WAVELENGTH-SPECIFIC RESONANCE-ENHANCED MULTIPHOTON IONIZATION FOR ISOMER DISCRIMINATION VIA FRAGMENTATION AND METASTABLE ANALYSIS

In this work, it is shown that resonance-enhanced multiphoton ionizati on (REMPI) at two wavelengths, 266 and 193 nm, can be used to produce fragmentation from which isomers can be distinguished. In particular, metastable ion energy analysis using a reflectron time-of-flight devic e is used to analyze the metastable ion products arising from REMPI-in duced fragmentation during the flight time to the detector. It is demo nstrated that metastable ion production is particularly sensitive to v ariations in wavelength and that unique metastable ions are produced a t the correct choice of wavelength that allows easy discrimination of isomer pairs. Such discrimination is demonstrated for the isomers Leu- Tyr and Ile-Tyr at 193 nm, while it is shown that the isomers cannot b e distinguished at 266 nm. Likewise, it is shown that the isomer pair Trp-Ala and Ala-Trp can be discriminated on the basis of their metasta ble fragmentations at 266 nm but not at 193 nm. Further, a comparison of fragmentation by REMPI is made to electron impact (EI). It is demon strated that metastable ions cannot be easily detected by EI for these compounds. It appears that metastable ion detection is strongly enhan ced in REMPI due to the narrow-band excitation of the ion manifold and the relative sensitivity which may be up to 6 orders of magnitude gre ater in REMPI than EI under comparable conditions.