A spurious [CH5]− ion: Problems with negative chemical ionization quadrupole instrument

It is shown that in a quadrupole mass spectrometer set up for negative chemical ionization, positive ions can be formed between the ionization chamber and the analyser both by electron impact and by chemical ionization. These ions will be registered together with negative ions formed in the ion source if the instrument is equipped with a positive and a negative conversion dynode and may reach the secondary electron multiplier even if the negative conversion dynode is disconnected.     

Structural identification of fatty acid methyl esters by collision spectra of their [M ? H]− species

Negative ion chemical ionization linked to collisionally activated decomposition (CAD) experiments has proved to be an efficient analytical tool in mass spectrometric characterization of fatty acids. The CAD mass-analysed ion kinetic energy spectra of [M ? H]^? species, obtained by OH^? reaction with a selection of six C₁₈ fatty acid methyl esters, reveal useful correlations with the original structure of the neutrals, giving evidence of both chain branching and double bond positions.     

O− and OH− chemical ionization of some fatty acid methyl esters and triacylglycerols

O^? and OH^? react with fatty acid methyl esters (FAMES) under chemical ionization conditions both as Bronsted bases to form [M - H]^? and as nucleophiles to form the carboxylate ion RCOO^?. O^? shows a much greater tendency to react as a nucleophile than does OH^?. The [M - H]^? ions fragment by elimination of CH³OH, with unsaturation in certain positions in the fatty acid hydrocarbon chain promoting this elimination for unknown reasons. The reaction of O^? and OH^? with triacylglycerols leads to [M - H]^?, characteristic of the molecular mass, and to carboxylate ions characteristic of the fatty acid(s) present in the lipid. The presence of the three ester functions in the lipids greatly enhances the formation of carboxylate ions compared to the FAMES.     

Acetonitrile covalent adduct chemical ionization tandem mass spectrometry of non-methylene-interrupted pentaene fatty acid methyl esters

Acetonitrile covalent adduct chemical ionization tandem mass spectrometry (CACIMS/MS) has shown to be an efficient method for the identification of double‐bond position in homoallylic, conjugated and several polyene non‐methylene‐interrupted (NMI) fatty acid methyl esters. However, it has not been thoroughly evaluated for NMI highly unsaturated fatty acids (HUFA) with more than four double bonds. Docosahexaenoic acid (DHA)‐rich single cell oil (DHASCO^®; Martek Biosciences, Corp.) was partially hydrogenated (partially hydrogenated DHASCO; PHDO) producing ten novel 22:5 and 22:6 HUFA isomers. In single‐stage MS, the ratio of [M+54]⁺/[M+54‐32]⁺ for the 22:5 and 22:6 isomers indicated the presence of homoallylic or partially conjugated double‐bond systems. The CACIMS/MS spectra revealed six 22:5 isomers with diagnostic ions corresponding to the homoallylic 22:5n‐6 and 22:5n‐3 isomers, and four distinct NMI 22:5 isomers. Diagnostic ions for four 22:6 isomers were identical to the native DHA illustrating that CACIMS/MS is sensitive to double‐bond position but not geometry. Three gas chromatography (GC) peaks for partially conjugated 22:6 isomers were also detected and clearly distinguishable from homoallylic 22:6 isomers, but their CACIMS/MS spectra did not yield prominent ions indicative of double‐bond position, possibly due to co‐elution. Overall, CACIMS/MS was effective for determining double‐bond position in NMI 22:5 isomers. Further investigations are warranted to evaluate and determine fragmentation patterns for partially conjugated and NMI 22:6 HUFA. Copyright © 2011 John Wiley & Sons, Ltd.     

[M + 11]+˙ and [M + 11]−˙ ions in the nitrogen positive and negative ion chemical ionization mass spectra of aromatic amines

The N₂ negative ion chemical ionization (NICI) mass spectra of aniline, aminonaphthalenes, aminobiphenyls and aminoanthracenes show an unexpected addition appearing at [M + 11]^?˙. This addition is also observed in the N₂ positive chemical ionization (PCI) mass spectra. An ion at [M – 15]^? is found in the NICI spectra of aminoaromatics such as aniline, 1- and 2-aminonaphthalene and 1- and 2-aminoanthracene. Ion formation was studied using labeled reagents, variation of ion source pressure and temperature and examination of ion chromatograms. These experiments indicate that the [M + 11]^?˙, [M – 15]^?˙ and [M + 11]^+˙ ions result from the ionization of analytes altered by surface-assisted reactions. Experiments with ¹⁵N₂, [¹⁵N] aniline, [2,3,4,5,6-²H₅] aniline and [¹³C₆] aniline show that the [M + 11]^?˙ ion corresponds to [M + N – 3H]^?˙. The added nitrogen originates from the N₂ buffer gas and the addition occurs with loss of one ring and two amino group hydrogens. Fragmentation patterns in the N₂ PCI mass spectrum of aniline suggest that the neutral product of the surface-assisted reaction is 1,4-dicyanobuta-1,3-diene. Experiments with diamino-substituted aromatics show analogous reactions resulting in the formation of [M – 4H]^?˙ ions for aromatics with ortho-amino groups. Experiments with methylsubstituted aminoaromatics indicate that unsubstituted sites ortho to the amino group facilitate nitrogen addition, and that methyl groups provide additional sites for nitrogen addition.     

