M. Kraft

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.

Spectroscopy in the gas phase with GaAs/AlGaAs quantum-cascade lasers.

We demonstrate what we believe is the first application of the recently developed electrically pumped GaAs/AlGaAs quantum-cascade lasers in a spectroscopic gas-sensing system by use of hollow waveguides. Laser light with an emission maximum at 10.009 microm is used to investigate the mid-infrared absorption of ethene at atmospheric pressure. We used a 434-mm-long silver-coated silica hollow waveguide as a sensing element, which served as a gas absorption cell. Different mixtures of helium and ethene with known concentrations are flushed through the waveguide while the laser radiation that passes through the waveguide is analyzed with a Fourier-transform infrared spectrometer. The experimentally obtained discrete ethene spectrum agrees well with the calculated spectrum. A detection threshold of 250 parts per million is achieved with the current setup.

A Mid-Infrared Sensor for Monitoring of Chlorinated Hydrocarbons in the Marine Environment

Abstract As part of the European research project SOFIE - “Spectroscopy using Optical Fibres in the Marine Environment”, a portable sensor system for chlorinated hydrocarbons in seawater is being developed. This novel analytical tool for real-time in-situ monitoring of a particularly important class of seawater pollutants consists of a robust, miniaturised FT-IR spectrometer in a sealed aluminium pressure vessel and a fibre optic sensor head. In a laboratory set-up using an ATR-crystal as a simplified sensor head, the effect of potentially interfering substances, both of natural and anthropogenic origin, on the sensor response was tested. It was found that the sensor readings for a specific analyte are not susceptible to aliphatic and aromatic components as well as other chlorinated hydrocarbons up to concentrations well above the average levels to be encountered in the oceans. The same applies for the parameters salinity and turbidity. Consequently, the proposed sensor system should be well suited for real-world sub-sea applications.

Sensor head development for mid-infrared fibre-optic underwater sensors

In the process of developing a mid-infrared fibre-optic sensor for operation in the marine environment, a number of parameters influencing the sensor performance have to be taken into consideration. Optical engineering and spectroscopy are the key sciences when designing an optical sensor. Other disciplines strongly influencing the design considerations for this application are marine engineering and hydrodynamics. The development and performance of a sensor head for application in the marine environment is discussed in this paper. Following optical, chemical, hydrodynamic and stability considerations, a U-shaped sensor head was found to be the most suitable sensor geometry. A prototype of such a sensor head has been built, attached to a specially designed underwater FT-IR spectrometer and subjected to simulated real-world flume-tank tests under controlled conditions. In combination with a compatible IR spectrometer, this sensor head allowed the qualitative and quantitative detection of a range of environmentally relevant analytes in water down to ppb concentration levels. Furthermore, it could be demonstrated that the sensitivity can be increased significantly by using tapered fibres for the sensor head.

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.

Improved MOEMS based ultra rapid Fourier transform infrared spectrometer

We present an improved FTIR spectrometer using a novel MOEMS actuator and discuss in detail the properties of the MOEMS component and the resulting FT-IR sensor device. Spectral resolution and the spectral range allow making use of the inherent multi-analyte detection capabilities giving the spectroscopy platform an advantage over singlewavelength IR sensors. With its further miniaturization potential due to its MOEMS core, this compact, energy efficient and robust spectrometer can thus act as transducer for portable and ultra-lightweight spectroscopic IR sensors, e.g. all purpose hazardous vapor sensors, sensors for spaceborne and Micro-UAV based IR analysis, and many more.

GaAs/AlGaAs quantum cascade laser – a source for gas absorption spectroscopy

Abstract We report on an application of the recently developed electrically pumped GaAs/AlGaAs quantum cascade lasers in gas spectroscopy. Laser light with emission maximum at 10.009 μ m is used to investigate the absorption in side bands of the vibrational spectrum of ethene at atmospheric pressure. Different mixtures of helium and ethene with known concentrations are flushed through an absorption cell. Laser radiation passing through the gas absorption cell is analyzed using a Fourier transform spectrometer. The laser spectrum is modulated by the ethene absorption. The experimentally obtained discrete spectrum is compared with the Hitran database, showing full agreement.

Determination of stress in silicon wafers using Raman spectroscopy

With a strong industrial trend towards using thin silicon in semiconductor devices, process legacy-induced stresses are matter of increasing practical importance. A key problem here is a lack of suitable metrology equipment for measuring inherent substrate material stresses in the manufacturing line. To overcome this, the use of Raman microspectrometry as a tool for measuring stress levels and distributions quantitatively on entire productive wafers was researched. Combining model cases, theoretical considerations and real-world samples, it could be shown that Raman can provide the necessary analytical accuracy and reliability, allowing to relate ensuing stress states e.g. to different wafer thinning process parameters.

Real-time detection of flame-retardant additives in polymers and polymer blends with NIR imaging spectroscopy

The detection of flame retardants is critical for the recycling of polymers. To investigate the possibility of reliable purefraction sorting, a sample set containing a wide range of relevant polymers and polymer blends containing various practically relevant flame-retardant additives was produced and investigated. NIR point spectra were acquired with an FTIR laboratory spectrometer and hyper-spectral NIR images were obtained using a spectrograph-based hyper-spectral imaging system. The laboratory spectrometer measurements were used to assign spectral features to the corresponding chemical compounds and derive a chemometric model that can be used to detect flame-retardant additives. The hyperspectral NIR images were used to adapt the chemometric model to the spectral features present in the hyper-spectral image data for the real-time detection.