Influence of Droplets with Net Charge in Inductively Coupled Plasma Atomic Emission Spectroscopy and Implications for the Easily Ionizable Element Chemical Matrix Effect

In the generation of an aerosol by pneumatic nebulization, concurrent with the production of a distribution of droplet sizes is a second distribution of net charge on the droplets. The influence that the droplets with net charge exert, spatially, on Ca(II) emission in the inductively coupled plasma (ICP) has been examined in the absence and presence of NaCl, an easily ionized element chemical matrix. Tertiary aerosol droplet size distributions were also measured. A mesh positioned in the spray chamber was used to establish an electric field between it and the grounded solution being nebulized. A 200 V dc potential applied to the mesh discriminated against the transport of droplets with net charge to the plasma. For samples that contained 100 mM NaCl, the full width at half-maximum of the Ca(II) emission profile viewed at 10 mm above the load coil was significantly reduced with 200 V applied to the mesh. The source of off-axis analyte emission enhancement low in the plasma in the presence of a NaCl matrix has been attributed to desolvation of the droplets with net charge beyond the coulomb fission limit, causing progeny droplets to be created within the aerosol. Droplets with net charge comprise ∼ 50% of the aerosols studied in this work, and these droplets do have a role in determining the analyte emission profiles in the inductively coupled plasma.