Sheathing of the liquid sampling - Atmospheric pressure glow discharge microplasma from ambient atmosphere and its implications for optical emission spectroscopy

The use of a borosilicate glass cylinder to sheath the operational area of the liquid sampling-atmospheric glow discharge (LS-APGD) microplasma from ambient atmosphere and its effects on optical emission spectroscopy (OES) is described. A positive influence of the glass sheath on analyte and background emission characteristics is realized, along with minor drawbacks in terms of analytical performance (i.e., repeatability) and physical restraint. Sheathing the plasma significantly reduces the ambient atmosphere background spectral contributions, especially those of nitrogen species, resulting in improved measurements of interfered Pb (I) and Ag (I) transitions. Qualitatively, the background reduction by means of glass sheathing is comparable to that of a post-acquisition, mathematical background subtraction method. In addition, the noise associated with atmospheric background species is reduced as well, unlike what is observed in the post-acquisition background correction. Limits of detection (LOD) improvements for silver, lead, and cadmium are observed, while those of sodium and lithium were compromised. By virtue of the alleviation of N-2 emission from the background, the LOD of lead improved by multiple orders of magnitude after sheathing the plasma. Future development will be focused toward both improving the analytical performance of the LS-APGD-OES and better understanding the mechanism of the improvements seen by sheathing the plasma.