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

本文提出了一种与激光辐照相结合的控制火花放电的分析方法，其独特之处在于可以通过激光辐照来固定火花放电的点火点。由于火花放电很容易在特定的表面位置，如非金属夹杂物，称为选择性放电，有时会偏离火花放电光学发射光谱中的一些元素的浓度的平均值。因此，通过使试样表面的燃点稳定化，可以提高分析精度和精密度。在使用具有圆形尖端的钨探针和铁板作为对电极和试样电极时，在空气中几mm距离的电极之间很难诱发火花放电，并且在我们的实验条件下需要小于0.7mm的更近的间距来产生火花放电。通过引入激光照射，电极间距可以扩展到3.5mm，同时保持稳定的火花放电，我们还将钨针电极水平移动到距离激光诱导等离子体击穿位置2mm以内，无论火花通道的长度如何，都可以在针电极和击穿位置之间保持火花通道。结果表明，火花放电的起始位置可以通过激光辐照来控制，采用该组合技术，Fe I 373.5nm的发射强度比单独用火花放电光谱法获得的发射强度提高了1.5倍。这可能是由于除了通过单独的火花放电获得的采样量之外，通过激光烧蚀获得的采样量的附加增量。

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.