New Developments in Portable Fast Gas Chromatography.

Abstract

Currently commercially available portable gas chromatographs are suitable only for the analysis of volatile organic compounds, and do not include temperature programming capability. This dissertation presents the development of a novel direct at-column heater design and a pressure driven injection system that is compatible with semi-volatile analytes. The collinear and coaxial at-column heater designs incorporate heater and temperature sensing elements placed along the column. Resistive heating controls the column temperature via a feedback circuit and a software selected program. Linear temperature programming ramps up to 14$\sp\circ$C/s were obtained, with a total power consumption of less than 100 Watt. The accuracy and precision of the temperature control were satisfactory. Relative standard deviations of the retention times were less than 0.3% for temperature ramps from 0.5$\sp\circ$C/s to 10$\sp\circ$C/s. Peak shapes were symmetric, indicating good linearity. Comparison with a conventional oven showed that peaks eluted from the directly heated column with slightly longer retention times ($<$10%), and an up to 50% increase in peak width. Long range and short range temperature gradients along the column were identified as possible causes for the observed efficiency losses. The injection system designed in this work is based on the deviation of flows via solenoid valves that are located far away from the actual injector. An in-line pre-concentration trap serves for analyte enrichment and reduction of the detection limit. The injector permits the modulation of the injection band width. The fastest injection band achieved was 30 ms. It significantly reduced the influence of extra-column effects on column efficiency observed with the split injector. Long injection bands (30-60 seconds) permitted the integration of compounds at the head of the column, prior to the column temperature program, reducing detection limits for those compounds. Retention time reproducibility was not affected by the pressure driven injector. However, quantitative reproducibility was less than 20%. This aspect of the injector needs further study and improvement. The injector and column assembly developed in this work, together with a small FID, permitted the construction of a prototype portable gas chromatograph suitable for the fast analysis of volatile and semi-volatile organic compounds.