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Extraction of CO 2 from air samples for isotopic analysis and limits to ultra high precision δ 18 O determination in CO 2 gas
Author(s) -
Werner Roland A.,
Rothe Michael,
Brand Willi A.
Publication year - 2001
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.487
Subject(s) - quartz , chemistry , analytical chemistry (journal) , oxygen , contamination , liquid nitrogen , glass tube , mineralogy , metallurgy , capillary action , environmental chemistry , composite material , ecology , materials science , organic chemistry , biology
The determination of δ 18 O values in CO 2 at a precision level of ±0.02‰ (δ‐notation) has always been a challenging, if not impossible, analytical task. Here, we demonstrate that beyond the usually assumed major cause of uncertainty – water contamination – there are other, hitherto underestimated sources of contamination and processes which can alter the oxygen isotope composition of CO 2 . Active surfaces in the preparation line with which CO 2 comes into contact, as well as traces of air in the sample, can alter the apparent δ 18 O value both temporarily and permanently. We investigated the effects of different surface materials including electropolished stainless steel, Duran® glass, gold and quartz, the latter both untreated and silanized. CO 2 frozen with liquid nitrogen showed a transient alteration of the 18 O/ 16 O ratio on all surfaces tested. The time to recover from the alteration as well as the size of the alteration varied with surface type. Quartz that had been ultrasonically cleaned for several hours with high purity water (0.05 µS) exhibited the smallest effect on the measured oxygen isotopic composition of CO 2 before and after freezing. However, quartz proved to be mechanically unstable with time when subjected to repeated large temperature changes during operation. After several days of operation the gas released from the freezing step contained progressively increasing trace amounts of O 2 probably originating from inclusions within the quartz, which precludes the use of quartz for cryogenically trapping CO 2 . Stainless steel or gold proved to be suitable materials after proper pre‐treatment. To ensure a high trapping efficiency of CO 2 from a flow of gas, a cold trap design was chosen comprising a thin wall 1/4″ outer tube and a 1/8″ inner tube, made respectively from electropolished stainless steel and gold. Due to a considerable 18 O specific isotope effect during the release of CO 2 from the cold surface, the thawing time had to be as long as 20 min for high precision δ 18 O measurements. The presence of traces of air in almost all CO 2 gases that we analyzed was another major source of error. Nitrogen and oxygen in the ion source of our mass spectrometer (MAT 252, Finnigan MAT, Bremen, Germany) give rise to the production of NO 2 at the hot tungsten filament. NO 2 + is isobaric with C 16 O 18 O + ( m/z 46) and interferes with the δ 18 O measurement. Trace amounts of air are present in CO 2 extracted cryogenically from air at −196 °C. This air, trapped at the cold surface, cannot be pumped away quantitatively. The amount of air present depends on the surface structure and, hence, the alteration of the measured δ 18 O value varies with the surface conditions. For automated high precision measurement of the isotopic composition of CO 2 of air samples stored in glass flasks an extraction interface (‘BGC‐AirTrap’) was developed which allows 18 analyses (including standards) per day to be made. For our reference CO 2 ‐in‐air, stored in high pressure cylinders, the long term (>9 months) single sample precision was 0.012‰ for δ 13 C and 0.019‰ for δ 18 O. Copyright © 2001 John Wiley & Sons, Ltd.