Monochromatic spatial imaging of the liquid sampling – Atmospheric pressure glow discharge: Effects of gas flow on spatial profiles of analyte and background species

The liquid sampling – atmospheric pressure glow discharge (LS-APGD) microplasma was paired with a monochromatic imaging spectrometer (MIS) to interrogate relevant plasma species/elements. The coupling of the LS-APGD source to the MIS allows for the generation of spatial emission profiles at specific analyte and background species' wavelengths. A major goal of this work was to utilize these emission profiles to gain an understanding of the species' location in the plasma as well as the effects of plasma gas flow on these emission profiles. It was found that as sheath gas and counter gas flow rates were increased, there were visible differences in intensity and distribution of emission from the monitored species in the plasma. Silver was used as the representative (test) analyte for this work due to its good sensitivity, wavelength positioning, and ability to provide continuity for comparison to previous works. The majority of analyte emission was found to occur at the tip of the solution electrode, where solutes are introduced to the plasma, whereas various background species emitted throughout the plasma. In addition to aiding in the understanding of plasma operation based on spatial emission characteristics, these studies also aided in optimizing the gas flow parameters for analyte emission intensity, signal-to-background (S/B), and signal-to-noise (S/N) values. The optimized gas flow parameters were found to be 0.7 and 0.1 L min−1 for the sheath and counter gas respectively.