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A four level population rate equations model is described and is used to interpret published resonant ionisation data for the Yb atom. Three different laser wavelengths were used in the experiment for the excitation steps. The ion-yield graphs a s a function of laser fluences are well reproduced. From the analysis, the Einstein coefficient for the transition of the 17 3P2 state to the 7 3S1 state is determined to be 9940 s-1 with an uncertainty of 10%. The sensitivity of multi-photon processes to a time structure in the laser pulse is studied and important differences in ion yield are calculated for excitation steps involving simultaneous absorption of two or more photons.

A modern ion trap offers several advantages when using full scan tandem mass spectrometry (MS/MS) instead of selected-reaction monitoring (SRM); these advantages are discussed and a comparison of SRM and full scan MS/MS is made. An example of the application of full scan MS/MS to mixture analysis is given. © 1997 by John Wiley & Sons, Ltd. Received 3 April 1997; Revised 22 May 1997; Accepted 24 May 1997 Rapid. Commun. Mass Spectrom. 11, 1151-1153 (1997 The conjoint use of liquid chromatography and atmospheric pressure ionization (API) mass spectrometry is now recognized as the method of choice for the high performance quantitation of pharmaceuticals. Since API sources usually generate a single pseudomolecular ion, there is potential for ambiguity in peak identification due to isobaric chemical interference. For this reason, selected-reaction monitoring (SRM) performed on a triple quadrupole mass spectrometer, has become the preferred quantitative tool of the pharmaceutical industry.Modern API ion traps, in common with triple quadrupole instruments, can perform SRM experiments.1 One characteristic that distinguishes the modern ion trap from triple quadrupole instruments is the fact that sensitivity is not sacrificed while performing a full-scan tandem mass spectrometry (MS/MS) experiment when compared with an SRM experiment. In addition, since the full-scan MS/MS spectrum obtained is both at unit mass resolution and at high sensitivity, quantitative experiments are normally performed in the full-scan MS/MS mode on the modern ion trap.2,3 A number of advantages accrue as a result.First, the ion or ions to be used for quantitation may be selected either before or after acquisition, since a full product spectrum is obtained, and can be altered without the need for further analysis. The nett effect is the same as if several SRM experiments had been performed. Second, the ability to perform full-scan MS/ MS at sensitivities equivalent to SRM facilitates fast method development; a significant issue in this application. Third, since a full-scan product spectrum is obtained, the spectra obtained can be used to rule out false positives. Finally, the prospect of analyzing more than one component simultaneously, by fragmenting all the parent ions at once and monitoring different product ions in the full-scan MS/MS spectrum, now becomes a real possibility. This paper explores the advantages of using full-scan MS/MS for quantitative analysis. EXPERIMENTALThe liquid chromatographic system comprised a vacuum degasser, P4000 pump and AS3000 auto sampler (Thermo-Separation Products, San Jose, CA, USA). A 3.3 cm ϫ 4.6 mm C8 column (Supelco, Bellefonte, PA, USA), was run at 0.85 mL/min with 60% methanol, 39% water (Baxter, Muskegon, MI, USA) and 1% acetic acid (Curtin Matheson Scientific Inc., Houston, TX, USA).Hydrocortisone, prednisolone (Sigma, St. Louis, MA, USA) and d 3 -hydrocortisone (Cambridge Isotope Laboratory, Andover, MA, USA) were obtained for the experiments described. All samples were prepared in 60% methanol + 39% ...