UBC Theses and Dissertations
Development of a capacitively coupled plasma as a gas chromatographic detector Huang, Degui
This work has two objectives: first, to develop an atmospheric pressure radio frequency capacitively coupled plasma (CCP) as a detector for gas chromatography (GC) and, secondly, to understand the excitation process in the CCP from a fundamental point of view. In the process of developing the CCP as a gas chromatographic detector, the design of a CCP torch useful as an atomic emission detector for GC has involved several stages of evolution. Initially, two plasma torches, a cylindrical one with concentric electrodes and a rectangular one with two parallel electrodes, were designed and their performance evaluated. It was found that the parallel plate torch which utilized a rectangular-bore quartz tube had more stable emission signals, which was attributed to less gas flow turbulence. Therefore the rectangular torch was chosen for further experiments. Another consideration of using the rectangular torch was that it was easier to interface it with a capillary column from a gas chromatograph. With a flow of make-up gas consisting of the plasma gas, the outlet flow of resolved analytes from the GC column was entrained into the discharge. This configuration rninimized the dead volume so that the high resolution provided by the capillary column would not be degraded by the plasma torch volume. One problem encountered in the initial work was that arcing occurred occasionally through air between the electrodes which often damaged the plasma torch even though quartz tubes enclosed the electrodes to isolate them from the air. The reason for this is believed to be due to the high voltage from the power supply. In order to make the torch more amendable to testing, a demountable version was designed such that different quartz tube and electrode dimensions could be evaluated. Also, the operating frequency was shifted from 27.12 to 13.56 MHz. One advantage of using the 13.56 MHz RF power supply was that the electrodes did not have to be enclosed in an insulator and could be exposed to air without external arcing around the outside of the torch because the power supply provides lower output voltage. The torch performance was evaluated for the determination of organotin compounds. One difficulty still encountered, however, was heating of the parallel plate electrodes which yielded unstable plasma conditions at times. As a result, a new version of the CCP torch was designed by utilizing water cooled electrodes. With this configuration and the 13.56 MHz power supply, the electrode could contact the discharge tube directly and the plasma was formed without arcing. The efficiency of power transfer to the plasma was increased significantly. Furthermore, the plasma created was extremely stable. Both metal and non-metal elements could be excited sufficiently using the modified configuration and excellent results have since been obtained. The detection limits for F, CI, Br, I, S, and C were determined to be 10 to 78 pg/s. The fulfillment of the second objective was carried out on the modified water-cooled plasma torch as mentioned above. Spatial distributions associated with the CCP which arise as a result of the transverse power coupling geometry were obtained. Some clues as to possible excitation processes in the CCP have been gleaned from the study of the relative intensities of lines for several non-metals. Basically, electron impact is the major excitation mechanism in the CCP. Excitations by the helium or argon metastables may play very a minor role. Charge transfer excitation reactions were found between He₂⁺ and CO and N₂.
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