Mass spectrometric discrimination of structural isomers without prior chromatographic separation is challenging due to the similar fragmentation patterns of isomeric molecules. To overcome this challenge, this presentation introduces femtosecond time-resolved mass spectrometry (FTRMS) as a means to quantify the composition of isomer mixtures. FTRMS uses a pair of time-delayed femtosecond laser pulses to induce molecular ionization and controlled dissociation, resulting in distinct fragmentation patterns as the time delay between the two pulses is scanned. This “pump-probe” technique takes advantage of the unique relaxation dynamics of each isomer immediately after vertical ionization, which results in distinct ion yield dynamics for each isomer. Upon quantification of ion yield dynamics in individual isomers, the measured ion yield dynamics in any isomer mixture can be resolved into fractional contributions from each isomer. As an illustration, FTRMS is used to quantify mixtures of nitrotoluene isomers, which are known environmental contaminants near manufacturing facilities for explosives such as TNT. FTRMS can quantify binary mixtures of para/meta-nitrotoluene and para/ortho-nitrotoluene with average error of 5% and ternary mixtures of para/meta/ortho-nitrotoluene with average error of 7%.

Quantifying the composition of structural isomer mixtures with femtosecond time-resolved mass spectrometry

ACS Fall 2021

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