# Mid-Infrared Laser Spectroscopy Detection and Quantification of Explosives in Soils Using Multivariate Analysis and Artificial Intelligence

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## Abstract

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## 1. Introduction

^{−1}. This spectral window corresponds to the fingerprint region of the HE samples deposited on painted metal car doors. Other studies have demonstrated diverse QCL approaches to the rapid identification and characterization of HEs, given the routine requirements of security checks [40,41].

## 2. Materials and Methods

#### 2.1. Reagents

#### 2.2. Sample Preparation

#### 2.3. Soil Characterization

#### 2.4. Data Acquisition and QCL System

^{−1}, 1111 to 1178 cm

^{−1}, and 1178 to 1600 cm

^{−1}The spectral linewidth was < 2 cm

^{−1}and the scan time was approximately 1.5 s for each of the diodes. The average power typically varied between 0.5 and 10 mW across the entire tuning range of ≈ 600 cm

^{−1}with 100:1 Transverse Electromagnetic Mode (TEM

_{00}) polarization and a beam divergence of < 2.5 mrad in the x-axis and < 5 mrad in the y-axis. The spectrometer had a 7.6-cm diameter ZnSe lens, which was used to focus the MIR beam to collect the reflected light and focus the light onto a thermoelectrically cooled mercury–cadmium–telluride (MCT) detector. The wavelength accuracy and precision were 0.5 and 0.2 cm

^{−1}, respectively. The spectroscopic system worked best at a distance to the target of 15 ± 3 cm, with each laser producing an elliptical spot with diameters of 4 and 2 mm in the same space at a distance of 15 cm due to the difference of beam divergence in the axes (Galan-Freyle et al. [30]).

#### 2.5. Data Quantification Analysis

^{2}), which indicates the percentage of variance present in the true component values reproduced by the PLS regression model. In contrast, the sensitivity (SEN) of multivariate methods can be estimated as the net analyte signal (NAS) [57] at a unit concentration as follows [58]:

**b**‖,

**b**denotes the regression vector of the PLS model. The analytical sensitivity (

**γ**) can be defined similarly to univariate calibrations [59] as

_{o})

^{1/2},

_{o}denotes the distance of the predicted sample to the mean of the calibration set at zero concentration and Δ(α,β) is a statistical parameter correlated with the α and β probabilities of falsely stating the presence or absence of the analyte. Δ(α,β) = 3.3 was used to compute the LOD values providing that the value for the degrees of freedom was > 25.

^{2})/(N − 1))

^{1/2},

#### 2.6. Pattern Recognition Analysis

#### 2.7. Artificial Intelligence Scheme

## 3. Results and Discussion

#### 3.1. Quantification Analysis

^{−1}). In this study, VN was used as a preprocessing step as it proved to be better than the other tested preprocessing steps except for KBr. Other preprocessing steps were tested, such as mean centering (MC), linear offset subtraction, straight-line subtraction, minimum and maximum normalization, multiplicative scatter correction, first derivative, second derivative, and no preprocessing step. When applying VN, the average intensity is calculated first, and then this value is subtracted from the spectrum. Next, the sum of the squared intensities is calculated, and the spectrum is divided by the square root of this sum. Models for DNT in KBr were generated to evaluate the detection in the absence of interferences: one at high concentrations (0–20%) and another at low concentrations (0–3%). The errors for these models are listed in Table 1. The most effective preprocessing method for the KBr models was MC. This result suggests that VN is a suitable preprocessing step only when interferences from the matrix are present. In a model free from the interfering matrix (KBr), applying VN for generating samples with low concentrations had the same signal intensity as samples with high concentrations. This is reflected in the lousy prediction and high uncertainty for samples with low concentrations (see Supplementary Materials: Figure S3).

#### Figures of Merit (FM)

**b**, which, in turn, were derived from the spectra and their respective concentrations of standards. VN directly affects the magnitude ‖

**b**‖ and value of SEN as a consequence. However, a better parameter for sensitivity can be obtained by calculating γ because this parameter is only affected by instrumental noises. The noise level was measured by collecting 20 spectra of a blank (target) and calculating an average of the standard deviations for all wavenumbers. The resulting noise for the models was different when considering 20 normalized spectra with VN (see Table 3). Otherwise, the γ-values calculated from the two types of models were very similar, which is an indication that γ is not affected by VN preprocessing or any other types of preprocessing.

