Mapping capability of linear correlation statistics for characterization of complex materials using laser-induced breakdown spectroscopy

Novel Use of a Static Modification of Two-Dimensional Correlation Analysis. Part I: Comparison of the Secondary Structure Sensitivity of Electronic Circular Dichroism, FT-IR, and Raman Spectra of Proteins

Two-dimensional (2D) correlation coefficient analysis is employed to classify and characterize spectral variations among heavily overlapped near-infrared spectra of pellets and films of three kinds of polyethylene (PE), high-density (HD), low density (LD), and linear low-density (LLD) polyethylene, and five kinds of ivory signature seals. The sample-sample (SS) 2D correlation maps are used for classification while the wavenumber-wavenumber (WW) 2D correlation maps are used for determining spectral variation among the above materials. Both correlation maps are obtained by multiplying the original data with themselves. It is found that the NIR spectra of pellets and films of HD PE are clearly different from those of LD PE and LLD PE, while the NIR spectra of five kinds of ivory seals yield easily discernable squares in the SS correlation maps. The background variation is thought to be behind the differentiation of the PE samples because the WW correlation maps do not indicate appearance of new bands. The correlation results are compared with those of principal component analysis (PCA). This study is a novel application of 2D correlation coefficient analysis which reveals that a comprehensive description of demanding spectral systems is achievable by utterly simple mathematical means because 2D correlation maps are obtained via a single mathematical operation.

Laser induced breakdown spectroscopy (LIBS) of a hydrocarbon flame and the rocket motor simulator plume is reported. The flame and plume were seeded with various types of elements. The LIBS signal of the seeded element goes down drastically in the presence of the flame in comparison to the signal obtained from the aerosols of the elements. Similarly the LIBS signal of the trace elements was very weak in the luminous flame and the simulator plume in comparison to the measurement outside it. The intensity of the LIBS signal from the trace elements present in the plume/flame was found to be dependent on the process of seeding, the transition probability and the decay time of the background emission. Ultimately LIBS was found to be more sensitive than atomic emission spectroscopy (AES) in detecting the trace elements in the simulator plume. AES was used to estimate the temperature of the plume by comparing the experimental and simulated emission spectra of OH radicals. This study establishes LIBS as an improved health monitoring system over AES for the plume studied here.

Effective Laser-Induced Breakdown Spectroscopy (LIBS) Detection Using Double Pulse at Optimum Configuration

P analysis in fertilizers using LIBS.

Laser-induced breakdown spectroscopy (LIBS) technology for metal and nonmetallic detection accuracy is the key technology to be solved in LIBS measurement, Due to metal elements and non-metallic elements in the lively, atomic structure and the degree of excitation of the laser are totally different, so the laser induced plasma evolution and spectral intensity are absolutely different. Among the many factors that affect measurement accuracy, the single and double pulse of the laser has a great influence on the measurement accuracy of metal and non-metal, they both have their own advantages, but also have their own shortcomings. In order to compare the effect of SP-LIBS and DP-LIBS on the measurement results of different elements, in this experiment, we put the metal element aluminum and non-metallic element carbon as the sample, the laser energy as a variable, using the high-speed camera shooting SP- LIBS and DP- LIBS plasma images. Using the spectral analyzer to record the spectral intensity of the elements, by calculating the relative RSD of the signal intensity and comparing the spectral intensity and the signal stability for different elements, develop an optimized experimental program. The experimental results show that under the same energy condition, the metal aluminum ion image under the DP- LIBS and the non-metallic carbon ion image under the SP- LIBS are the most suitable images. By considering the stability of the line intensity and the signal stability, we find that the sensitivity and stability of the signal strength of the metal elements under the double pulse are better than that of the single pulse, and for the non-metallic element, the single pulse laser is better than the double pulse.

Flame-assisted plasma modulation to improve the raw signal quality for laser-induced breakdown spectroscopy

We investigate the use of a second laser with a selected wavelength to improve the limit of detection (LoD) of trace elements in the Laser‐Induced Breakdown Spectroscopy (LIBS) technique. We consider the combination of LIBS with Laser‐Induced Fluorescence (LIF), in which the second laser is used to excite trace elements in the plasma. The influence of the main experimental parameters on the trace elements LIF signal, namely the ablation fluence, the excitation energy, and the inter‐pulse delay, was studied experimentally and a physical interpretation of the results was presented. For illustrative purpose we considered detection of Pb in brass samples and in water. The plasma was produced by a Q‐switched Nd:YAG laser and then re‐excited by a nanosecond optical parametric oscillator laser. We found out that the optimal conditions for our experimental set‐up were obtained for relatively weak ablation fluence of 2–3 J/cm2 and inter‐pulse delay of 5–10 μs. Using the LIBS‐LIFS technique, a single‐shot LoD for detection of lead of about 1.5 part per million (ppm) was obtained for solids and 0.5 ppm for liquids. These LoDs represent an improvement of about two orders of magnitude with respect to LIBS. We also discuss resonance‐enhanced LIBS (RELIBS), in which the second laser excites the main plasma component instead of the impurities. For the set of parameters used the RELIBS, Pb signal does not differ significantly from the LIBS signal except at low ablation fluence.

