Valery Bulatov

Study of matrix effects in laser plasma spectroscopy by shock wave propagation

Abstract The origin of analytical matrix effects in laser plasma spectroscopy was investigated. We focused on matrix effects in sand/soil mixtures and attempted to explain the increase in the spectral response of trace elements (at constant concentration) with sand percentage. First, it was found that the energy coupled in the plasma, and which can be calculated from the propagation of the laser induced shock wave, indeed characterizes the matrix. A simple experimental setup for such measurements was suggested. Our results indicate that previous explanations of the matrix effects in this system may not be correct. We suggested that the main matrix effects were attributed to the depth of the laser-induced crater, which was correlated to a portion of the laser energy that penetrates into sand particulates and does not cause direct ablation. This explanation holds when no other effects are present (e.g. grain size distribution). The hypothesis was validated by experimental data.

Aerosol analysis by cavity-ring-down laser spectroscopy

The cavity-ring-down technique was applied for aerosol detection. The experimental set-up was based on a pulsed dye laser pumped with the third harmonic of an Nd:YAG laser. Validation of the method was performed using calibrated aerosol flows, all under ambient conditions. The method was exemplified with non-absorbing aerosols, such as NaCl and CuCl 2·2H2O, of various sizes and concentrations. The results were used for the evaluation of the corresponding aerosol extinction coefficients as a function of size, shape and index of refraction. The thus obtained aerosol extinction efficiencies were compared to theoretical models. Good agreement with theory was observed for NaCl aerosols, while the results for CuCl2·2H2O particulates required averaging over particle size and over the orientation dependent index of refraction. The actual sensitivity currently achieved was as low as an extinction coefficient of 8 ×10 −8 cm −1 , which means detection capability of about six water micro-particulates per cm 3 . The ultimate theoretical performance of this method for aerosol detection was estimated as an extinction coefficient of 1.4 × 10 −12 cm −1 , corresponding to about 100 micro-particulates per m 3 . These figures indicate that this method has the

Spectroscopic imaging of laser-induced plasma

Spectroscopic imaging provides 2D images with full spectral resolution at each pixel. Thus, chemical imaging of an object, as well as other useful information, can be obtained. An imaging spectroscopy method in the visible range is presented and applied to laser plasma. This is a powerful research tool with numerous possible applications. This study is focused on spectroscopic imaging of laser-produced plasmas, and such spectral images (full spectrum at each pixel) are presented for the first time. Detailed information on optical and geometrical effects are obtained, and an insight to the optimization of the laser plasma spectroscopy method is achieved. The size and the spatial shape of the plasma, which can be used for matrix effect compensation, are measured. Similarity maps and classification maps of laser-induced plasma are obtained for the first time. These maps are used for allocation of chemical components in the plasma. The signal to noise ratio maps of the spectra obtained from laser-induced plasmas are provided. These surfaces possess a clear maximum, indicating that there is a preferred site in the plasma, where the emitted light provides the best signal to noise ratio. The performance of the current method is limited by the lack of temporal resolution, although it can be extended by a proper temporal gating.

On-line remote prediction of gasoline properties by combined optical methods

Abstract On-line prediction of 10 gasoline properties, such as research and motor octane numbers, vapor pressure, API gravity, aromatic contents, etc., are carried out on-line by a remotely operated detector coupled to the main control unit by communication optical fibers. This information is of considerable importance since it is needed for process monitoring and for the preparation of final petrochemical products of well defined properties. The currently available spectroscopic methods for predicting gasoline properties, which are based on near infrared (NIR) (and recently on Fourier transform infrared (FTIR) spectrometry) coupled with chemometric algorithms, provide limited performance (e.g. 0.4 standard error of prediction (SEP), for octane number). We propose an improvement of the performance by applying combined optical methods. In this study we combine on-line information from a short-wave NIR photodiode array spectrometer (700–1000 nm) with laser induced fluorescence (LIF) spectra obtained with a PC-plugged in linear CCD spectrometer. UV excitation is performed by harmonics of a compact and low-cost Nd:YAG laser, and the system is remotely operated through optical fibers. A comparison between the octane number predictions by NIR spectrometry and by LIF from third and fourth harmonics, is provided. It is shown that the addition of the fluorescence information improves octane number prediction (

Fast aerosol analysis by Fourier transform imaging fluorescence microscopy

Fourier transform imaging spectroscopy was combined with fluorescence microscopy and a cooled CCD detector for fast analysis of aerosols contaminated with polycyclic aromatic hydrocarbons (PAHs). Aerosols were collected on glass fiber filters and inspected, for the first time, by this imaging technique, which provides a full fluorescence spectrum at each pixel. Mapping of PAH contamination was carried out and used for identification and quantification of the compounds. Quantification limits (based on 95% confidence intervals of calibration plots) in the 10 ng cm(-)(2) range on filter are reported, which corresponds to 20 ng m(-)(3) in air, integrated in 1 min. The absolute detection limit (on filter) is estimated as low as 0.25 pg, corresponding to an air concentration of 0.5 pg m(-)(3), integrated in 1 min. The method is examined for analysis of monocomponent contamination and for simple mixtures. After a proper automation, this method has the potential to provide in situ and on-line results regarding particulate airborne PAH contaminations.

