Analysis of oxosteroids by nano-electrospray mass spectrometry of their oximes

A method for the analysis of neutral oxosteroids by electrospray mass spectrometry is described. The oxosteroids are converted into their oximes by treatment with hydroxyammonium chloride in aqueous methanol. Intense peaks corresponding to protonated oxime molecules are observed in nano-electrospray mass spectra. The detection limits for the oximes of progesterone, pregnenolone and dehydroepiandrosterone were 2.5, 5 and 25 pg/microL, respectively, approximately 20 times lower than for the underivatised steroids. The signal intensities were proportional to the concentration of the steroids in the range of 500 to 2.5 pg/microL. Fragmentation by collision-induced dissociation (CID) was studied using oximes of 28 model steroids carrying an oxo group at C-3, C-17 or C-20. Some of the steroid oximes were labelled with deuterium or (15)N. Fragment ions were observed which yielded useful structural information. Upon CID, protonated oximes of 3-oxo-Delta(4)-steroids produced abundant ions by cleavage through the B-ring and by loss of the side chain, while protonated oximes of saturated 3-oxosteroids did not give abundant ions by cleavage through the B-ring. Protonated oximes of 20-oxosteroids unsubstituted at C-21, C-17 or C-16 produced a characteristic ion at m/z 86 containing the side chain, C-16 and C-17. Protonated oximes of steroids containing only a 17-oxo group gave fewer ions of diagnostic value. Coupled with the selective isolation of steroid oximes from a biological matrix this method of derivatisation and CID may be used for the analysis of neutral oxosteroids in biological samples.     

Mass spectrometry in structural and stereochemical problems—CLXIV. Electron impact promoted fragmentation of some thiosemicarbazones

The mass spectral fragmentation of thiosemicarbazone derivatives of some typical aliphatic, alicyclic and aromatic aldehydes and ketones has been rationalized using high resolution mass spectrometry supplemented by deuterium labeling. Thiosemicarbazone derivatives of carbonyl compounds yield mass spectra which have little in common with those generated by their semicarbazone analogs.     

Mass spectra of some syn- and anti-aromatic aldoximes

The mass spectra of eight pairs of geometric isomers of aromatic oximes and four other single aromatic oximes are reported. The loss of H₂O, HO and HCN are major fragmentations from the molecular ion of all the benzaldoximes studies; however, the loss of HCN from the molecular ion did not occur in the oximes of 9-phenanthraldehyde, 1-naphthaldehyde and 2-naphthaldehyde. The halogen substituted benzaldoximes eliminate HCNO and H₂CNO forming an additional fragmentation pathway from the molecular ion. In general, variations were found for each pair of syn- and anti-oximes but no consistent patterns could be found in the spectra for all the syn-isomers versus all of the anti-isomers. The geometric isomerism of four oximes previously reported in the literature have been established for the first time as anti-m-bromobenzaldoxime, syn-9-phenanthraldoxime, syn-1-naphthaldoxime and syn-2-naphthaldoxime. Three new oxime acetates were prepared and their mass spectra are discussed.     

Secondary ion mass spectra of quaternary pyridine aldoximes

The secondary ion mass spectra (s.i.m.s.) of the most widely used therapeutic monoquaternary pyridine aldoximes and diquaternary pyridine aldoximes, as well as some related monoquaternary ammonium salts and neutral oximes, are reported. The monoquaternary derivatives of the oximes yield prominent intact cations which provide molecular weights, and fragmentation patterns which are dominated by even-electron ions. The diquaternary oximes investigated do not give dications, but rather undergo charge separation reactions sometimes accompanied by intramolecular aromatic substitution, and they sometimes yield monoquaternaries by expulsion of a proton. The s.i. mass spectra are structure-specific for the monoquaternary salts, allowing isomer distinction in the cases examined. Quaternary salts can be quantified by s.i.m.s., and low detection limits (less than 50 ng) are demonstrated here for the oxime salts; thus s.i.m.s. is an appropriate analytical technique for the title compounds. Procedures of derivatization which convert the neutral amines to ionic compounds, such as quaternization with alkyl halides, provide a simple means of obtaining high-quality s.i.m.s.     

