Laser-induced acoustic desorption (LIAD) was successfully coupled to a conventional atmospheric pressure chemical ionization
(APCI) source in a commercial linear quadrupole ion trap mass spectrometer (LQIT). Model compounds representing a wide variety
of different types, including basic nitrogen and oxygen compounds, aromatic and aliphatic compounds, as well as unsaturated
and saturated hydrocarbons, were tested separately and as a mixture. These model compounds were successfully evaporated into
the gas phase by using LIAD and then ionized by using APCI with different reagents. From the four APCI reagent systems tested,
neat carbon disulfide provided the best results. The mixture of methanol and water produced primarily protonated molecules,
as expected. However, only the most basic compounds yielded ions under these conditions. In sharp contrast, using APCI with
either neat benzene or neat carbon disulfide as the reagent resulted in the ionization of all the analytes studied to predominantly
yield stable molecular ions. Benzene yielded a larger fraction of protonated molecules than carbon disulfide, which is a disadvantage.
A similar but minor amount of fragmentation was observed for these two reagents. When the experiment was performed without
a liquid reagent (nitrogen gas was the reagent), more fragmentation was observed. Analysis of a known mixture as well as a
petroleum cut was also carried out. In summary, the new experiment presented here allows the evaporation of thermally labile
compounds, both polar and nonpolar, without dissociation or aggregation, and their ionization to predominantly form stable
molecular ions. 相似文献
Efforts to analyze trace levels of cyclic peroxides by liquid chromatography/mass spectrometry gave evidence that acetonitrile suppressed ion formation. Further investigations extended this discovery to ketones, linear peroxides, esters, and possibly many other types of compounds, including triazole and menadione. Direct ionization suppression caused by acetonitrile was observed for multiple adduct types in both electrospray ionization and atmospheric pressure chemical ionization. The addition of only 2% acetonitrile significantly decreased the sensitivity of analyte response. Efforts to identify the mechanism were made using various nitriles. The ion suppression was reduced by substitution of an acetonitrile hydrogen with an electron-withdrawing group, but was exacerbated by electron-donating or steric groups adjacent to the nitrile. Although current theory does not explain this phenomenon, we propose that polar interactions between the various functionalities and the nitrile may be forming neutral aggregates that manifest as ionization suppression.
An internal standard system has been developed for a mass spectrometer equipped with an atmospheric pressure chemical ionization source. The system has been used to overcome sensitivity drift problems that are commonly encountered when the spectrometer is used for long-term environmental monitoring. Additionally, the internal standard has been used to correct sensitivity changes induced by the matrix being analyzed. Principal components of the system are a low concentration internal standard source and a flow-delivery system for introducing the standard to the reagent gas delivery stream of the spectrometer. Following an experiment, real-time data are downloaded to a personal computer where internal standard correction and data analysis are performed. Application of the internal standard to the measurement of nicotine and pyridine is demonstrated. 相似文献
Laser-induced acoustic desorption (LIAD) combined with ClMn(H(2)O)(+) chemical ionization (CI) was tested for the analysis of nonpolar lipids and selected steroids in a Fourier-transform ion cyclotron resonance mass spectrometer (FT-ICR). The nonpolar lipids studied, cholesterol, 5α-cholestane, cholesta-3,5-diene, squalene, and β-carotene, were found to solely form the desired water replacement product (adduct-H(2)O) with the ClMn(H(2)O)(+) ions. The steroids, androsterone, dehydroepiandrosterone (DHEA), estrone, estradiol, and estriol, also form abundant adduct-H(2)O ions, but less abundant adduct-2H(2)O ions were also observed. Neither (+)APCI nor (+)ESI can ionize the saturated hydrocarbon lipid, cholestane. APCI successfully ionizes the unsaturated hydrocarbon lipids to form exclusively the intact protonated analytes. However, it causes extensive fragmentation for cholesterol and the steroids. The worst case is cholesterol that does not produce any stable protonated molecules. On the other hand, ESI cannot ionize any of the hydrocarbon analytes, saturated or unsaturated. However, ESI can be used to protonate the oxygen-containing analytes with substantially less fragmentation than for APCI in all cases except for cholesterol and estrone. In conclusion, LIAD/ClMn(H(2)O)(+) chemical ionization is superior over APCI and ESI for the mass spectrometric characterization of underivatized nonpolar lipids and steroids. 相似文献
