Simple defocus laser irradiation to suppress self-absorption in laser-induced breakdown spectroscopy (LIBS).

This study introduces a novel and simple way to suppress the self-absorption effect in laser-induced breakdown spectroscopy (LIBS) by utilizing a defocusing laser irradiation technique. For this purpose, a Nd:YAG laser with a wavelength of 1,064 nm and repetition rate of 10 Hz with energy in the range of 10 mJ-50 mJ was used. The laser irradiation was focused by using a 150-mm-focal-length plano-convex lens onto the sample surface under defocusing of approximately -6 mm. Potassium chloride (KCl) and sodium chloride (NaCl) pellet samples were used to demonstrate this achievement. When the defocus position is adjusted to -6 mm for KCl and NaCl samples, the self-reversal in the emission lines of K I 766.4 nm, K I 769.9 nm, Na I 588.9 nm, and Na I 589.5 nm vanish. Meanwhile, the FWHM values of K I 766.4 and K I 769.9 nm are 0.29 nm and 0.23 nm, respectively, during -6 mm defocus laser irradiation, as opposed to 1.24 nm and 0.86 nm under tight focus laser irradiation. Additionally, this work demonstrates that, when the laser energy is changed between 10 and 50 mJ, no self-reversal occurs in the emission lines when -6 mm defocus laser irradiation is applied. Finally, a linear calibration curve was generated using KCl at a high concentration ranging between K concentrations from 16.6% to 29%. It should be noted that, even at such high K concentrations, the calibration curve is still linear. This means that self-absorption is almost negligible. This simple change in defocus laser irradiation will undoubtedly contribute to the suppression of the self-absorption phenomenon, which disrupts LIBS analytical results.

High sensitivity hydrogen analysis in zircaloy-4 using helium-assisted excitation laser-induced breakdown spectroscopy.

High-sensitivity detection of hydrogen (H) contained in zircaloy-4, a commonly used material for nuclear fuel containers, is crucial in a nuclear power plant. Currently, H detection is performed via gas chromatography, which is an offline and destructive method. In this study, we developed a technique based on metastable excited-state He-assisted excitation to achieve excellent quality of H emission spectra in double-pulse orthogonal laser-induced breakdown spectroscopy (LIBS). The production of metastable excited-state He atoms is optimized by using LiF as sub-target material. The results show a narrow full-width-at-half-maximum of 0.5 Å for the H I 656.2 nm emission line, with a detection limit as low as 0.51 mg/kg. Thus, using this novel online method, H in zircaloy-4 can be detected efficiently, even at very low concentrations.

Unusual parallel laser irradiation for suppressing self-absorption in single pulse laser-induced breakdown spectroscopy.

This study demonstrates a new approach for suppressing the self-absorption effect in single-pulse laser-induced breakdown spectroscopy (LIBS) using unusual parallel laser irradiation. A nanosecond Nd:YAG laser with a wavelength of 1064 nm was fired parallel to and focused at a very close distance of 1 mm to the sample surface. The experiment was carried out in air at atmospheric pressure. In this configuration, the sample was ablated by a shockwave generated from the air breakdown plasma formed near the sample surface. Under this condition, we successfully obtained spectra of the resonance emission line for high concentration K (K I 766.4 nm and K I 769.9 nm) that are free from self-reversal and weakly affected by the self-absorption. Furthermore, the quantitative analysis results for the element K showed that a linear calibration curve over a wide concentration range could be achieved, which indicates the effectiveness of this technique in reducing the self-absorption effect and improving the analytical performance of ordinary single-pulse LIBS.

Suppression of self-absorption in laser-induced breakdown spectroscopy using a double pulse orthogonal configuration to create vacuum-like conditions in atmospheric air pressure.

