Progressive Cellular Architecture in Microscale Gas Chromatographs for Broad Chemical Analyses
Creators: name: Liao, Weilin
Publisher: name: My University
Year: 2021
Subjects: subject: Microscale gas chromatograph
subject: Electrical Engineering
subject: Engineering

description: Detection and identification of volatile organic compounds (VOCs) are needed in many applications, such as environmental monitoring, homeland security, and industrial pollutant monitoring. A gas chromatograph (GC) is an instrument that separates, detects, and identifies VOCs of interests by the characteristic individual retention time of each VOC. While conventional GCs provide gold-standard performance, they are benchtop instruments not designed for portability. Microscale gas chromatographs (µGCs) incorporate miniaturized GC components that are not only miniature and lightweight, but also manufacturable at low cost. These features facilitate portability and allow exploration of new architectures where the miniaturized components are arranged in ways that are conventionally difficult or impossible. This work investigates a µGC architecture that provides partial separation of the sampled analytes into groups during sampling, and subsequently further separation of each group using a short separation column tailored for each group. This architecture, named the progressive cellular architecture (PCA), incorporates a series of heterogeneous cells, each of which includes a preconcentrator and a separation column, which are tailored for a specific group of compounds within the overall range. The preconcentrators are connected in series during sampling to provide partial separation. During separation, each preconcentrator is sequentially heated to desorb its collected compounds into the corresponding column for separation. The overall benefit of the PCA is to provide a broad chemical analysis in an energy efficient manner. To evaluate the vapor collection and separation provided by the PCA, a 3-cell subsystem (PCA3mv) was built and tested. Quantitative analysis was performed with various polar and nonpolar compounds within a mass range of 32-212 g/mol and a vapor pressure range of 0.005-231 mmHg including alkanes, alcohols, aromatics, and phosphonates. The next generation of the PCA that is studied is the multi-sensing progressive cellular architecture (MPCA). The MPCA incorporates three types of detectors with distinct detection principles within each cell, allowing critical µGC components to be monolithically integrated within a single chip. The MPCA chip is designed to integrate three preconcentrators, three separation columns, six capacitive detectors, three arrayed integrated photoionization detectors, and a carrier gas filter. Finite element analysis was performed to analyze the thermal isolation between the components. The MPCA chip was assembled within a system, named MPCA1.0, that also incorporated valve manifolds, a bi-directional pump module, and the supporting electronics. The system was evaluated by sampling, separating, and detecting 22 analytes including alkanes, aldehydes, ketones, aromatics, and phosphonate esters. As an enhanced version of the MPCA, the MPCA1.1 system, is investigated to remove the need for a valve at the sample inlet by balancing the internal gas flow with a secondary separation pump downstream of the detectors. This change avoids any interference of the valves with the analytes, hence eliminating the need for any customized valves. Consequently, the overall performance and manufacturability of the µGC are improved. For further enhancement, the MPCA2.0 system enables closed loop pressure control and an internal reference to increase system reliability and repeatability. Projected from the experimental results, the limit of detection and limit of identification for 30-minute sampling were in the range of 0.00003-0.0475 mg/m3 and 0.0001-0.071 mg/m3, respectively. Over a tested range of 0-80% relative humidity in the sample, the systems showed &lt;10% variation in the responses.