Application of modulation techniques to atomic emission spectrometry with inductively-coupled radio-frequency plasma and radio-frequency glow discharge plasma

Three novel measuring systems for modulation technique to be applied to atomic emission spectrometry are reviewed. A plasma gas modulation technique is employed for an improvement in the detection sensitivity in inductively-coupled radio-frequency plasma atomic emission spectrometry (ICP-AES). Both an amplitude modulation technique and a bias current modulation technique are effective for obtaining better analytical performance in radio-frequency glow discharge optical emission spectrometry (GD-OES). These techniques produce cyclically-changing components in the emission signals by modulating the excitation processes occurring in each plasma. The detection with a lock-in amplifier plays an important role in these measuring systems; the modulated component of the emission intensities can be selectively detected whereas any noises are completely removed. Furthermore, the background levels are generally less effected by the plasma modulation. These benefit yield the emission signals having larger signal-to-noise ratios as well as larger signal-to-background ratios and therefore the analytical results with better precision and sensitivity.