Development of Micro Gas Chromatographs and Micro Photoionization Detectors

Abstract

Micro gas chromatographs are miniaturized analytical instruments developed based on gas chromatography (GC) and Micro-Electro-Mechanical Systems technologies for fast, on-site, volatile compound analysis. The small footprint of these devices enables portability, low power consumption, and fast analysis time. Due to these features, micro gas chromatographs can be applied to environmental monitoring, homeland security, and online quality tracking. This dissertation describes the development of prototype instruments as well as studies of core components such as micro preconcentrators, microcolumns, Deans switches, and micro vapor detectors. Two nondestructive photoionization detectors have been developed and verified to have ultra-low detection limits (few pg) as well as rapid response rates. The micro flow-through photoionization detector (micro-PID) integrates microfluidic channels and ionization chambers on monolithic chips, providing ‘on column detection’ for analytes. The micro-PID can be used at fluidic interfaces between each dimension in multidimensional GC systems for obtaining additional information for gas routing control and signal reconstruction. Helium dielectric barrier discharge photoionization detectors use helium plasma as an ionization source to emit photons with energies up to 17.5 eV. Thus, these detectors can serve as universal vapor sensors and will expand the application scope of micro GC systems. These detectors were used in an application on formaldehyde detection, which was studied with a heart cutting two-dimensional (2D) micro GC. Target compounds can be transferred into the second-dimensional system for further precise separation and detection. Analysis of other air pollution-related, toxic compounds was performed by the first-dimensional column and micro-PID simultaneously. The formaldehyde low detection limit of the novel 2D system was tested to be sub-ppb (V/V) level with 6 min of sampling. The design, fabrication, and evaluation of separation microcolumns are also discussed in this dissertation. A stationary phase thickness gradient coated microcolumn was developed to generate high-resolution chromatograms by introducing a peak focusing effect. Additionally, a hybrid adhesive connection method was developed for high-temperature applications and evaluated to be leak free and volatile compound free up to 350 °C even after thermal cycling. By integrating all miniaturized components, including micro preconcentrators, microcolumns, and micro-PIDs, an ultra-compact micro GC system (147 g in weight and 0.14 L in volume) was constructed. By scaling down the system, we achieved high separation performance and low power consumption, which clearly separated 14 compounds in 3 minutes.