A review of the use of laser-induced breakdown spectroscopy for bacterial classification, quantification, and identification

The use of laser-induced breakdown spectroscopy to determine the elemental composition of bacterial cells has been described in the peer-reviewed literature since 2003. Fifteen years on, significant accomplishments have been reported that have served to clarify and underscore the areas of bacteriological investigation that LIBS is well-suited for as well as the challenges that yet remain to be faced. This review will attempt to summarize the state of the field by surveying the available body of knowledge. The early days of these experiments, roughly from 2003 to 2007, in which many of the most fundamental experiments were initially conducted will be described. The more in-depth investigations that followed in the subsequent decade will then be detailed. Many important aspects of performing LIBS on bacterial cells were reported on and are summarized here including: the use of chemometric algorithms for statistical classification of unknown spectra; the influence of the mounting substrate on classification; the effect of the testing gas atmosphere and the choice of bacterial cell growth nutrient medium on the measured LIBS spectrum; the efficacy of a LIBS-based test as a genus-level or strain–level discrimination test; the ability of LIBS to determine the cell titer or concentration of cells in the initial sample; the effects that possible contaminations or interferents within the sample would have on the LIBS spectrum; the influence that environmental stresses the cells may be exposed to during growth and the state of reproductive health of the cells could have on the LIBS spectrum; the use of standoff or remote apparatus to minimize the risk to the operators during bacteriological identification of unknown specimens; and the combination of other optical modalities with LIBS to enhance the sensitivity or specificity of identification. Lastly, tables are provided which summarize both every species of bacteria ever tested with LIBS as well as the major lessons learned by the community through 15 years of careful investigation

Combining Laser-Induced Breakdown Spectroscopy (LIBS) and Visible Near-Infrared Spectroscopy (Vis–NIRS) for Soil Phosphorus Determination

Conventional wet chemical methods for the determination of soil phosphorus (P) pools, relevant for environmental and agronomic purposes, are labor-intensive. Therefore, alternative techniques are needed, and a combination of the spectroscopic techniques—in this case, laser-induced breakdown spectroscopy (LIBS)—and visible near-infrared spectroscopy (vis-NIRS) could be relevant. We aimed at exploring LIBS, vis-NIRS and their combination for soil P estimation. We analyzed 147 Danish agricultural soils with LIBS and vis-NIRS. As reference measurements, we analyzed water-extractable P (Pwater), Olsen P (Polsen), oxalate-extractable P (Pox) and total P (TP) by conventional wet chemical protocols, as proxies for respectively leachable, plant-available, adsorbed inorganic P, and TP in soil. Partial least squares regression (PLSR) models combined with interval partial least squares (iPLS) and competitive adaptive reweighted sampling (CARS) variable selection methods were tested, and the relevant wavelengths for soil P determination were identified. LIBS exhibited better results compared to vis-NIRS for all P models, except for Pwater, for which results were comparable. Model performance for both the LIBS and vis-NIRS techniques as well as the combined LIBS-vis-NIR approach was significantly improved when variable selection was applied. CARS performed better than iPLS in almost all cases. Combined LIBS and vis-NIRS models with variable selection showed the best results for all four P pools, except for Pox where the results were comparable to using the LIBS model with CARS. Merging LIBS and vis-NIRS with variable selection showed potential for improving soil P determinations, but larger and independent validation datasets should be tested in future studies

Soil Nutrient Detection for Precision Agriculture Using Handheld Laser-Induced Breakdown Spectroscopy (LIBS) and Multivariate Regression Methods (PLSR, Lasso and GPR)

