Enantiomer-specific analysis of multi-component mixtures by correlated electron imaging–ion mass spectrometry

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.

Universality of photoelectron circular dichroism in the photoionization of chiral molecules

“…This sets quite a limitation on the range of systems that can be studied, and makes data interpretation challenging for example when multiple resonances are hit by the excitation process. Nevertheless, REMPI-PECD is considered as a promising technique and a great metrology tool, enabling for instance accurate determination of enantiomeric excess [30,31]. In this work, we have bridged the gap between the generality of one-photon XUV PECD and the versatility of femtosecond laser sources by demonstrating that PECD could be observed in non-resonant multiphoton/tunnel ionization.…”

Determination of accurate electron chiral asymmetries in fenchone and camphor in the VUV range: sensitivity to isomerism and enantiomeric purity

“…However, as compared to this latter method, the i 2 PEPICO experiment provides the capability for ion mass--tagged PECD measurement. This allows for the simultaneous enantiomer specific analysis of multi--component mixtures which was proposed and demonstrated for a mixture of chiral and non--chiral species --spurious achiral decomposition product of alanine 15 --and for prepared mixtures of chiral compounds by Rafie Fanood et al 30 . In that later case an e.e.…”

“…The benchmarking versus a conventional chromatographic analytical method, highlights the interest and high accuracy, in the % range, of PECD as a precise enantiomeric analytical tool in the gas phase. Especially when combined in with TOF mass spectrometry of the photoions, recorded in coincidence with the photoelectrons, via the so--called PECD--PICO scheme, we stress the ability to analyze a mixture of compounds simultaneously and to provide highly accurate relative ee measurement of each of these compounds, now with significantly improved accuracy over the first demonstrations of this capability 15,30 . As compared to the well--established GC type of technique, PECD is a direct method with no derivatization, nor use of reagents, directly applicable in the gas phase, and which provides additional electronic information, as a multi--dimensional chiroptical probe (photon energy, electron energy, coincident ion mass).…”

Identifying and Understanding Strong Vibronic Interaction Effects Observed in the Asymmetry of Chiral Molecule Photoelectron Angular Distributions

“…Such developments are now attracting considerable interest because of the possibilities for time-resolved chiral probing, and convenient lab-based analytical applications that are thereby opened up. [23][24][25] In particular, the use of resonantly enhanced multiphoton ionization (REMPI), as opposed to single VUV photon ionization, creates new opportunities for increased vibrational selectivity in PECD, as ionization then occurs out of a prepared vibrational level or wavepacket. [26] Besides permitting greater control of vibrational excitation and mode specificity, REMPI brings with it opportunities for selective isomer and/or conformer specific ionization.…”