Negative thermal ionization mass spectrometry of main group elements: Part 1. 5th group: phosphorus, arsenic and antimony

Abstract

A systematic investigation of the formation of MO − x ions ( x = 1, 2 or 3) for the elements P, As, and Sb using negative thermal ionization mass spectrometry is presented. A single-filament ion source and the use of BaO to reduce the electron work function of the rhenium filament material were found to be the optimum conditions for ionizing the above mentioned 5th main group MO x compounds. Ion currents at the detector side of 10 −12 A and more were obtained for PO − 3, PO − 2, AsO − 2, and SbO − 2. SbO − was detected as a minor abundant ion. Owing to the low electron affinity of the elements no atomic ions were measured. No significant dependence of the PO − 2 and PO − 3 ion currents on the different phosphorus compounds was found. Within the 5th main group a tendency was observed on going from phosphorus to antimony in the formation of negative thermal MO − x ions with lower x values. It was possible to determine the oxygen isotope abundances from phosphate and arsenate samples of natural isotopic composition. Measuring PO − 2 and AsO − 2 ions, relative standard deviations of 2% for the least abundant 17O isotope, 0.3% for 18O and 0.001–0.002% for 16O were obtained. However, possible oxygen isotope exchanges between the phosphorus compound and other oxygen atoms in the ion source (for example from BaO or residual gases) can influence the measured isotope ratio. The isotopic abundances of antimony were determined by measuring SbO − 2 ions from a K[Sb(OH) 6] sample with relative standard deviations of 0.08%. The antimony results provide additional data for recalculating the atomic weight of this element as 121.763 ± 0.005.