Ali Khumaeni

New Technique for the Direct Analysis of Food Powders Confined in a Small Hole Using Transversely Excited Atmospheric CO2 Laser-Induced Gas Plasma

Taking advantage of the differences between the interactions of transversely excited atmospheric (TEA) CO2 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 CO2 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.

Rapid Analyses of Tiny Amounts of Powder Samples Using Transversely Excited Atmospheric CO2 Laser-Induced Helium Gas Plasma with the Aid of High-Vacuum Silicon Grease as a Binder on a Metal Subtarget

Rapid quantitative analyses of powder samples available in tiny amounts have successfully been conducted by utilizing a transversely excited atmospheric (TEA) CO2 laser-induced He gas plasma. In this study, 4 mg of powder sample was homogeneously mixed with 4 mg of high-vacuum silicon grease and the silicon grease–mixed powder sample (SMP) was painted on a metal surface, which serves as a subtarget. The grease functions to strongly bind the powder and to suppress blow-off of the powder particles. When a TEA CO2 laser (750 mJ, 10.6 μm, 200 ns) was directly focused on the metal subtarget in He gas at 1 atmosphere, a hightemperature He gas plasma was induced, producing a profusion metastable He atoms. It is assumed that the powder particles together with the silicon grease were vaporized to be effectively atomized and excited through metastable He atoms. The result revealed that this technique can be widely employed in the rapid semi-quantitative analyses of powder samples present in minute amounts. A quantitative analysis of loam soil containing different concentrations of Cu was successfully demonstrated, resulting in a good linear calibration curve. The detection limits of Cr and Pb in loam soil were approximately 4 and 13 mg/kg, respectively. Also, we confirmed that this technique can be applied to check the quality of commercial products such as gold film (Au foil), mineral supplement tablets, and prestigious cosmetic powders.

Excitation Mechanism of H, He, C, and F Atoms in Metal-Assisted Atmospheric Helium Gas Plasma Induced by Transversely Excited Atmospheric-Pressure CO2 Laser Bombardment

To clarify the excitation mechanism of hydrogen in transversely excited atmospheric-pressure (TEA) CO2 laser-induced helium gas plasma, atomic emission characteristics of H, C, F, and He were studied using a Teflon sheet (thickness of 2 mm) attached to a metal subtarget. The TEA CO2 laser (750 mJ, 200 ns) was focused on the Teflon sheet in the surrounding He gas at 1 atm. Atomic emissions of H, C, F, and He occurred with a long lifetime, a narrow spectrum width, and a low-background spectrum. The correlation emission intensity curves of H–He and F–He indicated a parabolic functions. To explain the emission characteristics, we offered a model in which helium metastable atoms (He*) play an important role in the excitation processes; namely, atoms collide with helium metastable atoms (He*) to be ionized by the Penning effect, and then recombine with electrons to produce excited states, from which atomic emissions occur.

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.

Food powder analysis by using transversely excited atmospheric CO2 laser-induced plasma spectroscopy

A direct and sensitive analysis of food powder sample has successfully been carried out by utilizing the special characteristics of pulsed transversely excited atmospheric (TEA) CO2 laser. In this study, a food powder was placed in a container made of copper plate and covered by a metal mesh. The container was perpendicularly attached on a metal surface. A high-temperature luminous plasma was induced on a metal surface 5 mm above the mesh. Once the plasma was produced, a strong shock wave was induced, blowing-off of the powder from the container to enter into the plasma to be dissociated and excited. By using this method, a semi-quantitative analysis of food powder was made. The detection limits of Cr in the powdered agar and Cd in the powdered rice were 9 mg/kg and 50 mg/kg, respectively.

Rapid identification of elements in liquid by using pulse carbon dioxide laser-induced plasma spectroscopy

Identification of major elements in milk liquid has been rapidly carried out by using pulse carbon dioxide (CO2) laser. Experimentally, a milk liquid was poured on a Cu metal plate to make a thin film on the metal surface. The liquid was then dried at 100°C for 5 minute. A thin film of milk liquid was produced with a thickness of around 0.1 mm. The Cu plate only functions as a metal subtarget. A pulse CO2 laser was irradiated on a metal surface to induce a luminous plasma. The atomization of liquid and excitation processes of atoms from the film sample happen in the plasma region. Various major elements in milk liquid, namely Ca, Mg, Na, and K can successfully be detected. Therefore, the method has high-prospect for major elemental analysis in liquid sample target.Identification of major elements in milk liquid has been rapidly carried out by using pulse carbon dioxide (CO2) laser. Experimentally, a milk liquid was poured on a Cu metal plate to make a thin film on the metal surface. The liquid was then dried at 100°C for 5 minute. A thin film of milk liquid was produced with a thickness of around 0.1 mm. The Cu plate only functions as a metal subtarget. A pulse CO2 laser was irradiated on a metal surface to induce a luminous plasma. The atomization of liquid and excitation processes of atoms from the film sample happen in the plasma region. Various major elements in milk liquid, namely Ca, Mg, Na, and K can successfully be detected. Therefore, the method has high-prospect for major elemental analysis in liquid sample target.

