A combination of mass shifts arising from known structural variations of neutral precursors, and accurate m a s measurements of key fragment ions, has led to a proposed systematization of the lowenergy fragmentation mechanisms for MH' ions of actinomycins. These rationalizations are used in a predictive manner to interpret fragment ion spectra, of unknown impurities in actinomycin standards, in terms of structure. Both the strengths and limitations of this approach are emphasized. This work has uncovered actinomycin variants in which one of the threonine residues is substituted by serine, a structural variation which does not appear to have been reported previously.
Reversed-phase liquid chromatography (RPLC) was combined with atmospheric pressure chemical ionization mass spectrometry (APCI-MS), via a heated pneumatic nebulizer interface, for the determination of the polycyclic aromatic sulfur heterocycle (PASH) content of samples obtained by the fractionation of an extract of a pond sediment contaminated by coke-oven residues. Some of the samples produced by the fractionation procedure contained large amounts of other polycyclic aromatic compounds (PACs) which co-eluted with the compounds of interest, making it difficult to obtain mass spectra suitable for compound identification and verification. Therefore, the use of tandem mass spectrometry (MS/MS), as a selective method for the identification of target analytes in complex matrices, was investigated. Initially, PASH standards were injected into the mass spectrometric system by flow injection and their collisionally induced dissociation mass spectra recorded. From these results, it was possible to select ions suitable for selected reaction monitoring (SRM) experiments on both the PASH standards (to establish detection limits and also retention times which could be used to identify these compounds) and the fractions (to establish the possible presence of the selected PASHs in the fractions). The RPLCSRM experiments led to a tentative identification of some of the PASH standards in the fractions. However, the use of multiple reaction monitoring experiments allowed the positive identification of dibenzothiophene, phenanthrol4,5-bcdl thiophene, phenanthro [3,4-b] thiophene and benzo [ bl naphthol 2,341 thiophene in the fractions, along with several of their isomers. Quantification of the PASH standards by RPLC-SRM in the extracts found them to be present at high levels.
Capillary column supercritical fluid chromatography (SCF) was combined with atmospheric pressure chemical ionization (APCI) mass spectrometry through a heated pneumatic-nebulizer interface, originally developed for liquid chromatographylmass spectrometry (LCIMS), and subsequently modified for use with supercritical fluid chromatography/mass spectrometry (SFC/MS). A high pressure syringe pump was used to pass SFC-grade carbon dioxide, used as the mobile phase, through a capillary SFC column which was contained in an oven, maintained at 100 "C. The eluent passed through the heated pneumatic-nebulizer interface into the ionization region of the mass spectrometer with the aid of the flow of nebulizing gas. The system was optimized using benz[a]anthracene, and then applied to analyze a standard mixture of polycyclic aromatic compounds (PACs) as well as complex mixtures of PACs obtained by the fractionation of a pond sediment contaminated by coke-oven residues. A detection limit of 40 pg was established for chrysene.The mass spectrometer is the ideal detector for chromatography due to its inherent specificity, selectivity, and sensitivity.'-3 The direct interfacing of supercritical fluid chromatography (SFC) with mass spectrometry may provide significant advantages over liquid chromatography/mass spectrometry (LC/MS) because of the relative ease of mobile phase evaporation and removal in SFC. Potential application areas for SFC/MS are in the analysis of high molecular weight compounds that are of limited volatility, and of thermally labile compounds for which gas chromatography (GC) is either impossible or impracticable.' The operation of an SFC/MS interface requires that the mobile phase undergo a pressure reduction prior to ionization within the mass spectrometer. For capillary SFCIMS, the sensitivity obtained using electron ionization (EI) is one or two orders of magnitude less than that obtained using chemical ionization (CI);4 in any event, mass spectra that contain ions resulting from both EI and CI are often obtained due to the high C 0 2 pressures in the EI ion s o~r c e .~ Most EI ion sources are relatively gas-tight and are designed to operate at low flow rates or with gases that produce spectra similar to EI spectra (i.e., charge-transfer from He as in GUMS). By using a more open source design to minimize the pressure in the ionization volume, CI spectral contributions in capillary SFC/MS can be rninimi~ed.~ Detection limits of 20 pg have been reported for the capillary SFC/MS analysis of pyrene.'For the development of an effective SFC/MS interface, three requirements have to be fulfilled. First, the gas flow rates generated by volatilization of the mobile phase have to be handled by the interface (this is obviously easier for capillary SFC/MS than for packed + NRCC # 34889.
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