Effect of the duty ratio on FFT power spectrum of the emission signal excited by square-wave-pulsed glow discharge plasma

An advanced data-processing method using a fast Fourier transform (FFT) analyzer, associated with a modulation of DC voltage applied to a plasma excitation source, is suggested in glow discharge optical emission spectrometry (GD-OES). A pulsation of the DC voltage can be employed effectively for analysis of several specified samples, such as a ultra-thin layer to suppress the sputtering rate; however, it might worsen the analytical performance due to a decrease in the emission intensity. The FFT analyzer has an ability to disperse electronic signals by frequency to yield a power spectrum, and it is thus employed to select the component at a particular frequency. In pulsed GD-OES, data analysis based on FFT algorism enabled the signal component of an analyte emission line to be separated at a pulse frequency from the overall emission signal having any other frequencies, when the pulsed discharge voltage could control the sampling and excitation processes of the analyte in the plasma. The power spectrum of a Cu I emission line strongly depended on the duty ratio of the pulsed DC voltage, because each signal component at higher-order overtone frequencies contributed to the spectrum in a different manner. At a duty ratio of 20%, a summation of the FFT components from the fundamental frequency to higher-order harmonic frequencies could be effective to obtain larger signal intensities with smaller variances. The FFT component of the Cu I signal summed up to the eighth harmonic frequency was 3.8-times as large as the component of the fundamental frequency, whereas the RSD was improved from 2.32% to 0.20%.