Chemical Ionization Mass Spectrometry of Steroids

Abstract

The chemical ionization mass spectra of a group of commonly occuring steroids are discussed. Intense signals (base peak) are observed in the molecular ion region for hydrocarbons and ketones. Functional group elimination is observed with sterols and especially their methyl and TMSi ethers, with the result that base peaks for these compounds are found at m/e values well below the expected molecular ion region. These observations are compared with those from the corresponding electron impact spectra.     

Thermal analysis of crude oils and comparison with SIMDIST and TBP distillation data

Four commercial Saudi Arabian crude oils were characterized by thermogravimetry (TG) and differential thermal analysis (DTA). These crude oils, Arab Berri (AB), Arab Light (AL), Arab Medium (AM) and Arab Heavy (AH), were also subjected to the traditionally employed true boiling point (TBP) distillation and simulated distillation (SIMDIST). The TG/DTA data show that the hydrocarbons present in these crude oils fall into four groups: the volatiles, the low molecular weight, the medium molecular weight and the high molecular weight compounds. These four types of hydrocarbons were observed to display certain trends, such that the volatile and low molecular weight hydrocarbons increased, while the medium and high molecular weight hydrocarbons decreased with the lightness of the crude. The volatile contents of AB, AL, AM and AH crude oils up to 280°C were 50.1, 42.2, 42.3 and 38.5 mass percent, respectively. This confirms that AB is the lightest of these crude oils with maximum volatile content. The mass percentage loss from the TG results is in good agreement with the percentage distilled from TBP (ASTM D 2892) and SIMDIST. During evaporation, the TG mass loss follows a similar trend to those of the TBP and SIMDIST results and thus behaves like distillation. During the oxidative degradation, the TG curve shows a higher mass loss as compared to the distillation data. The higher deviation of the TG mass loss and percentage distilled at the higher-temperature end of the curve may be attributed to the higher content of asphaltenes and carbonaceous material present in AH as compared to the AB crude oil. At around 200°C, the TG mass loss curve intersects the TBP and SIMDIST curves and shows a derivation from distillation behaviour. This intersection temperature of the TG and distillation curves is observed to decrease with the heaviness of the crude and can be an indication of the onset of thermal degradation of hydrocarbons present in the crude oil. On the whole, the TG data closely resemble the distillation results.     

Mass spectrometry of onium compounds—IV: Diazonium salts

The zwitterions of diazotized ortho and para aminophenol are very volatile and give strong peaks in the mass spectra corresponding to the molecular ion. The m-isomer, which cannot be stabilized through a simple quinonoid structure, gives rise to a complex spectrum through decomposition and coupling reactions. The zwitterions of diazotized anthranilic and sulphanilic acids did not give the molecular ion peak.     

Infrared matrix-assisted laser desorption/ionization of polycyclic aromatic hydrocarbons with a sulfolane matrix

Infrared matrix-assisted laser desorption/ionization (IR-MALDI) of the polyaromatic hydrocarbons (PAHs) anthracene, benzo[a]pyrene, and dibenz[a,h]anthracene was performed using a 10.6-microm CO2 laser and a liquid matrix. Sulfolane (tetrahydrothiophene 1,1-dioxide) was found to be an effective matrix for PAH ionization: mass spectra obtained with a sulfolane matrix contain an intense molecular ion peak; interference from PAH fragment and matrix peaks is negligible in all cases. The main limitation of the sulfolane matrix is sample evaporation after 3 to 5 min in vacuum. This sample lifetime can be increased to between 15 and 30 min using a 2:1 (v/v) mixture of sulfolane and glycerol, but the resulting spectra have greater matrix interference and decreased shot-to-shot signal stability.     

Electron impact mass spectrometry of 2,5-bis(p-R-phenyl)thiophenes

The mass spectra of a series of 2,5-bis(p-R-phenyl)thiophenes are reported and discussed. The fragmentation patterns and mechanisms postulated have been confirmed by the determination of the metastable peaks. All the compounds possess the molecular ion as the base peak.     

Analysis of amino acids as N-dithiocarbamic derivatives. A comparison of electron impact and field ionization mass spectra

Amino acids were converted to their N-dithio-alkylcarbamate derivatives by reaction with carbon bisulfide in basic solution followed by reaction with ethyl bromide to form an ethyl thioester. The derivatives are sufficiently volatile for mass spectrometric analysis via the direct inlet probe, without the need for further esterification of the carboxylic acid group. The electron impact spectra exhibit relatively intense molecular in peaks and structurally diagnostic fragment ions. The general features of the electron impact and field ionization mass spectra are compared.     

