PDI-PIXE-MS: Particle Desorption Ionization Particle-Induced X-Ray Emission Mass Spectrometry

Abstract

Incident ions, from a Van de Graaff accelerator, in the MeV energy range, deposit their energy into the near surface of a sample. This, in turn, causes atomic, molecular, cluster and fragment ion species to be desorbed and ionized, while simultaneously emitting characteristic elemental X-rays. The multielemental X-rays provide qualitative elemental information, which may be deconvoluted and fit to a theoretical X-ray spectrum, generated by a quantitative analysis program, GUPIX, while the atomic, molecular, cluster, and fragment ion species are identified using a quadrupole mass spectrometer. This methodology directly links elemental determinations with chemical speciation.The development of this particle desorption ionization particle induced X-ray emission mass spectrometer, the PDI-PIXE-MS (or PIXE-MS) instrument, which has the ability to collect both qualitative multielemental X-rays and mass spectral data is described. This multiplexed instrument has been designed to use millimeter-sized MeV particle beams as a desorption ionization (PDI) and X-ray emission (PIXE) source. Two general methods have been employed, one simultaneous and the other sequential. Both methods make use of a novel X-ray/ion source developed for use with the quadrupole mass spectrometer used in these experiments. The first method uses a MeV heavy ion particle beam, typically oxygen, to desorb and ionize the sample, while simultaneously producing characteristic multielemental X-rays. The resulting molecular, cluster, and fragment ions are collected by the mass spectrometer, and the X-rays are collected using a Si-PIN photodiode detector in conjunction with a multichannel analyzer (MCA). Heavy ions of N+, O+, O+2, Ar+, and Kr+ have been investigated, although heavy ion X-ray and mass spectra have focused on the use of oxygen particle beams. The second method is performed by first collecting the X-ray data with a MeV ion beam of He+ ions, then desorbing and ionizing the sample species with a MeV particle beam of heavy ions, producing good ion yields, for mass spectral data collection. The potential development of a scanning microprobe instrument, that would provide micron-scale, imaged, multielemental, and molecular and fragment ion chemical information is being investigated through the development of this prototype PIXE-MS instrument.