The effect of gas flow entrainment on the gas sampling, ion sampling, and ion detection processes in inductively coupled plasma mass spectrometry (ICP-MS) has been investigated. Isolated, single droplets of sample from a monodisperse dried microparticulate injector (MDMI) were used in conjunction with time-resolved ICP-MS, photographs of ion cloud movement, and time-gated imaging using a gateable, intensified charge-coupled device (ICCD) detector mounted on an imaging spectrometer. The results indicate that gas flow entrainment into the sampling orifice can have a significant effect on the plasma gas velocities as far as 7 mm from the sampling orifice. The effects are most pronounced within 3 mm of the sampling orifice. The trends in these results are consistent with theoretical calculations. Photographic images show that plasma gas initially as far as 3 mm off axis adopts a curved path into the sampling orifice. Time-resolved emission images of Sr+ ion clouds approaching the sampling orifice demonstrate the entrainment process and significant distortion of the ion cloud as it flows into the sampling orifice. Spatial maps of La+ ICP-MS signals were acquired as a function of distance from the vaporization point and distance from the plasma axis. The results suggest that gas entrainment has a significant effect on the spatial path of ions in the plasma and that accurate radially resolved spatial mapping of plasmas using mass spectrometry may not be possible. The widths of radially resolved La+ ICP-MS signal peaks do not change significantly when ions are sampled 2 mm from the vaporization point compared to 5 mm away. In contrast, ICP-MS signals measured on axis as a function of time clearly show broadening due to diffusion. These observations suggest that some detected ions may have originated from offaxis locations in the plasma.
Ian I. Stewart, Carl E. Hensman, and John W. Olesik, "Influence of Gas Sampling on Analyte Transport within the ICP and Ion Sampling for ICP-MS Studied Using Individual, Isolated Sample Droplets," Appl. Spectrosc. 54, 164-174 (2000)