Hadamard Transform Photothermal Deflection Imaging.

Abstract

A photothermal deflection densitometer based on CW laser illumination was characterized. The system was used for quantitative analysis of compounds separated by thin layer chromatography. First application of pulsed photothermal deflection spectroscopy to solid samples was demonstrated. A new densitometer for thin layer chromatography based on this method was described. A technique was shown to be suitable for direct measurements in the ultraviolet region where most biological compounds absorb. A pulsed excimer laser was used to produce a transient thermal refractive index gradient. With low energy pulses, typically less than 2 mJ, the system was more sensitive than any currently existing thin layer chromatography reflectance or photothermal densitometer. The signal was linear with the amount of analyte for about three orders of magnitude. A new technique, Hadamard matrix coding of the heating beam, was developed to avoid the intense illumination of the surface needed for sensitive measurements and minimize unwanted sample transformations. Hadamard masks were used to encode a line-focused laser beam to obtain spatially multiplexed photothermal detection signals. The spatial distribution and relative quantities of samples on silica were recovered by Hadamard transformation of the matrix of the encoded signals. The sensitivity and linear dynamic range of the Hadamard system was tested by using an expanded and line-focused Ar('+) laser beam. Results were comparable to single point measurements at similar delivered powers. The power delivered to each sample spot was about two orders of magnitude smaller than in a single point measurement. Therefore, sample decomposition, photoisomerization and photochemical transformations are eliminated. These problems are commonly encountered when samples are illuminated with intense laser illumination are voided. Hadamard masks with 125-509 elements and 25-200 (mu)m element widths were used to study the dynamic range and the resolution capability of the system. The effects of mask aperture width, mask motion, mask construction technology and analog-to-digital converter dynamic range were investigated. Spatial resolution was found to depend on element width and mask motion. The technique was to encode information efficiently. A 12 bit A/D converter was shown to be adequate for most situations.