Lars Landström

Impact of data reduction on multivariate classification models built on spectral data from bio-samples

Multivariate data analysis methods have been used to evaluate single shot spectral data, obtained by laser induced breakdown spectroscopy (LIBS), from ten different biological samples (simulants and possible interferents in Biological Warfare Agent (BWA) detection applications). Spectral data as echellograms (2D CCD images) and extracted 1D spectra were used and the classification performance was studied as the number of input variables was altered. Principal component analysis (PCA) indicated a possibility to separate the samples due to spectral differences, and partial least squares discriminant analysis (PLS-DA) was applied to study the predictability in more detail. For full resolution 1D spectra, a normalization of the data mainly resulted in visual effects in the PCA score-plots without significant effect in predictability by the PLS-DA models, however, normalization improved the predictability if the amount of variables were heavily reduced. A quite strong data (variable) reduction could be performed on both the 1D and 2D data without losing significant predictability. Using similar amounts of variables, the prediction models performed better using the echellograms directly compared to the extracted 1D spectra. The problem of spectral data shift (relative ‘database’ spectra) was also investigated, where already small shifts cause the models to fail. However, after a selection of important variables and allowing certain regions for these variables, the impact of shift on predictability could be reduced.

Detection and monitoring of CWA and BWA using LIBS

Results related to laser induced breakdown spectroscopy (LIBS) as an analytical tool for applications regarding CWA and BWA detection/monitoring will be presented and discussed in this paper. A ‘real-time’ aerosol analysis set-up using LIBS on single μm-sized particles (sampled from ambient air into a particle stream) has been developed and evaluated. Here, a two-stage triggering unit ensures a high hit-rate of the sampled aerosol particles and the optical emission from the laser induced plasma is collected and coupled into an echelle spectrometer equipped with an intensified CCD detector. Each CCD image (echellogram), optimally originating from a single μm-sized particle, is then analyzed and the result treated by an alarm algorithm built from a database using multivariate data analysis. The database signatures of simulant agents and interferents were obtained in controlled atmospheres (aerosol chamber/wind tunnel) as well as from measurements in different ambient background. The LIBS bioaerosol system with alarm algorithm was also tested in ‘real-life’ settings (subway station) during simulant dispersions. Painted surfaces have also been analyzed by LIBS to obtain information about residues of organophosphates on, or within, the paint. Depth analysis has been performed, which illustrated the possibility to monitor diffusion and penetration behavior of neat CWAs and simulant chemicals in the paint layer by following the intensity of phosphorous emission lines in single shot LIBS spectra as function of number of laser pulses. In addition, LIBS analysis was also performed after simple ethanol decontamination procedures, after which P emission lines still could be observed. The possibilities and challenges associated with the different set-ups and applications will be briefly discussed in connection with the presented results.

Experimental examination of ultraviolet Raman cross sections of chemical warfare agent simulants

Laser induced Raman scattering from the commonly used chemical warfare agent simulants dimethyl sulfoxide, tributyl phosphate, triethyl phosphonoacetate was measured at excitation wavelengths ranging from 210 to 410 nm using a pulsed laser based spectrometer system with a probing distance of 1.4 m and with a field of view on the target of less than 1mm. For the purpose of comparison with well explored reference liquids the Raman scattering from simulants was measured in the form of an extended liquid surface layer on top of a silicon wafer. This way of measuring enabled direct comparison to the Raman scattering strength from cyclohexane. The reference Raman spectra were used to validate the signal strength of the simulants and the calibration of the experimental set up. Measured UV absorbance functions were used to calculate Raman cross sections. Established Raman cross sections of the simulants make it possible to use them as reference samples when measuring on chemical warfare agents in droplet form.

Spectroscopic investigation of substrates contaminated by chemical warfare agents

Laser induced breakdown spectroscopy (LIBS) and Attenuated Total Reflection Fourier Transform Infrared (ATR-FTIR) spectroscopy have been evaluated for the purpose of analyzing contaminated and decontaminated substrates, such as Si wafers and different Chemical Agent Resistant Coating (CARC) paint systems. As contaminants, sarin (GB) and soman (GD) were studied as well as two simulant organophosphate compounds, trimethyl phosphate (TMP) and tributyl phosphate (TBP). First, the clean substrates were analyzed to obtain reference spectra and to reveal any interfering spectral overlap. Phosphorous lines were used to indirectly follow the occurrence of organophosphates via LIBS, which was also utilized for depth profile analysis into the substrates. Infrared absorption spectroscopy was also applied as a tool to monitor any residual contaminants. As expected, results showed that no, or below the detection limit, diffusion of the contaminants occurred into Si or the CARC paints. However, on top of and within a simple cover paint it was possible to detect P emission lines by LIBS, and ATR-FTIR revealed the presence of, e.g., C–O–P, P–F, and PO vibrational bands, indicating GB and GD and/or breakdown products. The LIBS technique also showed promising results for monitoring diffusion processes within one paint matrix. Rinse and elution steps performed on the non-CARC paint had little effect as decontamination procedures.

