Mid-infrared Wave Research Articles

Floral Classification of Honey Using Mid-Infrared Spectroscopy and Surface Acoustic Wave Based z-Nose Sensor

Fourier transform infrared spectroscopy (FTIR) and z-Nose were used as screening tools for the identification and classification of honey from different floral sources. Honey samples were scanned using microattenuated total reflectance spectroscopy in the region of 600-4000 cm(-1). Spectral data were analyzed by principal component analysis, canonical variate analysis, and artificial neural network for classification of the different honey samples from a range of floral sources. Classification accuracy near 100% was achieved for clover (South Dakota), buckwheat (Missouri), basswood (New York), wildflower (Pennsylvania), orange blossom (California), carrot (Louisiana), and alfalfa (California) honey. The same honey samples were also analyzed using a surface acoustic wave based z-Nose technology via a chromatogram and a spectral approach, corrected for time shift and baseline shifts. On the basis of the volatile components of honey, the seven different floral honeys previously mentioned were successfully discriminated using the z-Nose approach. Classification models for FTIR and z-Nose were successfully validated (near 100% correct classification) using 20 samples of unknown honey from various floral sources. The developed FTIR and z-Nose methods were able to detect the floral origin of the seven different honey samples within 2-3 min based on the developed calibrations.

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

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.

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.

Thermal effects limitations in mid-infrared continuous wave optical parametric oscillators

Continuous-wave (cw) optical parametric oscillation (OPO) is a stringent test for the assessment of optical quality of nonlinear mid-infrared materials, because of the low intracavity loss (≤1% per round-trip) required to balance the weak cw parametric gain. We observed for the first time parametric oscillation at 2.53 μm (idler) and 1.265 μm (signal) using chalcopyrite AgGaS2 in a triply resonant configuration leading to a pump threshold of 90 mW at λp~845 nm. Strong thermal effects induced by the residual absorption of the crystal are reported and analyzed. In particular, long-term (>30 min) stable single mode-pair operation is observed owing to a passive thermal servo of the cavity optical path length. The effect of thermal lensing under cw operation is also analyzed. We compare the performance of the AgGaS2-TRO with those of a periodically poled lithium niobate (PPLN) OPO, subject to weaker thermal effects.

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.

Mid-infrared evanescent wave sensors - a novel approach for subsea monitoring

The sustained health of the marine ecosystems can be seen as a research task of substantial importance, since increasing pollution of the oceans poses a serious threat to the global water resources. Hence, understanding of the physical, chemical and biological parameters determining the condition of the oceans demands persistent monitoring of the aquatic environment. This requires the development of advanced screening instrumentation capable of continuous in situ and real-time monitoring of hazardous pollutants. Remotely operated optical sensor systems able to withstand harsh measurement conditions represent a promising alternative to conventional methods involving extensive sampling procedures. Thus, interest in emerging novel sensor techniques has substantially increased during the last decade. This paper summarizes first achievements towards the development of mid-infrared fibre-optic evanescent wave sensors operating in the spectral range 3-20 µm in seawater. This wavelength region is particularly attractive for optical sensing, since molecule-specific information due to the stimulation of ground vibrational modes of organic substances is provided. First results focusing on the detection of chlorinated hydrocarbons reveal the feasibility of this approach, with detection limits in the low ppb concentration range in artificial seawater. As well as the design of a miniaturized subsea sensor module based on Fourier transform infrared spectroscopy, possibly interfering parameters such as salinity and turbidity are investigated under laboratory conditions.