Manfred Karlowatz

Sol-gel based mid-infrared evanescent wave sensors for detection of organophosphate pesticides in aqueous solution

Abstract This work demonstrates the application of organically modified sol–gels as recognition layers combined with mid-infrared evanescent wave sensors for in situ detection of nitrated organics in aqueous media. Sol–gels were prepared by acid-catalyzed copolymerization of phenyltrimethoxysilane (PTMOS) and tetramethoxysilane (TMOS) and were spin-coated onto ZnSe attenuated total reflection (ATR) waveguides. These sensors were investigated with respect to their enrichment properties of selected organophosphates, i.e. parathion, fenitrothion and paraoxon, respectively, and their capability of suppressing interfering water background absorptions. Figures of merit are derived from calibration curves determined to assess sensitivity and reproducibility of the applied sensor system. It can be concluded that sol–gel coated infrared optical sensors enable reproducible detection of organophosphates down to the sub-ppm concentration range. Furthermore, measurement of spiked river water samples demonstrates feasibility as remote field sensor system. Once the required sensitivity is achieved, sol–gel based mid-infrared evanescent wave sensors have the potential of being an alternative to commonly applied biosensors for detection of organophosphates in environmental analysis, since they provide superior mechanical and chemical stability during application relevant periods of time.

In situ sensing of volatile organic compounds in groundwater: First field tests of a mid-infrared fiber-optic sensing system

A prototype mid-infrared sensor system for the determination of volatile organic pollutants in groundwater was developed and tested under real-world conditions. The sensor comprises a portable Fourier transform infrared spectrometer, coupled to the sensor head via mid-infrared transparent silver halide fiber-optic cables. A 10 cm unclad middle section of the 6-m-long fiber is coated with ethylene propylene copolymer in order to enrich the analytes within the penetration depth of the evanescent field protruding from the fiber sensor head. A mixture of tetrachloroethylene, dichlorobenzene, diethyl phthalate, and xylene isomers at concentrations in the low ppm region was investigated qualitatively and quantitatively in an artificial aquifer system filled with Munich gravel. This simulated real-world site at a pilot scale enables in situ studies of the sensor response and spreading of the pollutants injected into the system with controlled groundwater flow. The sensor head was immersed into a monitoring well of the aquifer system at a distance of 1 m downstream of the sample inlet and at a depth of 30 cm. Within one hour, the analytes were clearly identified in the fingerprint region of the IR spectrum (1300 to 700 cm−1). The results have been validated by head-space gas chromatography, using samples collected during the field measurement. Five out of six analytes could be discriminated simultaneously; for two of the analytes the quantitative results are in agreement with the reference analysis.

New Frontiers for Mid-Infrared Sensors: Towards Deep Sea Monitoring with a Submarine FT-IR Sensor System

A sub-sea deployable fiber-optic sensor system for the continuous determination of a range of environmentally relevant volatile organic compounds in seawater has been developed. The prototype of a robust, miniaturized Fourier transform infrared (FT-IR) spectrometer for in situ underwater pollution monitoring was designed, developed, and built in our research group. The assembled instrument is enclosed in a sealed aluminium pressure vessel and is capable of maintenance-free operation in an oceanic environment down to depths of at least 300 m. The whole system can be incorporated either in a tow frame or a remotely operated vehicle (ROV). A suitable fiber-optic sensor head was developed, optimized in terms of sensitivity and hydrodynamics, and connected to the underwater FT-IR spectrometer. Due to a modular system design, various other sensor head configurations could be realized and tested, ensuring facile adaptation of the instrument to future tasks. The sensor system was characterized in a series of laboratory and simulated field tests. The sensor proved to be capable of quantitatively detecting a range of chlorinated hydrocarbons and monocyclic aromatic hydrocarbons in seawater down to the low ppb (μg/L) concentration range, including mixtures of up to 6 components. It has been demonstrated that varying amounts of salinity, turbidity, or humic acids, as well as interfering seawater pollutants, such as aliphatic hydrocarbons or phenols, do not significantly influence the sensor characteristics. In addition, the sensor exhibits sufficient long-time stability and a low susceptibility to sensor fouling.

Chemically Tapered Silver Halide Fibers: An Approach for Increasing the Sensitivity of Mid-Infrared Evanescent Wave Sensors:

In this work an innovative etching technique for tapering silver halide fibers is introduced. As silver halides form soluble complexes with thiosulfate in aqueous solution, the fiber can be chemically tapered by an etching process, which also warrants a high quality of the fiber surface. The evanescent field sensitivity of thus obtained tapered fibers was raised by more than one order of magnitude, demonstrated by calibration curves of tetrachloroethylene in hexane recorded with a tapered sensor fiber coupled to a Fourier transform infrared (FT-IR) spectrometer.

