Jyrki Kauppinen

Multicomponent analysis of ft-ir spectra

In multicomponent analysis we have the IR spectrum of an unknown mixture of gases, for which we have no knowledge of the constituent gases, not to mention their partial pressures. Instead we know a large set of library spectra of pure molecular gases, measured in known pressures and with the same interferometer as the unknown sample. By using these pure spectra we ought to be able to calculate the partial pressures of the pure gases in the mixture, with error limits. The errors in the obtained values arise from the measurement noise in the spectra. Because the calculation of such partial pressures that best explain the mixture spectrum is a relatively simple task, we shall consider it only briefly and will concentrate on the calculation of their error limits. We shall also consider the optimal choice for a resolution that will provide error limits as small as possible and the application of the non-negativity constraint for the partial pressures. In addition to gas spectra, all the calculations apply to spectra of noninteracting liquids as well.

Diode laser-based photoacoustic spectroscopy with interferometrically-enhanced cantilever detection

A novel sensitive approach to detect weak pressure variations has been applied to tunable diode laser-based photoacoustic spectroscopy. The sensing device consists of a miniature silicon cantilever, the deflection of which is detected with a compact Michelson-type interferometer. The photoacoustic system has been applied to the detection of carbon dioxide (CO2) at 1572 nm with a distributed feedback diode laser. A noise equivalent sensitivity of 2.8 x 10-10 cm-1WHz-1/2 was demonstrated. Potential improvements of the technique are discussed.

Line-shape distortions in misaligned cube corner interferometers

Line-shape distortions caused by the misalignment of the moving cube mirror in Fourier transform spectrometers have been described. A method of studying and correcting these distortions is presented. By using this method we can estimate the accuracy of the line position, which is especially important in high-resolution Fourier transform spectroscopy. The method is verified in simulations, and in practice it has been used to align the Oulu Fourier transform spectrometer.

Photoacoustic Gas Analysis Using Interferometric Cantilever Microphone

Abstract Theoretical considerations and a simple but realistic model of the function of the cantilever‐based photoacoustic trace gas system are presented. The essential features of the cantilever dynamics, thermal characteristics, and noise models are derived. Some other related constructions are shown with the practical implementations of the real system.

Optimization of a Microphone for Photoacoustic Spectroscopy

The principles of optimizing a microphone for a photoacoustic detector in the nonresonant mode of operation are presented. Performance of the constructed optical cantilever microphone is compared to the commercial electret microphone by measuring the photoacoustic spectrum of methane with both systems. The sensitivity of the presented photoacoustic detector in the nonresonant mode of operation is 100 times higher with the optical cantilever microphone than with the electret microphone.

Instrumental Resolution Considerations for Fourier Transform Infrared Gas-Phase Spectroscopy:

Instrumental resolution has a significant effect on the performance of Fourier transform infrared (FT-IR) spectrometers used for gasphase analysis. Low-resolution FT-IR spectroscopy offers some valuable advantages compared with the traditional high-resolution FT-IR gas-phase spectroscopy, especially in nonlaboratory environments. First, high signal-to-noise ratio (SNR) spectra can be acquired in field conditions without the use of traditional liquid nitrogen-cooled detectors. Second, the dynamic range for quantitative analysis is larger for low-resolution spectroscopy than for high-resolution due to the lower absorbance values and lower noise levels. Third, spectral analysis speed is increased and data storage requirements are substantially reduced. The purpose of this study was to investigate the effect of instrumental resolution on FT-IR gas-phase analysis. The effects of spectral resolution on sensitivity, selectivity, accuracy, precision, spectral overlap, dynamic range, and nonlinearity are separately discussed.

Fabrication and characterization of an ultrasensitive acousto-optical cantilever

A cantilever-type silicon device for sensing changes in pressure has been designed, fabricated and characterized. The microfabrication process is based on two-sided etching of silicon-on insulator (SOI) wafers. The rectangular cantilevers are 9.5 µm thick, and cover an area of a few square millimeters. The cantilevers are surrounded by thick and tight frames, since on the three free sides there are only narrow, micrometer sized air gaps between the cantilever and the frame. This design and excellent mechanical properties of single crystal silicon enable sensitive detection of time-dependent gas pressure variations, i.e. acoustic waves. The mechanical properties of the cantilever have been characterized by analyzing its dynamic behavior. The resonance frequency and the mechanical vibrational mode patterns have been determined using finite-element method (FEM) simulations and laser interferometry. These results are found to be in good agreement with each other. Initially this mechanical door-like cantilever was designed to be used in ultra-high sensitivity photoacoustic gas sensing, but it can also be applied quite generally in various kinds of sound wave detection schemes.

Spectral line-shape distortions in Michelson interferometers due to off-focus radiation source

When the radiation source in a Michelson interferometer is placed in front of or behind the focal plane of the collimator, distortions arise in the spectral line shapes. The appearance and behavior of these distortions in a cube-corner interferometer are treated in this work. We shall also present a simple and fast method to calculate the true line position of a distorted line, and an easy rule of thumb to connect the off-focus shift and the amount of distortion. Calculation of the phase-error curves in the signal domain is treated as well. This calculation gives a possibility of correcting the distorted interferogram.

New Method for Spectral Line Shape Fitting and Critique on the Voigt Line Shape Model

In this work we introduce a new method for testing spectral line shape models and optimizing the parameters in any parametric model. Given some general parametric line shape and a piece of a spectrum, the method finds the optimal values for the parameters and gives a number which tells how well the spectrum under consideration is explained by that model. The number of spectral lines under analysis may be more than one, and their exact locations need not be known. This characteristic follows from the property that the method does not need the information about line positions and amplitudes at all. Thus, in the absence of a singlet line, a set of overlapping lines can also be analyzed. The analysis is carried out in the signal domain by utilizing linear prediction. Application examples of the method to a molecular spectrum measured in gas phase are given. The results suggest that the Voigt line shape, despite its common use, is not a correct model in molecular spectroscopy. Its limitations become evident when one is trying to enhance the resolution by linear prediction, which requires detailed knowledge of the line shape. Instead a stochastic model, which is also tested, turns out to be rather promising.

Large aperture cube corner interferometer with a resolution of 0.001 cm −1

The interferometer of the Fourier transform spectrometer at the University of Oulu has been modified so that the maximum instrumental resolution is better than 10(-3) cm(-1). The resolution of the previous interferometer was 4.5 x 10(-3) cm(-1). The present interferometer consists of large cube corner mirrors and a large Mylar beam splitter. Each corner mirror has been made with three flat mirrors on an adjustable supporting frame. The interferometer was already in practical use in 1985. The first spectra (H(2)O, CO(2), N(2)O, OCS) recorded on this interferometer have been presented in HANDBOOK OF INFRARED STANDARDS WITH SPECTRAL MAPS AND TRANSITION ASSIGNMENTS BETWEEN 3 AND 2600 microm, G. Guelachvili and K. Narahari Rao, Eds. (Academic, New York, 1986).