气体流量与射频功率对电感耦合等离子体温度分布的影响

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

针对以电感耦合等离子体(ICP)为激发光源的分析仪器,研究ICP的温度空间分布对样品电离和激发具有重要意义。本研究建立了ICP的二维轴对称模型,利用有限元方法解算磁流体力学方程组,得到ICP的最高温度和温度空间分布,并研究了气体(辅助气、冷却气)流量和射频功率对ICP的最高温度和温度空间分布的影响。结果表明,辅助气、冷却气的流量及射频功率几乎不会改变ICP的最高温度(约10 000K),但会改变ICP的温度空间分布;辅助气流量的增大有助于ICP中心通道的形成,样品通过中心通道,有利于样品的原子化和电离;对于矩管直径为20mm的ICP,未通入样品时,运行时的射频功率在理论上不能超过1 600 W,而实际上空载功率可能更低。

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 localthermodynamic 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 Win theory,while the actual RF power may be lower.