Sub-target effect in film analysis using TEA CO2 laser-induced plasma

A TEA CO2 laser beam (500 mJ, 200 ns) was focused on film samples, under low pressure surrounding gas at around 1 Torr. It has been found that to generate the laser plasma for the sample of oil or powder, the sample should be attached in the form of thin film on the surface of a metal, such as copper plate, which acts as a sub-target. The plasma has favorable characteristics for spectrochemical analysis due to its low background and sharp line spectrum, and hence an optical multichannel analyzer (OMA) without gated function can be used for spectrum acquisition. Using the sub-target effect we have performed for analyses on water molecular layer and water impurities, where the water was condensed by heating process or electrolysis process on the sub-target so that impurities in the water were attached to the metal surface. It should be emphasized that in this case the sub-target itself has never been ablated and no any damaged on its surface. Another application of the sub-target effect is used for the analysis of oil contamination in soils. We have succeeded to detect clearly the emission line of C I 247.9 nm from the carbon as a major constituent of the oil. To derive the carbon emission intensity coming only from oil, compensation was made to cancel the contribution from other organic species using the emission of Ca, which inherently contains in other organic species in soil. As result, a good linear relationship between carbon emission intensity and oil concentration was obtained.