Self-absorption, which is known to severely disturb identification of the emission peak intensity in emission-based spectroscopy, was first studied using ordinary single pulse laser-induced breakdown spectroscopy (LIBS). It was found that severe self-absorption, with an evident self-reversal, occurs in the resonance emission lines of high concentration Na, K, and Al, and thus it is impossible to obtain the linear calibration curve required for quantitative analysis. To overcome this problem, we introduce a double pulse orthogonal technique in which the first laser is fired in a parallel orientation at a varied distance of 2-6 mm from the sample surface. It is well known that the strong shock wave generated by this laser irradiation temporarily creates a vacuum-like condition immediately in front of the sample surface. This action is followed by a second laser irradiation oriented perpendicular to the sample surface. The sample ablated by the second laser irradiation expands following the shockwave excitation process in the vacuum-like air atmosphere created by the first laser. The obtained spectra of the resonance emission lines of high concentration Na, K, and Al are free from the self-reversal and weakly affected by the self-absorption effect. A linear calibration curve that intercepts near zero point for K element over a wide concentration range is also demonstrated in this study. This simple modification is considered notably helpful in overcoming the self-absorption that occurs in ordinary single pulse atmospheric pressure LIBS.

Suppression of self-absorption effect in laser-induced breakdown spectroscopy by employing a Penning-like energy transfer process in helium ambient gas.

A unique approach for achieving total suppression of the self-absorption effect in laser-induced breakdown spectroscopy (LIBS) has been demonstrated employing a previously published technique of laser-induced plasma spectroscopy utilizing a helium (He) metastable excited state (LIPS-He*).This achievement was attained by the use of the He metastable excited state (He*) and a Penning-like energy transfer mechanism for the delayed excitation of the ablated analyte atoms. KCl and NaCl samples showed the disappearance of the self-absorption emission lines of K I 766.4 nm, K I 769.9 nm, Na I 588.9 nm, and Na I 589.5 nm, and the FWHM values of K I 766.4 and Na I 588.9 nm were found to be 0.8 nm and 0.15 nm, respectively, by LIPS-He* as compared to 4.8 nm and 1.4 nm, respectively, by single-laser operation. A standard Al sample also showed the total disappearance of the self-absorption emission lines Al I 394.4 nm and Al I 396.1 nm. The FWHM of Al I 396.1 nm was 0.12 nm when LIPS-He* was employed compared to 0.44 nm when a single laser was used. A remarkable linear calibration line with zero intercepts was also obtained for high-concentration Al samples (87.0%, 93.0% and 99.8%). Thus, it is established that the self-absorption effect can be completely neglected when excitation through He* is employed in LIBS.

Underlying Physical Process for the Unusual Spectral Quality of Double Pulse Laser Spectroscopy in He Gas.

This study is aimed at elucidating the physical processes responsible for the excellent spectral qualities in terms of full width at half-maximum (fwhm) and signal-to-noise (S/N) ratio shown in a special double pulse laser-induced spectroscopy. Apart from the use of atmospheric He ambient gas, the achievement is due to the first laser for generating He gas plasma and the subsequent use of the second laser pulse for target ablation, in opposite order of the two-laser operations in conventional double pulse LIBS. This setup allows adjustments of the many experimental parameters to yield the optimal condition resulting in 0.03 nm fwhm and around 1000× S/N ratio of Cu I 521.8 nm and far surpasses the spectral qualities obtained by other techniques. This is obtained by allowing the crucial separation of the target plasma from the He gas plasma and thereby enabling the He-assisted excitation (HAE) to play its full and unique role of nonthermal excitation, taking advantage of metastable excited He atoms in the He plasma and the Penning-like energy transfer process. This excellent performance is further verified by its successful application analysis of Cr in low alloy steel samples, with the presence of smooth linear calibration lines, signifying the absence of the self-absorption effect well-known in ordinary LIBS.

H-D Analysis Employing Low-Pressure microjoule Picosecond Laser-Induced Breakdown Spectroscopy.

An experimental study is conducted in search of the much needed experimental method for practical and minimally destructive analysis of hydrogen (H) and deuterium (D) in a nuclear power plant. For this purpose, a picosecond (ps) Nd:YAG laser is employed and operated with 300-500 µJ output energies in a variety of ambient gases at various gas pressures. The sample chamber used is specially designed small quartz tube with an open end that can be tightly fitted to the sample surface. It is found that ambient Ar gas at reduced pressure of around 0.13 kPa gives the best spectral quality featuring fully resolved H and D emission lines with clearly detectable intensities and practically free from surface water interference. The D emission intensities measured from zircaloy plates containing various concentrations of D impurity are shown to yield a linear calibration line with extrapolated zero intercept, offering its potential application to quantitative analysis. The estimated detection limit of less than 10 ppm is well below the sensitivity limit of around 600 ppm required for the regular inspection of zircaloy tubes in a heavy water nuclear power plant. The use of the exceedingly low laser energy is shown to offer an additional advantage of minimum destructive effect marked by the resulted tiny craters of about 5 µm diameter with 25 µm depth. These results promise the potential development of the desired alternative analytical tool for regular in situ and real time inspection of the zircaloy tubes in a heavy water power plant.