Comparison of electron impact and chemical ionization mass spectra of some arenetricarbonylchromium complexes

The mass spectrometric behavior of a) the tricarbonylchromium complexes of a series of aromatic hydrocarbons, b) the dimethyldiphenyl compounds of the Group IV elements (i.e., diphenylpropane, dimethyldiphenylsilane, etc.) and c) the mono- and bis-tricarbonylchromium complexes of these ligands under electron impact and chemical ionization conditions are reported. The MH⁺ ion is base peak for all of the simple arenetricarbonylchromium complexes using chemical ionization, whereas [M — 3 CO]⁺ or ⁵²Cr⁺ dominate the spectra with electron impact ionization. The chemical ionization spectra of the series of Group IV element ligands do not exhibit signals in the molecular ion region, the base peak being [M — Ph]⁺. [M — CH₃]⁺ is the electron impact base peak for each of the ligands except the lead-containing compound, for which the base peak is ²⁰⁸Pb⁺. The mono-tricarbonylchromium complexes yield chemical ionization molecular ion clusters, but their base peaks arise via fragmentation of the Group IV element—aromatic ring bonds. Electron impact ionization spectra of the mono complexes are characterized by losses of CO and the production of Cr⁺ ions, neither of which occurs with chemical ionization. For the series of bis-tricarbonylchromium complexes, an MH⁺ ion is prominent only in the chemical ionization spectrum of the diphenylpropane complex. The electron impact induced spectra of the bis-tricarbonylchromium complexes are similar to those of the mono-complexes in that loss of CO is a prominent feature.     

Mass spectral fragmentation pattern of 2,2′-Bipyridyls. Part IX. 6-Alkoxy-2,2′-bipyridyls

The mass spectral fragmentation patterns of 6-methoxy-, 6-ethoxy- and 6-propoxy-2,2 ′-bipyridyls are reported. The base peaks in the spectra of both the 6-methoxy and 6-ethoxy compounds are due to the M-lion of 6-methoxy-2,2′-bipyridyl, while the base peak with 6-propoxy-2,2- bipyridyl is due to a species formed by loss of C₃H₆ from the molecular ion.     

木香挥发性化学成分的气相色谱-质谱分析

采用水蒸气蒸馏法从木香中提取挥发油,用气相色谱-质谱联用仪对木香挥发油进行化学成分的分析。利用化学计量学方法对重叠峰进行分辨,对各个色谱峰定性,并用色谱峰总体积积分和归一化法获得各化合物的相对含量。共鉴定了60种化合物,占挥发油总成分的97%以上。其中主要有土木香内酯、巴西菊内酯、(Z)6,(Z)9-十五二烯-1-醇等。     

Evaluation of mass discrimination effects in the quantitative analysis of polydisperse polymers by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry using uniform oligostyrenes

Mass discrimination effects in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) were quantitatively investigated using equiweight and equimolar mixtures of uniform polystyrene (PS) oligomers. Uniform PS oligomers were separated by preparative super-critical fluid chromatography (SFC) from commercial standard PS samples. The separated PS oligomers, with degrees of polymerization n = 2–25, have absolutely no molecular weight distributions. Equiweight and equimolar mixtures of uniform PS oligomers were accurately prepared by weighing by microbalance, and their spectra were recorded using a MALDI-TOF mass spectrometer. In the lower molecular weight region (less than about 10³) the oligomers with lower molecular weights give lower mass spectral intensities, with no correlation with laser power. In contrast, higher laser powers yield a decrease of mass spectral intensities in the higher molecular weight region. These results clearly show that mass discrimination effects occur at lower and higher molecular weights depending on the laser power, and provide quantitative information about the discrimination. Using the data on equiweight and equimolar mixtures of PS oligomers, it was possible to calibrate the MALDI-TOF mass spectral data for an analysis of molecular weight distribution of a standard monodisperse PS sample with number-averaged molecular weight of 10³, and to compare it with the molecular weight distribution measured by analytical SFC. The result from the calibrated MALDI-TOF mass spectrum, however, does not agree perfectly with that from the SFC results, because undetectable peaks in MALDI-TOF mass spectra at lower and higher molecular weights could not be included in the calibration of peak intensities. Copyright © 2001 John Wiley & Sons, Ltd.     

