High sensitive determination by the use of laser multi-photon ionization.

1. 激光光电离电流分析。溶液中芳香分子的电流信号由电子的快信号和离子的慢信号组成。对于摩尔吸收率大的分子，电流信号大。它的激光波长依赖性（双光子电离光谱）显示出与吸收光谱相似的结构，但在较短波长区域，由于较大的过剩能量增加了电离效率。溶液中高灵敏度分析利用每个分子的双光子电离光谱确定了高灵敏度测定的最佳波长。对己烷中几种芳烃进行了高震检测。芘的检出限为20 ppt和337 nm，苝的检出限为50 ppt和383 nm，萘的检出限为20 ppt和407 nm。极性溶剂的暗电流大，检出限较低。利用染料激光器的二次谐波激发，对环境氮气中的芳香烃进行了检测。苯的检出限为9 ppb，甲苯为8 ppb，苯胺为0.8 ppb。采用热蒸发法测定固体芳烃。其他应用激光可以聚焦在一个小点上，方便应用于高性能液相色谱系统的检测器。此外，激光光电离技术还可以测定金属表面的少量芳香烃。已经对这些课题进行了初步实验。

1. Analysis of the laser photoionization current.The current signal of th aromatic molecules in solution consists of a fast signal of electrons and a slow signal of ionz. The current signal is large for those molecule with large molar absorptivities. Its laser wavelength dependence (two-photon ionization spectrum) shows structure similar to the absorption spectrum, but it increases in the shorter wavelength region due to the increase in the ionization efficiency for a larger excess energy.2. High sensitive analysis in solutionThe best wavength for a high sensitive determination has been decided by the use of the two-photon ionization spectrum of each molecule. The high seisitve detection of several aromatic hydrocarbons have been carried out in hexane. The detection limits and detection wavelengths are 20 ppt and 337 nm for pyrene, 50 ppt and 383 nm for perylene, and 20 ppt and 407 nm for naphthacene. The detection limits are considerably lower in polar solvents because of their large dark current.3. High sensitive analysis in gas phaseThe aromatic hydrocarbons in ambient nitrogen gas were detected with excitation by the second harmonics of the dye lasers. The detection limits are 9 ppb for benzene, 8 ppb for toluene, and 0.8 ppb for aniline. The solid aromatic hydrocarbons were determined by thermal evaporation techniques.4. Other applicationsThe laser can be focused in a small spot and is convenient for an application to a detector of a high-performance liquid chromatography system. Also, a small amount of aromatic hydrocarbons on metal surface can be determined by the laser photoionization technique. Preliminary experiments on these subjects have been carried out.