^{−1}) provides an estimation of the minimum concentration difference (resultant) discernible by the model considering the instrumental noise as the only source of error. In the case of NAT-S Low, γ was 0.003% for the model with VN preprocessing and 0.002% for the model without preprocessing, with the difference being statistically insignificant. It is not possible to make a comparison of the sensitivities between the modes with matrix and without matrix because the magnitude of ‖

**b**‖ depends on the number of signals and number of latent variables (LV) (see Supplementary Materials: Figure S5). For the models of spectra with many signals (BC, SYN-S, and NAT-S), the magnitude of ‖

**b**‖ is higher than that for the models with low-intensity signals (KBr models). A better FM for this comparison is the LOD. For the models with the matrix (BC, SYN-S, and NAT-S), the LOD values are close to that of the model without the matrix (KBr models). Curves for the samples of low concentrations of DNT in NAT-S were generated to determine whether the employed concentration range influenced the LOD values of the curves. In these cases, the LOD decreased from 0.8% to 0.3%. DNT concentrations between 0.3% and 0.8% in the NAT-S Low model can be quantified with higher uncertainties and higher probabilities of false positives and missed detections because RSD should be between 10% and 33%.

^{2}(100 points) for each sample was generated. Two samples comprised the same soil as that used in the NAT-S models and were contaminated with 10% of DNT. The first sample consisted of a simple mixture (NAT-S-M). The second sample involved mixing and macerating the components (NAT-S-MM). The concentrations were predicted using the NAT-S model with three preprocessing methods: VN, MC, and no preprocessing. The map for the NAT-S model with VN is included in Supplementary Materials: Figures S6–S9, while the predictions for the 100 points for each map using different preprocessing methods are shown in Figure 4a,b. VN preprocessing applied to both samples (NAT-S-M and NAT-S-MM) resulted in better results, with the predicted values being close to the true value on average (10% DNT, see Figure 4c). MC preprocessing worked better in NAT-S-MM than NAT-S-M. This indicates that while the macerated process homogenized the size of the particles, MC was not able to compensate for the difference in the particle size. In contrast, the VN preprocessing method was able to compensate for these differences. The prediction of DNT in NAT-S-M shows peaks of high DNT concentrations (>10% DNT). This is because the particles of DNT were not homogenized. The models with no preprocessing provided bad predictions due to the difference in the baseline of the spectra. The above discussion demonstrates that VN preprocessing corrects the spectral variation due to changes in the particle size. To demonstrate the change in spectrum with the particle size, the tested soil was sieving for three particle sizes (d): d > 0.85 mm, 0.85 mm > d > 0.25 mm, and d < 0.25 mm. One hundred spectra were acquired at various locations on the sample surface for each value of d, and the average and standard deviation of the spectra were determined (see Supplementary Materials: Figures S10 and S11). The background offset spectral decreases with d, whereas the standard deviation increases with d; however, this pattern is not consistent throughout the spectra. It is higher in the 1000–1200-cm

^{−1}and 1400–1600-cm

^{−1}regions. This can be explained by the fact that MC is not able to correct the background offset spectral completely in contrast to VN; VN is better because it scales the spectrum to unit vectors, whereas MC only changes the baseline.

^{2}(100 points) were generated with the %DNT predicted from the NAT-S model using VN preprocessing. The mappings are present in Supplementary Materials: Figure S12, while predictions for each point are illustrated in Figure 4d. It can be noticed from the figure that the predictions were lower than the true values. This indicates that the NAT-S model quantifies below the true value of 10% DNT; however, it is capable of predicting the existence of an explosive in a soil. This indicates that the technique should be used in known soil to have good quantification.

^{−1}. PETN and RDX are nitro aliphatic explosives. TNT is very similar to DNT but considered to be a more challenging interference. Predictions for these samples were generated using the calibration curves for DNT/NAT-S. While the predicted values should have been zero or close to zero because the samples did not contain DNT, the average predicted concentrations were 8.8% (BA), 3.1% (IBP), -8.0% (PETN), 2.3% (RDX), and 25.8% (TNT) (see Table 4). The objective was to measure the model’s capability of discriminating against these interferences.

