Beyond monoisotopic accurate mass spectrometry: ancillary techniques for identifying unknown features in non-targeted discovery analysis

Abstract

High-resolution mass spectrometry (HR-MS) is an important tool for performing non-targeted analysis for investigating complex organic mixtures in human or environmental media. This perspective demonstrates HR-MS compound identification strategies using atom counting, isotope ratios, and fragmentation pattern analysis based on ‘exact’ or ‘accurate’ mass, which allows analytical distinction among mass fragments with the same integer mass, but with different atomic constituents of the original molecules. Herein, HR-MS technology is shown to narrow down the identity of unknown compounds for specific examples, and ultimately inform future analyses when these compounds reoccur. Although HR-MS is important for all biological media, this is particularly critical for new methods and instrumentation invoking exhaled breath condensate, particles, and aerosols. In contrast to standard breath gas-phase analyses where 1 mass unit (Da) resolution is generally sufficient, the condensed phase breath media are particularly vulnerable to errors in compound identification because the larger organic non-volatile molecules can form identical integer mass fragments from different atomic constituents which then require high-resolution mass analyses to tell them apart.

How do cancer-sniffing dogs sort biological samples? Exploring case-control samples with non-targeted LC-Orbitrap, GC-MS, and immunochemistry methods

“…We have recognized this effect in prior work investigating exogenous exposures and cytokines response. Within-sample patterns were explored using heatmapping and intraclass correlation coefficients in previous articles ( Pleil et al 2011a , Stiegel et al 2015 , Pleil et al 2018 ).…”

“…The analyses are performed without preconception, and the methodology is broad; the general idea is to test for as many chemicals as possible in biological media (blood, breath urine, etc) and derive descriptive statistical models from the resulting data. High-resolution (HR-MS) instruments, either with time-of-flight or Orbitrap mass analyzers, are the workhorse tools for this approach because HR-MS provides added dimensions for identifying unknown features via accurate mass measurements, isotopic prediction of chemical formula, and structural elucidation via fragmentation; we have previously discussed specific methods here ( Pleil and Isaacs 2016 , Pleil et al 2018 ). The lack of a pre-determined analyte list allows the data to be queried at leisure for any compounds or features determined to be of interest after the fact ( Pleil and Stiegel 2013 , Rager et al 2016 , Milman and Zhurkovich 2017 , Newton et al 2018 , Sobus et al 2018 ).…”

Mass spectrometry-based aerosolomics: a new approach to resolve sources, composition, and partitioning of secondary organic aerosol

“…In recent years, non-target analysis (NTA) of UHPLC-HRMS measurements has become a powerful tool that builds peak lists of all detected compounds in complex samples, and determines the molecular formula based on the exact mass and isotopic pattern. Furthermore, MS 2 -spectra can be compared to fragmentation libraries and enable database-assisted identification of compounds (Ditto et al, 2018;Ma et al, 2022;Pereira et al, 2021;Pleil et al, 2018). However, there are currently no established databases of atmospheric SOA tracers, which can be applied on measurements of ambient PM 2 .5 filter samples.…”

Mass spectrometry-based Aerosolomics: a new approach to resolve sources, composition, and partitioning of secondary organic aerosol

“…In recent years, non-target analysis (NTA) of UHPLC-HRMS measurements has become a powerful tool that builds peak lists of all detected compounds in complex samples and determines the molecular formula based on the exact mass and isotopic pattern. Furthermore, MS 2 spectra can be compared to fragmentation libraries and enable database-assisted identification of compounds (Ditto et al, 2018;Ma et al, 2022;Pereira et al, 2021;Pleil et al, 2018). However, there are currently no established databases of atmospheric SOA tracers which can be applied to measurements of ambient PM 2.5 filter samples.…”