Electron ionization mass spectral fragmentation of deoxynivalenol and related tricothecenes

Careful analysis of the electron impact (EI) mass spectral data obtained for the trimethylsilyl (TMS) ethers of known trichothecene mycotoxins of the deoxynivalenol group permitted the construction of a database useful for the identification of these mycotoxins directly from a gas chromatography/mass spectrometry (GC/MS) run. Structures of the ions at m/z 103, 117, 147 and 191 were elucidated by high-resolution mass spectrometry (HRMS) and a fragmentation scheme was suggested. The relative abundances of these ions in the mass spectra of the trichothecenes allowed a fast structural diagnosis during analysis of biological matrices. A new mycotoxin of this group, 3-acetylnivalenol, was tentatively identified by using MS data interpretation only.     

Hydrogen and trimethylsilyl transfers during EI mass spectral fragmentation of hydroxycarboxylic and oxocarboxylic acid trimethylsilyl derivatives

This paper, describing electron ionization mass spectral fragmentation of some hydroxycarboxylic and oxocarboxylic acid trimethylsilyl derivatives, focuses on the formation of fragment ions resulting from the interactions between the two functionalities of these compounds. These interactions result in the formation of fragment ions at [CH2=C(OTMS)2]+., [CH2=CHC(OTMS)=OTMS]+, [M-31]+, [M-105]+, and [M-RCHO]+. in the case of hydroxycarboxylic acid trimethylsilyl derivatives of formula RCHOTMS(CH2)nCOOTMS and at [RC(OTMS)=CH2]+., [RC(=OTMS)CH=CH2]+, and [M-RC(=O)CH2]+ in the case of oxocarboxylic acid trimethylsilyl esters of formula RC(=O)(CH2)nCOOTMS. Some of these fragmentations appeared to be sufficiently specific to be used to characterize these compounds. Several fragmentation pathways involving trimethylsilyl and hydrogen transfers were proposed to explain the formation of these different fragment ions and were substantiated by deuterium labeling.     

Electron ionization mass spectral fragmentation of derivatized 4,5- and 5,6-epoxysterols

The electron ionization (EI) mass spectral fragmentation of derivatized 4,5- and 5,6-epoxysterols was investigated. Interesting fragmentation processes involving a transannular cleavage of the epoxide ring after transfer of the trimethylsilyl group are significant in the case of 4,5-epoxysterol trimethylsilyl ethers (affording abundant fragment ions at m/z 403 and 404). Different pathways, which have been substantiated by deuterium labelling, are proposed in order to explain the formation of these ions. In contrast, this transfer is not significant in the case of 5,6-epoxysterol trimethylsilyl ethers. The EI mass spectra of these latter compounds appear to be very complex and to differ slightly according to the stereochemistry of the epoxy group. Acetate and trifluoroacetate derivatives of 4,5-epoxysterols display interesting EI mass spectra dominated by a fragment ion at m/z 332 resulting from cleavage of the steroid ring A.     

Electron ionization mass spectral fragmentation study of sulfation derivatives of polychlorinated biphenyls

Background  

Polychlorinated biphenyls are persistent organic pollutants that can be metabolized via hydroxylated PCBs to PCB sulfate metabolites. The sensitive and selective analysis of PCB sulfate monoesters by gas chromatography-mass spectrometry (GC-MS) requires their derivatization, for example, as PCB 2,2,2-trichloroethyl (TCE) sulfate monoesters. To aid in the identification of unknown PCB sulfate metabolites isolated from biological samples, the electron impact MS fragmentation pathways of selected PCB TCE sulfate diesters were analyzed and compared to the fragmentation pathways of the corresponding methoxylated PCBs.     

Effects of bifunctional interactions in the chemical ionization mass spectrometry of carboxylic acids and methyl esters

The methane chemical ionization mass spectra of dodecan-1, 10-dioic acid and the methyl ester show enhanced water and methanol elimination relative to monofunctional analogs. Elimination of acetic acid from 1,10-diacetoxydecane, however, was reduced relative to 1-acetoxydecane. This appears to be the result of bifunctional interactions. The steric requirement of this interaction is shown in an homologous series of dicarboxyl compounds. The change in interfunctional separation is shown to affect the proton and ammonium ion affinities in the ammonia chemical ionization spectra of the dicarboxyl compounds in a manner explained in terms of proton and ammonium ion complex formation. The observation is extended to various other functional groups.     

Trimethylsilyl transfer during electron ionization mass spectral fragmentation of some omega-hydroxycarboxylic and omega-dicarboxylic acid trimethylsilyl derivatives and the effect of chain length

The electron ionization (EI) mass spectral fragmentation of omega-hydroxycarboxylic and omega-dicarboxylic acid trimethylsilyl derivatives was investigated. The mass spectra of these compounds exhibited fragment ions resulting from classical fragmentation of the trimethylsilyl ether and ester groups, and others resulting from the interactions between the two functionalities (m/z 147, 204, 217, [M-31](+) and [M-105](+) in the case of omega-hydroxycarboxylic acid derivatives and m/z 147, 204, 217 and [M-131](+) in the case of omega-dicarboxylic acid derivatives). Several fragmentation pathways were proposed to explain the formation of these different fragment ions. It is proposed that the ions at m/z 204 and 217 are formed via an initial trimethylsilyl transfer between the ether and the ester group or between the two ester groups. This transfer appeared to be more favoured in the case of omega-dicarboxylic acid derivatives and to be dependent on the chain length. A more efficient transfer was in fact observed for compounds with a relatively long alkyl chain. In the case of shorter omega-hydroxycarboxylic and omega-dicarboxylic acid trimethylsilyl derivatives the formation of the ions at m/z 204 and 217 suffers strongly from competition from production of the ion at m/z 147.     