Precision agriculture (PA) strongly relies on spatially differentiated sensor information. Handheld instruments based on laser-induced breakdown spectroscopy (LIBS) are a promising sensor technique for the in-field determination of various soil parameters. In this work, the potential of handheld LIBS for the determination of the total mass fractions of the major nutrients Ca, K, Mg, N, P and the trace nutrients Mn, Fe was evaluated. Additionally, other soil parameters, such as humus content, soil pH value and plant available P content, were determined. Since the quantification of nutrients by LIBS depends strongly on the soil matrix, various multivariate regression methods were used for calibration and prediction. These include partial least squares regression (PLSR), least absolute shrinkage and selection operator regression (Lasso), and Gaussian process regression (GPR). The best prediction results were obtained for Ca, K, Mg and Fe. The coefficients of determination obtained for other nutrients were smaller. This is due to much lower concentrations in the case of Mn, while the low number of lines and very weak intensities are the reason for the deviation of N and P. Soil parameters that are not directly related to one element, such as pH, could also be predicted. Lasso and GPR yielded slightly better results than PLSR. Additionally, several methods of data pretreatment were investigated

Laser-Induced Plasma and its Applications

The laser irradiation have shown a range of applications from fabricating, melting, and evaporating nanoparticles to changing their shape, structure, size, and size distribution. Laser induced plasma has used for different diagnostic and technological applications as detection, thin film deposition, and elemental identification. The possible interferences of atomic or molecular species are used to specify organic, inorganic or biological materials which allows critical applications in defense (landmines, explosive, forensic (trace of explosive or organic materials), public health (toxic substances pharmaceutical products), or environment (organic wastes). Laser induced plasma for organic material potentially provide fast sensor systems for explosive trace and pathogen biological agent detection and analysis. The laser ablation process starts with electronic energy absorption (~fs) and ends at particle recondensation (~ms). Then, the ablation process can be governed by thermal, non-thermal processes or a combination of both. There are several types of models, i.e., thermal, mechanical, photophysical, photochemical and defect models, which describe the ablation process by one dominant mechanism only. Plasma ignition process includes bond breaking and plasma shielding during the laser pulse. Bond breaking mechanisms influence the quantity and form of energy (kinetic, ionization and excitation) that atoms and ions can acquire. Plasma expansion depends on the initial mass and energy in the plume. The process is governed by initial plasma properties (electron density, temperature, velocity) after the laser pulse and the expansion medium. During first microsecond after the laser pulse, plume expansion is adiabatic afterwards line radiation becomes the dominant mechanism of energy loss

Laser-induced breakdown spectroscopy applied to pasture, titanium, and bioplastics

Precision agriculture is a farming practice that makes production more efficient. Farmers are able to treat infield variability optimising efficiency, growth, and yield by tailoring the time, rate, and type of fertilizer that is applied. This reduces costs, waste, and environmental side effects such as runoff and leaching caused by overfertilization. Precision agriculture technology measures the nutritional status of crops to inform what, and where, nutrients are needed. The sensors need to be precise, discriminative, and work in real time to ensure that optimal windows for nutrition are not missed. These sensor systems provide aerial imaging, and crop, or soil, colour index maps. A technology that has proven effective on some agricultural specimens is laserinduced breakdown spectroscopy (LIBS). LIBS is an optical emission technique that utilizes a high-powered pulsed laser to create a plasma on the sample surface. As the plasma cools, photons are emitted at distinct wavelengths corresponding to the elemental composition in the plasma, which should represent the sample. This thesis investigates using LIBS as a sensor for precision agriculture. Multiple chemometric methods have been used on the pasture spectra to build calibration models. There are large deviations between spectra belonging to a single sample. This is due to surface inhomogeneity, particle size, lens-to-sample distance, temperature fluctuations between plasmas, and other causes. Temperature corrections were investigated using Boltzmann plots, Saha-Boltzmann plots, and intensity ratios. With limited success in mitigating the variations in pasture spectra, LIBS was used to investigate non-aqueous systems. The ability to selectively sinter the surface of injection moulded titanium was examined. Titanium metal injection moulding allows the creation of complex metal parts that are lightweight, biocompatible, and costs less than machining. Multiple LIBS pulses produced sintering in the ablation crater of injection moulded titanium by sufficiently heating the titanium particles so that fusion occurred. The spectra from LIBS can be used to monitor the extent to which the surface is sintered by measuring the reduction in carbon emissions. An autofocus system, based on the triangulation method, was used to minimise variations caused by lens-to-sample distance (LTSD). With the success of sintering titanium, LIBS was used to investigate non-aqueous organic systems. Employing LIBS to discriminate bioplastics from regular plastics was explored in recycle waste streams. If bioplastics are present in the recovery process of regular plastics the resulting product contains impurities. This study was undertaken to determine the feasibility of incorporating bioplastics in the curbside pickup of recyclables in New Zealand. The common recyclables are plastics, glass, tin cans, and aluminium cans. The setup was designed to emulate a one-shot LIBS detection system in a recycling plant. Models were created using k nearest neighbours and soft independent modelling class analogy from the spectra. 100 % discrimination between bioplastics and regular plastics was achieved. An autofocus system, combining dual lasers, was used to overcome the occlusions produced by sample geometry

