100 Years of Photoionization: Resonance Enhanced Multiphoton Ionization (REMPI) and Single Photon Ionization (SPI) Mass Spectrometry for On-line Process and Product Analysis

Photo Ionization (PI) in the vacuum has specific advantages for mass spectrometric applications that the inventor of PI (F. Mohler, PNAS 12, 1926, 494) could not have foreseen. A key feature of PI as ion source in mass spectrometry is the ability to softly (no fragmentation) ionize molecules. Furthermore, the selection of the photon energy of the light used for PI opens up the possibility of compound-selective ionization. The introduction of more intense VUV light sources for single-photon ionization (SPI), the invention of (pulsed) lasers that enabled resonance-enhanced multiphoton ionization (REMPI) of molecules, and finally advances in mass spectrometry analysis technology enabled the development of a variety of instruments and applications of photoionization mass spectrometry (PIMS) from the late 1970s onwards. Although PI is more suitable for specialized mass spectrometric applications, an impressive variety of PIMS instruments and applications have been developed in the last decades.
This lecture on the occasion of PI's 100th anniversary deals with the development of analytical PIMS systems, e.g., for environmental monitoring or industrial process analytics. For example, analysis of coffee roasting gases for taste prediction, direct detection of marine pollution from seawater or the characterization of industrially relevant catalytic reactions by PIMS is presented.
On the other hand, the hyphenation of PIMS with other analytical instruments, such as chromatography or thermal analysis as well as the development of novel mass spectrometric concepts using PI methods is discussed. An example is the direct coupling of thermo-gravimetry (TG) to study the decomposition of polymers. PI in the C-trap region of an Orbitrap mass analyzer or on-line REMPI detection of aromatic molecules from individual laser desorbed aerosol particles in aerosol single particle mass spectrometry are only selected examples for new mass spectrometric concepts utilizing PI.