Incompletely desolvated droplets in argon inductively coupled plasmas: their number, original size and effect on emission intensities

Abstract

The presence of incompletely desolvated droplets in the Ar ICP results in changes in Ca I emission intensity as large as a factor of 20 and variations in the Ca II emission intensity as great as a factor of 2.5. Most of the signal fluctuations are due to the effect of vaporizing droplets or their vaporization products on analyte atoms and ions produced from previously vaporized and atomized droplets. Above the normal analytical zone some peaks in the time-resolved analyte emission intensity result from vaporizing, desolvated particles. The number of incompletely desolvated droplets in the plasma varies from more than 15000 per second low (5 mm above the load coil) in the plasma to less than 100 per second high (above the normal analytical zone) in the plasma. The number of incompletely desolvated droplets in the plasma is a strong function of the center (nebulizer) gas flow rate as well as the applied power. Emission intensities from locations within about 1.4 mm of a vaporizing droplet are dramatically affected. Ca I emission intensities are enhanced near vaporizing droplets while Ca II emission intensities are depressed. The size of droplets at the exit of the spray chamber that produce detectable laser light scattering signals and emission intensity fluctuations are greater than about 12 μm in diameter. The size of droplets that are incompletely desolvated in the plasma depends on the applied power and center (nebulizer) gas flow rate. The results have far reaching implications on fundamental understanding of processes controlling emission intensities in Ar ICPs. Potential biases introduced into time-integrated measurements due to the presence of incompletely vaporized droplets must be considered.