Double-Modulation Technique for a Multielemental Voigt Detector

Abstract

A double-modulation technique (modulation of light source and magnetic field) was applied to a multielemental detector in the Voigt configuration. This was intended to overcome the problem that even in the absence of analyte atoms source radiation leaks through the polarizers in the orthogonal configuration, resulting in an undesirable offset signal. Four hollow-cathode lamps as light sources were operated with sinusoidal currents at four different frequencies (several hundred Hz) while the magnetic field was modulated at 60 Hz. As a result, side-band waves appeared at f₀±60 Hz, where f₀ was the frequency of the light source modulation. The signals from a solar-blind photomultiplier tube were converted into a power spectrum in a frequency domain after a fast Fourier transformation (FFT), followed by the detection of each side-band peak. Thus, multielemental Voigt signals were selectively discriminated without interference of the offset signal peaks at the light modulation frequencies.