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Analysis of Chirality by Femtosecond Laser Ionization Mass Spectrometry
The distinction of enantiomers poses an important challenge in chemical analysis. Classical approaches are based on the difference of the absorption coefficients for left and right-circularly polarized light, which constitutes the Circular Dichroism (CD). Conventional CD spectrometers are based on one photon absorption, and are inherently not mass-selective.
To combine CD measurements with LIMS the ion yields for left and right circularly polarized laser pulses, YLCP and YRCP respectively, are applied to define the CD in ion yield as
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In 2009 we were able to demonstrate the first observation of circular dichroism using femtosecond laser ionization mass spectrometry (fs-LIMS).[1] Furthermore we have investigated the influence of the pulse duration on the CD in ion yield. For this purpose (R)-(+)- and (S)-(-)-3-methyl-cyclopentanone has been ionized with femtosecond laser pulses with a central wavelength of 311 nm. CD values of up to 0.15 have been observed as shown in fig. 1. The decreasing CD in ion yield for a decreasing laser pulse length can be described my means of theoretical calculations.[2].
Fig. 1: Circular dichroism measured in the multiphoton-ionization of (R)-(+)- and (S)-(-)-3-methyl-cyclopentanone around 311 nm as a function of the pulse duration.
To enhance the analytical feasibility of this chirality analysis approach, we investigated whether the observation of CD effects in laser ionization experiments is restricted to resonant conditions. More specifically, we studied the CD in multiphoton ionization for (R)- and (S)-propylene oxide. Interestingly, the observed sign of the CD in ion yield cannot be explained by resonant excitation and is different for parent and fragment ions as shown in fig. 2, respectively.[3, 4]
Fig. 2: CD in ion yield for (R)- and (S)- propylene oxide for the parent and different fragment ions.
Most recently, we have demonstrated the simultaneous measurement of photoion circulardichroism (PICD) and photoelectron circular dichroism (PECD) in photoelectron-photoion coincidence experiment.
Fig. 3 Graphical abstract from ref. [5]
Literature:
[1] H.G. Breunig, G. Urbasch, P. Horsch, J. Cordes, U. Koert, K.-M. Weitzel, ChemPhysChem, 8, 1199-1202, (2009), http://dx.doi.org/10.1002/cphc.200900103
[2] P. Horsch, G. Urbasch, K.-M. Weitzel, D. Kröner, PCCP, 13, 2378–2386, (2011), http://dx.doi.org/10.1039/c0cp01903h
[3] P. Horsch, G. Urbasch, K.-M. Weitzel, Z. Phys. Chem., 225, 587-594, (2011), http://dx.doi.org/10.1524/zpch.2011.0103
[4] P. Horsch, G. Urbasch, K.-M. Weitzel, Chirality, 24, 684-690, (2012), http://dx.doi.org/10.1002/chir.22037
[5] S. Lehmann, K.-M. Weitzel, PCCP, 22, 13707-1312. (2020) https://doi.org/10.1039/D0CP01376E