Variable aperture extraction lens for ion beam investigation in inductively coupled plasma-mass spectrometry†
A variable aperture was introduced into a commercially available sector field multicollector inductively coupled plasma-mass spectrometer. A diameter-variable aperture allows an in situ study of the radial isotopic composition within the ion beam. Additional information on the intensity distribution could be gained. The elements boron, cadmium and lead, covering a wide mass range, were investigated. In contrast to earlier experiments [Kivel et al., Spectrochimica Acta Part B: Atomic Spectroscopy, 2012, 76, 126–132], the current setup allows for lower element concentration levels in the samples and a drastically reduced measurement time. A significant radial dependence of the isotopic composition within the ion beam was observed for cadmium and lead, whereas for boron, such dependence could not be detected. The beam profiles however show a systematic trend towards smaller beam diameters for higher masses. Even though the beam diameter is dependent upon the mass of the ion, the transmission into the mass spectrometer can be considered almost complete. Thus, a contribution to mass discrimination by space-charge induced beam broadening and energy-selective ion transmission, at least within the boundaries studied here, can be excluded.