Effect of Gas Volume Flow Rate and RF Power on the Temperature Distribution of Inductively Coupled Plasma

Mass spectrometer which takes inductively coupled plasma (ICP) as ion source has been widely used in the field of chemical elemental analysis. ICP has become the most popular ion source because of its high sensitivity, precision and repeatability. The process of ICP experiments showed that the temperature of ICP is difficult to measure directly, because the effect of gas flow rate and power of temperature distribution can not been obtained. The numerical simulation method and the finite element method were adopted to establish the two-dimensional axisymmetric magnetohydrodynamics model of ICP, which was used to study the temperature characteristics of plasma at different gas flow rates and RF power. Assuming that the ICP was in the state of local thermodynamic equilibrium(LTE), because the plasma could be considered as a kind of conductive fluid. The plasma satisfied the mass continuity equation, the momentum equation, the energy conservation equation and the Maxwell equation systems. And the hydrodynamic equation and the Maxwell equation were coupled by the Lorentz force. Ignoring its turbulence effect, the plasma was considered to be optically thin. And ignoring the energy dissipation and the pressure work in the energy equation, the flow of the plasma was considered to be laminar flow. The temperature spatial distribution of ICP under different conditions had been obtained by calculating the equation systems of magnetohydrodynamics. The results showed that the auxiliary gas, the cooling air flow rate and the RF power can hardly change the maximum temperature (about 10 000 K) of the ICP, but they can change the temperature spatial distribution of the ICP. The increase of auxiliary gas flow rate can help the formation of the ICP central channel to allow the sample to stay in the plasma for a longer period of time, which is conducive to the atomization and ionization of the sample and is extremely favorable for the sample analysis. Before entering samples，the plasma formed in the rectangle tube of 20 mm diameter, the RF power should not be greater than 1 600 W in theory, while the actual RF power may be lower.