Influence of Gas Sampling on Analyte Transport within the ICP and Ion Sampling for ICP-MS Studied Using Individual, Isolated Sample Droplets

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 off-axis locations in the plasma.

Reduced-Pressure Inductively Coupled Plasma Mass Spectrometry for Nonmetallic Elements

A reduced-pressure argon inductively coupled plasma (ICP) is interfaced to a mass spectrometer to evaluate its possibility of increasing the sensitivity of nonmetallic elements. An electrostatically shielded water-cooled torch is used for the investigation of the secondary discharge at the sampling orifice. Iodine vapor is continuously introduced into the torch as an analyte by using a peristaltic pump. The effects of plasma operating parameters such as gas flow rate, pressure, and power on the intensities of background and iodine ions are studied. It is shown that when the pressure is less than about 30 Torr, an intensive secondary discharge occurs at the sampling orifice if the torch shield is not grounded. The background ion intensity and secondary discharge effect decrease with increasing pressure. The pressure in the torch has an important effect on both polyatomic and analyte intensities. At about 130 Torr of torch pressure, the iodine signal is more than one order of magnitude higher than that obtained at atmospheric pressure, which suggests that low-pressure ICP provides a sensitive ion source for the elements with high ionization potential.

Optical Determination of Stagnation Temperature Behind a Gas Sampling Orifice

A technique has been developed for measuring the temperature during a transient combustion event. It combines the features of atomic resonance absorption and direct sampling to produce a relatively simple, intrusive diagnostic technique to obtain time-resolved measurements. In this study, a propagating hydrogen/air flame was used to provide a rapid temperature increase. A small fraction of krypton was added to the reactants and the absorption of resonant radiation at 123.5 nm was recorded downstream of the sampling orifice within a flow tube. Conversion from absorption measurements to temperature values was performed using a computer model of the radiation source and the absorption by the sample. The model of the source was validated by comparing predicted and recorded spectra of hydrogen Lyman-α emissions, while the absorption model for the sampled gas was tested by comparing the temperatures predicted by krypton absorption measurements with those recorded at a range of known temperatures. The direct sampling atomic resonance technique minimizes time-history distortions inherent in other direct sampling techniques, and is capable of tracking local temperatures during the passage of a propagating flame front.