An evaluation of alumina reaction tube conditioning for high-precision 2H/1H isotope measurements via gas chromatography/thermal conversion/isotope ratio mass spectrometry.
BACKGROUND
The condition of the pyrolysis reactor is very important for obtaining stable, precise hydrogen isotopic ratios using gas chromatography/thermal conversion/isotope ratio mass spectrometry (GC/TC/IRMS). However, few studies of the conditioning process have been conducted, and little is known about the best methods for high-precision hydrogen isotope analysis.


METHODS
We investigated δ(2)H variations and observed the changes in carbon coating using six different conditioning methods for the pyrolysis alumina tube: (i) no treatment; (ii) conditioning with 4 μL hexane; (iii) conditioning with 2 μL hexane; (iv) conditioning with 2 μL hexane followed by backflushing overnight; (v) conditioning with 10 s of backflushing with methane; (vi) conditioning with 3 s of backflushing with methane.


RESULTS
Conditioning the alumina tube can improve the pyrolysis efficiency of organic compounds because a moderate amount of carbon acts as a catalyst in high-temperature regions of the alumina tube. Carbon actually flows in the tube and is difficult to confine to the high-temperature region. Insufficient amounts of carbon in the high-temperature regions lead to incomplete pyrolysis of organic compounds and lower δ(2)H values due to kinetic fractionation of hydrogen isotopes. In contrast, excess hexane or methane can lead to higher δ(2)H values, probably due to enrichment of deuterium in the hydrocarbon residue.


CONCLUSIONS
The δ(2)H values obtained by Method 6 are closest to the TC/EA δ(2)H values and are more precise than those obtained by other methods, perhaps because this method introduces a moderate, consistent amount of carbon with each sample injection.
State Key Lab of Loess and Quaternary Geology, IEE, CAS, Xi'an 710075, China.
Yunning
Y
Cao
Weiguo
W
Liu
Peter E
PE
Sauer
Zheng
Z
Wang
Zheng-hua
ZH
Li