火焰原子吸收对K元素激光诱导击穿光谱测量的影响

当使用激光诱导击穿光谱技术（LIBS）测量火焰场内碱金属元素时，等离子体内碱金属原子所发出的LIBS信号，会受到等离子体外火焰中基态碱金属原子的吸收，影响测量精度。基于Beer Lambert定律和CH4 空气火焰场内气态含K物质的热力学平衡原理，建立了火焰场内K元素LIBS信号的原子吸收模型，并分析了实际生物质颗粒燃烧K元素释放浓度范围内，火焰气氛、K元素浓度分布以及总K浓度对火焰原子吸收效率的影响。研究发现，随着O2/CH4的摩尔比值的增加，火焰中热力学平衡状态下K原子占总K的比例从约25%逐步降低，火焰原子吸收效率也从86.8%逐步降低。当O2/CH4的摩尔比值大于2时，火焰尾气中会存在剩余O2，此时火焰内K原子的吸收效率均低于13%。同时，火焰中K元素浓度分布以及总K浓度的合理调整亦对火焰原子吸收效率具有降低作用。在此基础上，提出了创造氧化性气氛、调整K浓度分布来降低火焰原子吸收效率和提高LIBS测量精度的解决途径。

Influence of flame atomic absorption on measurement of K using laser-induced breakdown spectroscopy

When the measurement of alkali metals is performed using laser induced breakdown spectroscopy (LIBS) in flame，the emission of the alkali atoms in the plasma can be absorbed by the alkali atoms outside the plasma in the flame，influencing the LIBS measurement accuracy. Based on the Beer Lambert law and the thermodynamic equilibrium calculation，a flame atomic absorption model covering the concentration range of K released during practical biomass combustion was developed，and the influences of the atmosphere of the flame，the K distribution and the total K concentration on the flame atomic absorption efficiency were analyzed. It is found that，with the increase of the O2/CH4 ratio，the equilibrium molar fraction of atomic K in all K containing species decreases from approximate 25%，leading to the decrease of the flame atomic absorption efficiency from 86.8%. When the O2/CH4 molar ratio ratio exceeds 2，excess O2 exists in the flame, and the flame atomic absorption efficiency is always less than 13%. Meanwhile，by proper adjustment of K distribution and the total K concentration，the flame atomic absorption efficiency can also be reduced. Based on this，it is proposed that to reduce the flame atomic absorption efficiency and facilitate the K LIBS measurement accuracy in flame，an oxidizing flame atmosphere and a proper K distribution profile should be created.