Spectrometrie de Masse du Dimethoxy-2,6 Naphtalene et de Certains de ses Derives

The mass spectra of 2,6-dimethoxynaphthalene and 14 of its derivatives are reported. The fragmentation patterns proposed to account for the main peaks observed are frequently based on the presence of the appropriate metastable peaks. The fragmentation is initiated by the cleavage of an O? CH₃ bond. The molecular ion is the most intense ion in each case and it does not appear to directly eliminate a methoxy group or a substituent directly attached to the naphthalene ring. With NO₂ as a substituent the elimination of nitric oxide from the molecular ion has been observed and confirmed by peak matching.     

Etude en Spectrometrie de Masse d'une Serie d'Anhydride du Type Acetoxy-3 Bicyclo[2.2.2]octene-2 Dicarboxylique-5,6

A series of 3-acetoxy bicyclo[2.2.2]oct-2-ene 5,6 dicarboxylic anhydrides with different substituents is studied by high resolution mass spectrometry. The spectra obtained are complicated and can only be differentiated by the presence of certain peaks and their variable relative abundances. We have almost entirely explained these peaks by the use of classic degradation mechanisms. We have applied the explanations advanced by Cimarusti and Wolinsky for a series of analogues.The degradation scheme is very similar for all the products, and is unaffected by the substituents and their relative position.     

Mass Spectra of Furfuryl Aldoximes and Ketoximes

The mass spectra of 8 furfuryl aldoximes and ketoximes were studied under EI and CI conditions. Complicated fragmentation patterns were obtained in EI conditions, including formation of a furfuryl cation radical, furfuryl cation, and [M-17]⁺. The relatively simple patterns of cleavage resulted in few major ion peaks contributed from the adducts, protonation products, and dehydration products under CH₄-CI and NH₃-CI conditions. Comparison with the spectra of their isomeric amides indicated no evidence that the isomerization took place from oximes before fragmentation in both ionization methods.     

Quantum chemical electron impact mass spectrum prediction for de novo structure elucidation: Assessment against experimental reference data and comparison to competitive fragmentation modeling

We investigate the success of the quantum chemical electron impact mass spectrum (QCEIMS) method in predicting the electron impact mass spectra of a diverse test set of 61 small molecules selected to be representative of common fragmentations and reactions in electron impact mass spectra. Comparison with experimental spectra is performed using the standard matching algorithms, and the relative ranking position of the actual molecule matching the spectra within the NIST‐11 library is examined. We find that the correct spectrum is ranked in the top two matches from structural isomers in more than 50% of the cases. QCEIMS, thus, reproduces the distribution of peaks sufficiently well to identify the compounds, with the RMSD and mean absolute difference between appropriately normalized predicted and experimental spectra being at most 9% and 3% respectively, even though the most intense peaks are often qualitatively poorly reproduced. We also compare the QCEIMS method to competitive fragmentation modeling for electron ionization, a training‐based mass spectrum prediction method, and remarkably we find the QCEIMS performs equivalently or better. We conclude that QCEIMS will be very useful for those who wish to identify new compounds which are not well represented in the mass spectral databases.     

Characterization of polysiloxanes with different functional groups by time-of-flight secondary ion mass spectrometry

Polydimethylsiloxane (PDMS), polyhydromethylsiloxane (PHMS), and polymethylphenylsiloxane (PMPhS) have been studied by TOF-SIMS to investigate effects of functional group changes on polymer fragmentation mechanisms. Cyclic fragments are observed in the low mass range spectra of PDMS and PHMS, but not in the spectrum of PMPhS. Effects of functional group substitution on the fragmentation mechanisms of polysiloxanes are evident in the high mass range spectra (>1000 Da). Peaks of oligomers cationized by silver dominate the high mass range of the spectra of all low molecular weight polysiloxanes. However, fragmentation patterns of these samples are different. Neutral cyclic fragments cationized by silver are identified in the high mass range of the spectra of PDMS and PHMS, but not in the spectrum of PMPhS. The major fragments of PHMS and PMPhS are [oligomer-14+Ag]⁺. The PHMS spectrum also shows peaks [oligomer-28+Ag]⁺. These distinctive fragmentation patterns can be used to identify the polysiloxanes.     

