Markus W. Sigrist

Atmospheric pollution monitoring using CO2-laser photoacoustic spectroscopy and other techniques

The subject of air pollution monitoring is reviewed. First, we discuss the main conventional techniques currently applied to the detection of gaseous air pollutants, as well as the state of the art of novel detection schemes. We distinguish between nonspectroscopic and spectroscopic methods. Spectroscopic techniques are of primary interest since they offer several advantages, e.g., the simultaneous monitoring of numerous substances. Photoacoustic (PA) spectroscopy represents a promising spectroscopic technique due to its intrinsically high sensitivity, the large dynamic range, and the comparatively simple experimental arrangement. Emphasis is put on detection selectivity which often may restrict the range of applications of the technique for pollution monitoring due to the lack of powerful, continuously tunable IR laser sources. The theoretical aspects of PA spectroscopy with respect to trace gas detection and multicomponent analysis are thus discussed in detail. Some characteristics of a portable PA system built by us are outlined in order to illustrate the usefulness of such systems.

Laser‐generated stress waves in liquids

The generation of laser‐induced stress waves in liquids by the vaporization process and the thermoelastic effect was studied experimentally. A high‐speed camera and special high‐sensitivity stress transducers with a response time of a few nanoseconds have been used for these investigations. The experimental results obtained for water, n‐heptane, and carbon tetrachloride are discussed. For the first time, the individual contributions of vaporization and the thermoelastic effect on stress generation are separated. In addition, tunable high‐frequency acoustic waves, with frequencies up to 60 MHz, have been generated in water by the impact of a laser pulse exhibiting longitudinal mode beating. Since existing theories on the thermoelastic generation of acoustic waves do not yield satisfactory agreement with our experimental data, a new spherical model is proposed, where the transient heating caused by the laser impact, is represented by the three‐dimensional heat pole. This solution of the equation of heat con...

Cavity-ring-down principle for fiber-optic resonators: experimental realization of bending loss and evanescent-field sensing

A novel measurement principle for fiber-optic sensing is presented. Use of a cavity-ring-down scheme enables measurements of minute optical losses in high-finesse fiber-optic cavities. The loss may be induced by evanescent-field absorption, fiber bending, fiber degradation, Bragg gratings, or any other effect that might change the fiber transmission or cavity reflector properties. The principle is proved to be rather insensitive to ambient perturbations such as temperature changes. A high-sensitivity measurement of loss due to bending is presented as a proof-of-principle. With a cavity finesse of 627 a sensitivity for induced loss of 108 ppm (4.68 x 10(-4) dB) is achieved. Preliminary measurements of evanescent-field absorption are also discussed.

Trace gas monitoring by laser photoacoustic spectroscopy and related techniques (plenary)

Trace gas sensing systems have to meet several requirements like high detection sensitivity and selectivity, multicomponent capability, field suitability, etc. In this respect devices based on tunable lasers combined with appropriate detection schemes are attracting great interest. This report reviews recent developments demonstrating the potential of such systems. The performance of various spectroscopic systems with different lasers (gas lasers, nonlinear optical sources like optical parametric oscillators and difference frequency generation, near-infrared external cavity diode lasers, quantum cascade lasers) and different detection schemes (photoacoustic, multipass transmission, cavity ringdown) is discussed and illustrated with examples from various areas. Applications include laboratory analyses of multicomponent samples with isotopic selectivity, field measurements in ambient urban and rural air or even at volcanic sites, as well as investigations in the area of biology and medical diagnostics.

Photoacoustic spectroscopy with quantum cascade distributed-feedback lasers

We present photoacoustic (PA) spectroscopy measurements of carbon dioxide, methanol, and ammonia. The light source for the excitation was a single-mode quantum cascade distributed-feedback laser, which was operated in pulsed mode at moderate duty cycle and slightly below room temperature. Temperature tuning resulted in a typical wavelength range of 3cm(-1)at a linewidth of 0.2cm(-1). The setup was based on a Herriott multipass arrangement around the PA cell; the cell was equipped with a radial 16-microphone array to increase sensitivity. Despite the relatively small average laser power, the ammonia detection limit was 300 parts in 10(9)by volume.

Glucose sensing in human epidermis using mid-infrared photoacoustic detection

No reliable non-invasive glucose monitoring devices are currently available. We implemented a mid-infrared (MIR) photoacoustic (PA) setup to track glucose in vitro in deep epidermal layers, which represents a significant step towards non-invasive in vivo glucose measurements using MIR light. An external-cavity quantum-cascade laser (1010–1095 cm−1) and a PA cell of only 78 mm3 volume were employed to monitor glucose in epidermal skin. Skin samples are characterized by a high water content. Such samples investigated with an open-ended PA cell lead to varying conditions in the PA chamber (i.e., change of light absorption or relative humidity) and cause unstable signals. To circumvent variations in relative humidity and possible water condensation, the PA chamber was constantly ventilated by a 10 sccm N2 flow. By bringing the epidermal skin samples in contact with aqueous glucose solutions with different concentrations (i.e., 0.1–10 g/dl), the glucose concentration in the skin sample was varied through passive diffusion. The achieved detection limit for glucose in epidermal skin is 100 mg/dl (SNR=1). Although this lies within the human physiological range (30–500 mg/dl) further improvements are necessary to non-invasively monitor glucose levels of diabetes patients. Furthermore spectra of epidermal tissue with and without glucose content have been recorded with the tunable quantum-cascade laser, indicating that epidermal constituents do not impair glucose detection.

