Using laser absorption spectrometry for the measurement of stable isotopes of atmospheric CO₂ instead of the traditional isotope ratio mass spectrometry method decreases sample preparation time significantly, and uncertainties in the measurement accuracy due to CO₂ extraction and isobaric interferences are avoided. In this study we present the measurement performance of a new dual-laser instrument developed for the simultaneous measurement of the δ¹³C, δ¹⁸O and δ¹⁷O of atmospheric CO₂ in discrete air samples, referred to as the Stable Isotopes of CO₂ Absorption Spectrometer (SICAS). We compare two different calibration methods: the ratio method, based on the measured isotope ratio and a CO₂ mole fraction dependency correction, and the isotopologue method, based on measured isotopologue abundances. Calibration with the ratio method and isotopologue method is based on three different assigned whole-air references calibrated on the VPDB (Vienna Pee Dee Belemnite) and the WMO 2007 (World Meteorological Organization) scale for their stable isotope compositions and their CO₂ mole fractions, respectively. An additional quality control tank is included in both methods to follow long-term instrument performance. Measurements of the quality control tank show that the measurement precision and accuracy of both calibration methods is of similar quality for δ¹³C and δ¹⁸O measurements. During one specific measurement period the precision and accuracy of the quality control tank reach WMO compatibility requirements, being 0.01 ‰ for δ¹³C and 0.05 ‰ for δ¹⁸O. Uncertainty contributions of the scale uncertainties of the reference gases add another 0.03 ‰ and 0.05 ‰ to the combined uncertainty of the sample measurements. Hence, reaching WMO compatibility for sample measurements on the SICAS requires reduction of the scale uncertainty of the reference gases used for calibration. An intercomparison of flask samples over a wide range of CO₂ mole fractions has been conducted with the Max Planck Institute for Biogeochemistry, resulting in a mean residual of 0.01 ‰ and −0.01 ‰ and a standard deviation of 0.05 ‰ and 0.07 ‰ for the δ¹³C measurements calibrated using the ratio method and the isotopologue method, respectively. The δ¹⁸O could not be compared due to depletion of the δ¹⁸O signal in our sample flasks because of storage times being too long. Finally, we evaluate the potential of our Δ¹⁷O measurements as a tracer for gross primary production by vegetation through photosynthesis. Here, a measurement precision of <0.01 ‰ would be a prerequisite for capturing seasonal variations in the Δ¹⁷O signal. Lowest standard errors for the δ¹⁷O and Δ¹⁷O of the ratio method and the isotopologue method are 0.02 ‰ and 0.02 ‰ and 0.01 ‰ and 0.02 ‰, respectively. The accuracy results show consequently results that are too enriched for both the δ¹⁷O and Δ¹⁷O measurements for both methods. This is probably due to the fact that two of our reference gases were not measured directly but were determined indirectly. The ratio method shows residuals ranging from 0.06 ‰ to 0.08 ‰ and from 0.06 ‰ to 0.1 ‰ for the δ¹⁷O and Δ¹⁷O results, respectively. The isotopologue method shows residuals ranging from 0.04 ‰ to 0.1 ‰ and from 0.05 ‰ to 0.13 ‰ for the δ¹⁷O and Δ¹⁷O results, respectively. Direct determination of the δ¹⁷O of all reference gases would improve the accuracy of the δ¹⁷O and thereby of the Δ¹⁷O measurements.

中文翻译：

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同时测量大气 CO ₂的δ ¹³ C、δ ¹⁸ O 和δ ¹⁷ O——双激光吸收光谱仪的性能评估

使用激光吸收光谱法测量大气CO ₂的稳定同位素，而不是传统的同位素比质谱法，显着缩短了样品制备时间，避免了由于CO ₂萃取和同量异位素干扰引起的测量精度不确定性。在这项研究中，我们展示了一种新型双激光仪器的测量性能，该仪器用于同时测量离散空气样品中大气CO ₂的δ ¹³ C、δ ¹⁸ O和δ ¹⁷ O，称为CO的稳定同位素₂吸收光谱仪 (SICAS)。我们比较了两种不同的校准方法：比率法，基于测得的同位素比和CO ₂摩尔分数依赖性校正，以及同位素法，基于测得的同位素体丰度。使用比率法和同位素体法校准是基于三个不同的指定全空气参考，在 VPDB（Vienna Pee Dee Belemnite）和 WMO 2007（世界气象组织）尺度上校准它们的稳定同位素组成和CO ₂摩尔分数，分别。两种方法都包含一个额外的质量控制罐，以跟踪长期仪器性能。质量控制罐的测量表明，对于δ ¹³ C和δ ¹⁸ O测量，两种校准方法的测量精度和准确度具有相似的质量。在一个特定的测量周期内，质量控制罐的精度和准确度达到 WMO 兼容性要求，δ ¹³ C为 0.01 ‰，δ ¹⁸ O为 0.05 ‰. 参考气体标度不确定度的不确定度贡献使样品测量的组合不确定度再增加 0.03 ‰ 和 0.05 ‰。因此，要在 SICAS 上实现 WMO 对样本测量的兼容性，需要降低用于校准的参考气体的标度不确定性。已与马克斯普朗克生物地球化学研究所进行了大范围CO ₂摩尔分数范围内的烧瓶样品的比对，得出平均残差为 0.01 ‰ 和-0.01  ‰，δ的标准偏差为 0.05 ‰ 和 0.07 ‰分别使用比率法和同位素体法校准的¹³ C测量值。该δ ¹⁸ Ø由于储存时间太长，我们的样品瓶中δ ¹⁸ O信号耗尽，因此无法进行比较。最后，我们评估了我们的Δ ¹⁷ O测量作为植被通过光合作用进行的总初级生产的示踪剂的潜力。在此，<0.01  ‰的测量精度是捕捉Δ ¹⁷ O信号季节性变化的先决条件。δ ¹⁷ O和Δ ¹⁷ O 的最低标准误差比值法和同位素体法的分别为0.02‰和0.02‰以及0.01‰和0.02‰。因此，精度结果表明，对于两种方法的δ ¹⁷ O和Δ ¹⁷ O测量结果都过于丰富。这可能是因为我们的两种参考气体不是直接测量而是间接确定的。比率方法显示δ ¹⁷ O和Δ ¹⁷ O结果分别为0.06 ‰ 至0.08 ‰ 和0.06 ‰ 至0.1 ‰ 的残差。同位素体法显示δ ¹⁷ O 的残差范围为 0.04 ‰ 至 0.1 ‰ 和 0.05 ‰ 至 0.13 ‰和Δ ¹⁷ O结果，分别。直接确定所有参考气体的δ ¹⁷ O将提高δ ¹⁷ O的准确度，从而提高Δ ¹⁷ O测量的准确度。