Quantitative analysis of higher hydrocarbons in natural gas using coupled solid-phase extraction / supercritiacal fluid extraction with on-line GC analysis

Characterisation of natural gas with respect to the hydrocarbon content is a challenging analytical problem due to the extremely low concentrations and the complexity of the matrix. In this publication a method is described for fully on-line preconcentration and analysis of n-nonane and higher hydrocarbons in natural gas. The system developed here is based on preconcentration of the components of interest on an octadecyl silica solid phase extraction material. The preconcentrated components are desorbed using supercritical carbon dioxide and are transferred on-line to a capillary GC for analysis. With this approach fully automated, unattended analysis of natural gas samples is possible. Sub-ppm concentrations of hydrocarbons can be measured with good repeatabilities. Reliable calibration can be obtained by introduction of a calibration standard using an LC injection valve incorporated in the system.