Light and Molecular Ions: The Emergence of Vacuum UV Single-Photon Ionization in MS

Abstract

Thanks to recent technological advances and singlephoton ionization's (SPI's) ability to detect all organics,Because the human eye does not detect IR radiation, we cannot see a warm object in the dark without an IR camera or other "transformation method". MS has the same problem: it cannot directly detect the objects of intereststhe analyte moleculessbut only their ionized counterparts. Therefore, the transformation methods that ionize the analyte molecules are of particular importance to MS. The invention of new ionization methods such as MALDI or ESI for MS detection of biomolecules has had an enormous influence on the development of MS and its applications in analytical chemistry.However, many ionization methods result in the formation not only of intact molecular ions but also of ion fragments. This is particularly true for electron impact (EI) ionization, the classical ionization method for smaller molecules ranging in mass up to ∼500 Da. EI uses electrons of 70-eV kinetic energy, near where maximal ionization efficiency occurs, 1 which makes it a "hard" ionization technique because the analytes are heavily fragmented upon ionization. The molecular peak is often barely visible in the mass spectra of labile compounds, although the fragment patterns do supply information about the presence of distinct functional groups in the investigated molecules. Furthermore, the fragment patterns of unknown substances can be used for a statistically based identification by comparison with EI-MS library data. The benefits of the latter feature made MS with EI the routine spectrometric detection method for GC. However, the frequent absence of information on the molecular weight of labile compounds is a severe disadvantage of EI, hampering the identification of unknowns. Furthermore, for complex mixtures of organic compounds, EI-MS requires a preseparation method such as GC; otherwise, the overlapping complex fragment patterns cannot be deconvoluted into mass spectra of individual compounds.These shortcomings of EI have led to a continuing demand for robust fragmentation-free or "soft" ionization methods for MS. Several soft ionization techniques have been developed, including methods based on chemical ionization (CI), field ionization (FI), and photoionization (PI). More recently, MALDI and ESI emerged and became the dominant soft ionization methods for polar

Long-term litter decomposition controlled by manganese redox cycling

“…3E). Synchrotron-LDPI is a soft-ionization mass spectrometry technique that is particularly sensitive for lignin-derived compounds (29,30). Inspection of the resulting mass spectra showed that one set of peaks at lower mass-tocharge (m/z) ratios ( P ar + ; m/z = 340-370) gains intensity with increasing litter age whereas the intensity of a second set of peaks at higher m/z ratios ( P ar − , m/z = 396-414) declines (Fig.…”

“…To confirm that these mass fragments (i.e., m/z = 340, 370, 396, 399, 400, 406, and 414) correspond to aromatic structures, we determined ionization energies (IEs) for each of the fragments in a second LDPI experiment as previously described (31,32) and detailed in SI Materials and Methods. This method exploits the fact that aromatic ring structures have lower ionization energies (≤8.5 eV) than other organic moieties (>9 eV) (29,30). Because ionization energies of these mass fragments ranged from 7.3 to 8.5 eV (Table S2), we attributed these peaks to aromatic moieties.…”

“…To identify molecular changes in aromatic components of the litter (lignins and tannins), synchrotron-based laser desorption post ionization (LDPI) mass spectrometry of fresh needles and litter layers was performed on a modified time-of-flight secondary ion mass spectrometer (TOF.SIMS V; IonTOF) coupled to a synchrotron UV light port at beamline 9.0.2 of the Advanced Light Source (ALS) (29,31). To identify peaks in the resulting mass spectra that corresponded to aromatic structures, ionization energies for each of the most prominent mass peaks were determined as described in ref.…”

“…31 and detailed in SI Materials and Methods. Based on the low IEs of larger aromatic systems (29,30), peaks with IEs of less than 8.5 eV were attributed to aromatic moieties.…”

Rapid Identification and Quantification of Linear Olefin Isomers by Online Ozonolysis-Single Photon Ionization Time-of-Flight Mass Spectrometry

“…Single photon ionization (SPI) is another attractive soft ionization technique because of its low degree of fragmentation with simple spectrum interpretation compared with conventional EI method [26,27], especially for complex samples (e.g., gasoline and diesel [28,29], cigarette smoke [30], and volatile organic compounds (VOCs) [31][32][33]). The coupling of SPI with time-of-flight mass spectrometer (SPI-TOFMS) has the advantages of both rapid detection speed and simple spectra analysis [34,35].…”