In 1969, Spectrametrics introduced the first commercially available direct current plasma system which used a right angle plasma as an excitation source and a crossed prism echelle spectrometer as the analytical detector. This system, known as the SpectraJet I, suffered from poor light though put as well as a substantial interference of the analyte signal by the plasma background column due to the right angle nature of the arc geometry. The dc argon plasma is initiated by arcing a high-voltage spark from cathode to anode. Once ignited, the plasma is sustained by a relatively low voltage, about 50 volts with a current of about seven ampers. Samples are nebulized and introduced into the excitation area in aerosol form.
Supplementary argon flows around the cathode and anode to maintain a stable discharge.
It is important to note that when this source is used with the echelle system, only light from the excitation region is focused into the spectrometer. The intense continuum from the plasma column and the plume is not observed. This design resulted in improved stability and better detection limits than previously reported for dc plasmas. The sample excitation region and photometric observation area of the SpectraJet III is centered uniquely in the crook of the Y where spectral contribution from the plasma continuum is minimal.