Development and use of portable gas chromatograph in gas analysis

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Title: Development and use of portable gas chromatograph in gas analysis
Author: Tse, Yu Tat
Contributor: University of Helsinki, Faculty of Science, Department of Chemistry
Publisher: Helsingin yliopisto
Date: 2018
Language: eng
Thesis level: master's thesis
Discipline: Analytical Chemistry
Analyyttinen kemia
Analytisk kemi
Abstract: The literature part of this thesis contains the review of development of portable gas chromatograph (GC) and its application in gas analysis. The scope includes portable capillary GC and chip-based GC. Gas chromatography is a separation technique based on different retention behavior of compounds in stationary phase. The use of portable GC enables chemists to carry out rapid on-site chemical analysis. Rapid, on-site analyses are valuable in fields such as air quality monitoring, emergency reaction and forensic application. Studies have shown that performance of portable GC analysis in these fields was as promising as conventional, bench-top GC analysis. The aims of portable GC development were mainly improved separation efficiency, faster analysis, greater portability, reduced power consumption, increased autonomous time and lower detection limit. Different components of portable GC are reviewed: they are separating channels/columns and stationary phases, temperature programming system, pre-concentrator, injector and detector. Semi-packed column and materials with great surface-area-to-volume ratio as stationary phase support were researched to increase surface area of retention. An improved separation efficiency was observed. Multi-channel capillary chips were fabricated to increase sample capacity of the column. Resistive heating was used in portable GC to provide high heating rate. This enables high efficiency separation in fast GC analysis. Efforts were made to reduce power consumption of the heating system to increase portable time. Using ambient air as the carrier gas eliminate the need of helium gas tank in the portable GC system. Researches were done to overcome the limitations of using ambient air. Vacuum-outlet GC technique was used to speed up the analysis. Air purification method was discussed to provide stable supply of clean air. Stability of stationary phase in ambient air was compared. A pre-concentrator is always used to lower the detection limit of gas analysis. Solid-phase microextraction (SPME) devices were commonly used. Micro-fabricated pre-concentrators were designed to enrich the analyte on-line prior to sample injection. The type of adsorbent in pre-concentrator and methods to achieve selectivity were discussed. Miniaturized detectors reported in portable GC were reviewed. Changes in the detector design were made to enhance signal quality and sensitivity in various detectors. They were made very small and light to increase portability of the GC. At last, portable GCxGC system is also mentioned. GCxGC has higher separation power than one-dimensional GC system. It allows chemist to separate analytes from complicated matrixes. Pneumatic and thermal modulation that transfer analyte bands from column to column was described. The advantages of adaptive GCxGC were also explained. The experimental part of this thesis describes a standard gas generation system of volatile organic compounds (VOCs) and its use in VOCs quantitation with internal standard, using SPME arrow as the sampler. The standard generation system was based on diffusion of analyte vapour through a deactivated capillary out of a GC vial. The vapour was carried away by nitrogen gas then diluted in various mixing ratio with nitrogen gas. The standard gas generation system can produce gas standard from any compound with high vapour pressure. The concentration of gas standard generated was validated with liquid standard. After choosing the appropriate sampling time with SPME arrow, calibration curves were constructed with conventional GC-MS and portable capillary GC-MS. Internal standard, octanal in this experiment, was generated using similar method. At various dilution factor of VOC standard, the peak area of internal standard was similar despite fluctuations. It showed the mass of internal standard extracted on the SPME arrow was relatively constant in different points on the calibration curves. Calibration curves of the VOCs with internal standard showed an improved correlation coefficient compared to calibration curves of the same VOCs without internal standard. It showed the use of internal standard in the can compensate the errors during sampling procedures. For real sample analysis, VOCs emitted from lemon sample was analyzed using the system to estimate emission rate of VOCs from lemon sample. Possible add-ons to the system were also discussed to make the system portable and reduce the uncertainty arising from variation of temperature and humidity in air during air sampling.

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