Mass spectrometry (MS) can identify isomers, but the process is complex and often requires advanced techniques. High-resolution MS instruments and new fragmentation methods have enabled the differentiation of some isomers, such as amino acid residues in proteins and peptides. For example, isomers like aspartic acid and isoaspartic acid, leucine and isoleucine, and D/L enantiomers can be distinguished using these advanced MS techniques Hurtado, 2012. However, the subtle structural differences between isomers often pose significant challenges, necessitating the development of specialized methods.
Tandem mass spectrometry (MS/MS) has proven effective in identifying isomers, particularly when coupled with high-performance liquid chromatography (HPLC). For instance, ion trap and triple quadrupole MS/MS have been used to identify isomers of perfluorooctane sulfonate (PFOS) by analyzing their fragmentation patterns and retention times Langlois, 2006. Additionally, infrared ion spectroscopy (IRIS) combined with MS has shown promise in distinguishing positional isomers, such as ortho-, meta-, and para-isomers, by probing diagnostic vibrations specific to each isomer Outersterp, 2020.
Despite these advancements, MS alone often struggles to distinguish isomers due to their identical mass-to-charge ratios. Techniques like gas chromatography-mass spectrometry (GC-MS) with soft ionization, and derivatization strategies like mCPBA epoxidation, have been developed to enhance isomer identification. These methods allow for the analysis of isomer-specific fragments and the spatial mapping of isomers in complex biological samples Furuhashi, 2017; Kuo, 2019. Moreover, laser control MS has been used to achieve quantitative identification of geometric and structural isomers by exploiting differences in their fragmentation patterns induced by shaped femtosecond laser pulses Cruz, 2007.
In summary, mass spectrometry can identify isomers, but it often requires advanced techniques such as high-resolution MS, tandem MS, IRIS, and specialized ionization methods. These techniques enhance the ability to distinguish isomers by analyzing their unique fragmentation patterns and diagnostic vibrations.