Binary alkali metal compounds with the zintl anions [Ge9]4− and [Sn9]4−

The binary germanides M₁₂Ge₁₇ and M₄Ge₉ (M ? Na, K, Rb, Cs) and the stannides M₁₂Sn₁₇ and M₄Sn₉ (M ? K, Rb, Cs) were identified by a combination of direct synthesis, thermogravimetric analysis, vibrational spectroscopy, X-ray powder data and single crystal structure analysis. The M₁₂E₁₇ phases contain the cluster anions [E₉]^4? and [E₄]^4? in the ratio 1:2, forming a hierarchical structure with the cluster anions at the atomic positions of the hexagonal Laves phase MgZn₂. Like the M₄E₄ phases, the M₄Ge₉ compounds are hierarchical derivatives of the cubic Cr₃Si structure but with [Ge₉]^4? anions. The thermogravimetric analyses give strong evidence for the existence of at least one more phase with [E₉]^4? and [E₄]^4? clusters and of the clathrate phases M₆E₁₃₆ in addition to the well-known M₈E₄₄□₂ chlathrates.     

Collision-induced dissociation of negative ions in the gas phase: [Aryl S]−, [aryl SO]− and [aryl SO2]−

Collision-induced dissociation of the ions [ArS]^?, [ArSO]^? and [ArSO₂]^? has uncovered a rich and varied ion chemistry. The major fragmentations of [ArS]^? are complex and occur without prior ring hydrogen scrambling: for example, [C₆H₅S]^?→[C₂HS]^? and [HS]^?; [p-CD₃C₆H₄S]^?→[C₆H₄S]^?˙, [CD₃C₄S]^? and [C₂HS]^?. In contrast, all decompositions of [C₆H₅CH₂S]^? are preceded by specific benzylic and phenyl hydrogen interchange reactions. [ArSO₂]^? and [ArSO₂]^? ions undergo rearrangement, e.g. [C₆H₅SO]^?→[C₆H₅O]^? and [C₆H₅S]^?; [C₆H₅SO₂]^?→[C₆H₅O] ^?. The ion [C₆H₅CH₂SO]^? eliminates water, this decomposition is preceded by benzylic and phenyl hydrogen exchange.     

Studies of structure and mechanism in acetonitrile chemical ionization tandem mass spectrometry of polyunsaturated fatty acid methyl esters

Recently it has been shown that acetonitrile chemical ionization tandem mass spectrometry (CI-MS/MS) is a rapid, on-line means to determine double bond position in fatty acid methyl esters (FAME). The mechanism of this gas phase condensation reaction has been studied. Evidence of the (1-methyleneimino)-1-ethenylium ion (m/z 54), formed upon the reaction of acetonitrile with itself, adding across the double bond in a [2 + 2] cycloaddition reaction is observed. When this nascent complex undergoes collision-induced dissociation, two diagnostic ions emerge. One of these ions results from loss of the hydrocarbon end of the FAME, whereas the other ion results from loss of the methyl ester end, and when considered together, the diagnostic ions localize the positions of the double bonds in the FAME. Several labeling and MS/MS/MS experiments on the two diagnostic ions were performed to determine a plausible fragmentation mechanism of the stable (1-methyleneimino)-1-ethenylium–FAME complex. The first generation product ions, or diagnostic ions, appear to be formed though a charge-driven mechanism, whereas the second generation product ions are formed via charge-remote fragmentations. Plausible mechanisms for the formation and subsequent dissociation of the diagnostic ions are presented for the monounsaturated, diunsaturated, and polyunsaturated (3 or more double bonds) FAME.     