Prediction of electron ionization mass spectra of <Emphasis Type="Italic">О</Emphasis>-alkyl methylphosphonothionofluoridates

Electron ionization mass spectra of poorly studied toxic alkyl methylphosphonothionofluoridates and alkyl methylphosphonofluoridates are discussed. It is demonstrated that the compounds are decomposed in accordance with the general scheme of fragmentation of monofunctional organic compounds RX (X is a functional group), proposed previously. At the same time, noticeable differences between the corresponding mass spectra are found. The most important difference occurs in their alkene subspectra containing a peak of alkene ion [R–H]^+? and peaks of its decay products. A method was developed for the simulation of mass spectra of unknown alkyl methylphosphonothionofluoridates by transforming available mass spectra of their oxygen analogues.     

High-resolution mass spectra of anti- and syn-isomers of acylferrocene oximes

High-resolution mass spectra of anti- and syn-acylferrocene oximes were examined at various ionizing voltages and fragmentation patterns of the oximes are presented. Significant differences between the anti- and syn-isomers were found in the fragmentation behaviour of the low voltage (15 to 20 eV) spectrum of formylferrocene oxime.     

Etude en Spectrometrie de Masse de bis Heterocycles Ethers du Resorcinol

The mass spectra of some heterocycles, resorcinol ethers, are examined. We have interpreted the results in terms of the fragmentations of monoheterocycles. No high resolution measurements were made. When there is one benzylic ketone, the decomposition process suggests that the functional heterocycle is cleaved before the other. Each heterocycle appears to participate on the resulting ions unlike in the case of fragmentations of non-functional heterocycles.     

Stereochemical applications of mass spectrometry: 1—The utility of electron impact and chemical ionization mass spectrometry in the differentiation of stereoisomeric benzoin oximes and phenylhydrazones

A study of the electron impact and chemical ionization (H₂, CH₄, and iso-C₄H₁₀) mass spectra of stereoisomeric benzoin oximes and phenylhydrazones indicates that while the former can be distinguished only by their chemical ionization mass spectra the latter are readily distinguishable by both their electron impact and chemical ionization mass spectra. The electron impact mass spectra of the isomeric oximes are practically identical; however, the chemical ionization spectra show that the E isomer forms more stable [MH]⁺ and [MH? H₂O]⁺ ions than the Z isomer for which both the [MH]⁺ and [MH? H₂O]⁺ ions are relatively unstable. In electron impact the Z-phenylhydrazone shows a lower [M]⁺˙ ion abundance and more facile loss of H₂O than does the E isomer. This more facile H₂O loss also is observed for the [MH]⁺ ion of the Z isomer under chemical ionization conditions.     

A metastable ion study of the electron impact fragmentation of some substituted 2,6-pyrido-containing crown ethers

Extensive networks of metastable ions link the major peaks in the electron impact mass spectra of two crown ethers containing 2,6-pyrido units. High-resolution mass measurements and the metastable peaks allow the elucidation of the fragmentation pathways. The spectra are influenced more by the presence of aromatic substituents than by the 2,6-pyrido units.     

Fragmentation of oxime and silyl oxime ether odd‐electron positive ions by the McLafferty rearrangement: new insights on structural factors that promote α,β fragmentation

The McLafferty rearrangement is an extensively studied fragmentation reaction for the odd‐electron positive ions from a diverse range of functional groups and molecules. Here, we present experimental and theoretical results of 12 model compounds that were synthesized and investigated by GC‐TOF MS and density functional theory calculations. These compounds consisted of three main groups: carbonyls, oximes and silyl oxime ethers. In all electron ionization mass spectra, the fragment ions that could be attributed to the occurrence of a McLafferty rearrangement were observed. For t‐butyldimethylsilyl oxime ethers with oxygen in a β‐position, the McLafferty rearrangement was accompanied by loss of the t‐butyl radical. The various mass spectra showed that the McLafferty rearrangement is relatively enhanced compared with other primary fragmentation reactions by the following factors: oxime versus carbonyl, oxygen versus methylene at the β‐position and ketone versus aldehyde. Calculations predict that the stepwise mechanism is favored over the concerted mechanism for all but one compound. For carbonyl compounds, C–C bond breaking was the rate‐determining step. However, for both the oximes and t‐butyldimethylsilyl oxime ethers with oxygen at the β‐position, the hydrogen transfer step was rate limiting, whereas with a CH₂ group at the β‐position, the C–C bond breaking was again rate determining. n‐Propoxy‐acetaldehyde, bearing an oxygen atom at the β‐position, is the only case that was predicted to proceed through a concerted mechanism. The synthesized oximes exist as both the (E)‐ and (Z)‐isomers, and these were separable by GC. In the mass spectra of the two isomers, fragment ions that were generated by the McLafferty rearrangement were observed. Finally, fragment ions corresponding to the McLafferty reverse charge rearrangement were observed for all compounds at varying relative ion intensities compared with the conventional McLafferty rearrangement. Copyright © 2012 John Wiley & Sons, Ltd.     

