Study of a toroidal argon and a cylindrical helium microwave induced plasma for analytical atomic emission spectrometry-I. Configurations and spectroscopic properties

Abstract

The instrumental and operating conditions are described under which a toroidal argon and a diffuse helium low power microwave induced plasma (MIP) discharge can be realized in a TM 010 resonator. When the MIP is coupled to graphite furnace atomization and a quartz tube with 4 and 5mm internal diameter is used, a toroidal argon MIP can be sustained at 85 and 100 W and with 1.6 and 2.71/min outer gas + wetted carrier gas, respectively. In the case of pneumatic nebulization with a Meinhard nebulizer operated at 1.21/min, no outer gas is used and the toroidal MIP can be sustained in a quartz capillary of 4mm internal diameter at 110W. A diffuse helium MIP can be sustained in a quartz capillary with 4–5 mm internal diameter, a total helium flow of 0.1–0.21/min and a microwave power in the range of 120–180 W. These values apply to the use of graphite furnace evaporation but also to aerosols generated by pneumatic nebulization which pass through a heated graphite furnace. The intensities of OH-bands, argon and analyte lines are minimum in the centre of the toroidal argon MIP. However, the signal-to-background ratios (SBR) for an iron atom line, for instance, are maximum in the centre. In the case of the diffuse helium plasma all species have maximum intensities in the centre of the discharge. Therefore, the SBR for an iron atomic line is fairly constant over the whole discharge diameter. Rotational temperatures measured from the intensity distribution of the rotational lines in the OH bands for the case of a 100 W toroidal argon MIP are 2000–2700 K and are minimum at the centre of the discharge. In the case of a 150 W helium MIP, however, the rotational temperatures (2200–2600 K) are maximum in the centre of the discharge.