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Abstract
Simultaneous, enantiomer-specific identification of chiral molecules in multi-component mixtures is extremely challenging. Many established techniques for single-component analysis fail to provide selectivity in multi-component mixtures and lack sensitivity for dilute samples. Here we show how enantiomers may be differentiated by mass-selected photoelectron circular dichroism using an electron–ion coincidence imaging spectrometer. As proof of concept, vapours containing ∼1% of two chiral monoterpene molecules, limonene and camphor, are irradiated by a circularly polarized femtosecond laser, resulting in multiphoton near-threshold ionization with little molecular fragmentation. Large chiral asymmetries (2–4%) are observed in the mass-tagged photoelectron angular distributions. These asymmetries switch sign according to the handedness (R- or S-) of the enantiomer in the mixture and scale with enantiomeric excess of a component. The results demonstrate that mass spectrometric identification of mixtures of chiral molecules and quantitative determination of enantiomeric excess can be achieved in a table-top instrument.
Highlights
Simultaneous, enantiomer-specific identification of chiral molecules in multi-component mixtures is extremely challenging
Recent work has highlighted the role for enantiomer-specific detection of, for example, the monoterpene limonene in analysing biogenic emissions of VOCs2,3
Much progress has been made in the analysis of mixtures of volatile organics (VOCs) in such areas using direct injection mass spectrometry[4,5], where ‘soft’ ionization techniques can obviate the need for prior chromatographic separation, a particular advantage for field and in vivo studies
Summary
Simultaneous, enantiomer-specific identification of chiral molecules in multi-component mixtures is extremely challenging. Mass spectrometric techniques for determination of singlecomponent enantiomeric excess (e.e.) relevant to pharmaceutical development have, developed greatly over the past decade[6,7] and rely on interactions with a known chiral reference reagent to discriminate enantiomers. These reference compounds have to be carefully chosen in advance according to the specific target species, and have themselves to be sourced in enantiopure form. By eliminating the often indeterminate contributions from solvation[14], gas phase studies of biomolecules can be of considerable value for developing fundamental understanding[15] and permit the weak interactions responsible for chiral molecular recognition to be directly examined[16,17]
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