Pulsed CO2 laser-induced gas plasma spectroscopy based on single beam splitting for trace metal analysis on a material surface

ABSTRACT Laser-induced gas plasma spectroscopy based on pulsed CO2 laser beam splitting has been applied to the problem of trace film analysis on the silicon surface. In this study, 2.1 J of laser energy (70% of the laser beam) was focused at a 10-degree incidence on a metal mesh attached to a sample surface containing trace metal elements in order to produce a gas plasma. The remaining part of the laser beam (approximately 30% or 0.9 J) was employed to vaporize a film which had been deposited on the material by focusing the laser beam 3 cm under the surface. In this scheme, the vaporized metal film moves into the gas plasma region, in which the dissociation and excitation takes place. Our measurements show that the detection of Cr on the silicon surface can be made with high sensitivity. The limit of detection of Cr in the silicon material was approximately 7.5 × 1012 atom/cm2.

Time-resolved plasma imaging in microwave-assisted laser-induced breakdown spectroscopy

To study the dynamics of luminous plasma induced by a pulse laser and microwaves (MWs), time resolved imaging of microwave-assisted laser-induced plasma was carried out. In this study, a luminous plasma was induced by a second harmonic Nd:YAG laser (532 nm, 8 ns, 5 J) on calcium oxide (Ca2O3) pellet at low pressure (5 Torr) of Ar gas. The luminous plasma was enhanced by intensified microwaves (MWs) generated from a magnetron. The image of plasma induced by laser has different features from the plasma induced by the laser with MWs. For the case of laser only, the plasma lifetime is approximately 50 μs, the plasma size at 5 μs is approximately 3 mm, and the neutral Ca emission is much brighter than the ionic Ca emission. When the MWs was introduced into the laser-induced plasma, the plasma emission was enhanced, namely the plasma lifetime was elongated of approximately 8 times, the plasma size at 5 μs was enlarged of approximately 3 times, and the ionic Ca emission is much brighter than the case of neutral Ca emission, which indicated that the plasma temperature is much higher than the case of laser only. Therefore, the plasma induced by the microwave-assisted laser can be effectively used for the analysis of elemental composition in materials with high sensitivity.

Synthesis and characterization of high-purity gold nanoparticles by laser ablation method using low-energy Nd:YAG laser 1064 nm

High-purity gold nanoparticles (GNPs) has been successfully synthesized by using laser ablation method utilizing low-power neodymium yttrium aluminum garnet (Nd:YAG) laser at the fundamental wavelength. Experimentally, pulse laser beam (Nd:YAG laser, 1064 nm, 7 ns, 30 mJ) was directed and focused onto a high-purity gold sheet (99.95%), which was placed into a pure liquid of deionized water, to produce GNPs colloid. Dark-red color colloid of high-purity GNPs was successfully synthesized. The GNPs had a spherical shape with an average diameter of 23.5 nm and standard deviation of 6.4 nm. The surface plasma resonance was centered at wavelength maximum at 520 nm.

Rapid Detection of Oil Pollution in Soil by Using Laser-Induced Breakdown Spectroscopy

Detection of oil pollution in soil has been carried out using laser-induced breakdown spectroscopy (LIBS). A pulsed neodymium-doped yttrium aluminum garnet (Nd:YAG) laser (1,064 nm, 8 ns, 200 mJ) was focused onto pelletized soil samples. Emission spectra were obtained from oil-contaminated soil and clean soil. The contaminated soil had almost the same spectrum profile as the clean soil and contained the same major and minor elements. However, a C–H molecular band was clearly detected in the oil-contaminated soil, while no C–H band was detected in the clean soil. Linear calibration curve of the C–H molecular band was successfully made by using a soil sample containing various concentrations of oil. The limit of detection of the C–H band in the soil sample was 0.001 mL/g. Furthermore, the emission spectrum of the contaminated soil clearly displayed titanium (Ti) lines, which were not detected in the clean soil. The existence of the C–H band and Ti lines in oil-contaminated soil can be used to clearly distinguish contaminated soil from clean soil. For comparison, the emission spectra of contaminated and clean soil were also obtained using scanning electron microscope-energy dispersive X-ray (SEM/EDX) spectroscopy, showing that the spectra obtained using LIBS are much better than using SEM/EDX, as indicated by the signal to noise ratio (S/N ratio).