Richard A. Crocombe

Digital Signal Processing for Step-Scan Fourier Transform Infrared Photoacoustic Spectroscopy:

A software-based digital signal processing (DSP) method using the data system processor has been developed to demodulate the photoacoustic responses of a sample to the fundamental phase modulation frequency and its harmonic frequencies (up to the ninth harmonic) in step-scan FT-IR photoacoustic measurements, without the use of any additional hardware. The DSP algorithm and its sampling depth multiplexing advantages are compared to conventional hardware demodulation. Comparison of results from the DSP method to those from hardware demodulators are shown at both the phase modulation frequency and the harmonics, and application of the DSP method to step-scan photoacoustic measurements with phase modulation is discussed as it applies to obtaining depth profile information in heterogeneous materials.

FT-IR imaging of polymers: an industrial appraisal

Chemical structure and conformation and physical property anisotropy at the microscopic level can have a major influence on the macroscopic performance characteristics of polymer products. Images based on infrared spectral differences and changes generated using FT-IR microscopy techniques are becoming increasingly used to highlight both chemical structure variations and morphology gradients within polymer articles. This paper illustrates potential industrial applications to polymer characterisation and analysis using mid-infrared FT-IR microscopy systems fitted with focal plane array (FPA) detectors.

The Quantitative Analysis of a Model Fermentation Broth

There has been interest in the quantitative infrared spectroscopy of aqueous solutions for many years, but these studies have not been numerous, because even qualitative applications of this type have been regarded as difficult. This has been due to the poor sensitivity of dispersive infrared instruments, combined with the high-infrared absorptivity of water. Past applications have been largely limited to concentrated solutions in thinpathlength transmission cells, but some attenuated total reflectance [ATR] work has been reported. However, the greater sensitivity of Fourier transform instruments, combined with the use of high-throughput ATR liquid cells, enables quantitative measurements to be made in aqueous solutions at and below the 1 % level. A typical ATR cell, giving about 10 reflections, provides a reproducible, effective pathlength of about 12 microns in the mid-infrared when used with aqueous solutions. The crystal may be constructed from Germanium or ZnSe, and the accessory may be designed as a flowcell for continuous monitoring purposes. FT-IR liquid cell ATR has been used in conjunction with a number of studies of biological systems, and has recently been employed in the quantitative analysis of ethanol in gasohol. In this paper, the analysis of a model fermentation broth by liquid cell ATR is described.

Generalized Two-Dimensional Fourier Transform Infrared Photoacoustic Spectral Depth-Profiling Analysis

This paper reports the first application of a generalized two-dimensional (G2D) correlation method in photoacoustic spectral depth-profiling analysis of laminate/heterogeneous samples. In this method, photoacoustic magnitude spectra at different modulation frequencies are used to generate two-dimensional (2D) correlation spectra. The relative spatial origins of correlated signals are then determined from the signs of the corresponding contours. The unique features and advantages of this technique over previously reported depth-profiling methods are demonstrated and discussed. These include simplification of step-scan photoacoustic phase-modulation experiments as well as 2D correlation-analysis rules, avoidance of phase-related negative-band ambiguities, and enhancement of both spectral and depth resolutions, etc. The recently developed software-based digital signal-processing (DSP) technique for step-scan photoacoustic measurements offers an efficient means (sampling-depth multiplexing advantage) to collect within a single scan all necessary data for this type of 2D correlation analysis. Index Headings: Photoacoustic; PAS; Two-dimensional; Step-scan; FT-IR; Depth profiling; Polymer films; DSP.

Design and performance of a dynamically aligned step-scan interferometer

In the past decade advances in microprocessors have been applied in FT-IR spectrometers to deliver higher data system performance at substantially reduced cost. Now, advances in microprocessors and signal processing technology are being applied to overcome current performance limits in the FT-IR optical bench, and considerably expand their functionality. This paper describes the design and performance of a new generation of research-grade interferometer, the 896, using state-of-the-art technology. The major enhancements are: rapid auto-align (allowing interchanging of beamsplitters); dynamic alignment (giving active stabilization and allowing high performance operation over a wide variety of scan conditions); enhanced stability rapid-scan mode with general purpose kinetic software; step-scan mode (0.004 Hz to 800 Hz) with variable phase modulation; and dynamic range extension.

Applications of a dynamically aligned, rapid- and step-scan interferometer

Step-scanning FT-IR has recently attracted increasing attention, and has novel applications in photoacoustic spectroscopy, time-resolved spectroscopy, and in multiple modulation experiments like vibrational dichroism (where clear separation of the frequencies is critical) and 2-D IR studies. The design and performance of a new dynamically-aligned interferometer which is capable of kinetic scan rates, conventional rapid-scanning, and also step-scan operation have been described. This paper discusses applications of this interferometer.

Chemical imaging cameras: the first steps

This paper describes the use of a fast readout MCT focal plane array for spectrochemical imaging.

Performance and applications of a mid-infrared step-scan FT-IR spectroscopic imaging system

An IR spectroscopic imaging technique has been developed which combines a step-scan Fourier transform (FT) Michelson interferometer with focal plane array (FPA) image detection. The first reports utilized an indium antimonide FPA in the spectral region 3950-1975 cm-1. Mercury-cadmium- telluride (MCT) focal plane arrays are now commercially available, giving access to the spectral range 3950-800 cm-1, thus greatly broadening the application of the technique for chemical analysis in the mid IR region. This paper describes some of the instrumental considerations as well as applications in the use of an MCT array detector, for novel mid-IR spectroscopic imaging applications.

Frequency-resolved, phase-resolved and time-resolved step-scan Fourier transform infrared photoacoustic spectroscopy

This paper demonstrates, compares and discusses frequency-resolved, phase-resolved and time-resolved step-scan Fourier transform infrared photoacoustic spectroscopic approaches used in depth profiling chemical analysis of layered polymeric samples.

Advances in Photoacoustic Step-scan FT-IR Spectroscopy

An integrated system for FT-IR photoacoustic depthprofiling experiments is described, including data-visualization techniques. An example using a multilayer polymer is presented. Extensions of the current method to other spectral regions and using digital signal-processing are discussed.