Variable Pathlength Cavity Spectroscopy Development of an Automated Prototype

Abstract

ABSTRACT VARIABLE PATHLENGTH CAVITY SPECTROSCOPY DEVELOPMENT OF AN AUTOMATED PROTOTYPE by Ryan Andrew Schmeling The University of Wisconsin-Milwaukee, 2016 Under the Supervision of Professor Joseph H. Aldstadt III Spectroscopy is the study of the interaction of electromagnetic radiation (EMR) with matter to probe the chemical and physical properties of atoms and molecules. The primary types of analytical spectroscopy are absorption, emission, and scattering methods. Absorption spectroscopy can quantitatively determine the chemical concentration of a given species in a sample by the relationship described by Beer’s Law. Upon inspection of Beer’s Law, it becomes apparent that for a given analyte concentration, the only experimental variable is the pathlength. Over the past ~75 years, several approaches to physically increasing the pathlength have been reported in the literature. These have included not only larger cuvettes and novel techniques such as Differential Optical Absorption Spectroscopy, but also numerous designs that are based upon the creation of an optical cavity in which multiple reflections through the sample are made possible. The cavity-based designs range from the White Cell (1942) to Cavity Ring-Down Spectroscopy (O'Keefe and Deacon, 1998). In the White Cell approach, the incident beam is directed off-axis to repeatedly reflect concave mirror surfaces. Numerous variations of the White Cell design have been reported, and it has found wide application in infrared absorption spectroscopy in what have become to be known as “light pipes”. In the CRDS design, on the other hand, highly reflective dielectric mirrors situated for on-axis reflections result in the measurement of the exponential decay of trapped light that passes through the exit mirror. CRDS has proven over the past two decades to be a powerful technique for ultra-trace analysis ( In this thesis, a novel approach to ultra-trace analysis by absorption spectroscopy is described. In this approach known as Variable Pathlength Cavity Spectroscopy (VPCS), a high finesse optical cavity is created by two flat, parallel, dielectric mirrors — one of which is rotating. Source light from a pulsed dye laser (488 nm) enters the optical cavity in the same manner as in Cavity Ring-Down Spectroscopy (CRDS), i.e., by passing through the cavity entrance mirror. However, unlike CRDS in which the mirrors are fixed, concave, and mechanically unaltered, the cavity exit mirror contains a slit (1.0 mm diameter) that is rotated at high speed on an axle, thereby transmitting a small fraction of the trapped light to a photomultiplier tube detector. In this approach, unlike CRDS, absorbance is measured directly. In previous prototype designs of the VPCS instrument, instrument control (alignment) and data acquisition and reduction were performed manually; these functions were both inefficient and tedious. Despite this, the VPCS was validated in "proof of concept" testing, as described with a previous prototype (Frost, 2011). Frost demonstrated that the pathlength enhancement increased 53-fold compared to single-pass absorption measurements in monitoring NO2 (g) at part-per-billion levels. The goal of the present work is to improve upon the previous prototype (“P4”) that required manual alignment, data collection, and data reduction by creating a completely automated version of VPCS — i.e., the “P5” prototype. By developing source code in LabVIEW™, demonstration that the VPCS can be completely controlled in an automated fashion is described. Computationally, a Field-Programmable Gate Array is used to automate the process of data collection and reduction in real-time. It is shown that the inputs and outputs of the P5 instrument can be continuously monitored, allowing for real-time triggering of the source laser, collection of all data, and reduction of the data to report absorbance. Furthermore, it is shown that the VPCS can be automatically aligned — also in real-time on the order of microseconds — to a high degree of precision by using servo-actuators that adjust the beam position based upon the input from a sensitive CCD camera. With the implementation of this hardware and LabVIEW code, more precise data collection and reduction is done. With this new fully automated design, the instrument characteristics (e.g., to include factors such as rotation speed, off-set angle, and pathlength variation) can improve the enhancement by ~130-fold vs. single-pass absorption measurements.