ABSTRACT
Timely and accurate identification of harmful bacterial species in the environment is paramount for preventing the spread of diseases and ensuring food safety. In this study, laser-induced breakdown spectroscopy technology was utilized, combined with four machine learning methods - KNN, PCA-KNN, RF, and SVM, to conduct classification and identification research on 7 different types of bacteria, adhering to various substrate materials. The experimental results showed that despite the nearly identical elemental composition of these bacteria, differences in the intensity of elemental spectral lines provide crucial information for identification of bacteria. Under conditions of high-purity aluminum substrate, the identification rates of the four modeling methods reached 74.91%, 84.05%, 85.36%, and 96.07%, respectively. In contrast, under graphite substrate conditions, the corresponding identification rates reached 96.87%, 98.11%, 98.93%, and 100%. Graphite is found to be more suitable as a substrate material for bacterial classification, attributed to the fact that more characteristic spectral lines are excited in bacteria under graphite substrate conditions. Additionally, the emission spectral lines of graphite itself are relatively scarce, resulting in less interference with other elemental spectral lines of bacteria. Meanwhile, SVM exhibited the highest precision rate and recall rate, reaching up to 1, making it the most effective classification method in this experiment. This study provides a valuable approach for the rapid and accurate identification of bacterial species based on LIBS, as well as substrate selection, enhancing efficient microbial identification capabilities in fields related to social security and military applications.
ABSTRACT
Self-absorption-free laser-induced breakdown spectroscopy (SAF-LIBS) can directly obtain the applicable quasi-optically thin lines by determining the optimal acquisition delay time according to the intensity ratio of doublet lines at specific transition wavelength of the analyzed elements, thus eliminating the influence of self-absorption on quantitative results. In quantitative analysis of samples with a certain content range, the key to the convenient application of this technique is to rapidly select the suitable doublet lines for the element to be analyzed. The theoretical analysis shows that the evolution trend of doublet intensity ratio is monotonous under the assumptions that the plasma is uniform and in local thermal equilibrium (LTE) and the area density (Nl) is a constant, which is also confirmed by the experimental results of Cu and Al. Thus, a rapid spectral line selection criterion for SAF-LIBS applications is derived: only when the doublet intensity ratios measured at the initial and final stages of plasma induced by the boundary sample with the highest element content lie on both sides of the theoretical ratio, the doublet lines can reach quasi-optically thin during plasma evolution and are suitable for SAF-LIBS measurements. This new criterion is helpful to promote the practicality and industrial application of SAF-LIBS technology.
ABSTRACT
A resonance/non-resonance, doublet-based, self-absorption-free, laser-induced breakdown spectroscopy (SAF-LIBS) technique is proposed for greatly expanding the measurement range of quantitative elemental analysis by using a quasi-optically thin line. The quasi-optically thin spectral line is obtained by matching the measured doublet atomic lines' intensity ratios with the theoretical one, and the applicable measurement range is expanded by utilizing the resonance and non-resonance lines. The specific calibration process consists of two parts: the nonlinear LIBS calibration and the linear SAF-LIBS calibration. For quantitative measurements, the approximate content of the unknown sample is determined first by using the LIBS calibration curve, and then the SAF-LIBS spectra and the resonance or non-resonance calibration curve that corresponds to the predetermined content are used for further implementing the quantitative analysis. Univariate quantitative analysis results of Cu show that this resonance/non-resonance doublet-based SAF-LIBS technique not only captures the quasi-optically thin spectral line in a wide range of elemental content, but also possesses high correlation coefficients of calibration curves, small relative errors of measurement and low limits of detection. The applicability and limitations of this technique are also discussed, and the evolution as well as the related major determinants of self-absorption are analyzed by taking advantage of the spatial-temporal evolution images of plasma emissivity.
ABSTRACT
We studied intensity fluctuations of a single photon source relying on the pulsed excitation of the fluorescence of a single molecule at room temperature. We directly measured the Mandel parameter Q(T) over 4 orders of magnitude of observation time scale T by recording every photocount. On time scale of a few excitation periods, sub-Poissonian statistics is clearly observed and the probablility of two-photons events is 10 times smaller than Poissonian pulses. On longer times, blinking in the fluorescence, due to the molecular triplet state, produces an excess of noise.