Identification and detection of phosphorylated proteins by laser induced breakdown spectroscopy

Thesis (Master)--Izmir Institute of Technology, Chemistry, Izmir, 2011Includes bibliographical references (leaves: 53-56)Text in English; Abstract: Turkish and Englishx, 56 leavesLaser-Induced Breakdown Spectroscopy (LIBS) is an optical atomic emission spectroscopic technique that uses an energetic laser source to generate a luminous plasma. Spectrochemical analysis of the light emitted from the plasma reveals information about the elemental composition of the sample. Phosphorylation is an important regulatory mechanism that activates or deactivates many proteins and enzymes in a wide range of cellular process. Identification and detection of phosphoproteins have a crucial importance in phosphopeptide mapping. This study is based on the assessment of the capabilities and limitations of LIBS as a quick and simple method for in-gel identification and determination of phosphorylated proteins, specifically casein and ovalbumin before mass spectrometric analysis for the elucidation of phosporylation sites. For this purpose, an optical LIBS set-up was constructed from its commercially available parts and the system was optimized for LIBS analysis of polyacrylamide gels. Nd:YAG laser operating at 532 nm wavelength and at 10 Hz frequency was used to create plasma on dry gel surfaces. Emitted light from a luminous plasma was analyzed and detected by an Echelle type spectrograph containing Intensified CCD, detector. With this study, LIBS detection of phosphorous proteins after electrophoretic separation of phosphorylated proteins has been shown, for the first time. After SDS-PAGE gel separation process, phosphoproteins were recognized from prominent P(I) lines (at 253.5 nm and 255.3 nm) in a plasma formed by the focused laser pulses on the gel, just in the center or in the vicinity of the electrophoretic spot. Spectral emission intensity of P(I) lines from LIBS data has been optimized with respect to laser energy and detector timing parameters by using standard Na2HPO4. It has been shown that phosphorylated proteins (casein and ovalbumin in mixture) can be identified by LIBS after both coomassie brilliant blue and silver staining procedures. Technique shows a great promise in microlocal spotting of phosphorylated proteins in gel before MS analysis for the determination of the phosphorylation sites

Simultaneous double pulse enhanced laser induced breakdown spectroscopic detection of trace heavy metals in herbs

