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

Abstract

Pulses of analyte and matrix ions from individual drops are measured simultaneously using a twin quadrupole inductively coupled plasma mass spectrometer (ICP-MS). The sample solution is introduced by monodisperse dried microparticulate injection (MDMI). At modest Pb concentrations (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 shapes of the Li(+) pulses are attributed to space charge effects in the extraction system and ion optics of the mass spectrometer. A qualitative depiction for this behavior is proposed, in which the Li(+) ions are deflected out of the preferred ion path and then refocused by the ion optics. 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 the first two ion lenses, especially the extractor lens. This observation shows that the part of the matrix effect that occurs in the ion optics takes place mainly in the first two lenses.