SPATIAL LOCATION OF THE SPACE-CHARGE EFFECT IN INDIVIDUAL ION CLOUDS USING MONODISPERSE DRIED MICROPARTICULATE INJECTION WITH A TWIN QUADRUPOLE INDUCTIVELY-COUPLED PLASMA-MASS SPECTROMETER

Pulses of analyte and matrix ions from individual drops are measured s imultaneously using a twin quadrupole inductively coupled plasma mass spectrometer (ICP-MS). The sample solution is introduced by monodisper se dried microparticulate injection (MDMI). At modest Pb concentration s (500 ppm), a shoulder on the leading edge of the Li+ signal appears. At higher matrix concentrations (1000 to at least 1500 ppm), a dip in the leading edge of the Li+ signal develops. These changes in the sha pes of the Li+ pulses are attributed to space charge effects in the ex traction system and ion optics of the mass spectrometer. A qualitative depiction for this behavior is proposed, in which the Li+ ions are de flected out of the preferred ion path and then refocused by the ion op tics. Part of the Li+ ion cloud is driven ahead of the Pb+ cloud, and part is trapped behind the Pb+ cloud. The result is a shoulder on the leading edge of the Li+ signal. With the Pb matrix present, the shapes of the analyte ion pulses are sensitive to the voltages applied to th e first two ion lenses, especially the extractor lens. This observatio n shows that the part of the matrix effect that occurs in the ion opti cs takes place mainly in the first two lenses.