Substituent effects in the mass spectral fragmentation of aryl esters

The mass spectra of m- and p-substituted phenyl acetates, phenyl propionates, phenyl chloroacetates and phenyl fluoroacetates have been determined. The fragmentation of aryl esters is affected by acyl substituents as well as by aryl substituents. Esters having acyl groups of low ionization potential show greater changes in fragmentation because of aryl substituents than those having acyl groups of high ionization potential. Each series has a fairly definite crossover point where fragmentation changes from predominant rearrangement to predominant cleavage.     

Thermal and mass spectrometric fragmentation of esters of deuterated long chain carboxylic acids

A method combining thermal fragmentation and mass spectrometry for the determination of the position of double bonds in an unsaturated ester is presented. The thermal fragmentation of methyl esters of deuterated long chain carboxylic acids yields a homologous series of olefins plus a homologous series of unsaturated esters. The positions of the deuterium atoms in the original ester are revealed by the deuterium content of its fragments as determined by mass spectrometry. Therefore, the positions of double bonds of a polyunsaturated acid can be determined by pyrolysis after saturation by deuterium. The structures of the unsaturated fragments are ascertained by mass spectrometric method, and the formation of the ion [M – 32] in the mass spectrometric fragmentation of unsaturated methyl esters is studied by means of deuterium labeling.     

Cobalt-catalyzed reductive carbonylation of methyl esters to acetaldehyde and carboxylic acid

The reaction of methyl esters with synthesis gas and a Co—LiI catalyst results in the formation of anhydrous acetaldehyde and a carboxylic acid in very high yield. At 180 °C and 5000 psig, acetaldehyde is produced from methyl acetate at a rate of 7 M h⁻¹ and >95% selectivity.     

Atmospheric pressure chemical ionization mass spectrometry and in-source fragmentation of lutein esters

Negative-ion atmospheric pressure chemical ionization (APCI) mass spectrometry and in-source collisionally induced dissociation (CID) were employed to obtain structural information of lutein esters from marigold extract. Both molecular ions and structurally significant fragments corresponding to the loss of fatty acids were observed in high abundance in the current study. Six lutein diesters including lauroylmyristoyl-lutein (LML), dimyristoyl-lutein (dML), myristoylpalmitoyl-lutein (MPL), dipalmitoyl-lutein (dPL), palmitoylstearoyl-lutein (PSL) and distearoyl-lutein (dSL) were characterized in a marigold flower extract. Breakdown curves (plots of relative ion abundance vs. internal energy) of three lutein diesters were established by monitoring the relative ion abundance of molecular and fragment ions at different cone voltages during negative-ion APCI-LC/MS analysis.     

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.     

Reaction of ADP with amino acid methyl esters mediated by trimethylsilyl chloride

Reaction of ADP with amino acid methyl esters mediated by trimethylsilyl chloride in pyridine produced adenosine 5'-phosphoramidates in good yields under mild conditions, it is interesting that nucleophilic attack of amino acid methyl esters only occurred on alpha-phosphorus of ADP.     

Retention behaviour of carboxylic acid methyl esters in supercritical fluid chromatography

The study on retention behavior in supercritical fluid chromatography (SFC) is necessary to understand the mechanism of the various interactions in SFC. The retention of SFC in carboxylic acid methyl ester/polymethylsiloxane/CO2 system was studied systematically and the retention behavior of this kind of compounds under various typical operation conditions was described using the method of an alternative unified theory of chromatographic retention. The results illustrated that expression: Ink.= a + b/T + cp + dp/T + ep2/T can be used to describe quantitatively the retention behavior of carboxylic acid methyl ester/polymethylsiloxane/CO2 system in the ranges of reduced density from 0.549 to 1.411. It was also found that the entropy of solute in stationary phase is dependent on the density of supercritical fluid (SF) under typical operating conditions of SFC.     

Characterization of electrospray ionization mass spectrometry for N-diisopropyloxyphosphoryl dipeptide methyl esters

A systematic study of the fragmentation pattern of N-diisopropyloxyphosphoryl (DIPP) dipeptide methyl esters in an electrospray ionization (ESI) tandem mass spectrometry (MS/MS) was presented. A combination of accurate mass measurement and tandem mass spectrometry had been used to characterize the major fragment ions observed in the ESI mass spectrum. It was found that the alkali metal ions acted as a fixed charge site and expelled the DIPP group after transferring a proton to the amide nitrogen. For all the N-phosphoryl dipeptide methyl esters, under the activation of a metal ion, the rearrangement product ion at m/z 163 was observed and confirmed to be the sodium adduct of phosphoric acid mono-isopropyl esters (PAIE), via a specific five-membered penta-co-ordinated phosphorus intermediate. However, no rearrangement ion was observed when a beta-amino acid was at the N-terminal. This could be used to develop a novel method for differentiating isomeric compounds when either alpha- or beta-amino acid are at the N-terminus of peptides. From the [M+Na]+ ESI-MS/MS spectra of N-phosphoryl dipeptide methyl esters (DIPP Xaa1 Xaa2 OMe), the peaks corresponding to the [M+Na Xaa1 C3H6]+ were observed and explained. The [M+Na]+ ESI-MS/MS spectra of N-phosphoryl dipeptide methyl esters with Phe located in the C-terminal, such as DIPPValPheOMe, DIPPLeuPheOMe, DIPPIlePheOMe, DIPPAlaPheOMe and DIPPPhePheOMe, had characteristic fragmentation. Two unusual gas-phase intramolecular rearrangement mechanisms were first proposed for this fragmentation. These rearrangements were not observed in dipeptide methyl ester analogs which did not contain the DIPP at the N-terminal, suggesting that this moiety was critical for the rearrangement.