Inductively coupled plasma mass spectrometry with a twin quadrupole instrument using laser ablation sample introduction and monodisperse dried microparticulate injection
The focus of this dissertation is the use of a twin quadrupole inductively coupled plasma mass spectrometer (ICP-MS) for the simultaneous detection of two m/z values;The twin quadrupole ICP-MS is used with laser ablation sample introduction in both the steady state (10 Hz) and single pulse modes. Steady state signals are highly correlated and the majority of flicker noise cancels when the ratio is calculated. Using a copper sample, the isotope ratio 63Cu+/65Cu+ is measured with a relative standard deviation (RSD) of 0.26%. Transient signals for single laser pulses are also obtained. Copper isotope ratio measurements for several laser pulses are measured with an RSD of 0.85%;Laser ablation (LA) is used with steel samples to assess the ability of the twin quadrupole ICP-MS to eliminate flicker noise of minor components of steel samples. Isotopic and internal standard ratios are measured in the first part of this work. The isotope ratio 52Cr+/53Cr+ (Cr present at 1.31%) can be measured with an RSD of 0.06% to 0.1%. For internal standard elements, RSDs improve from 1.9% in the Cr+ signal to 0.12% for the ratio of 51V+ to 52Cr+. In the second part of this work, one mass spectrometer is scanned while the second channel measures an individual m/z value. When the ratio of these two signals is calculated, the peak shapes in the mass spectrum are improved significantly;Pulses of analyte and matrix ions from individual drops are measured simultaneously using the twin quadrupole ICP-MS with 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 (1500 ppm), a dip in the leading edge of the Li+ signal becomes apparent. Space charge effects are consistent with the disturbances seen. A model for this behavior is proposed. In this model, deflection of Li+ by space charge causes part of the ion cloud to be driven ahead of the Pb+ and part to be trapped behind the Pb+ cloud resulting in a shoulder on the Li+ signal.