Field desorption mass spectrometry of 1-methylpyridinium salts

From the discovery of field ionization to field desorption and liquid injection field desorption/ionization-mass spectrometry-A journey from principles and applications to a glimpse into the future

The discovery of the ionizing effect of strong electric fields in the order of volts per Ångstrom in the early 1950s eventually led to the development of field ionization-mass spectrometry (FI-MS). Due to the very low ion currents, and thus, limited by the instrumentation of the 1960s, it took some time for the, by then, new technique to become adopted for analytical applications. In FI-MS, volatile or at least vaporizable samples mainly deliver molecular ions, and consequently, mass spectra showing no or at least minor numbers of fragment ion signals. The next major breakthrough was achieved by overcoming the need to evaporate the analyte prior to ionization. This was accomplished in the early 1970s by simply depositing the samples onto the field emitter and led to field desorption-mass spectrometry (FD-MS). With FD-MS, a desorption ionization method had become available that paved the road to the mass spectral analysis of larger molecules of low to high polarity and even of organic salts. In FD-MS, all of these analytes deliver spectra with no or at least few fragment ion peaks. The last milestone was the development of liquid injection field desorption/ionization (LIFDI) in the early 2000s that allows for sample deposition under the exclusion of atmospheric oxygen and water. In addition to sampling under inert conditions, LIFDI also enables more robust and quicker operation than classical FI-MS and FD-MS procedures. The development and applications of FI, FD, and LIFDI had mutual interference with the mass analyzers that were used in combination with these methods. Vice versa, the demand for using these techniques on other than magnetic sector instruments has effectuated their adaptation to different types of modern mass analyzers. The journey started with magnetic sector instruments, almost skipped quadrupole analyzers, encompassed Fourier transform ion cyclotron resonance (FT-ICR) and orthogonal acceleration time-of-flight (oaTOF) analyzers, and finally arrived at Orbitraps. Even interfaces for continuous-flow LIFDI have been realized. Even though being niche techniques to some degree, one may be confident that FI, FD, and LIFDI have a promising future ahead of them. This Account takes you on the journey from principles and applications of the title methods to a glimpse into the future.

Multiple stage pentaquadrupole mass spectrometry for generation and characterization of gas-phase ionic species. The case of the PyC2H 5 (+·) isomers

Eleven isomers with the PyC2H 5 (+·) composition, which include three conventional (1-3) and eight distonic radical cations (4-11), have been generated and in most cases successfully characterized in the gas phase via tandem-in-space multiple-stage pentaquadrupole MS(2) and MS(3) experiments. The three conventional radical cations, that is, the ionized ethylpyridines C2H5-C5H4N(+·) (1-3), were generated via direct 70-eV electron ionization of the neutrals, whereas sequences of chemical ionization and collision-induced dissociation (CID) or mass-selected ion-molecule reactions were used to generate the distonic ions H2C(·)-C5H4N(+)-CH3 (4-6), CH3-C5H4N(+)-CH 2 (·) (7-9), C5H5N(+)-CH2CH 2 (·) (10), and C5H5N(+)-CH(·)-CH3 (11). Unique features of the low-energy (15-eV) CID and ion-molecule reaction chemistry with the diradical oxygen molecule of the isomers were used for their structural characterization. All the ion-molecule reaction products of a mass-selected ion, each associated with its corresponding CID fragments, were collected in a single three-dimensional mass spectrum. Ab initio calculations at the ROMP2/6-31G(d, p)//6-31G(d, p)+ZPE level of theory were performed to estimate the energetics involved in interconversions within the PyC2H5 (+·) system, which provided theoretical support for facile 4⇌7 interconversion evidenced in both CID and ion-molecule reaction experiments. The ab initio spin densities for the a-distonic ions 4-9 and 11 were found to be largely on the methylene or methyne formal radical sites, which thus ruled out substantial odd-spin derealization throughout the neighboring pyridine ring. However, only 8 and 9 (and 10) react extensively with oxygen by radical coupling, hence high spin densities on the radical site of the distonic ions do not necessarily lead to radical coupling reaction with oxygen. The very typical "spatially separated" ab initio charge and spin densities of 4-11 were used to classify them as distonic ions, whereas 1-3 show, as expected, "localized" electronic structures characteristic of conventional radical ions.

Diagnostic positive and negative ion fast atom bombardment mass spectrometry of low-molecular-weight ammonium compounds of marine origin

Five ammonium compounds--betaine, taurine, homarine, trigonelline and tetramethylammonium chloride--were investigated by positive and negative ion fast atom bombardment (FAB) mass spectrometry. Observation of the parent cation [M + H]+ together with series of clusters (e.g. [nM + H]+, n = 2-5) and fragmentation patterns in the positive FAB mode may be used diagnostically as a fingerprint of the compound. The shifts in mass and relative intensity by addition of trichloroacetic acid and metal salts offer corroborating information of the identity of a compound in a mixture. The FAB negative ion spectra were characterized by clusters incorporating the anion (e.g. [M + Cl]-) but are generally less informative than the spectra obtained in the positive mode.

Degradation pathway of pralidoxime chloride in concentrated acidic solution

The degradation products of pralidoxime chloride (1) (X- = Cl-) in concentrated aqueous solutions (less than or equal to 50% w/v) were identified using one or more methods: HPLC, polarography, voltammetry, MS and/or NMR. The products found were the 2-cyano-, 2-carboxamido- and 2-carboxy-1-methyl-pyridinium chlorides, 1-methyl pyridinium chloride, cyanide ion, ammonia and carbon dioxide. 1-Methyl-2-pyridone was indirectly identified by the presence of cyanide ion. The degradation rate increased with increasing pH values between pH 1 and 3.2 and with increasing concentrations between 1 and 50% w/v pralidoxime chloride. The results suggest that 1 (X- = Cl-) is dehydrated by a hydroxyl-ion catalyzed reaction ot the nitrile 2 which is hydrolyzed to either the pyridone 6 and cyanide ion or to 2-carboxamido-1-methyl-pyridinium chloride 3. The amide is hydrolyzed to give the 2-carboxy derivative 4 which finally is decarboxylated to give 1-methylpyridinium chloride 5.