Electron Ionization and Atmospheric Pressure Photochemical Ionization in Gas Chromatography-Mass Spectrometry Analysis of Amino Acids
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The mass spectra of tert-butyldimethylsilyl (TBDMS) derivatives of 17 amino acids were obtained using electron ionization (EI) and atmospheric pressure photochemical ionization (APPhCI) mass spectrometry. The APPhCI mass spectra for all of the derivatives except arginine were shown to consist of only molecular [M](+.) and quasimolecular [MH](+) ions whereas, in the case of EI, the compounds in question underwent a drastic fragmentation. The application of APPhCI to gas chromatography-mass spectrometry enables a reliable identification of the TBDMS derivatives of amino acids in a mixture, even if its components are only partially resolved, due to the unique molecular masses for each compound. Comparison of the respective positive-ion chemical ionization (PICI) mass spectra available in the literature with APPhCI spectra has shown that, in the case of PICI, unlike APPhCI, noticeable fragmentation occurs.Keywords:
Fragmentation
Atmospheric-pressure chemical ionization
Polyatomic ion
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Field desorption
Desorption electrospray ionization
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A high-performance liquid chromatograph with mass spectrum detection (HPLC-MS/APCI) method has been established for simultaneous determination of ten major bioactive components of Naodesheng injection including safflor yellow A, puerarin, daidzein, ginsenosides (Rg1, Rg2, Rb1, Rd, Re, Rh1), and notoginsenoside R1. The separations were carried out with a Luna C18 column (5 μm, 150×4.6 mm, Phenomenex, U.S.A.) with a stepwise gradient elution of the mobile phase consisting of water (0.1% of formic acid, v/v)–methanol (0 min, 70 : 30; 8 min, 30 : 70; 20 min, 10 : 90) at a flow-rate of 0.8 ml/min. The proposed method was applied to analyze five various Naodesheng injections and produced data with acceptable linearity, repeatability, precision and accuracy having lower limits of quantitation (LLOQs) of 0.02—0.2 μg. The calibration curves were linear in respective range for all compounds, all of them with coefficients of determination above 0.9900. The intraday precessions were less than 5.0%. The proposed method is accurate, sensitive and simple, a useful alternative for routine analysis in the quality control of Traditional Chinese Medicine.
Atmospheric-pressure chemical ionization
Thermospray
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Abstract Orthogonal acceleration time‐of‐flight (oa‐TOF) mass spectrometry (MS) was coupled to gas chromatography (GC) to measure ion yields (ratio of ion counts to number of neutrals entering the ion source) and signal‐to‐noise ( S/N ) in the electron ionization (EI) mode (hard ionization) as well as in the soft ionization modes of chemical ionization (CI), electron capture negative ion chemical ionization (NICI) and field ionization (FI). Mass accuracies of the EI and FI modes were also investigated. Sixteen structurally diverse volatile organic compounds were chosen for this study. The oa‐TOF mass analyzer is highly suited for FI MS and provided an opportunity to compare the sensitivity of this ionization method to the more conventional ionization methods. Compared to the widely used quadrupole mass filter, the oa‐TOF platform offers significantly greater mass accuracy and therefore the possibility of determining the empirical formula of analytes. The findings of this study showed that, for the instrument used, EI generated the most ions with the exception of compounds able to form negative ions readily. Lower ion yields in the FI mode were generally observed but the chromatograms displayed greater S/N and in many cases gave spectra dominated by a molecular ion. Ion counts in CI are limited by the very small apertures required to maintain sufficiently high pressures in the ionization chamber. Mass accuracy for molecular and fragment ions was attainable at close to manufacturer's specifications, thus providing useful information on molecular ions and neutral losses. The data presented also suggests a potentially useful instrumental combination would result if EI and FI spectra could be collected simultaneously or in alternate scans during GC/MS. Copyright © 2009 John Wiley & Sons, Ltd.
Ambient ionization
Field desorption
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Abstract The application of gas chromatography chemical ionization mass spectrometry to the determination of a variety of alkyl alkylphosphonates, phosphonofluoridates, phosphonothiolates and an amidophosphorocyanidate is described. Comparison is made between the electron ionization and chemical ionization mass spectrometry of these compounds. Chemical ionization mass spectrometry is shown to enhance the capability for identification, especially when a limited sample is available. Results indicate that methane is highly useful for obtaining protonated molecular ions and association ions (formed by the transfer of a reactant ion to a sample molecule) as well as meaningful fragment ions. Ionizing ethylene and isobutane gives protonated molecular ions as base peaks for all of the compounds studied, including those where a lower abundance of the [MH] + ion is found via methane chemical ionization mass spectrometry. Ethylene is superior to isobutane on the basis of its effectiveness for serving as both a carrier and a reagent gas and gives better sensitivity. Although not an intrinsic part of this present study, analytical sensitivities in the subnanogram range were found.
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Various refinery streams in the naphtha boiling range, which have appreciable aromatic content, were analyzed for alkylated thiophenes to the hundred part per million level by a variety of quadrupole and triple quadrupole mass spectrometry procedures. Several separation, ionization, and analyzing techniques were compared to develop the most efficient and sensitive method for this analysis. The study compares the use of gas and open column liquid chromatography, electron impact and ammonia chemical ionization, and the use of a single mass analyzer vs. multiple mass analyzer techniques. Electron impact ionization combined with normal single quadrupole mass spectrometry was not sensitive to the low ppm level, even after the separation. Minor fragmentation peaks of alkyl aromatics still resulted in significant interferences. Ammonia chemical ionization, both in the negative and positive ion modes, enhanced the sensitivity for alkylthiophenes by increasing the response for these compounds by a factor of ten. Mass spectrometry/mass spectrometry has been previously applied to the study of sulfur compounds in hydrocarbon matrices using chemical ionization. However, in the current study with naphtha range materials, the daughter ion scanning mode under electron impact ionization was found to provide the most promising results. The level of detection was only slightly lessmore » than single quadrupole chemical ionization analyses, but was much better than the triple quadrupole chemical ionization analyses.« less
Atmospheric-pressure chemical ionization
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