# Analysis of Salt Mixture Contamination on Insulators via Laser-Induced Breakdown Spectroscopy
## 3 Results and Discussion  
### Spectral Characteristics for Na\({}_{2}\)Co\({}_{3}\) and NaHCO\({}_{3}\)  
Figure 2 shows the whole spectra for samples #1-1 to #1-6 at an energy intensity of 3.8 J/cm\({}^{2}\). The intensity of the spectra increased with increasing concentrations of Na. The Na content in Na\({}_{2}\)CO\({}_{3}\) was higher than that in NaHCO\({}_{3}\) when the sample had the same amount of substance. Therefore, the spectral intensity of Na\({}_{2}\)CO\({}_{3}\) was higher than that of NaHCO\({}_{3}\), which was demonstrated in the range of 588 nm to 589 nm. As Na\({}_{2}\)CO\({}_{3}\) and NaHCO\({}_{3}\) were similar in element composition, except for H, the characteristic spectral wavelengths were very similar in these samples, i.e., either they were pure compounds (#1-1 and #1-6) or a mixture of two compounds (#1-2 through #1-5). In Figure 3, the spectral resonance lines (588.995 nm, 589.592 nm) and nonresonant lines (568.297 nm, 568.859 nm) for Na were selected from Figure 2. Self-absorption was found in the resonance lines (588.995 nm, 589.592 nm) at a laser fluence of 3.8 J/cm\({}^{2}\), as shown by a significant decrease in signal intensity. The variation in the Na concentration in different samples contributed to the self-absorption phenomenon, i.e., the higher the concentration, the more notable the self-absorption. This susceptibility existed because the laser ablated the target surface, and plasma was generated by the trailing edge  
Figure 2: Spectra intensity of samples #1-1 to #1-6 at an energy intensity of 3.8 J/cm\({}^{2}\) shown in black, red, navy blue, green, light blue, and purple, respectively. of the laser pulse and disappeared during condensation. Moreover, the temperature of the entire illuminant was not uniform. After Na atoms are excited by the emission wavelength, the generated photons were absorbed by other Na atoms when passing through the low-temperature region of the plasma, resulting in changes in the intensity and profile of the Na spectrum. The influence of Na concentration on the self-absorption effect could be determined by comparing the contours of the Na resonance lines. The relationship between the Na content, according to the atomic lines (Na I 588.995 nm, Na I 568.859 nm), and the average relative spectral intensity as a function of laser energy intensity is shown in Figure 4. As the laser energy density increased, the spectral intensity corresponding to each spectral line was enhanced significantly. At a laser fluence of 6.4 J/cm\({}^{2}\), as the concentration increased, the intensity first increased and then decreased, which was due to the strong self-absorption effect. This result demonstrated that the range of Na content was limited when using calibration curves to analyze the linear relationship between Na concentration and laser fluence. However, reducing the laser energy density per pulse and selecting the ion line reduced the atomic emission intensity because of the weak self-absorption effect. It prolonged the dynamic range of the concentration measurements, which allowed the Na atomic lines (Na I 588.995 nm, Na I 568.859 nm) to maintain a good linear relationship at a laser energy of 1.9 J/cm\({}^{2}\). Therefore, when the concentration was held constant, reducing the laser energy, and selecting the proper Na ion line would extend the linear analysis range and improve the accuracy of Na determination using linear calibration curves. For the elements tested, a good linear relationship was the basis of the artificial data methods used to process the LIBS spectral data. This method could also be used as a preprocessing method when employing the artificial intelligence algorithm to determine the relationship between the concentration of other elements and spectral intensity. The intensity and variation trends of spectral lines were considered to analyze the influence of the Na salt mixture on the calibration results. The linear calibration coefficient (R\({}^{2}\)) for Na was obtained with a laser fluence of 3.8 J/cm\({}^{2}\).