Synthesis and three-dimensional structure of 2-arylidene-3-oxoquinuclidine oximes

2-Arylidene-3-oxoquinuclidine oximes were obtained from the products of condensation of 3-oxoquinuclidine with aromatic aldehydes. It is shown that three of the four theoretically possible isomers (isomerism involving the C=C and C=N bonds) are formed; their thermal stability was studied. The structures of the isomeric oximes was established on the basis of data from the ¹H and ¹³C NMR spectra and were confirmed by data from chromatographic mass spectrometry and the UV and IR spectra.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 5, pp. 666–674, May, 1981.     

缩胺基脲衍生物HeI PES和电子结构的研究

利用HeI紫外光电子能谱(PES)首次得到一组缩胺基脲衍生物的PES谱图。并利用RHF(6-31G^*^*)计算了它们的电子结构,解析了实验谱,分析和说明与实验电离能所对应的分子轨道的特征。论证了计算结果与实验值间很好的相符关系。并通过实验和计算结果,讨论了这类化合物取代基的影响和为何缺乏生物活性。     

Etude de la Fragmentation en Spectrographie de Masse de Spiropyrannes Benzodithioliques. Influence de la Position et de la Nature des Substituants

The mass spectra of 12 benzodithiolic spiropyrans are reported. Their fragmentation pathways have been determined by exact mass and metastable transition measurements. The nature of the fragmentation is strongly dependent upon the position and nature of the substituents. A comparison is made with their photo and thermochemical decompositions.     

Photodissociation of alkane ions in the source of a mass spectrometer: A potential analytical technique

Alkane ions generated by 70 eV electron ionization are irradiated in the source of a reversed geometry, double focusing mass spectrometer by 2.41 eV (514.5 nm) photons from an argon ion laser. The ions in the chamber are extracted, accelerated to 6 keV and spectra obtained by scanning the field of the magnet. The laser light is mechanically chopped and changes in the peak heights of the mass spectra resulting from photodissociation of ions in the ion chamber are recorded using a phase-sensitive detector to give what are referred to as photoinduced spectra. The peaks for the molecular ions of alkanes are large relative to other peaks in the photoinduced spectra, a situation which contrasts with that for the mass spectra. The relative photodissociation cross-sections of several alkanes, C₆? C₁₀, C₁₂ and C₁₆, have been measured and those of linear and branched [C₁₀H₂₂]⁺˙ ions are compared. Some of the characteristic features of the photoinduced spectra make the identification of the sample simpler than from mass spectra. The method is applied to the analysis of some alkane mixtures and its potential in analytical work discussed.     

Artifacts in four-sector tandem mass spectrometry

Several types of artifacts were shown to be present in 4-sector tandem collision-induced dissociation (CID) mass spectra. In CID spectra of protonated peptides produced by liquid secondary-ion mass spectrometry (LSIMS), peaks corresponding to successive losses of matrix molecules from the precursor ion were observed. In addition, peaks corresponding to MH+ ions of smaller peptides that were also present in the sample/matrix mixture in greater abundance than the selected precursor ion were observed. Both of these types of artifact peaks were shown to originate from the 'peak-at-every-mass' chemical noise at the same nominal mass as that selected by the first 2 sectors (MS1). These noise ions are transmitted through to the collision cell and produce fragments that are analysed and detected in the next 2 sectors (MS2). A second, unrelated, kind of artifact was found to be due to decompositions in the second field-free region of MS2 in an EBEB geometry machine. These artifacts, which are detectable over only a very limited mass range when using a conventional single-point detector, can be present over a much greater mass range when an array detector is used and when the collision cell is floated above ground potential. A clear understanding of the origins of all peaks in a CID spectrum is important in order to have a firm foundation for interpretation, manual or computer-aided, of the spectra of unknown compounds.