In this work, the Stark effect is shown to be mainly responsible for wrong elemental allocation by automated laser-induced breakdown spectroscopy (LIBS) software solutions. Due to broadening and shift of an elemental emission line affected by the Stark effect, its measured spectral position might interfere with the line position of several other elements. The micro-plasma is generated by focusing a frequency-doubled 200 mJ pulsed Nd/YAG laser on an aluminum target and furthermore on a brass sample in air at atmospheric pressure. After laser pulse excitation, we have measured the temporal evolution of the Al(II) ion line at 281.6 nm (4s(1)S-3p(1)P) during the decay of the laser-induced plasma. Depending on laser pulse power, the center of the measured line is red-shifted by 130 pm (490 GHz) with respect to the exact line position. In this case, the well-known spectral line positions of two moderate and strong lines of other elements coincide with the actual shifted position of the Al(II) line. Consequently, a time-resolving software analysis can lead to an elemental misinterpretation. To avoid a wrong interpretation of LIBS spectra in automated analysis software for a given LIBS system, we recommend using larger gate delays incorporating Stark broadening parameters and using a range of tolerance, which is non-symmetric around the measured line center. These suggestions may help to improve time-resolving LIBS software promising a smaller probability of wrong elemental identification and making LIBS more attractive for industrial applications.

A comparative study of single-and double-pulse laser induced breakdown spectrometry (LIBS) was carried out to detect the silver element in silver jewelry samples. One Nd:YAG was adopted in the experiment to generate two pulses which were set in an orthogonal configuration. The results show that, for double-laser LIBS, average diameter (3μm) of craters on the sample surface is much smaller and the intensity of silver emission signals are magnified by a factor of 40 compared with the single-pulse LIBS. Due to the superiority in high sensitivity and low energy consumption, double-pulse LIBS presents a bright application prospect in rapid on-site undamaged analysis of precious metal jewelry.

This study focuses on the discrimination of extracted animal fats in liquid form using laser induced breakdown spectroscopy (LIBS) technique assisted with principal component analysis (PCA). The interaction of laser and liquid sample produces liquid splashing due to strong shock wave effect and subsequently generates lower intensity of LIBS signals. LIBS difficulties in liquid are resolved using paper substrate to enhance LIBS emission intensity. Laser pulse from Q-switched Nd:YAG laser with energy of 220 mJ and frequency of 1 Hz was used to ablate extracted animal fats. The obtained LIBS spectra of extracted animal fats were further evaluated using PCA. LIBS spectra are compressed and visualised as data points in the score plot of PCA. PCA results demonstrated that data points from different extracted animal fats were clustered separately in the score plot with variance greater than 90%. The findings show LIBS system assisted with PCA was capable to differentiate various extracted animal fats.

We report a relatively simple configuration of laser-induced breakdown spectroscopy (LIBS) that is suitable for gas flow diagnostics with increased spatial resolution, signal intensity, and stability. In this optical configuration, two laser beams are generated by splitting a single laser beam, and then they are focused and crossed orthogonally at the detection volume from two different optical paths. Different from dual-pulse LIBS, this LIBS configuration uses only one laser source, and thus is of relatively low cost. Several advantages were found for this simple beam-crossing LIBS when it was demonstrated in air in the present work, particularly on signal enhancement and stabilization, confining plasma volume, and controlling plasma position. Both of the latter two advantages are relevant to spatial resolution improvement of LIBS in gases, which has rarely been discussed in previous reports. An enhancement factor of 2 was found for atomic hydrogen, nitrogen, and oxygen emissions with respect to conventional LIBS. Another advantage is that the position of breakdown can be precisely controlled through adjustment of the propagation of the two beams, also resulting in smaller plasma volume and stable emission intensity. Furthermore, the technique is moderately tolerant to dust particles neutrally present in the environment, avoiding the spark occurring at a position out of the detection volume. Beyond LIBS, the new configuration has other potential applications, e.g., laser-induced ignition, which is also briefly discussed.

Multivariate approach to the chemical mapping of uranium in sandstone-hosted uranium ores analyzed using double pulse Laser-Induced Breakdown Spectroscopy

We present a new type of handheld laser-induced breakdown spectroscopy (LIBS) spectrometer for developing mobile atomic spectroscopy solutions for real world applications. A micro diode-pumped passive Q-switched solid-state laser with high repetition rate of well above 1 kHz in comparison to 1-10 Hz as used in a traditional LIBS instrument is employed to produce a train of laser pulses. The laser beam is further fast scanned over a pre-defined area, hence generating several hundreds of micro-plasmas per second at different locations. Synchronized miniature CCD array spectrometer modules collect the LIBS signal and generate LIBS spectra. By adjusting the integration time of the spectrometer to cover a plurality of periods of the laser pulse train, the spectrometer integrates the LIBS signal produced by this plurality of laser pulses. Hence the intensity of the obtained LIBS spectrum can be greatly improved to increase the signal-to-noise ratio (SNR). This unique feature of the high repetition rate laser based LIBS system allows it to measure elements at trace levels, hence reducing the limit of detection (LOD). The increased signal intensity also lessens the sensitivity requirement for the optical spectrometer. In addition, the energy of the individual laser pulse can be reduced in comparison to traditional LIBS system to obtain the same signal level, making the laser pulse less invasive to the sample. The typical measurement time is within 1 second. Several examples of real world applications will be presented.

Simultaneous measurements of fuel concentration and temperature in gas jets by laser induced breakdown spectroscopy