Practical and highly sensitive elemental analysis for aqueous samples containing metal impurities employing electrodeposition on indium-tin oxide film samples and laser-induced shock wave plasma in low-pressure helium gas.

We have conducted an experimental study exploring the possible application of laser-induced breakdown spectroscopy (LIBS) for practical and highly sensitive detection of metal impurities in water. The spectrochemical measurements were carried out by means of a 355 nm Nd-YAG laser within N2 and He gas at atmospheric pressures as high as 2 kPa. The aqueous samples were prepared as thin films deposited on indium-tin oxide (ITO) glass by an electrolysis process. The resulting emission spectra suggest that concentrations at parts per billion levels may be achieved for a variety of metal impurities, and it is hence potentially feasible for rapid inspection of water quality in the semiconductor and pharmaceutical industries, as well as for cooling water inspection for possible leakage of radioactivity in nuclear power plants. In view of its relative simplicity, this LIBS equipment offers a practical and less costly alternative to the standard use of inductively coupled plasma-mass spectrometry (ICP-MS) for water samples, and its further potential for in situ and mobile applications.

Spectral and dynamic characteristics of helium plasma emission and its effect on a laser-ablated target emission in a double-pulse laser-induced breakdown spectroscopy (LIBS) experiment.

A systematic study has been performed on the spectral characteristics of the full spectrum of He emission lines and their time-dependent behaviors measured from the He gas plasmas generated by a nanosecond neodymium-doped yttrium aluminum garnet laser. It is shown that among the major emission lines observed, the triplet He(I) 587.6 nm emission line stands out as the most prominent and long-lasting line, associated with de-excitation of the metastable triplet (S = 1) excited state (1s(1) 3d(1)). The role of this metastable excited state is manifested in the intensity enhancement and prolonged life time of the Cu emission with narrow full width half-maximum, as demonstrated in an orthogonal double-pulse experiment using a picosecond laser for the target ablation and a nanosecond laser for the prior generation of the ambient He gas plasma. These desirable emission features are in dire contrast to the characteristics of emission spectra observed with N2 ambient gas having no metastable excited state, which exhibit an initial Stark broadening effect and rapid intensity diminution typical to thermal shock wave-induced emission. The aforementioned He metastable excited state is therefore responsible for the demonstrated favorable features. The advantage of using He ambient gas in the double-pulse setup is further confirmed by the emission spectra measured from a variety of samples. The results of this study have thus shown the potential of extending the existing laser-induced breakdown spectroscopy application to high-sensitivity and high-resolution spectrochemical analysis of wide-ranging samples with minimal destructive effect on the sample surface.

A comparative study of pressure-dependent emission characteristics in different gas plasmas induced by nanosecond and picosecond neodymium-doped yttrium aluminum garnet (Nd:YAG) lasers.

An experimental study has been performed on the pressure-dependent plasma emission intensities in Ar, He, and N2 surrounding gases with the plasma induced by either nanosecond (ns) or picosecond (ps) yttrium aluminum garnet laser. The study focused on emission lines of light elements such as H, C, O, and a moderately heavy element of Ca from an agate target. The result shows widely different pressure effects among the different emission lines, which further vary with the surrounding gases used and also with the different ablation laser employed. It was found that most of the maximum emission intensities can be achieved in Ar gas plasma generated by ps laser at low gas pressure of around 5 Torr. This experimental condition is particularly useful for spectrochemical analysis of light elements such as H, C, and O, which are known to suffer from intensity diminution at higher gas pressures. Further measurements of the spatial distribution and time profiles of the emission intensities of H I 656.2 nm and Ca II 396.8 nm reveal the similar role of shock wave excitation for the emission in both ns and ps laser-induced plasmas, while an additional early spike is observed in the plasma generated by the ps laser. The suggested preference of Ar surrounding gas and ps laser was further demonstrated by outperforming the ns laser in their applications to depth profiling of the H emission intensity and offering the prospect for the development of three-dimensional analysis of a light element such as H and C.

