Technology Profile

Photoacoustic Spectroscopy (PAS) was invented by Alexander Graham Bell in 1880 during his research to develop the photophone, a device that communicated via light waves. Bell was successful in demonstrating PAS and the photophone but practical devices for PAS and optical communications have taken approximately a hundred years to develop to the point of great utility.

FASTER, LOWER COST. FTIR photoacoustic spectroscopy continues to become a more cost effective and easy to use technique with expanded capabilities for both depth profiling and spectral range. In the last ten years, FTIR-based PAS performance has improv ed markedly. Ten years ago it was not uncommon to signal-average overnight to obtain a single useful FTIR photoacoustic spectrum. Today a polymer spectrum for quantitative identification purposes can be obtained with a single scan and searched against a spectral library to produce the identification in a total analysis time of 5 seconds. This speed combined with the little or no sample preparation aspect of PAS makes the technique a natural for at-line or laboratory analyses where sample throughput spee d is important. The need for instrument economy is also addressed by the newer low end FTIR/PAS systems.

DEPTH-PROFILING IN MICRONS. FTIR/PAS performance levels for depth-profiling have also improved dramatically due to the introduction of step-scan FTIR spectrometers which are now available from Bio-Rad, Bruker, and Nicolet. These new modestly priced re search grade instruments allow both the magnitude and phase of the photoacoustic signal to be easily analyzed. This facilitates illucidation of depth dependent molecular information due to gradient or layered compositions. FTIR/PAS data can be plotted s o that the relative depths of absorption can be identified for specific infrared absorbance bands. This aids in determining the ordering of layers in layered materials on a micron dimensional scale which is not accessible with an infrared microscope. Th ese scanning capabilities combined with advances in signal processing allow sensing depth to be varied from less than a micrometer to over 100 micrometers in polymers. The expanded mirror velocity range of the new generation of research grade spectromete rs also opens the door to measurements from the ultraviolet through the far infrared spectral regions. This is especially advantageous for PAS measurements because the technique's minimal sample preparation requirement and nondestructive capability have always made it applicable in many areas of chemical and biological research.

EASY-TO-USE MULTISAMPLER. In addition to the performance/price advances of FTIR/PAS, molecular spectroscopists will find that the current MTEC Model 300 photoacoustic detector has advanced significantly in ease-of-use over earlier models. The MTEC 300 is installed by sliding it into the standard FTIR sample slide mount fixture. A purge coupler allows a closed purge path to the detector. Sample insertion and purge operations are controlled by a single lever. Power is supplied by an international de sk-top power supply. All of the features of the MTEC 300 including optional diffuse reflectance, transmittance, and microsampling capabilities are unsurpassed in FTIR sampling technologies. This combination of capabilities permits the model 300 to measu re a spectrum of nearly any condensed sample without regard to opacity, surface character, or reflectivity. The spectra produced can be used for both qualitative and quantitative analyses.

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