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The use of acetonitrile as a solvent for acid-base, electrochemical, and spectroscopic studies has markedly increased over the last decade. Its high dielectric constant (37.5 at 20 °C), low UV cutoff (less than 200 nm), and low proton availability contribute to its favorable properties. However, it is difficult to purify, particularly with respect to compounds such as acrylonitrile and benzene which have boiling points close to that of acetonitrile.The classical method for the purification of acetonitrile consisting of repeated fractional distillations from phosphoric oxide gives a relatively pure product. However, this treatment is time-consuming and causes extensive polymerization

The behavior in atmospheric pressure chemical ionization of selected model polycyclic aromatic compounds, pyrene, dibenzothiophene, carbazole, and fluorenone, was studied in the solvents acetonitrile, methanol, and toluene. Relative ionization efficiency and sensitivity were highest in toluene and lowest in methanol, a mixture of molecular ions and protonated molecules was observed in most instances, and interferences between analytes were detected at higher concentrations. Such interferences were assumed to be caused by a competition among analyte molecules for a limited number of reagent ions in the plasma. The presence of both molecular ions and protonated analyte molecules can be attributed to charge-transfer from solvent radical cations and proton transfer from protonated solvent molecules, respectively. The order of ionization efficiency could be explained by incorporating the effect of solvation in the ionization reactions. Thermodynamic data, both experimental and calculated theoretically, are presented to support the proposed ionization mechanisms. The analytical implications of the results are that using acetonitrile (compared with methanol) as solvent will provide better sensitivity with fewer interferences (at low concentrations), except for analytes having high gas-phase basicities. (J Am Soc Mass Spectrom 2008, 19, 1926 -1941) © 2008 Published by Elsevier Inc. on behalf of American Society for Mass Spectrometry T he analysis of polycyclic aromatic compounds (PACs) in petroleum and environmental samples has been the subject of many investigations [1-3], due to their impact on the environment, refining processes, and product quality. However, success is often hindered by the number and low concentration of these compounds in such samples, as well as by the lack of standards. Gas chromatography (GC) and GC coupled with electron ionization (EI) mass spectrometry (GC/ MS) are normally employed for samples containing robust, thermally stable analytes [4,5]. Atmospheric pressure ionization methods [6]-electrospray ionization (ESI) [7], atmospheric pressure chemical ionization (APCI) [8], and atmospheric pressure photoionization (APPI) [9]-in conjunction with liquid chromatography (LC) have been used for samples of more problematic analytes.It is generally believed that ESI suffers more from nonlinear response and matrix effects than APCI. Linear response over a wide concentration range is an attractive feature of APCI and has been reported by several authors [10,11]. Recently, Roussis and Fedora compared the ability of APCI and ESI to quantify polar and ionic compounds in petroleum products [12]. They obtained linear ranges of three orders of magnitude for both techniques, and higher sensitivity for ESI. However, the ESI response was nonlinear over the concentration range of interest, and they recommended the use of APCI for quantitative LC/MS applications.In the course of a study of the application of APCI to the analysis of nitrogen-and particularly sulfurcontaining PACs in petroleum samples, we observe...

Software, available at no cost on the Internet, is described which uses polynomial expansion algorithms to calculate the isotope patterns for precursor ion, neutral loss, and MSn product ion tandem mass spectra. Such information is useful for determining product ion and neutral loss composition, identification of analytes in complex samples, deconvolution of overlapping spectra, and correction of peak heights or areas in quantitative analysis. The effect of less than unit mass resolution on the isotope patterns is described and experimental examples of the use of the software are presented.