The automated sample preparation system MixMaster for investigation of volatile organic compounds with mid-infrared evanescent wave spectroscopy

For efficient development assessment, and calibration of new chemical analyzers a large number of independently prepared samples of target analytes is necessary. Whereas mixing units for gas analysis are readily available, there is a lack of instrumentation for accurate preparation of liquid samples containing volatile organic compounds (VOCs). Manual preparation of liquid samples containing VOCs at trace concentration levels is a particularly challenging and time consuming task. Furthermore, regularly scheduled calibration of sensors and analyzer systems demands for computer controlled automated sample preparation systems. In this paper we present a novel liquid mixing device enabling extensive measurement series with focus on volatile organic compounds, facilitating analysis of water polluted by traces of volatile hydrocarbons. After discussing the mixing system and control software, first results obtained by coupling with an FT-IR spectrometer are reported. Properties of the mixing system are assessed by mid-infrared attenuated total reflection (ATR) spectroscopy of methanol-acetone mixtures and by investigation of multicomponent samples containing volatile hydrocarbons such as 1,2,4-trichlorobenzene and tetrachloroethylene. Obtained ATR spectra are evaluated by principal component regression (PCR) algorithms. It is demonstrated that the presented sample mixing device provides reliable multicomponent mixtures with sufficient accuracy and reproducibility at trace concentration levels.

Impact of shallow buried objects on the spectral properties of terrain features

This paper presents preliminary results of an investigation into the impact of buried objects on the environmental properties and electro-optical spectral characteristics of terrain features. This study focused on the analyses of various sensor information, including hyperspectral and thermal data, collected under a limited set of circumstances; these analyses include laboratory measurements and theoretical computations. A digital terrain model incorporating the relevant physical processes was also constructed to support these investigations. These analyses are particularly relevant to the detection of landmines and the exploitation of hyperspectral sensor data in this application. Results from these analysis efforts will be presented along with example spectral data and computational results.

The Influence of Wetting and Drying Cycles on Mid-Infrared Attenuated Total Reflection Spectra of Quartz: Understanding Spectroscopy of Disturbed Soil

Attenuated total reflection (ATR) spectroscopy is a well established optical technique investigating fundamental molecular vibrations in the mid-infrared (MIR) spectral regime for a wide variety of samples including liquids, thin films and powders. In the present study, first results simulating the influence of weathering processes on the spectral characteristics of soils are discussed. In particular, the effect of wetting and drying cycles on IR spectra of fine quartz (SiO2) powders has been investigated with ATR techniques. Resulting from a wetting and drying cycle, the sample spectra of quartz powders revealed significantly increased absorption intensities throughout the spectral region of interest (1400-600 cm-1). We hypothesize that this effect results from a higher packing density of the particles following the wetting procedure with the fines packed into interstitial spaces closer to the ATR waveguide surface. Moreover, a strong red shift of approx. 40 cm-1 of the absorption band assigned to asymmetric SiO4 stretching vibrations (1050 cm-1 to 1250 cm-1) could be observed. Both effects, increase in intensity and spectral shift, are reversed by mechanically disturbing the cemented powder after the wetting/drying cycle. Experiments with s- and p-polarized infrared radiation show similar (reversible) spectral shifts for this particular frequency range. It is expected that these findings will lead to better understanding of the spectral characteristics of soil in the mid-infrared spectral domain providing improved interpretation of data retrieved from disturbed soils e.g. potential landmine sites during hyperspectral imaging.

Phenomenological investigations for understanding spectral and polarimetric signatures of landmines

Georgia Tech is in the second year of a Multi-University Research Initiative designed to study the impact of environmental processes on optical signatures. In particular, this program is conducting phenomenological studies on hyperspectral and polarimetric signatures of various target classes in the visible and infrared wavebands. Initial research studies have focused on landmines and the impact of various environmental factors and processes (e.g., subsurface processes) on the resultant spectral infrared signatures. A variety of approaches have been employed in this research to gain a better understanding of the impact of the environment on the spectral and polarimetric characteristics of soil and landmine signatures. These approaches include theoretical analyses, physics-based signature modeling, field measurements, and laboratory studies. Results from these studies will be presented that underscore the importance of incorporating the subsurface processes into the signature analyses. The results of these analyses have been propagated to algorithm developers to permit the creation of more robust processing techniques based on these physical analyses and models. This paper will present an overview of the program, a review of the research investigations initiated over the past year, and a summary of the results from these initial investigations.