Quantitative deuterium analysis of titanium samples in ultraviolet laser-induced low-pressure helium plasma.

An experimental study of ultraviolet (UV) laser-induced plasma spectroscopy (LIPS) on Ti samples with low-pressure surrounding He gas has been carried out to demonstrate its applicability to quantitative micro-analysis of deuterium impurities in titanium without the spectral interference from the ubiquitous surface water. This was achieved by adopting the optimal experimental condition ascertained in this study, which is specified by 5 mJ laser energy, 10 Torr helium pressure, and 1-50 mus measurement window, which resulted in consistent D emission enhancement and effective elimination of spectral interference from surface water. As a result, a linear calibration line exhibiting a zero intercept was obtained from Ti samples doped with various D impurity concentrations. An additional measurement also yielded a detection limit of about 40 ppm for D impurity, well below the acceptable threshold of damaging H concentration in Ti and its alloys. Each of these measurements was found to produce a crater size of only 25 mum in diameter, and they may therefore qualify as nondestructive measurements. The result of this study has therefore paved the way for conducting further experiments with hydrogen-doped Ti samples and the technical implementation of quantitative micro-analysis of detrimental hydrogen impurity in Ti metal and its alloys, which is the ultimate goal of this study.

New technique for the direct analysis of food powders confined in a small hole using transversely excited atmospheric CO(2) laser-induced gas plasma.

Taking advantage of the differences between the interactions of transversely excited atmospheric (TEA) CO(2) lasers with metal and with organic powder, a new technique for the direct analysis of food powder samples has been developed. In this technique, the powder samples were placed into a small hole with a diameter of 2 mm and a depth of 3 mm and covered by a metal mesh. The TEA CO(2) laser (1500 mJ, 200 ns) was focused on the powder sample surfaces, passing through the metal mesh, at atmospheric pressure in nitrogen gas. It is hypothesized that the small hole functions to confine the powder particles and suppresses the blowing-off of sample, while the metal mesh works as the source of electrons to initiate the strong gas breakdown plasma. The confined powder particles are then ablated by laser irradiation and the ablated particles move into the strong gas breakdown plasma region to be atomized and excited; this method cannot be applied for the case of Nd:YAG lasers because in such case the metal mesh itself was ablated by the laser irradiation. A quantitative analysis of a milk powder sample containing different concentrations of Ca was successfully demonstrated, resulting in a good linear calibration curve with high precision.

Study of hydrogen and deuterium emission characteristics in laser-induced low-pressure helium plasma for the suppression of surface water contamination.

An experimental study was conducted in search of the experimental condition required for the much needed suppression of spectral interference caused by surface water in hydrogen analysis using laser-induced low-pressure helium plasma spectroscopy. The problem arising from the difficulty in distinguishing hydrogen emission from hydrogen impurity inside the sample and that coming from the water molecules was overcome by taking advantage of similar emission characteristics shared by hydrogen and deuterium demonstrated in this experiment by the distinct time-dependent and pressure-dependent variations of the D and H emission intensities from the D-doped zircaloy-4 samples. This similarity allows the study of H impurity emission in terms of D emission from the D-doped samples and thereby separating it from the H emission originating from the water molecules. Employing this strategy has allowed us to achieve the large suppression of water induced spectral interference from the previous minimum of 400 microg/g to the current value of 30 microg/g when a laser beam of 34 mJ under tight focusing condition was employed. Along with this favorable result, this experimental condition has also provided a much better (about 6-fold higher) spatial resolution, although these results were achieved at the expense of reducing the linear calibration range from the previous 4 300 microg/g to the present 200 microg/g.

Quantitative hydrogen analysis of zircaloy-4 in laser-induced breakdown spectroscopy with ambient helium gas.