FAB and chemical ionization in the production of [M+ H]+ and [M− H]− species from 2-aryl-2-methyl-1,3-dithianes

The unimolecular fragmentations of [M + H]⁺ and [M – H]^? ions from four 2-aryl-2-methyl-1,3-dithianes are described and clarified with the aid of deuterated derivatives. Comparison of the MIKE spectra of [M + H]⁺ species obtained under chemical ionization and fast atom bombardment (FAB) conditions reveals differences which are attributed to the different energetics involved in the two ionization processes. It is suggested that FAB is a ‘softer’ ionization technique but, at the same time, it provides, for the possibility of solvation, reaction sites not available in gas-phase protonation. [M – H]^? species and anionic fragments thereof were generally not obtained under FAB(?) conditions. [M – H]^? ions are readily produced in gas-phase reactions with OH^? via proton abstraction from C(4) or C(5), and from the 2-methyl substituent; and they fragment according to several reaction pathways.     

Identification of chlorinated fatty acids in fish by gas chromatography/mass spectrometry with negative ion chemical ionization of pentafluorobenzyl esters

This paper reports the development of a technique for identifying and confirming chlorinated fatty acids previously detected in fish by gas chromatography (GC) with halogen-specific detection (XSD). Fatty acid methyl esters (FAMEs) including chlorinated FAMEs within fractions of reversed-phase high-performance liquid chromatography of transesterified fish extracts were derivatized to pentafluorobenzyl esters, which were subjected to GC/mass spectrometry (MS) with negative ion chemical ionization (NICI). Pentafluorobenzyl esters displayed reasonably good GC characteristics, a very high ionization efficiency and a low degree of fragmentation. Chloride ion chromatograms extracted at m/z 35 and 37 from full scans were utilized for locating traces of chlorinated unknowns in the GC elution profile so that their mass spectra could be readily displayed. Significant ions displayed in the mass spectrum scanned in a narrow range of retention time where a chlorinated unknown was located were evaluated using ion chromatograms extracted at the m/z of these ions. The chromatographic peaks of those ions derived from the analyte were expected to center at that specific retention time, whereas those originating from matrix compounds were not. The isotopic patterns of chlorinated ions were also examined against their theoretical relative abundances. Using this approach, three metabolism-related dichloro fatty acids previously identified by GC/XSD in filet extracts of white sucker sampled downstream from a bleached kraft pulp mill were confirmed: dichlorooctadecanoic, dichlorohexadecanoic and dichlorotetradecanoic acids. In addition, an isomer of dichlorotetradecanoic acid was found in a reference fish sample. As sample preparation is critical in this application, improved conditions for hydrolysis and pentafluorobenzyl esterification are also discussed.     

Collision dissociation study of negative ion chemical ionization (OH−) fragments of bornyl C4?C5 esters

Collisional activation spectra show interesting and specific dissociations which demonstrate the retention of structural identity not only of carboxylate anions discussed in a previous paper, but also of other fragments obtained from bornyl C₄? C₅ esters under negative chemical ionization mass spectral conditions with OH^? as reactant ions.     

Temperature‐programmed capillary high‐performance liquid chromatography with atmospheric pressure chemical ionization mass spectrometry for analysis of fatty acid methyl esters

A new capillary high‐performance liquid chromatography method with atmospheric pressure chemical ionization mass spectrometry was developed for the analysis of fatty acid methyl esters and long‐chain alcohols. The chromatographic separation was achieved using a Zorbax SB‐C18 HPLC column (0.3 × 150 mm, 3.5 μm) with a mobile phase composed of acetonitrile and formic acid and delivered isocratically at a flow rate of 10 μL/min. The column temperature was programmed simply, using a common column oven. Good reproducibility of the temperature profile and retention times were achieved. The temperature programming during the isocratic high‐performance liquid chromatography run had a similar effect as a solvent gradient; it reduced retention times of later eluting analytes and improved their detection limits. Two atmospheric pressure chemical ionization sources of the mass spectrometry detector were compared: an enclosed conventional ion source and an in‐house made ion source with a glass microchip nebulizer. The enclosed source provided better detectability of saturated fatty acid methyl esters and made it possible to determine the double bond positions using acetonitrile‐related adducts, while the open chip‐based source provided better analytical figures of merit for unsaturated fatty acid methyl esters. Temperature‐programmed capillary high‐performance liquid chromatography is a promising method for analyzing neutral lipids in lipidomics and other applications.     

Mass-analyzed ion kinetic energy spectra of the [M + 1]+ ions of aliphatic esters produced by chemical ionization

To test the similarity of chemical ionization (CI) spectra of esters to the collision-induced decomposition mass-analyzed ion kinetic energy (CID-MIKE) spectra of protonated esters, the CID-MIKE spectra of the [M + 1]⁺ ions of nineteen aliphatic esters were studied. The major fragments produced in the two kinds of experiment are similar, but there are significant differences in the ions of high mass, which would reduce the usefulness of library searches of CI spectra to identify MIKE spectra of [M + 1]⁺ ions.     