A quality index for reference mass spectra

A numerical rating for evaluating the quality of reference mass spectra used for retrieval and interpretive applications is proposed. The overall quality index is the product of seven quality factors: (1) source of the spectrum; (2) ionization conditions; (3) higher molecular weight impurities; (4) illogical neutral losses; (5) isotopic abundance accuracy; (6) number of peaks; and (7) lower mass limit of peaks. For quality factor 6 the number of peaks of abundances ≥1% was found to correspond to 15 more than the number of atoms in the molecule with a correlation coefficient of 0.482. The program has been found useful in preparing a data base of 41429 mass spectra; for the majority of spectra judged to be unsatisfactory by the program this arose from a low quality factor 6 value (too few peaks).     

Field ionization mass spectrometry—I. Nucleosides

The field ionization mass spectra of a series of nucleosides are reported, and compared with spectra obtained by conventional electron-impact ionization. The latter are complex, with structurally significant molecular ion and sugar cleavage peaks often of low intensity or completely absent. In contrast, the field ion spectra are extremely simple, with all except guanosine (highest mass peak [M — 18]) showing intense molecular ion peaks, and the characteristic sugar (S) and base (B + H) cleavage products as the only other important fragments.     

Computer analysis of the cracking patterns of deuterated hydrocarbons

A generalized, statistical model for calculating the mass spectral fragmentation patterns of deuterated hydrocarbons of low molecular weight is presented. Isotopic differences in bond energies and probabilities of the bond breakage of hydrogen and deuterium are included. Only a single fitting parameter is used. This parameter seems to be consistent for similar species of hydrocarbons. Small statistical deviations from observed spectra for deuterated methanes, acetylenes, ethylenes, and ethanes are obtained when this model is used.     

Chemical ionization and electron impact mass spectra of thiosulfinates,thiosulfonates and sulfinyl sulfones

The chemical ionization and electron impact mass spectra of some thiosulfinates, thiosulfonates and sulfinyl sulfones have been studied. The electron impact mass spectra of four of the six thiosulfonates show molecular ions of less than 1%. Inconclusive evidence was obtained for sulfenyl sulfinate type intermediates in the electron impact spectra of thiosulfonates. The electron impact spectra of thiosulfonates were similar to those of thiosulfonates. The chemical ionization (isobutane) mass spectra of thiosulfinates and thiosulfonates generally show protonated molecular ions [MH]⁺ as base peaks and [MH+1]⁺ and [MH+2]⁺ peaks.     

Fast-atom bombardment mass spectrometry of conjugated benzo(a)pyrene metabolites

Fast-atom bombardment mass spectrometry has been used to obtain spectra of conjugated benzo(a)pyrene (bap) metabolites using a 1:1, glycerol + thioglycerol matrix, bap Glucuronides give positive- and negative-ion spectra with peaks due to [M + H]+ and [M - H]- ions and a major fragment peak (base peak) at [bap-OH]+ and [bap-O]-. bap Sulfates (sodium salts) give similar negative-ion spectra with [M - Na]- and [bap-O]- peaks, but the positive-ion spectra are dominated by sodium and glycerol adducts of the bap sulfates.     

Comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry peak sorting algorithm

We report a novel peak sorting method for the two-dimensional gas chromatography/time-of-flight mass spectrometry (GC x GC/TOF-MS) system. The objective of peak sorting is to recognize peaks from the same metabolite occurring in different samples from thousands of peaks detected in the analytical procedure. The developed algorithm is based on the fact that the chromatographic peaks for a given analyte have similar retention times in all of the chromatograms. Raw instrument data are first processed by ChromaTOF (Leco) software to provide the peak tables. Our algorithm achieves peak sorting by utilizing the first- and second-dimension retention times in the peak tables and the mass spectra generated during the process of electron impact ionization. The algorithm searches the peak tables for the peaks generated by the same type of metabolite using several search criteria. Our software also includes options to eliminate non-target peaks from the sorting results, e.g., peaks of contaminants. The developed software package has been tested using a mixture of standard metabolites and another mixture of standard metabolites spiked into human serum. Manual validation demonstrates high accuracy of peak sorting with this algorithm.     

