Datensatz

Zusammenfassung

The determination of δ¹⁸O values in CO₂ 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 CO₂. Active surfaces in the preparation line with which CO₂ comes into contact, as well as traces of air in the sample, can alter the apparent δ¹⁸O 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. CO₂ frozen with liquid nitrogen showed a transient alteration of the ¹⁸O/¹⁶O 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 CO₂ 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 O₂ probably originating from inclusions within the quartz, which precludes the use of quartz for cryogenically trapping CO₂. Stainless steel or gold proved to be suitable materials after proper pre-treatment. To ensure a high trapping efficiency of CO₂ 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 ¹⁸O specific isotope effect during the release of CO₂ from the cold surface, the thawing time had to be as long as 20 min for high precision δ¹⁸O measurements. The presence of traces of air in almost all CO₂ 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 NO₂ at the hot tungsten filament. NO₂+ is isobaric with C¹⁶O¹⁸O+ (m/z 46) and interferes with the δ¹⁸O measurement. Trace amounts of air are present in CO₂ 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 δ¹⁸O value varies with the surface conditions. For automated high precision measurement of the isotopic composition of CO₂ 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 CO₂-in-air, stored in high pressure cylinders, the long term (>9 months) single sample precision was 0.012‰ for δ¹³C and 0.019‰ for δ¹⁸O.