Investigation of Large Polychloride Anions: [Cl11]−, [Cl12]2−, and [Cl13]−

For decades the chemistry of polyhalides was dominated by polyiodides and more recently also by an increasing number of polybromides. However, apart from a few structures containing trichloride anions and a single report on an octachloride dianion, [Cl₈]^2?, polychlorine compounds such as polychloride anions are unknown. Herein, we report on the synthesis and investigation of large polychloride monoanions such as [Cl₁₁]^? found in [AsPh₄][Cl₁₁], [PPh₄][Cl₁₁], and [PNP][Cl₁₁]?Cl₂, and [Cl₁₃]^? obtained in [PNP][Cl₁₃]. The polychloride dianion [Cl₁₂]^2? has been obtained in [NMe₃Ph]₂[Cl₁₂]. The novel compounds have been thoroughly characterized by NMR spectroscopy, single‐crystal Raman spectroscopy, and single‐crystal X‐ray diffraction. The assignment of their spectra is supported by molecular and periodic solid‐state quantum‐chemical calculations.     

Gas-phase chemistry of electron deficient organometallic anions: the reactions of [Cr(CO)3]− ˙ and [Cr(CO)4]− ˙ with aldehydes,ketones, esters and ethers

Reactions in the gas phase of the 13- and 15-electron radical anions [Cr(CO)₃]^{? ˙} and [Cr(CO)₄]^{? ˙} with a series of 27 aldehydes, ketones, esters and ethers have been examined. Sequential alkane eliminations and metal-bonded CO ligand displacements were the principal reactions identified for the RCHO/[Cr(CO)₃]^{? ˙} systems with the latter reaction also common to the RCHO/[Cr(CO)₄]^{? ˙} systems. While [Cr(CO)₄]^{? ˙} was generally unreactive towards ketones R · R'CO, the principal products identified for [Cr(CO)₃]^{? ˙}/ketone reactions were the metal-decarbonylated species, respectively [R · R'CO · Cr(CO)_x]^{? ˙} with x = 0–3, and [R · (R' - H₂)CO · Cr(CO)₂]^{? ˙}. The reaction of [Cr(CO)₃]^{? ˙} with esters RCOOR' proceeds via metal insertion into the alkoxy C? O bond to give end products of the type [R'O · Cr · R(CO)₂]^? and [R'O? Cr(CO)₃]^? while the sole ionic products of dialkyl ether/[Cr(CO)₃]^{? ˙} reactions were identified as the alkoxytricarbonylchromium species [RO · Cr(CO)₃]^?.     

Quantification of trace fatty acid methyl esters in diesel fuel by using multidimensional gas chromatography with electron and chemical ionization mass spectrometry

Measurement of contamination of marine and naval diesel fuels (arising from product mixing or adulteration) with biodiesel or fatty acid methyl esters can be problematic, especially at very low levels. A suitable solution for this task for trace amounts of individual fatty acid methyl esters with resolution and quantification can be achieved by using a multidimensional gas chromatographic approach with electron and chemical ionization mass spectrometric detection. A unique column set comprising a 100 m methyl‐siloxane nonpolar first dimension column and high‐temperature ionic liquid column in the second dimension enabled identification of individual fatty acid methyl esters at below the lowest concentrations required to be reported in a diesel fuel matrix. Detection limits for individual fatty acid methyl esters compounds ranged from 0.5 to 5.0 mg/L, with excellent linearity up to 5000 mg/L and repeatability of the method from 1.3 to 3.2%. The method was applied to the analysis of diesel fuel samples with suspected biodiesel contamination. Contamination at 568 mg/L was calculated for an unknown sample and interpretation of the results permitted the determination of a likely source of the contamination.     

Polychloride Monoanions from [Cl3]− to [Cl9]−: A Raman Spectroscopic and Quantum Chemical Investigation

Polychloride monoanions stabilized by quaternary ammonium salts are investigated using Raman spectroscopy and state‐of‐the‐art quantum‐chemical calculations. A regular V‐shaped pentachloride is characterized for the [N(Me)₄][Cl₅] salt, whereas a hockey‐stick‐like structure is tentatively assigned for [N(Et)₄][Cl₂???Cl₃^?]. Increasing the size of the cation to the quaternary ammonium salts [NPr₄]⁺ and [NBu₄]⁺ leads to the formation of the [Cl₃]^? anion. The latter is found to be a pale yellow liquid at about 40 °C, whereas all the other compounds exist as powders. Further to these observations, the novel [Cl₉]^? anion is characterized by low‐temperature Raman spectroscopy in conjunction with quantum‐chemical calculations.