Time-resolved measurements of individual ion cloud signals to investigate space-charge effects in plasma mass spectrometry

A new approach to directly monitor space charge induced effects due to high concentrations of efficiently ionized elements in inductively coupled plasma mass spectrometry (ICP-MS) is described. The broadening of ion clouds produced from individual, monodisperse drops of sample is measured by using time-resolved ICP-MS. The extent of broadening due to high concentrations of Pb in the sample is related inversely to the analyte mass. For the lightest analyte investigated, Li+, the relative width of the time-resolved analyte peak increases and then shows a dip in the center as the Pb concentration is increased to 500 and then 1500 mu g/mL. The initial results of experiments that investigated chemical matrix effects as a function of concomitant species concentration, analyte mass, and sampling location in ICP-MS are consistent with space-charge effects.