In conventional quantitative mass spectrometry, it is common practice to vaporise sample mixtures and to maintain a reservoir of sample vapour at a constant pressure while a mass spectrum is being measured or recorded. This is achieved by allowing the vapour to leak into the mass spectrometer ion source at such a rate that no perceptible change in the pressure in the reservoir is caused. The ion currents at significant m/e values may then be related to the partial pressures of the components of the sample mixture in the reservoir, provided that the sensitivity of the instrument for each component has been determined by a prior calibration with a pure sample of that component. Such a technique is wasteful since less than I o/o of the total sample is consumed. Further, there is a restriction on the volatility of samples which can be analysed because it is necessary to provide a reservoir sample pressure of at least several microns in order that vapour may pass through the constriction into the ion source of the mass spectrometer.
In modern qualitative mass spectrometry, it is often necessary to study the behaviour of very small amounts of sample materials of very low volatility. Consequently, the conventional method cannot be used, and such materials are evaporated directly into the ion source from solid samples contained in a probe which can be adjusted to be close to the ionising electron beam. It is obvious that the partial pressure of the sample vapour must vary during the evaporation process and no prior calibrations of instrumental sensitivity are possible.
Recently, however, a technique was evolved1 in which the ion current at a significant m/e value was measured before, during and after evaporation of a known weight of sample. The area under the ion-current curve, i.e. the integrated ion current, could then be related directly to the weight of sample evaporated. Thus, provided that no reactions took place during an evaporation process, it was possible to measure the concentrations of the components of a mixture by a series of measurements of integrated ion-current curves at several m/e values. The very high sensitivity of this method was first demonstrated using nickel dimethylglyoximatel but the technique was extended to studies of the oxinates of the common metals2 and the rare earth+. It is the purpose of the present paper to extend the application of this technique to the chelates of holrnium with a number of substituted acetylacetones, to provide a method for the estimation of these chelates and to relate their mass spectrometric bshaviour to that observed in sublimation and gas chromatographic studies.