A new approach of quantitative analysis of liquid sample using laser ablation technique was developed. The liquid was immediately freezed using the mixture of dry ice and alcohol in weight ratio of 95% : 5%. As a result, an increase of the repulsion force from the sample surface will enable the generation of the laser-induced shock wave plasma which was difficult to carry out on liquid surface. The ice sample was then irradiated using Nd-YAG laser operated in its fundamental wavelength. In order to increase the signal to background ratio and to obtain a sharp atomic line spectra, helium gas was used instead of air. Dynamic characterization of the spatially integrated time profile of the Cu I 521.8 nm, Cu I 510.5 nm and Ha lines shows a shock excitation stage and cooling stage which is corresponded to our shock wave model even when the plasma was generated under atmospheric gas pressure. Further study of the time profile averaged temperature of the atmospheric plasma also shows an increase of temperature during the shock excitation stage followed by diminution of temperature during the cooling stage. An application of this technique was then applied to quantitative analysis of several liquid samples. A linear calibration curve which intercept at 0 point was obtained for all of the elements investigated in this study such as sodium, potassium, lithium, copper, silver, lead and aluminum. A detection limit of around 1 ppm was found for the above element. This new technique will contribute to a great extent of laser atomic emission spectrochemical analysis for liquid samples.
Hydrogen analysis of solid samples was made in high-pressure helium ambient gas by employing a modified configuration of the conventional laser induced breakdown spectroscopy (LIBS) method. Helium gas plasma and target plasma are generated together by focusing a single laser beam tightly above the target surface with simultaneously attacking the target in defocus mode. It is shown that the ablated hydrogen atoms from the target are immediately transported in the helium gas plasma region and excited by helium metastable atoms, which has long life time. This method can avoid the Stark broadening effect of hydrogen emission as well as overcome the undesirable mismatching effect, which is responsible for inefficient excitation of hydrogen in high-pressure region. It is further demonstrated that successful hydrogen analysis can be made at high pressure for sample with high boiling-point such as zircaloy, which is used as a container of uranium in nuclear power station.
A special technique for the modification of laser-induced breakdown spectroscopy (LIBS) has been developed to improve the spectral quality of hydrogen emission from solid sample in helium gas at atmospheric pressure. In this technique, the plasma was generated by focusing a fundamental Nd-YAG laser into a surrounding helium gas. The helium atoms excited to their metastable states would then serve to excite the atoms of the solid material vaporized by using another Nd-YAG laser. When properly synchronized, the resulted hydrogen emission line of H I 656.2 nm shows a dramatic improvement of emission intensity and spectral quality over what was obtained by conventional LIBS technique. This study further reveals that this improvement is mainly due to the role of metastable excited state in helium atom, which allows the delayed detection to be performed at a favorable moment when the charged particles responsible for the strong Stark broadening effect in the plasma have mostly disappeared.
An experimental study on gas analysis by means of laser-induced breakdown spectroscopy (LIBS) was conducted using a Nd-YAG laser (1,064 nm; 120 mJ, 8 ns) and helium host gas at atmospheric pressure on a sample of mixed water (H2O) and heavy water (D2O) in vapor form. It was shown that completely resolved hydrogen (Ha) and deuterium (Da) emission lines which are separated by only 0.179 nm could be obtained at a properly delayed detection time when the charged particles responsible for the strong Stark broadening effect in the plasma have mostly disappeared. It is argued that helium metastable excited state plays the important role in the hydrogen excitation process.
A Nd-YAG laser (1,064 nm, 120 mJ, 8 ns) was focused on various types of fossil samples, including fossilized buffalo horns (around 400,000 and 1 million years old, respectively) found in Sangiran, Indonesia. Such fossils represents an important starting point for tracing man’s origin and evolution during the Pleistocene era. Carbon emission was found to decrease significantly with the degree of fossilization and no carbon emission was found in a horn fossil dated at 1 million years. Some molecular band spectra were also found in all the fossils examined in this study. It was assumed that by combining information on carbon emission, hydrogen emission and molecular band spectra, that the degree of fossilization might be quantitatively calculated. Further results showed that silicon emission is not detected in old fossils, but it is present as a major constituent. This is probably due to the fact that silicon is strongly bound to other elements in old fossils and is ablated in the form of clusters. In order to prove the above hypothesis, a thin film of an old fossil was deposited on a silver plate substrate by means of a laser ablation technique. The resulting film was then irradiated and atomic emission lines of silicon were clearly detected. A comparative study of the low pressure plasma introduced in this study was conducted using the well known laser-induced breakdown spectroscopy (LIBS) technique and the results confirmed that operating conditions at atmospheric pressure are unfavorable for a fossil analysis.
An Nd-YAG laser (1,064 nm, 120 mJ, 8 ns) was focused on various types of solid organic samples such as a black acrylic plate (PMMA), a black poly-vinyl-chloride (PVC) plastic sheet and a Methoxy Polyaniline (M-PANI) film coated on the surface of a glass substrate, under a surrounding air pressure of 2 Torr. A modulated plasma technique was used to study the mechanism of excitation of the emission of the organic material. As a result, we conclude that ablated atoms and molecules are excited by a shock wave mechanism, similar to the case of hard samples such as metal. The ablation speed of hydrogen emission (H I 656.2 nm) was examined and the results show that the release speed of the ablated atoms is relatively low (less than Mach 10) and persists for a longer period of time (around 1ms); this phenomena can be understood by assuming that the soft target absorbs recoil energy, causing a low release speed of ablated atoms which would form the shock wave. This was overcome by placing a sub-target on the back of the soft sample so as to enhance the repelling force, thus increasing the release speed of the atoms. A possible application of the low pressure plasma on an organic solid was demonstrated in the detection of chlorine in a black poly-vinyl-chloride plastic sheet.
It is shown that remarkable improvements essential to quantitative spectrochemical analysis of hydrogen emissions from zircaloy samples were achieved when the low pressure surrounding air used in the previous experiment of Nd-YAG laser induced shockwave plasma was replaced by an inert gas. Using the high purity (99.999%) nitrogen gas at 1.5 Torr, a linear calibration curve of the H I 656.2 nm emission line was obtained with zero intercept from zircaloy samples prepared with various hydrogen concentrations. Further, when the surrounding nitrogen gas was replaced by helium gas, more than an order of magnitude enhancement was obtained on the signal to noise ratio, yielding a detection limit of less than 5 ppm.
An experiment was carried out to demonstrate the detection of a hydrogen emission line, H I 656.2 nm (Ha), in a plasma induced by a Q-switched Nd-YAG laser in a low pressure gas on various types of samples, such as zinc, a glass slide and a zircaloy tube. Contribution by surface water could be suppressed by a laser cleaning treatment and the resulting calibration curve obtained for zircaloy tube samples doped with various concentrations of hydrogen (0 ppm, 200 ppm, 540 ppm and 960 ppm) suggest potential applications to the quantitative analysis of hydrogen. A study of the dynamic process represented by the time profiles of the hydrogen emission, in comparison with those for zinc atomic emission, revealed a specific feature that is related to the small mass of hydrogen. This specific feature can be explained by the shock wave excitation mechanism in terms of new hypothetical process, namely a mismatch between the movement of ablated hydrogen atoms and the formation of the shock wave.