GC-MS for Analyte Identification in Isotope Ratio Mass Spectrometry (Combined GC-MS/IRMS)

近年来，我们一直在推进化合物特定同位素分析(CSIA)，以追踪有机地下水污染物的来源，并从化合物特定同位素数值中破译它们的自然转化。用气相色谱(GC)分离目标化合物，然后在线转化为CO2，利用同位素比值质谱仪(IRMS)直接进行13C/12C的同位素分析。虽然IRMS是唯一适合于确定准确的同位素比率的方法，但结构信息在燃烧到二氧化碳的过程中会丢失。因此，分析物的身份不能被独立确认。CSIA的早期应用受到的影响很小，因为它们针对的是受污染的地点，与背景有机物相比，这些地点的分析物浓度较高(mg/L)。因此，污染物在GC-IRMS色谱图中作为特征峰突出，并可以通过它们的保留时间可靠地识别。我们一直在努力将CSIA方法扩展到所谓的微污染物：农药、药品和消费者护理产品，这些污染物的浓度要小得多(从低微克/L到NAG/L范围)。因此，在GC-IRMS色谱图中，它们的峰通常嵌在有机物的共洗脱峰的连续组中，这导致了突出的研究挑战。(I)现场样品的分析保证了唯一的化合物鉴定，但经常显示色谱保留时间的细微变化，使其不能用于分析物鉴定。(Ii)同时，我们目前的研究工作将极大地受益于从结构上阐明这些干扰，以优化有针对性地消除它们的方法。因此，这项申请要求为关键仪器提供资金，以便与现有的IRMS仪器相结合：一个带有传统四极质谱计(QMS)的GC和一个在IRMS分析之前用于化合物转换的接口。该设备将允许分离GC流，以便一部分通过界面进入IRMS进行同位素分析，而另一部分被定向到QMS，以便能够同时进行结构鉴定。这一设置不仅将支持我们在自然丰度下的测量，这取决于目标分析物和背景干扰的识别。它还将有利于我们在国际原子能机构(原子能机构)的资助下开展的第二个研究途径：在不可能标记放射性物质的情况下，通过应用稳定的同位素标记脉冲来识别微污染物代谢物。我们将领导一个协调研究网络，在农业田间试验中追踪13C标记的抗生素、代谢物和抗生素耐药性的命运。在这里，GC-IRMS仪器将能够检测到13C标记的代谢物的色谱峰是大海捞针，而GC-MS分析将使这些未知代谢物的结构同时得到阐明。

Over recent years we have been advancing compound-specific isotope analysis (CSIA) to trace sources of organic groundwater contaminants, and to decipher their natural transformation from compound-specific isotope values. Gas chromatographic (GC) separation of target compounds is followed by online conversion to CO2 for direct isotope analysis of 13C/12C by isotope ratio mass spectrometry (IRMS). While IRMS is uniquely suited to determine accurate isotope ratios, structural information is lost during combustion to CO2. Hence, the identity of analytes cannot be confirmed independently. Early applications of CSIA were little affected, because they targeted contaminated sites, where analytes were present in high concentrations (mg/L) compared to background organic matter. Hence, pollutants stood out as characteristic peaks in GC-IRMS chromatograms and could be reliably identified by their retention times. We have been striving to expand the CSIA approach also to so-called micropollutants: pesticides, pharmaceuticals and consumer care products which occur in much smaller concentrations (low micrograms/L to nag/L range). In GC-IRMS chromatograms their peaks are, therefore, typically embedded in a contiuum of co-eluting peaks from organic matter leading to prominent research challenges. (i) Analysis of field samples warrants unique compound identification, yet frequently shows subtle changes in chromatographic retention times making them unreliable for analyte identification. (ii) At the same time, our current research efforts would greatly profit from structural elucidation of these interferences to optimize approaches for their targeted removal. This application, therefore, requests funding for crucial instrumentation to be combined with existing IRMS instrumentation: a GC with a conventional quadrupole mass spectrometer (qMS) and an interface for compound conversion prior to IRMS analysis. This equipment will allow splitting the GC flow so that one part goes through the interface into the IRMS for isotope analysis, while the other part is directed to the qMS to enable simultaneous structural identification. Not only will this setup support our measurements at natural abundance that depend on identification of target analytes and background interferences. It will also benefit a second research avenue that we pursue with funding by the International Atomic Energy Agency (IAEA): the identification of micropollutant metabolites by application of a stable isotope-labelled pulse where radioactive labelling is impossible. We will lead a Coordinated Research Network to trace the fate of 13C labelled antibiotics, metabolites and antibiotic resistance in an agricultural field experiment. Here, GC-IRMS instrumentation will be able to detect chromatographic peaks of 13C-labelled metabolites as “needles in the haystack”, while GC-MS analysis would enable simultaneous structural elucidation of these unknown metabolites.