GasBench Ⅱ顶空瓶内空气背景对<100 μg碳酸盐中碳氧同位素在线测定的影响及校正方法初探
Influence of the air background in GasBench Ⅱ vials on the measurements of C and O isotope of carbonates and a preliminary study of blank correction strategy
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摘要: 较为系统地研究了在特定排空时间下GasBench II顶空瓶内背景CO2的量和同位素组成变化,并检查了该空白CO2组分对碳酸盐C、O同位素在线测定的影响。结果发现,由于排空时间较短所导致的顶空瓶内剩余的空气CO2或由于排空方法不当所导致的外部空气少量回流都会对小样品量(<100 μg)碳酸盐C、O同位素测定结果产生显著影响。其影响程度取决于空白CO2的量和空白CO2与碳酸盐样品之间C、O同位素的差别大小。由此,对小样品量碳酸盐C、O同位素的测定结果进行了空白校正。当碳酸盐样品质量在20~100 μg,校正后δ13C 和δ18O值的标准偏差分别小于0.12‰ 和0.13‰,这证明了该空白校正方法的可行性。由于顶空瓶中空白CO2的量很小,所以对痕量CO2的量、δ13C和δ18O测定值的测定会存在一定波动,这对碳酸盐δ13C和δ18O校正值产生小于0.2‰的不确定度。采用准确测定的实验室空气CO2的δ13C和δ18O值则会大大减少该不确定度的影响。Abstract: In this study, we characterized the amounts and isotopic compositions of the blank CO2 remaining in vials after specific flushing times and examined how it influences the measured δ13C and δ18O values of carbonates. Results show that the blank CO2 remaining in the vials due to either incomplete removal of air caused by short flushing time or an input from an inherent source would significantly influence the measured C and O isotopic compositions for small-sized carbonates (e.g. <100 g). The extent of this influence depends on the amount of blank CO2 and the isotopic differences between the blank CO2 and the carbonates. Therefore, we performed blank corrections to the measured δ13C and δ18O values, especially for samples of small size. The corrected δ13C and δ18O values when sample weight is bigger than 20 μg have an overall precision of <0.12‰ and <0.13‰, respectively. This testifies the practicability of the described blank correction strategy. Although the variation for measurements of the amount, C and O isotope of blank CO2 could give rise to ~0.2‰ uncertainties for the corrected δ13C and δ18O values of small-sized carbonate samples, the uncertainties will be greatly reduced if we perform routine precise measurement on C and O isotope of lab air and use them for blank correction.
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