Atmospheric pressure microwave induced plasma ionization source for molecular mass spectrometry

An atmospheric pressure microwave induced plasma ionization (AP-MIPI) source design for molecular analysis is presented. It consists of three sections: the torch, the microwave cavity, and the vacuum interface. The torch uses an oscillating capillary nebulizer for introduction of aqueous solutions into the afterglow of the MIP. Samples introduced into the afterglow of the plasma are ionized by protonated solvent molecules in a fashion similar to the chemical ionization process. The ions are then transported into the mass spectrometer using a heated capillary tube. The combination of an improved torch design and the heated capillary tube provides a stable ionization source for direct aqueous solution introduction with flow rates of up to 0.5 mL/min. The system works well with solvent compositions similar to those used in high performance liquid chromatography. For most samples studied, the protonated molecules were the base peaks of the mass spectra. A linear dynamic range of at least two orders of magnitude and an upper limit of quantitation of 5 ng were observed for alanine. The technique was shown to maintain reproducibility and moderate loss of sensitivity (33% reduction) when an amino acid solution containing a sodium phosphate buffer was analyzed. The optimization of experimental parameters with respect to the torch and vacuum interface is discussed.