Progress in circular dichroism laser mass spectrometry.

Circular dichroism in ion yield has promising new potentials for chiral analysis. Our progress of its development is described here. Circular dichroism in ion yield is achieved by resonance-enhanced multiphoton ionization. The feasibility of circular dichroism spectroscopy and quantitative determination of circular dichroism by this method is demonstrated. Several excitation schemes have been applied using different types of lasers, which vary in wavelength and repetition rate. Progress to improve the statistical error and thus the lower limit of measurable circular dichroism is described. This is achieved by adding achiral compounds or racemic mixtures of chiral compounds to the sample gas as reference substances and ionizing them by the same laser pulse. Therefore, in the mass spectrum of every single laser pulse, ion signals of sample and reference species appear both being subject to the same kind of instrumental fluctuations (in particular of laser pulse energy). In another approach, a laser repetition rate of 200 Hz allowed averaging of large numbers of laser pulses.