Comparison of signal enhancement by co-existing carbon and by co-existing bromine in inductively coupled plasma mass spectrometry

Abstract

In inductively coupled plasma (ICP) mass spectrometry, signal enhancement by co-existing carbon results from multiple factors. To elucidate the signal enhancement factors, we compared the effects of co-existing carbon and co-existing bromine, which have similar ionization energies (C, 11.26 eV; Br, 11.81 eV). We eliminated the effect of sample introduction efficiency changes, which are considered to be one reason for signal enhancement, by using two nebulizers. The intensities of the P, I, S, As, Se and B signals were enhanced when a multi-element solution was introduced into the ICP from one nebulizer and a carbon solution was introduced from the other. No signal enhancement was observed by co-existing bromine. We focused on the bond energies of the oxides as a possible explanation for the difference between the results for carbon and bromine. Carbon oxide has a higher bond energy than bromine oxide, and therefore carbon reduces analyte oxides more readily than bromine does. We also considered the effects of the bond energies of the analyte oxides, as well as the effects of the degree of analyte ionization in the ICP, on signal enhancement. Signal enhancement was observed for analytes that were less than 60% ionized in the ICP and whose oxide bond energies exceeded 450 kJ mol⁻¹. Iodine was an exception; signal enhancement was observed for I (ionization degree, 29.85%), even though the bond energy of iodine oxide is only 240 kJ mol⁻¹. Therefore, charge transfer could not be eliminated as a cause of signal enhancement.