This experiment was carried out to address the need for overcoming the difficulties encountered in hydrogen analysis by means of plasma emission spectroscopy in atmospheric ambient gas. The result of this study on zircaloy-4 samples from a nuclear power plant demonstrates the possibility of attaining a very sharp emission line from impure hydrogen with a very low background and practical elimination of spectral contamination of hydrogen emission arising from surface water and water vapor in atmospheric ambient gas. This was achieved by employing ultrapure ambient helium gas as well as the proper defocusing of the laser irradiation and a large number of repeated precleaning laser shots at the same spot of the sample surface. Further adjustment of the gating time has led to significant reduction of spectral width and improvement of detection sensitivity to ~50 ppm. Finally, a linear calibration curve was also obtained for the zircaloy-4 samples with zero intercept. These results demonstrate the feasibility of this technique for practical in situ and quantitative analysis of hydrogen impurity in zircaloy-4 tubes used in a light water nuclear power plant.

Quantitative hydrogen analysis of zircaloy-4 using low-pressure laser plasma technique.

It is found in this work that variation of laser power density in low-pressure plasma spectrochemical analysis of hydrogen affects sensitively the hydrogen emission intensity from the unwanted and yet ubiquitous presence of ambient water. A special experimental setup has been devised to allow the simple condition of focusing/defocusing the laser beam on the sample surface. When applied to zircaloy-4 samples prepared with various hydrogen impurity concentrations using low-pressure helium surrounding gas, good-quality hydrogen emission lines of very high signal to background ratios were obtained with high reproducibility under weakly focused or largely defocused laser irradiation. These measurements resulted in a linear calibration line with nonzero intercept representing the residual contribution from the recalcitrant water molecules. It was further shown that this can be evaluated and taken into account by means of the measured intensity ratio between the oxygen and zirconium emission lines. We have demonstrated the applicability of this experimental approach for quantitative determination of hydrogen impurity concentrations in the samples considered.

Quantitative analysis of deuterium using laser-induced plasma at low pressure of helium.

It was proved that the analysis of deuterium can be conducted using laser-induced plasma spectroscopy. By selecting the appropriate surrounding gas, its pressure, and gating time of the detection system, it was shown that the emission lines of both hydrogen (H(alpha)) and deuterium (D(alpha)), separated by only 0.179 nm, can be fully resolved. A linear calibration curve was also obtained, indicating that this technique has the potential for quantitative analysis of deuterium. The minimum detection limit achieved in this stage of research was estimated to be 50 ppm. We have also shown that this technique can be used as a simple and rapid method for D and H analysis in solid samples.

Measurement of concrete strength using the emission intensity ratio between Ca(II) 396.8 nm and Ca(I) 422.6 nm in a Nd:YAG laser-induced plasma.

An experiment to investigate the potential of a laser-induced plasma method for determining concrete compressive strength was conducted by focusing a Nd:YAG laser on concrete samples with different degrees of compressive strength. This technique was developed in light of the role of the shock wave in the generation of a laser-induced plasma. It was found that the speed of the shock front depends on the hardness of the sample. It was also found that a positive relationship exists between the speed of the shock front and the ionization rate of the ablated atoms. Hence, the ratio of the intensity between the Ca(II) 396.8 nm and Ca(I) 422.6 nm emission lines detected from the laser-induced plasma can be used to examine the hardness of the material. In fact, it was observed that the ratio changes with respect to the change in the concrete compressive strength. The findings also show that the ratio increases with time after the cement is mixed with water.

Atomic hydrogen emission induced by TEA CO(2) laser bombardment on solid samples at low pressure and its analytical application.

Hydrogen emission has been studied in laser plasmas by focusing a TEA CO(2) laser (10.6 microm, 500 mJ, 200 ns) on various types of samples, such as glass, quartz, black plastic sheet, and oil on copper plate sub-target. It was found that H(alpha) emission with a narrow spectral width occurs with high efficiency when the laser plasma is produced in the low-pressure region. On the contrary, the conventional well-known laser-induced breakdown spectroscopy (LIBS), which is usually carried out at atmospheric air pressure, cannot be applied to the analysis of hydrogen as an impurity. By combining low-pressure laser-induced plasma spectroscopy with laser surface cleaning, a preliminary quantitative analysis was made on zircaloy pipe samples intentionally doped with hydrogen. As a result, a good linear relationship was obtained between H(alpha) emission intensity and its concentration.