Behaviour of analytical spectral lines emitted by plasma penciloperated in a continuous and in a pulsed mode

A modification of operating conditions by introduction of an admixture into the plasma gas stream was done for analytical application of plasma pencil – chemical analysis. Excitation capabilities using sets of standard water solutions containing Na+, Li+, Ca2+, Mg2+, Cu2+ and Zn2+ ions, construction of calibration dependences in the range 1–100 mg/l and calculation of instrumental detection limits for these elements (3-sigma) were done in the continuous and in the pulse mode. Aqueous solution of calibration standard (0.6 ml/min) was delivered into a Scott chamber with a concentric nebulizer with efficiency of about 6 %. The produced aerosol was introduced by argon (0.3 l/min) perpendicularly into the plasma pencil operated in flowing argon (4 l/min). Electrical input power in the range of 120–140 W coming into the plasma guaranteed a stability of the discharge with water and sample load. To create a pulsed mode a carrying wave 13.56 MHz was internally modulated by a rectangular wave 22 kHz with duty cycle 90 %. These modulation parameters guaranteed a high stability of the discharge. Rotational temperatures and electron number densities for the pencil under and without water load and sample load were calculated using OH spectra and H beta line and are very similar for both modes. Atomic lines of the elements above were well observable even if the rotational temperatures are relatively low about 1000 K. Intensities of the calibration lines in the continuous mode are lower than in the pulsed mode, linearity, range of linear concentrations and coefficients of determination of most of calibration lines are higher than in the continuous mode. Limits of detection are similar in both modes. Intensities of analytical signals, rotational temperatures and electron number densities along the discharge tube were acquired in 4 different positions in the range 11–15 cm from the aerosol entrance and an optimal position providing the best signal-to-noise ratio for each line intensity was established. These results indicate that the continuous mode should be rather replaced by the pulse mode in some analytical applications.