically bound oxygen (CxHyOz) can be thermally cracked in the pyrolysis
tube to CO, H2O, CO2 as the primary oxygen containing products, depending on
many parameters like temperature, chemical structure of the organic compound
(functional groups, C/O relation etc.), reaction time (static or dynamic
system), catalysts etc. These compounds will further react with an accessible
surplus of „active“ C to CO at elevated temperature. However, CO2 and H2O, and
probably also CO, can exchange oxygen isotopes with the silica or alumina
oxygen (exchange reactions of CO2 and H2O are described in the literature; I
could not find a corresponding paper on exchange of CO, but would expect (and
have seen) a similar behaviour for CO. Someone out there who could give me a
reference regarding the CO/quartz or CO/alumina oxygen isotope exchange
reaction??). Thus Ni shields or liners (introduced by Santrock and Hayes, or
e.g. Pt liner, see Taylor and Chen) are used in some setups to protect the
quartz wall of the reaction tube in order to avoid possible oxygen isotope
exchange reactions between the oxygen containing primary pyrolysis products and
the quartz oxygen and in order to prevent any pyrolytically produced H2 gas
from reacting with the quartz according to the reactions H2 + SiO2 <--> SiO +
H2O and H2O + C <--> H2 + CO at higher temp.).
It is remarkable that, in contrast to the oxygen, the carbon atom of the CO
once formed during pyrolysis seems unable to exchange its original carbon
isotope signature with the big surplus of carbon inside the pyrolysis tube (in
some cases e.g. vanillin, caffeine etc.: you can use the d13C of this CO to
measure intramolecular 13C/12C ratios of the C atoms to which oxygen was bound
in the original molecule, for details see e.g. Werner et al. and Dennis et
al.).

3. Memory effect
Memory effects are caused by the contamination of a sample of carbon monoxide
(your peak) with „oxygen“ from a previous analysis. Hopefully only small
amounts of oxygen from a previous sample remain in whatever form in the
pyrolysis sytem and its isotopic signature will influence the subsequent
measurements. „Whatever form“ may be involatile „oxygen complexes“ on the
surface of the carbon inside the tube or an exchangeable reservoir on or in the
quartz reactor etc. Also CaO and other rather stable oxides that remain in the
pyrolysis tube from incomplete reaction of previous e.g. carbonate samples can
be responsible for memory effects.
The isotopic signature of the oxygen blank from the carrier gas should stay
constant (it might change when you change your gas bottle). It will influence
every measurement, but has nothing to do with memory effects.

4. General remarks
Internationally accepted organic 18O standards are missing !!! Therefore
standardisation of pyrolysis results can grow to a big scale problem (see
Kornexl et al., 1999a). To solve the problem for now you must measure
internationally accepted 18O standards. However, only inorganic reference
materials are available at this time (e.g. water, carbonates).
Higher reaction temperatures are needed for nitrate, sulfate and carbonate
samples. By the way, can somebody tell me whether the conversion of H2O with C
to CO is quantitative in the „Farquhar et al. EA-Pyrolysis system“?
If you want to measure carbohydrates only you don´t need a maximum pyrolysis
temperature of ~1450°C. For a methodological reference see e.g. the Saurer et
al. Anal. Chem. paper which deals with the routine measurement of tree
cellulose samples using an EA-Pyrolysis system (< 1100°C). For references
dealing with organic substances other than carbohydrates see e.g. Breas et al.,
Dennis et al., Houerou et al., Rossmann et al. etc.
To measure also inorganic compounds (international standards !!) a „tube in
tube assembly“ (glassy carbon tube in an alumina tube) at a higher reaction
temperature (> 1250°C) is highly recommended. Glassy carbon is a very inert
substance and seems to stay „clean“ (no memory effect) unless a reactive
pyrolytic carbonaceous surface has built up. For references regarding
temperatures > or = 1300°C see e.g. Koziet; Kornexl et al.
Another problem in on-line systems will be to get your sample water-free in a
reproducible manner. E.g. cellulose shows relatively small hygroscopicity (only
to about 5 % water ?) as compared to e.g. fructose, which can be like syrup.
See here e.g. Koziet and Bricout. What about a lyophilized sample series of 32
samples in the autosampler carousel and an analysis time of 10 min for each
capsule?
Less important seems to be the problem of air enclosed in the sample capsules
(blank problem).

Hope these comments are useful. If there are any questions or remarks, please
contact me anytime on or off the list .



References:

O. Breas et al. (1998): Rapid Comm Mass Spectrom 12, 188
M.J. Dennis et al. (1998): J Anal Appl Pyrol 47, 95
G.D. Farquhar et al. (1997): Rapid Comm Mass Spectrom 11, 1554
G. Houerou et al. (1999): Rapid Comm Mass Spectrom 13, 1257
B.E. Kornexl et al. (1999a): Rapid Comm Mass Spectrom 13, 1248
B.E. Kornexl et al. (1999b): Rapid Comm Mass Spectrom 13, 1685
J. Koziet (1997): J Mass Spectrom 32,103
J. Koziet, E. Bricout (1997): 17, 403
A. Rossmann et al. (1998): Z Lebensm Unters Forsch A 207, 237
J. Santrock, J.M. Hayes (1986): Anal Chem 59, 119
M. Saurer et al. (1998): Anal Chem 70, 2074
J.W. Taylor, I.-J. Chen (1970): Anal Chem 42, 224
R.A. Werner et al. (1996): Anal Chim Acta 319, 159

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Dr. Roland A. Werner
Max-Planck-Institut fuer Biogeochemie
Tatzendpromenade 1a, 07745 Jena, Germany
Postanschrift:
Postfach 10 01 64, 07701 Jena, Germany
Tel.: +49-3641-643719, -643825
Fax:  +49-3641-643710
e-mail: [log in to unmask]

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