#### 3.2. Pattern Recognition Analysis

## 4. Conclusions

## Supplementary Materials

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 2.**Predicted values vs. real values of % DNT from partial least squares (PLS) models on the matrices: (

**a**) bentonite clay (BC); (

**b**) synthetic soil (SYN-S); (

**c**) natural soil (NAT-S); (

**d**) NAT-S to low concentration (NAT-S Low); (

**e**) potassium bromide (KBr) and (

**f**) KBr to low concentration (KBr Low).

**Figure 3.**DNT spectrum in the investigated soil matrices in K-M (K-M for T: K-M calculate from transmittance for red spectra): (

**a**) BC; (

**b**) SYN-S; (

**c**) NAT-S, and (

**d**) NAT-S Low.

**Figure 4.**Predictions for 100 pts. maps after applying VN, mean centering (MC), and no preprocessing: (

**a**) simple mixture (NAT-S-M); (

**b**) macerating the components (NAT-S-MM); (

**c**) Averages for the predictions of NAT-S-M and NAT-S-MM (

**d**) NAT2-S with VN preprocessing.

**Figure 5.**Dependence of the root-mean-square error of cross-validation (RMSECV) on LV for the considered interferences: the NAT-S optimization of the validation set (OPT-Val) and NAT-S models.

**Figure 6.**(

**a**) Probability distribution of the sum of ${\beta}_{j}$ (SUM (β)) for a simple binary discrimination model. (

**b**) Comparison of the tested machine learning (ML) methods in terms of their log-loss and accuracy.

**Table 1.**Accuracy, bias, and the number of latent variables (LV) for the studied models and matrices.

RMSEE | RMSECV | RMSEP | Bias | LV | |
---|---|---|---|---|---|

BC | 0.41 | 0.57 | 0.70 | −0.0007 | 11 |

SYN-S | 0.35 | 0.43 | 0.53 | −0.0069 | 7 |

NAT-S | 0.25 | 0.39 | 0.39 | 0.0010 | 10 |

NAT-S Low | 0.08 | 0.10 | 0.34 | 0.0100 | 7 |

KBr | 0.18 | 0.32 | 0.41 | −0.0044 | 10 |

KBr Low | 0.02 | 0.03 | 0.08 | −0.0003 | 9 |

**Table 2.**Relative standard deviation (RSD) and relative predictive determinant (RPD) values for the models generated.

RSDr | RSDh | RSDrd | RPD-CV | RPD-Test | |
---|---|---|---|---|---|

BC | 2.4 | 5.5 | 12.7 | 9.7 | 11.3 |

SYN-S | 1.5 | 5.3 | 9.1 | 9.2 | 16.2 |

NAT-S | 2.2 | 4.2 | 4.8 | 16.1 | 27.6 |

NAT-S Low | 3.2 | 13.8 | 32.9 | 9.1 | 12.6 |

KBr | 3.4 | 3.6 | 4.1 | 19.8 | 36.5 |

KBr Low | 5.7 | 6.2 | 11.9 | 27.5 | 50.1 |

**Table 3.**Sensitivity, analytical sensitivity, the limit of detection (LOD), and noise of the models, with and without vector normalization (VN) preprocessing.

