Mass spectrometric calibration of an atomizer/nebulizer for doping flames or discharge plasmas

Abstract

An atomizer or nebulizer provides a very versatile means of doping a flame or discharge plasma with a small amount (typically < 10 −5 mole fraction) of an additive. A wide variety of substances (e.g. salts of metallic halides) can be introduced as an aerosol by spraying an aqueous solution into the system, but the additive concentration must be calibrated. Optical calibration methods are difficult, and measurements of the atomizer effluent or depletion are unreliable. A conceptually simple and general method has been developed for calibration of the atomizer concentration in situ when the system can be sampled by a mass spectrometer. A solution is sprayed with the atomizer, which produces a distinctive type of ions in the system, either positive or negative. The same type of ions is then produced by introducing the vapour of a volatile substance of known vapour pressure using a small flow of a carrier gas through a gas saturator. By varying the saturator flow rate, the ion signal from the saturator is then matched to the ion signal from the atomizer. As the concentration introduced by the saturator is known, the calibration factor for the atomizer can be determined. An example is worked out for a premixed H 2O 2Ar flame having essentially no natural ionization, which is doped with an atomizer spraying a 0.05 M aqueous solution of KI to produce K + and I −. Then, with the atomizer spraying a 0.05 M KNO 3 solution, the I − atomizer signal is matched by an I − signal produced when C 6H 5I vapour is introduced by a tiny flow rate of argon through the saturator. If the ion chemistry is straightforward, the mass spectrometer could be replaced by a total ion collector. The method should be applicable to a variety of flames and also to discharge plasmas.