Abstract
Field ionization (FI), field desorption (FD), and liquid injection field desorption/ionization (LIFDI) provide soft positive ionization of gaseous (FI) or condensed phase analytes (FD and LIFDI). In contrast to the well-established positive-ion mode, negative-ion FI or FD have remained rare exceptions. LIFDI provides sample deposition under inert conditions, i.e., the exclusion of atmospheric oxygen and water. Thus, negative-ion LIFDI could potentially be applied to highly sensitive anionic compounds like catalytically active transition metal complexes. This work explores the potential of negative-ion mode using modern mass spectrometers in combination with an LIFDI source and presents first results of the application of negative-ion LIFDI-MS. Experiments were performed on two orthogonal-acceleration time-of-flight (oaTOF) instruments, a JEOL AccuTOF GCx and a Waters Micromass Q-TOF Premier equipped with LIFDI sources from Linden CMS. The examples presented include four ionic liquids (ILs), i.e., N-butyl-3-methylpyridinium dicyanamide, 1-butyl-3-methylimidazolium tricyanomethide, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, and trihexyl(tetradecyl)phosphonium tris(pentafluoroethyl)trifluorophosphate), 3-(trifluoromethyl)-phenol, dichloromethane, iodine, polyethylene glycol diacid, perfluorononanoic acid, anionic surfactants, a tetraphosphazene silanol-silanolate, and two bis(catecholato)silanes. Volatile samples were delivered as vapors via the sample transfer capillary of the LIFDI probe or via a reservoir inlet. Condensed phase samples were applied to the emitter as dilute solutions via the sample transfer capillary. The compounds either yielded ions corresponding to their intact anions, A−, or the [M–H]− species formed upon deprotonation. This study describes the instrumental setups and the operational parameters for robust operation along with a discussion of the negative-ion LIFDI spectra of a variety of compounds.
Highlights
Field ionization (FI) and field desorption (FD) are very soft ionization techniques in mass spectrometry that generally deliver intact positive molecular ions, M+, or adduct ions like [M+H]+ or [M+alkali]+ of molecular analytes [1,2,3,4]
The samples used here have been characterized by direct analysis in real time (DART) where one of the Ionic liquids (ILs) served as a reference compound for mass calibration [43] as IL cluster ions could be produced beyond m/z 6500 [44]
Four ILs with different anions were chosen as indicators of anion desorption in negative-ion liquid injection field desorption/ ionization (LIFDI) mode (Table 1)
Summary
Field ionization (FI) and field desorption (FD) are very soft ionization techniques in mass spectrometry that generally deliver intact positive molecular ions, M+, or adduct ions like [M+H]+ or [M+alkali]+ of molecular analytes [1,2,3,4]. Negative-ion field ionization (FI) was only realized in 1980 jointly by the groups of Nibbering and Röllgen who achieved M– ions and 2M– cluster ions of compounds of high positive electron affinity (EA) like tetracyanoethylene and some multiple chlorinated or brominated benzoquinones [7] They used 10-μm activated tungsten wires as emitters. Negative-ion FD of acidic organic compounds delivered [M–H]− and [M+Cl]− ions, respectively, when 10-μm tungsten wires were employed as emitters in combination with polyethylene oxide (PEO 4000) as the highly viscous solvent/matrix [9]. Dähling et al described the ionization by proton abstraction in negative-ion field desorption mass spectrometry of non-acidic compounds like saccharides and nucleotides from bare or activated tungsten wire emitters in the presence of PEO 4000 [8]. Negative-ion field desorption revitalized by using liquid injection field desorption/ionization-mass
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