Fundamental studies of atmospheric pressure electrical discharges for analytical spectrometry : towards the optimization of analytical ionization/excitation sources

Abstract

Electrical discharges have been critical to the advancement of analytical science for over a hundred years. Perhaps the most useful property of electrical discharges is that the resultant reactive species have exceptionally high potential energies (> 15 eV). These high energy reactive species are well suited for ionization (mass spectrometry, MS) and excitation (spectroscopy). Before 1973, many analytical electrical discharges, especially those used for molecular MS, were confined to high vacuum. High vacuum conditions are advantageous because a decrease the mean free path prevents an arc from forming, ambient gas flows and contamination do not affect the discharge, and there are no fluid dynamics to consider if mass transport is needed. Recently, numerous electrical discharges have been developed that operate at atmospheric pressure (AP) and are exposed to ambient conditions. Atmospheric pressure discharges are advantageous because they are less costly to build, sample compatibility is better, collisional cooling limits fragmentation of molecular species, and sample introduction is simpler. However, more work is needed to understand how AP ionization chemistry and mass transport at atmospheric pressure affect analytical results. In this work, chemical ionization processes were investigated for different types of electrical discharges and analytes with MS detection. Meanwhile, computational fluid dynamics and fluid flow visualization were used to better understand the role of mass transport from the ambient discharge source to the lower pressure MS inlet. Lastly, a variant of these electrical plasmas has been optimized for use as a source of vacuum ultraviolet radiation, for MS or optical spectroscopic detection. The results from these fundamental studies are used to suggest improvements that can be made in the design of ionization or excitation sources.