气相色谱-单光子电离飞行时间质谱的联用及在柴油组分表征中的应用

建立了一种气相色谱(GC)与单光子电离-飞行时间质谱(SPI-TOF MS)联用(GC/SPI-TOF MS)的分析方法。首先,设计了一种双层套管的传输管用于连接GC与SPI-TOF MS,实现了GC与单光子电离离子源的无缝连接。在此基础上,以n-十五烷标准品和苯/甲苯/二甲苯的标准气为对象,对电离源的重要电压参数进行了优化,得到了纯净的分子离子峰,实现了对各类有机物的快速和准确定性。最后,将该方法用于分析柴油中的挥发性与半挥发性有机物,获得了柴油组分的二维GC×SPI-TOF MS谱图。不需要复杂的谱图解析和数据处理,根据谱图中离子的质荷比(m/z)归纳了柴油的主要成分,包括脂肪烃、芳香烃和含量很低的苯并吡咯等含氮化合物;根据色谱的保留时间将柴油中的同分异构体区分开来。结果表明GC/SPI-TOF MS法是一种简单、有效的分析方法,非常适于柴油及复杂环境样品等的分析表征。     

Thermal stability of poly(N-vinyl-2-pyrrolidone-co-methacrylic acid) copolymers in inert atmosphere

The thermal stability of poly(N-vinyl-2-pyrrolidone-co-methacrylic acid) copolymers was studied by thermogravimetry and infrared spectroscopy in inert atmosphere. The thermogravimetric curves suggested that the effective degradation of both systems occurred in the temperature range 350–500 °C with more than 60% mass loss. At this temperature, the activation energy was in the range 160–200 kJ mol⁻¹ (average values), suggesting that the degradation occurred by a random scission of the chain. The FTIR results indicated that the main volatile products of degradation are CO₂, CO and hydrocarbons (unsaturated structures) with low molecular weight. Pure PVP also showed the formation of NH₃ which was apparently suppressed in the copolymer by the formation of large amounts of CO₂ and CO. The results suggested that the thermal stability of the copolymers was essentially associated with the N-vinyl-2-pyrrolidone monomer, losing stability when the percentage of methacrylic acid in the copolymer system was increased.     

Multi‐scale Gaussian/Haar wavelet strategies coupled with sub‐window factor analysis for an accurate alignment in nontargeted metabolic profiling to enhance herbal origin discrimination capability

Metabolic dataset can provide an overview of different herbal origin, which is conducted by some statistical procedures. Such results often deviate to a certain degree, due to peaks shifts in chromatographic signals. In order to solve this problem, an improved algorithm of combining sub‐window factor analysis with the mass spectrum information is proposed. The algorithm uses a peak detection approach derived either from multi‐scale Gaussian function or Haar wavelet to locate the peaks with different application scope; the candidate drift points at each peak are estimated by Fast Fourier transform cross correlation; Specifically, the best drift points at each candidate peaks are confirmed by sub‐window factor analysis and mass spectrum information in nontargeted metabolic profiling. Finally, the peak regions were aligned against a reference chromatogram, and the non‐peak regions were used linear interpolation. The chromatographic signals of 30 Bupleurum samples were aligned as an illustration of this algorithm, and they could be well distinguished using some statistical procedures. The result demonstrates that the presented method is stronger than other mass‐spectra based algorithms, when facing the alignment of some co‐eluted peaks.     

Effect of molecular weight on the crystalline structure of polytetrafluoroethylene as-polymerized

Melting and crystallization behavior of polytetrafluoroethylene as polymerized in emulsion and suspension is shown to depend on molecular weight. DSC heating curves for virgin PTFE with low molecular weight below 3 × 10⁵ have a single peak, whereas curves for higher molecular weight samples have double peaks. With increasing heating rate the areas of higher melting peaks become larger than the lower melting peaks. The morphology of polymer exhibiting double melting peaks is mainly folded ribbons or granular particles. The phenomenon of double melting is explained on the basis of two different crystalline states which correspond to the “fold regions” and the “linear segments” in a folded ribbon. The melting temperature of virgin PTFE is almost constant at ca. 330°C for molecular weights below 1 × 10⁶, and rises as the molecular weight increases above 1 × 10⁶. The heat of melting of virgin PTFE is nearly independent of molecular weight. On the basis of these results, we propose a model for melting and crystallization of low and high molecular weight PTFE and for the crystal structure.