Determination of N2O isotopomers with quantum cascade laser based absorption spectroscopy

We present an analytical technique based on direct absorption laser spectroscopy for high precision and simultaneous determination of the mixing ratios of the most abundant nitrous oxide isotopic species: (14)N(15)N(16)O, (15)N(14)N(16)O and (14)N(2) (16)O. A precision of 0.5 ??? was achieved for the site specific isotope ratios of N(2)O at 90 ppm using an averaging time of 300 s.

Laser fibre deterioration and loss of power output during photo-selective 80-w potassium-titanyl-phosphate laser vaporisation of the prostate.

BACKGROUND The potassium-titanyl-phosphate (KTP) laser technique for photo-selective vaporisation of the prostate (PVP) has been regularly improved over the last decade. Nonetheless, decreasing efficiency of tissue vaporisation during the course of the operation and macroscopic alterations of the laser fibres tip are regularly observed and seem to affect the outcome of this procedure. OBJECTIVE To investigate the course of power output and to determine the type and extent of fibre deterioration during PVP. DESIGN, SETTING, AND PARTICIPANTS Forty laser fibres were investigated during PVP in 35 consecutive patients with prostatic bladder outflow obstruction between January 2007 and August 2007 in a university hospital. INTERVENTION All patients underwent PVP performed by three different surgeons using the 80-W KTP laser. MEASUREMENTS Power output was measured at the beginning and regularly throughout PVP and throughout in vitro vaporisation without fibre-tissue contact. Microscopic documentation of the fibre tip was performed after the procedure. RESULTS AND LIMITATIONS Carbonisation and melting of the fibre tip was regularly visible and appeared to be more pronounced as more energy was applied. Additionally, 90% of the fibres showed a significant decrease of power output during PVP, resulting in an end-of-lifespan (ie, 275-kilojoule) median power output of 20% of the initial value. Final median power output after in vitro vaporisation was 83% of the starting value. The extent of the structural and functional changes might only be valid for the operative technique performed in this investigation. CONCLUSIONS Fibre deterioration caused significant reduction of power output during PVP. This finding is an explanation for the often observed decreasing efficiency of tissue ablation and may also be responsible for some of the typical drawbacks and complications of PVP. Hence, improvements in fibre quality are necessary to advance the efficiency of this technique.

Longitudinal resonant spectrophone for CO-laser photoacoustic spectroscopy

A photoacoustic (PA) system for monitoring gaseous air pollutants absorbing in the CO-laser range is presented. The characteristics of the CO laser and the interference caused by water-vapor absorption demand a special design of the PA cell and experimental setup. The optimum cell design was found by numerical simulation of the acoustic properties of various cell geometries. For this purpose a model using infinitesimal analogue acoustic impedances was developed. Based on a matrix formalism for fourterminals, a computer program was applied that permits the calculation of the frequency response of the PA signal amplitude at any position within a one-dimensional PA cell. Excellent agreement with experimental data is obtained. As a result, a new design for an acoustically resonant spectrophone with improved properties is presented. The response of the cell with aQ-factor of 52, operated at 555 Hz, is 2000 Pa cm/W.

Photoacoustic spectroscopy on trace gases with continuously tunable CO 2 laser

A novel photoacoustic (PA) system that uses a continuously tunable high-pressure CO(2) laser as radiation source is presented. A minimum detectable absorption coefficient of 10(-6) cm(-1) that is limited mainly by the desorption of absorbing species from the cell walls and by residual electromagnetic perturbation of the microphone electronics has currently been achieved. Although a linear dependence of the PA signal on the gas concentration has been observed over 4 orders of magnitude, the dependence on energy exhibits a nonlinear behavior owing to saturation effects in excellent agreement with a theoretical model. The calibration of the laser wavelength is performed by PA measurements on low-pressure CO(2) gas, resulting in an absolute accuracy of ± 10(-2) cm(-1). PA spectra are presented for carbon dioxide (CO(2)), ammonia (NH(3)), ozone (O(3)), ethylene (C(2)H(4)), methanol (CH(3)OH), ethanol (C(2)H(5)OH), and toluene (C(7)H(8)) in large parts of the laser emission range. The expected improvement in detection selectivity compared with that of studies with line-tunable CO(2) lasers is demonstrated with the aid of multicomponent trace-gas mixtures prepared with a gas-mixing unit. Good agreement is obtained between the known concentrations and the concentrations calculated on the basis of a fit with calibration spectra. Finally, the perspectives of the system concerning air analyses are discussed.