Dental Enamel Caries (Early) Diagnosis and Mapping by Laser Raman Spectral Imaging

Abstract A method for estimating the extent of tooth caries and providing imaging information based on Raman Spectral Imaging is suggested. This non‐destructive optical method is able to characterize and differentiate between normal enamel tooth surface, and initial and advanced tooth caries. Images and corresponding spectra were acquired from various tooth sites, and it was demonstrated that normal, white opaque, brown discoloured, and pitted tooth surfaces all have different distinct spectral features which characterize the different degrees of dental caries. Spectral analysis allows for detection of early changes in the surfaces of carious teeth, and the associated mapping capability allows for morphological characterization. It was found that the emission at 960 cm−1, which corresponds to PO stretching in the hydroxyapatite bond, is the most significant and can be used for diagnosis of caries. The emissions at 1070 cm−1 and at 590 cm−1 can also be applied, but are less accurate. The results suggest that this technique may be further developed and applied for clinical diagnosis of initial and more advanced demineralization processes of the enamel tooth surfaces.

Particulate material analysis by a laser ionization fast conductivity method. Water content effects.

Combined laser multiphoton ionization and fast-conductivity methods have been applied to probe organic contamination on wet particulate samples. This is a first attempt of testing such a technique for this purpose. A special emphasis has been made on establishing correlation between sample water content and detection of pyrene, which has been used as a probe contaminant in this study. Environmental (soil) and artificial samples (silica gel) have been studied. The experimental setup includes a pulsed N(2) laser and a fast-conductivity detection system, operated in an opened chamber under ambient conditions. The correlation between the observed photoionization signals and water content has been investigated in a slow-drying mode, where water has been gradually evaporated. Surface contamination of wet samples has been more efficient than moist ones. This was evident, in the case of soil samples, from the earlier appearance (at lower pyrene concentrations) of saturation-type features of the detected photoionization signals. Assumptions of a contamination model has enabled certain compensation for sample and matrix effects.

Laser two-photon ionization of pyrene on contaminated soils.

We report a first attempt to use the laser multiphoton ionization method for analysis of trace aromatic compounds on the surface of environmental (soils) and artificial (silica) samples. The measurement setup is composed of a N(2) pulsed laser and a fast conductivity detection system. The technique has been tested for detection of pyrene deposited on moist fine-powdered samples. The observed photoionization signals have indicated a gradual increase in the photoionization current and charge as a function of increasing concentration of pyrene/hexane solutions used for sample contamination. Contaminants have been analyzed in several (organic and inorganic) environmental samples, and a method to compensate for matrix effects is suggested. A contamination model is assumed and applied in order to renormalize all signals and provide an useful calibration plot. This calibration plot provides an upper estimate of pyrene LOD as 35 ng/g.

Classification and quantification of polycyclic aromatic hydrocarbons on quartz sand particles by direct Fourier transform imaging fluorescence

A new method for analysis of PAH contamination deposited on quartz sand was developed. It is based on direct observation of the UV-induced fluorescence using a microscope and a specially designed imaging spectrometer which provides full fluorescence spectra at each pixel. The method is fast and requires no sample preparation. Linear calibration plots were obtained for monocomponent contaminations and for simple PAH mixtures. Absolute detection limits of a few pg are reported, which correspond to bulk concentrations in the m gg ˇ 1 range. In addition to concentrations, the method provides microscopic chemical images of the contaminants, which may provide useful source characterization. # 1998 Elsevier Science B.V. All rights reserved.

Application of pulsed laser methods to in situ probing of highway originated pollutants

Abstract A possibility of a portable field kit for studying traffic originated pollutants based on laser induced fluorescence (LIF) or multi-photon ionization fast-conductivity (MPI-FC) has been addressed. The resulting instrumentation has been successfully tested on soil samples that were collected and classified according to their radial distance, R, from a busy highway. The sampling was done in the near-field range with R varying from 3 to 50 m. Organic material from the samples was transferred into hexane solutions by short-time shaking with pure hexane. These solutions were investigated by simple LIF and MPI-FC instruments, which can be engineered as portable units (e.g., a PC plugged-in spectrometer has been used, in which case both the signal excitation and detection were accessed by means of optical fibers). Detected traces showed a nearly 1 R fall-off of the LIF and MPI-FC experimental data as a function of the sample distance from the road. It is estimated that at 50 m from the road the contamination concentrations (about 90% aliphatics and 10% aromatics) are as high as 0.1 μg g−1. Both proposed methods provide good sensitivity at the relevant concentration range and may be applied for fast screening of traffic contamination of soils.