MONTE-CARLO OPTIMIZATION OF SIGNAL-TO-NOISE RATIO FOR ELECTROTHERMAL ATOMIZERS

This work uses Monte Carlo simulation models for Cu and Ag to study the change in the signal-to-noise ratio (SIN) with variations on the electrothermal atomizer length and diameter. A 5 x 5 grid corresponding to different lengths (2.0-4.0 cm) and diameters (0.30-0.70 cm) is filled with S/N data from the simulations. It is assumed that the S/N is shot-noise limited. The dosing hole diameter is held at 1.5 mm in all cases. A double-desorption type mechanism for Ag atomization was tested against experimental profiles and good agreement was achieved. Spherical microdroplets release Ag(g) with a fractional order (2/3) and an activation energy of 238 kJ mol-1. Ag(g) readsorbs as dispersed atoms on the wall upon collision. The secondary desorption is first order and has a lower activation barrier (117 kJ mol-1). The S/N predicted by the Monte Carlo simulation does not strongly depend on the heating rate or the nature of the analyte being determined. The optimum with respect lo geometry is broad. The optimum furnace diameter is near 0.50 cm. while the optimum furnace length is at the 4.0-cm limit used in these studies. Most commercial atomizers have diameters close to the optimum but shorter lengths, probably reflecting the pragmatic consideration that the longer tubes require larger power supplies and prolonged heating times which may substantially reduce the useful life of the furnace. Further increases in length may also accentuate atomizer non-isothermality which deters analytical improvement.