Division of Geological and Planetary Sciences , California Institute of Technology , Pasadena , California 91125 , United States.
Department of Earth and Environmental Sciences , Boston College , Chestnut Hill , Massachusetts 02467 , United States.
Anal Chem. 2020 Feb 18;92(4):3077-3085. doi: 10.1021/acs.analchem.9b04486. Epub 2020 Feb 3.
The stable isotopes of sulfate, nitrate, and phosphate are frequently used to study geobiological processes of the atmosphere, ocean, as well as land. Conventionally, the isotopes of these and other oxyanions are measured by isotope-ratio sector mass spectrometers after conversion into gases. Such methods are prone to various limitations on sensitivity, sample throughput, or precision. In addition, there is no general tool that can analyze several oxyanions or all the chemical elements they contain. Here, we describe a new approach that can potentially overcome some of these limitations based on electrospray hyphenated with Quadrupole Orbitrap mass spectrometry. This technique yields an average accuracy of 1-2‰ for sulfate δS and δO and nitrate δN and δO, based on in-house and international standards. Less abundant variants such as δO, δS, and δS, and the S-O "clumped" sulfate can be quantified simultaneously. The observed precision of isotope ratios is limited by the number of ions counted. The counting of rare ions can be accelerated by removing abundant ions with the quadrupole mass filter. Electrospray mass spectrometry (ESMS) exhibits high-throughput and sufficient sensitivity. For example, less than 1 nmol sulfate is required to determine O/S ratios with 0.2‰ precision within minutes. A purification step is recommended for environmental samples as our proposed technique is susceptible to matrix effects. Building upon these initial provisions, new features of the isotopic anatomy of mineral ions can now be explored with ESMS instruments that are increasingly available to bioanalytical laboratories.
硫酸盐、硝酸盐和磷酸盐的稳定同位素常用于研究大气、海洋和陆地的地球生物化学过程。传统上，这些和其他含氧阴离子的同位素通过转化为气体后用同位素质谱仪进行测量。这些方法在灵敏度、样品通量或精度方面容易受到各种限制。此外，没有通用的工具可以分析几种含氧阴离子或它们所含的所有化学元素。在这里，我们描述了一种新的方法，该方法基于电喷雾与四极杆轨道阱质谱的联用，有可能克服其中的一些限制。该技术基于内部和国际标准，硫酸盐 δS 和 δO 以及硝酸盐 δN 和 δO 的平均精度为 1-2‰。同时可以定量分析较少的变体，如 δO、δS 和 δS，以及 S-O“聚集”硫酸盐。同位素比值的观测精度受计数离子数量的限制。通过四极质量过滤器去除丰度离子，可以加速稀有离子的计数。电喷雾质谱 (ESMS) 具有高通量和足够的灵敏度。例如，在几分钟内，只需不到 1 nmol 的硫酸盐即可确定 O/S 比，精度为 0.2‰。建议对环境样品进行净化步骤，因为我们提出的技术容易受到基质效应的影响。在此基础上，具有越来越多的生物分析实验室可用的 ESMS 仪器，现在可以探索矿物离子同位素解剖的新特征。
