EFFECT OF ARGON PRESSURE ON SPECTRAL EMISSION OF A PLASMA PRODUCED BY A LASER MICROPROBE

The effect of an argon atmosphere on the self-absorption of emission spectra of a laser-induced plasma was studied with the use of a normal laser microprobe by changing the gas pressure. The degree of self-absorption was determined from the observed profiles of copper resonance lines using the curve-fitting method with the assumption of a Lorentzian profile. According to the results, self-absorption was reduced by decreasing the argon pressure and was eliminated at low pressures, the range of which depends on the analyte concentration. Studies of the spatially resolved spectrum show that the confining effect of the plasma by the argon atmosphere becomes effective at higher pressures, resulting in an increase in the emission intensity. As a result, there exists a moderate pressure at which self-absorption can be eliminated without losing too much intensity from reduction of confinement. At a pressure of around 150 Torr (1 Torr = 133.3 Pa) high emission intensity of the spectral line virtually free from self-absorption was obtained about 1.5 mm above the sample surface for the determination of copper (concentration 1-9.8%) in aluminium samples, and a linear calibration graph with a slope of unity was obtained.