Laser Induced Breakdown Spectroscopy (LIBS) is a versatile technique. It has enormous potential for in-situ elemental analysis of virtually any kind of material. Undesired fluctuations in analytical conditions seriously influence its quantitative measurements. Double Simultaneous Pulse (DSP) configuration is introduced in this work that can potentially minimize the influence of error factors and improve repeatability as well as detection limit. Its performance is compared with conventional Single Pulse (SP) LIBS configuration in a series of experiments performed under controlled environments of air and argon. Herbal samples Ficus Deltoidea, Phaleria Macrocarpa and Strobilanthes Crispus are utilized as investigative materials. Nd:YAG laser (1064 nm, 6 ns, 544 mJ) and Ocean Optics HR4000 spectrometer are employed for laser induced breakdown spectroscopic studies. Mg, Ca, Pb and Cu are quantified in samples through both configurations of LIBS. Better line profiles, emission intensities, signal-to-noise ratios (SNRs) and signal-to-background ratios (SBRs) are observed with DSP configuration. Electron density and plasma temperatures obtained from DSP configuration are slightly higher than those obtained with SP configuration. Comparatively, DSP configuration performs better in the estimation of heavy metal concentrations. Linear correlations of the calibration plots, limits of detection (LODs), relative standard deviation (RSD) and Errors of Prediction are generally improved. Minimum LODs obtained for Pb, Cu, Mg and Ca are 38.63 μg/g, 47.75 μg/g, 8.92 μg/g and 8.72 μg/g respectively. Minimum values of root mean square error of prediction (RMSEP) in the measurements, as yielded from SP and DSP configurations, are 83.13 μg/g and 43.13 μg/g respectively. Smaller errors and better repeatability are found as special traits of argon while smaller LOD is that of air environment. In this study DSP outperforms SP configuration at several fronts in both environments. It concludes that DSP can be a better alternative configuration of LIBS for quantification of heavy metals in herbal plants

A first simulation of soil-laser interaction investigation for soil characteristic analysis : simulation of soil-laser interaction investigation

Laser Induced Breakdown Spectroscopy (LIBS) is an important technique utilized in several areas including that of agriculture and space exploration. However, whilst LIBS provides a new way of analyzing chemical composition of targeted soils or rocks, the quality and repeatability of the results are affected by the terrain and soil conditions as a result of physical matrix effects which occur due to varying properties like specific heat and thermal conductivity. These physical and chemical matrix effects cause difficulties with quantitative LIBS analysis. Together with this, the diverse areas in which LIBS is utilized means that it can require varying conditions of ablation techniques. Therefore, it is prudent to investigate theoretically the effect of different soil characteristics on the ablation process. The work presented here is the first simulation based research on soil quality analysis using LIBS. Aiming to gain insights into the soil breakdown process, laser coupling, sample temperature and its sensing performance through simulation of the laser ablation of soil using finite element modelling software. The proposed model within COMSOL Multiphysics was designed and developed to study the influence of multiple nanosecond (ns) laser pulses on the surface of samples of soil with varying properties. The simulation results reveal the simulated soil sensing behaviour for the first time. The computational results were compared to those obtained from LIBS experiments conducted for the Argibot project at the University of Strathclyde

Signal Optimization and Enhancement of Laser-Induced Breakdown Spectroscopy for Discrimination of Bacterial Organisms

Bacterial pathogens can be differentiated via an elemental analysis techniqueknown as laser-induced breakdown spectroscopy (LIBS). This spectrochemical technique provides a near-instantaneous measurement of the elemental composition of a target. The aim of this work was to demonstrate the feasibility of LIBS for the rapid identification and discrimination of bacteria in simulated clinical specimens based on reproducible differences in the concentration of inorganic elements in bacterial cells. This research will describe the current experimental technique, including bacteria collection and mounting protocols, LIBS data acquisition, and spectral data analysis. These include methods for the collection, concentration, and separation of bacteria from unwanted biological matter, deposition of bacterial cells on a suitable ablation medium, the formation of high temperature laser-induced micro plasmas, collection, and analysis of the atomic emission spectra with a high-resolution spectrometer, and the differentiation of LIBS emission spectra from different bacterial species and genera using computerized chemometric algorithms. The construction of a spectral library database containing the LIBS emission spectra from hundreds of spectra obtained from highly diluted specimens of Staphylococcus epidermidis, Escherichia coli, Mycobacterium smegmatis, Pseudomonas aeruginosa, Enterococcus cloacae and sterile water control specimens is ongoing. Manipulation of this library with outlier elimination techniques, reduction of elemental contaminants contributing to extraneous background signals, and the addition of silver microparticles to enhance signal intensities are all being investigated to produce a standardized protocol that minimizes the bacterial limit of detection while maximizing classification accuracy