Photoelectron Spectra and Molecular Properties: Real-Time Gas Analysis in Flow Systems

For the analysis and optimization of numerous gas phase reactions in flow-systems, photoelectron spectroscopy has proven most valuable. This real-time measuring probe allows one to determine—with millimol quantities and within a few hours—the temperatures for different decomposition channels. Simultaneously, main products are characterized and, if need be, their yield can be improved. By careful performance of the experiment, short-lived and/or reactive molecules such as P₂, thioformaldehyde or silabenzene can be detected. PE-spectroscopic gas analysis is of particular advantage in the search for heterogeneous catalysts; they can be tested within a day using gas mixtures of varying composition over the temperature range from 300 K to 1300 K. In addition, PE-spectrometers are well-suited for on-line connection to computers; portable instruments for laboratory use are under development.     

Determination of ion mobility collision cross sections for unresolved isomeric mixtures using tandem mass spectrometry and chemometric deconvolution

Ion mobility (IM) is an important analytical technique for determining ion collision cross section (CCS) values in the gas-phase and gaining insight into molecular structures and conformations. However, limited instrument resolving powers for IM may restrict adequate characterization of conformationally similar ions, such as structural isomers, and reduce the accuracy of IM-based CCS calculations. Recently, we introduced an automated technique for extracting “pure” IM and collision-induced dissociation (CID) mass spectra of IM overlapping species using chemometric deconvolution of post-IM/CID mass spectrometry (MS) data [J. Am. Soc. Mass Spectrom., 2014, 25, 1810–1819]. Here we extend those capabilities to demonstrate how extracted IM profiles can be used to calculate accurate CCS values of peptide isomer ions which are not fully resolved by IM. We show that CCS values obtained from deconvoluted IM spectra match with CCS values measured from the individually analyzed corresponding peptides on uniform field IM instrumentation. We introduce an approach that utilizes experimentally determined IM arrival time (AT) “shift factors” to compensate for ion acceleration variations during post-IM/CID and significantly improve the accuracy of the calculated CCS values. Also, we discuss details of this IM deconvolution approach and compare empirical CCS values from traveling wave (TW)IM-MS and drift tube (DT)IM-MS with theoretically calculated CCS values using the projected superposition approximation (PSA). For example, experimentally measured deconvoluted TWIM-MS mean CCS values for doubly-protonated RYGGFM, RMFGYG, MFRYGG, and FRMYGG peptide isomers were 288.₈ Å², 295.₁ Å², 296.₈ Å², and 300.₁ Å²; all four of these CCS values were within 1.5% of independently measured DTIM-MS values.     

Analyse und Optimierung von Gasphasen-Reaktionen,XVII. Selenoketen

Analysis and Optimization of Gasphase Reactions, XVII. — Selenoketene The thermal decomposition of 1,2,3-selenadiazole in the gaseous phase has been investigated by photoelectron and mass spectroscopy. At temperatures above 720 K selenoketene is formed, the PE spectrum of which can be assigned based on ab initio SCF calculations as well as on radical cation state comparison with the iso(valence) electronic heterocumulenes H₂C  C  O and H₂C  C  S. The 4-phenyl derivative decomposes above 820 K forming phenylacetylene.