A PHOTON-COUNTING DYNAMIC DIGITAL LOCK-IN AMPLIFIER FOR BACKGROUND SUPPRESSION IN GLOW-DISCHARGE ATOMIC-EMISSION SPECTROMETRY

A photon counting dynamic digital lock-in amplifier, (PC-DDLIA), has b een developed for the suppression of Ar lines in glow discharge lamp a tomic emission spectrometry, (GDL-AES). The experimental set-up consis ts of a Grimm-type GDL, a prism-type scanning monochromator, photon co unting electronics, an Apple Ile computer with an interface card and a computer controllable high voltage power supply. The photon counting electronics are designed to convert the photon pulses to logic pulses. A discriminator is used to reject pulses below a threshold level. The high voltage power supply is modulated with a square waveform generat ed from DAC and photon pulses are counted synchronously by the timer/c ounter chip, versatile interface adaptor (VIA-6522) on the interface c ard of computer. The data are analyzed in two steps. In the ''learn mo de'', the GDL is modulated with a square waveform between 370 and 670 V and two spectra consisting of only Ar lines are obtained in a spectr al window between 287.1 and 290.0 nm. A new modulation waveform is com puted from these spectra which yields two overlapped spectra when the PC-DDLIA is scanned over the same spectral window. In the ''analysis m ode'' of data acquisition, a target material with the analyte element( s) in it is used and the spectrometer is scanned with a dynamically va rying rectangular waveform over the same spectral window. The net spec trum consists of pure atomic lines free from any Ar lines. The detecti on limit for the determination of Si (288.2 nm) in the presence of int erfering hr lines (288.1 and 288.4 nm) is found to be 0.083%, whereas suppression of Ar lines over the same spectral window lowers the detec tion limit to 0.013%.