Showing papers on "Atmospheric-pressure chemical ionization published in 1988"

Reduction of methylene blue during the ionization process

Abstract: Methylene blue is readily reduced during the process of ionization in methods as diverse as electron ionization, direct exposure probe electron ionization, direct exposure probe chemical ionization, solid sample secondary ion mass spectrometry, liquid sample secondary ion mass spectrometry and fast atom bombardment mass spectrometry. The reduction of methylene blue need not initially involve the abstraction of hydrogen from the solvent present in the latter two methods.

Characterization of polar substituted polycyclic aromatic compounds using high-resolution gas chromatography/mass spectrometry negative ion chemical ionization and positive and negative ion thermospray liquid chromatography/mass spectrometry.

Abstract: The sensitivity and selectivity of high-resolution gas chromatography/mass spectrometry in the negative ion chemical ionization mode with methane as reagent gas was evaluated for the characterization of polar substituted polycyclic aromatic compounds (PAC). The fragmentation patterns were affected by the nature of the substituent for polar substituted nitro-PAC that showed detection limits of 50 pg at full-scan acquisition. This technique has been applied to the characterization of polar high-performance liquid chromatographic fractions of diesel exhaust particulate (NBS Standard Reference Material 1650) and enabled the identification of 20 PAC of different chemical classes. Among them, hydroxynitro-, dinitro- and nitrosubstituted secondary amines were identified for the first time in diesel exhaust particulate. In addition, ‘filament-on’ thermospray (TSP) liquid chromatography/mass spectrometry (LC/MS) with positive and negative ions have been used for the characterization of similar polar compounds such as 2-nitroquinoline, 1,8-naphthalic anhydride, naphthalene sulphonic acid and 1,2-hydroxynitronaphthalene. LC analyses were performed on a reversed-phase system using either acetonitrile-water or methanol-water with 0.1 M ammonium acetate and 1% acetic acid as eluent. With negative ion TSP LC/MS a four- to fivefold loss in sensitivity was observed for naphthalene sulphonic acid compared with nitrohydroxy-PAC, that showed a minimum detectable amount of 50 ng in the reconstructed ion chromatogram.

Pulsed High-Pressure Liquid Injection of Biological Molecules into Supersonic Beam/Mass Spectrometry with Resonant Two-Photon Ionization Detection:

Abstract: Pulsed high-pressure liquid injection is used as a means of introducing thermally labile biological species into a time-of-flight mass spectrometer (TOFMS) system. The analyte species are then ionized by resonant two-photon ionization (R2PI) in the near ultraviolet. This method serves as a means of producing soft ionization or extensive fragmentation on the basis of the laser power and wavelength, without physical separation of the analyte from the effluent jet expansion. In addition, the pulsed liquid injection method allows introduction of the sample in methanol at 80°C without the extensive clustering that normally occurs in continuous expansions unless much higher temperature is utilized. With the use of this method the R2PI of catecholamines and indoleamines, as well as their metabolites, is studied in a TOFMS. The ability to obtain some spectral selectivity from jet expansions of liquid methanol is also demonstrated.

Application of high-performance liquid chromatography/atmospheric pressure ionization mass spectrometry for the analysis of non-volatile compounds

Abstract: The combination of a high-performance liquid chromatograph and a magnetic type mass spectrometer with an atmospheric ionization source has been applied for analyses of thermolabile compounds, i.e. antibiotics, amino acids and glycosphingolipids. Chromatography was performed on a 4 mm i.d. x 150 mm column packed with 5 μm ODS at a flow rate of 1 ml min−1. The liquid chromatography/atmospheric pressure ionization (LC/API) magnetic-type mass spectrometer provides not only an abundant pseudo-molecular ion but also fragment ions which give a structural information resulting from collision-induced dissociation at the differential pumping region of the liquid chromatograph/mass spectrometer interface. The isomers of kanamycins could be differentiated from each other on the basis of their mass spectra obtained at the relatively higher drift voltage applied. LC/API mass spectra of leucine and isoleucine were very similar, but it was easy to differentiate one from another by checking the existence of the ions at m/z 43 or m/z 57 in their mass spectra. The LC/API mass spectra of glucosylceramides showed [MH]+ ions of good abundance and significant fragment ions useful for chemical characterization at a proper drift voltage setting in a controlled and reproducible manner.

Sites of protonation in phenothiazines: methane and ammonia chemical ionization mass spectra

Abstract: Positive ion methane and ammonia chemical ionization mass spectra for ten phenothiazine derivatives are reported. The fragmentations observed in the chemical ionization mass spectra are rationalized in terms of the location of the added proton. High-resolution measurements are used to confirm empirical formulae of the ions in the mass spectra. Changes in the mass spectra with a change in the chemical ionization reagent gas from methane to ammonia are described. A comparison with positive ion secondary ion mass spectra of the same compounds show that the amount of fragmentation is higher in the secondary ion mass spectra, but the same types of ions are observed in spectra produced by both ionization methods.

Mass spectrometry with positive chemical ionization of some α-pinene and β-pinene derivatives

Abstract: Twenty-one pinanic compounds are analysed by ammonia chemical ionization mass spectrometry. This technique permits a clear characterization of each compound and an easy differentiation of stereoisomers. The major ion-molecule reactions are discussed.

Low-resolution accurate mass measurement in self-chemical ionization mass spectrometry

Abstract: Low-resolution (2000, 10% valley definition) accurate mass measurements using self-chemical ionization (self-CI) have been evaluated as an alternative to the conventional chemical ionization (CI) method. In conventional CI experiments a high pressure of reagent gas is required to induce ionization while in self-CI no reagent gas is used and the self-CI is produced presumably by molecular/fragment ion–molecule reactions. Nine compounds ranging in mass from 50–500 daltons were examined. Results obtained by the self-CI method indicate that the elemental composition assignment can be obtained simultaneously for the protonated molecule and/or molecular ion. It is also shown that perfluorokerosene can be used routinely in self-CI as an internal reference standard over a broad mass range (50–500 daltons). It is sometimes difficult to use a single reference standard in conventional low-resolution CI accurate mass spectrometry over a similar mass range.

Positive and negative ion mass spectrometry of antiepileptic hydantoins and their analogs.

Abstract: Positive ion electron impact (PIEI), positive ion chemical ionization (PICI), and negative ion chemical ionization (NICI) mass spectra of seven compounds of hydantoins and their analogs are presented; their probable fragmentation modes are also presented. In the PIEI mode, intensities of molecular ions differed according to different compounds. Cleavage at outsides of both carbonyl groups was commonly observed for all compounds. In the PICI mode, all compounds showed [M + 1]+ quasi-molecular ions constituting the base peaks. In the NICI mode, [M - 1]- quasi-molecular anions were the base peaks except for trimethadione and paramethadione. All negative spectra showed anions at m/z 42 due to [NCO]-; these peaks seem useful for screening of antiepileptics. An extraction procedure for the anti-epileptics from human urine or plasma, and their separation by gas chromatography (GC), are also presented to serve for their actual identification by GC/mass spectrometry.