Measurements of Raman scattering in the middle ultraviolet band from persistent chemical warfare agents

The very low Raman scattering cross section and the fluorescence background limit the measuring range of Raman based instruments operating in the visible or infrared band. We are exploring if laser excitation in the middle ultraviolet (UV) band between 200 and 300 nm is useful and advantageous for detection of persistent chemical warfare agents (CWA) on various kinds of surfaces. The UV Raman scattering from tabun, mustard gas, VX and relevant simulants in the form of liquid surface contaminations has been measured using a laboratory experimental setup with a short standoff distance around 1 meter. Droplets having a volume of 1 μl were irradiated with a tunable pulsed laser swept within the middle UV band. A general trend is that the signal strength moves through an optimum when the laser excitation wavelength is swept between 240 and 300 nm. The signal from tabun reaches a maximum around 265 nm, the signal from mustard gas around 275 nm. The Raman signal from VX is comparably weak. Raman imaging by the use of a narrow bandpass UV filter is also demonstrated.

Ultraviolet Raman scattering from V-agents

We present our latest experimental results on V-agent Raman scattering in the middle UV. The Raman scattering was examined using a pulsed tunable laser based spectrometer system. Neat droplets of the agents were placed on a silicon surface and irradiated with sequences of laser pulses. The Raman scattering was examined as a function of laser wavelength and accumulated exposure with a reduced level of exposure per pulse compared to our earlier investigations.

Raman imaging using fixed bandpass filter

By using fixed narrow band pass optical filtering and scanning the laser excitation wavelength, hyperspectral Raman imaging could be achieved. Experimental, proof-of-principle results from the Chemical Warfare Agent (CWA) tabun (GA) as well as the common CWA simulant tributyl phosphate (TBP) on different surfaces/substrates are presented and discussed.

Towards Fingermark Dating : A Raman Spectroscopy Proof-of-Concept Study

Fingermarks have, for a long time, been vital in the forensic community for the identification of individuals, and a possibility to non-destructively date the fingermarks would of course be beneficial. Raman spectroscopy is, herein, evaluated for the purpose of estimating the age of fingermarks deposits. Well-resolved spectra were non-destructively acquired to reveal spectral uniqueness, resembling those of epidermis, and several molecular markers were identified that showed different decay kinetics: carotenoids > squalene > unsaturated fatty acids > proteins. The degradation rates were accelerated, less pronounced for proteins, when samples were stored under ambient light conditions, likely owing to photo-oxidation. It is hypothesized that fibrous proteins are present and that oxidation of amino acid side chains can be observed both through Raman and fluorescence spectroscopy. Clearly, Raman spectroscopy is a useful technique to non-destructively study the aging processes of fingermarks.

Ultraviolet Raman scattering from persistent chemical warfare agents

Laser induced Raman scattering at excitation wavelengths in the middle ultraviolet was examined using a pulsed tunable laser based spectrometer system. Droplets of chemical warfare agents, with a volume of 2 μl, were placed on a silicon surface and irradiated with sequences of laser pulses. The Raman scattering from V-series nerve agents, Tabun (GA) and Mustard gas (HD) was studied with the aim of finding the optimum parameters and the requirements for a detection system. A particular emphasis was put on V-agents that have been previously shown to yield relatively weak Raman scattering in this excitation band.

Bioaerosol detection using single particle triggered LIBS

A prototype LIBS system, measuring on single aerosol particles sampled from ambient air, has been developed for BWA detection purposes. To further discriminate measurement sampling, a 405 nm induced fluorescence trigger stage has recently been incorporated. The induced fluorescence, as well as the scattered light, was measured on monodisperse NADH and NaCl aerosols in the ~1-7 μm range as well as on dispersions of various simulant bioaerosols and interferents. Finally, the discrimination of sampling for LIBS measurements was demonstrated on NADH particulates in high non-fluorescent aerosol background.