Almon P. Fisher

Quantum Cascade Laser-Based Photoacoustic Spectroscopy for Trace Vapor Detection and Molecular Discrimination

We report on the development of a microelectromechanical systems (MEMS)-scale photoacoustic sensor for the detection of trace gases. A mid-infrared quantum cascade laser (QCL) was used to determine detection limits for acetic acid, acetone, 1,4-dioxane, and vinyl acetate. The source was continuously tunable from 1015 cm−1 to 1240 cm−1, allowing for the collection of photoacoustic vibrational spectra for these gases. Exceptional agreement between the measured photoacoustic spectra and the infrared spectra for acetic acid, acetone, 1,4-dioxane, and vinyl acetate was observed. Partial least-squares (PLS) regression was used to develop an algorithm for classification of these compounds based solely on photoacoustic spectra.

Photoacoustic spectroscopy for trace vapor detection and molecular discrimination

Photoacoustic spectroscopy (PAS) is a useful monitoring technique that is well suited for trace gas detection. This method routinely exhibits detection limits at the parts-per-million (ppm) or parts-per-billion (ppb) level for gaseous samples. PAS also possesses favorable detection characteristics when the system dimensions are scaled to a microsystem design. Current research utilizes quantum cascade lasers (QCLs) in combination with micro-electromechanical systems (MEMS)-scale photoacoustic cell designs. This sensing platform has provided favorable detection limits for a standard nerve agent simulant. The objective of the present work is to demonstrate an extremely versatile MEMS-scale photoacoustic sensor system that is able to discriminate between different analytes of interest.

Wafer Level Assembly Methods for Complex Pathway Micro-Fluidic PCR Reactor

Micro-fluidic devices were fabricated that amplify DNA segments using the polymerase chain reaction (PCR). These devices employ a unique passive heating methodology that provides ultra-fast fluidic temperature transitions. The devices consist of alternating layers of thermally conductive and insulative layers. The highly thermally conductive layers consist of silicon etched to form vias and micro-channels. The thermally insulative layers were made of polyetherimide layers with laser ablated vias. Individual layers of materials were aligned and joined together to create this device. Novel joining technologies developed for the project included the epoxy specifically targeted for adhesive printing and the mechanical alignment methods. A custom formulated epoxy was created to give a submicron bond line that is controllable, strong, and highly resistant to aqueous and solvent exposure. The bonding temperature was less than 200degC, creating leak-free continuous micro-fluidic pathways. Biocompatible coatings were applied to the entire length of the 600 mm internal pathway before use. The device was used to successfully demonstrate PCR reaction times of less than 5 minutes; this is in comparison to the conventional methods which take several hours.