Investigation of Sample Atomization Using a Power-Modulated Radio-Frequency Glow Discharge Source

A power-modulated (pulsed), radio-frequency glow discharge source by atomic absorption spectrophotometry (rf-GD-AAS) atomizer was used to evaluate the roles of applied power, pressure, orifice diameter, duty cycle, and power-on time in the production of gas-phase sample atoms. As expected, the response of the modulated rf-GD-AAS source generally followed the same trends as those exhibited by the more common continuous-powering scheme. The effects of discharge power and pressure on the observed plasma emission and absorbance transients are presented. Use of small duty cycles and higher instantaneous powers, thus keeping the same overall average power as in the continuous mode, was shown to increase the production of ground-state atoms. However, using very high instantaneous powers may more efficiently populate excited states of the atomic species, thus decreasing the observed absorption signal for resonant transitions. Individual pulse transients were shown to be distorted if the plasma “on” times approached periods down to 2 ms. Plasma stabilization times for measurements taken in the “dark” portion of the pulse cycle (i.e., after pulse termination) were comparable to those obtained in the continuous mode (on the order of a few seconds) with the use of the same source and sample. Calibration curves were used to investigate the analytical utility of different temporal regions of the absorption transients with comparisons made between the plasma “on” and “off” portions of the cycle in the quantification of continuous plasma operation.