VN | ||||

LOD | SEN | γ | γ^{−1} | |

BC | 1.4 | 0.013 | 43 | 0.023 |

SYN-S | 1.2 | 0.016 | 53 | 0.019 |

NAT-S | 0.8 | 0.010 | 32 | 0.031 |

NAT-S Low | 0.3 | 0.087 | 289 | 0.003 |

Noise = 0.0003 | ||||

No Preprocessing | ||||

LOD | SEN | γ | γ^{−1} | |

BC | 2.3 | 1.48 | 39 | 0.025 |

SYN-S | 1.4 | 2.25 | 60 | 0.017 |

NAT-S | 1.0 | 1.36 | 36 | 0.028 |

NAT-S Low | 0.6 | 20.63 | 552 | 0.002 |

KBr | 0.6 | 0.51 | 14 | 0.072 |

KBr Low | 0.07 | 1.26 | 34 | 0.029 |

Noise = 0.037 |

NAT-S | NAT-S OPT-Val | |
---|---|---|

BA | 8.8 | 0.02 |

IBP | 3.8 | −0.02 |

PENT | −8.0 | 0.00 |

RDX | 2.3 | 0.02 |

TNT | 25.8 | 0.11 |

NAT-S | NAT-S OPT-Val | |
---|---|---|

R^{2} cal | 99.87 | 99.65 |

R^{2} val | 99.61 | 98.92 |

R^{2} test | 99.63 | 98.07 |

RMSEE | 0.245% | 0.425% |

RMSECV | 0.39% | 0.72% |

RMSEP | 0.39% | 0.88% |

Bias | 0.001 | −0.0016 |

LV | 10 | 19 |

ho | 0.06 | 0.021 |

LOD | 0.80% | 1.4% |

RPD | 16.1 | 9.61 |

**Table 6.**Confusion matrix and evaluation criteria for the binary discrimination model using SUM (β) parameters.

Precision | Recall | F1-Score | Support | Accuracy | Matrix of Confusion | ||
---|---|---|---|---|---|---|---|

Model | EXP | NONE | |||||

EXP | 0.768 | 1.000 | 0.869 | 1126 | 0.877 | 865 | 261 |

NONE | 1.000 | 0.793 | 0.884 | 997 | 0 | 997 |

Precision | Recall | f1-Score | Support | Accuracy | Confusion Matrix | ||||
---|---|---|---|---|---|---|---|---|---|

Model | DNT | NONE | RDX | TNT | |||||

DNT | 0.997 | 0.997 | 0.997 | 347 | 0.997 | 346 | 1 | 0 | 0 |

NONE | 1.000 | 0.996 | 0.998 | 564 | 0 | 564 | 0 | 0 | |

RDX | 0.993 | 1.000 | 0.997 | 149 | 1 | 0 | 148 | 0 | |

TNT | 0.990 | 1.000 | 0.995 | 100 | 0 | 1 | 0 | 99 | |

Test | DNT | NONE | RDX | TNT | |||||

DNT | 1.000 | 0.997 | 0.998 | 289 | 0.996 | 289 | 0 | 0 | 0 |

NONE | 0.993 | 1.000 | 0.997 | 433 | 0 | 430 | 1 | 2 | |

RDX | 0.993 | 0.993 | 0.993 | 139 | 1 | 0 | 138 | 0 | |

TNT | 1.000 | 0.981 | 0.990 | 102 | 0 | 0 | 0 | 102 |

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## Share and Cite

**MDPI and ACS Style**

Pacheco-Londoño, L.C.; Warren, E.; Galán-Freyle, N.J.; Villarreal-González, R.; Aparicio-Bolaño, J.A.; Ospina-Castro, M.L.; Shih, W.-C.; Hernández-Rivera, S.P.
Mid-Infrared Laser Spectroscopy Detection and Quantification of Explosives in Soils Using Multivariate Analysis and Artificial Intelligence. *Appl. Sci.* **2020**, *10*, 4178.
https://doi.org/10.3390/app10124178

**AMA Style**

Pacheco-Londoño LC, Warren E, Galán-Freyle NJ, Villarreal-González R, Aparicio-Bolaño JA, Ospina-Castro ML, Shih W-C, Hernández-Rivera SP.
Mid-Infrared Laser Spectroscopy Detection and Quantification of Explosives in Soils Using Multivariate Analysis and Artificial Intelligence. *Applied Sciences*. 2020; 10(12):4178.
https://doi.org/10.3390/app10124178

**Chicago/Turabian Style**

Pacheco-Londoño, Leonardo C., Eric Warren, Nataly J. Galán-Freyle, Reynaldo Villarreal-González, Joaquín A. Aparicio-Bolaño, María L. Ospina-Castro, Wei-Chuan Shih, and Samuel P. Hernández-Rivera.
2020. "Mid-Infrared Laser Spectroscopy Detection and Quantification of Explosives in Soils Using Multivariate Analysis and Artificial Intelligence" *Applied Sciences* 10, no. 12: 4178.
https://doi.org/10.3390/app10124178