Stabilization of spark discharge point on sample surface and enhancement of emission intensity by laser irradiation

提出了一种与激光照射相结合的分析用火花放电控制技术，其独特之处在于激光照射可以固定火花放电的发烧点。由于火花放电容易在特定的表面位置（如非金属夹杂物）开始，称为选择性放电，因此火花放电光学发射光谱法中某些元素的浓度有时会偏离其平均浓度。因此，稳定样品表面的点火点可以提高分析的准确度和精密度。当使用圆形钨笔和铁板作为计数器和样品电极时，在空气中这些距离为几mm的电极之间很难产生火花放电，在我们的实验条件下，需要更近的距离小于0.7 mm才能产生火花放电。通过引入激光照射，电极距离可延长至3.5 mm，同时保持稳定的火花放电。我们还将一个钨电极移动到距离激光诱导等离子体击穿位置2 mm以内的水平位置，无论火花通道的长度如何，都可以在触控电极和击穿位置之间保持火花通道。研究发现，激光照射可以控制火花放电的起始位置。采用火花放电光谱法，fei 373.5 nm的发射强度比单独使用火花放电光谱法的发射强度提高了1.5倍。这可能是由于除了单独通过火花放电获得的采样量外，激光烧蚀还增加了采样量。

A combined technique with laser irradiation is proposed to control spark discharge for analytical use, having a unique feature that firing points of the spark discharge can be fixed by laser irradiation. Because the spark discharge easily initiates at particular surface sites, such as non-metallic inclusions, called selective discharge, the concentration of some elements sometimes deviates from their average one in spark discharge optical emission spectrometry. Therefore, stabilization of firing points on a sample surface could improve the analytical accuracy and precision.When a tungsten stylus having a round-shaped tip and an iron plate were used as the counter and the sample electrodes, it was very difficult to induce spark discharges between these electrodes having distances of a few mm in air, and a closer spacing of less than 0.7 mm was required to generate the spark discharge under our experimental conditions. By introducing laser irradiation, the electrode distance could be extended up to 3.5 mm while keeping stable spark discharges.We also moved a tungsten stylus electrode to within 2 mm horizontally from the breakdown position of the laser-induced plasma The spark channel could be maintained between the stylus electrode and the breakdown position regardless of the length of the spark channels. It is found that the initiation site of a spark discharge can be controlled by laser irradiation.By using the present combined technique, emission intensity of Fe I 373.5 nm was enhanced 1.5 times brighter than the one obtained by a spark discharge spectrometry alone. This may be due to the additional increment of a sampling amount by laser ablation in addition to the one obtained by a spark discharge alone.