Pneumatic-modulation comprehensive two-dimensional gas chromatography for environmental analysis

Environmental petroleum hydrocarbon (PHC) monitoring is a major challenge. Analytical methods must be robust; operate with minimal user intervention; be suitable for remote field operation; and furnish analytical data that allows the different mechanisms of PHC environmental fate to be investigated. PHC are amenable to analysis by comprehensive two-dimensional gas chromatography (GCxGC). However, conventional GCxGC instrumentation relies on bulky thermal modulation systems. Thus alternative approaches based on fluidic modulation were investigated to determine their suitability for environmental PHC monitoring. First, a dynamic flow model, which maps carrier gas pressure and flow rate through the first-dimension separation column, the modulator sample loop, and the seconddimension column(s) in a fluidic modulation GCxGC system is described. The dynamic flow model assists design of a pneumatic modulation ensemble and leads to rapid determination of pneumatic conditions, timing parameters, and the dimensions of the separation columns and connecting tubing used to construct the GCxGC system. Three significant innovations are introduced, that were all uncovered by using the dynamic flow model, viz. i) a symmetric flow path‚ÄövÑvp modulator improved baseline stability, ii) appropriate selection of flow restrictors in the first dimension column assembly provides a generally more stable and robust system, and iii) these restrictors increase the modulation period flexibility of the GCxGC system. Next, a model was developed that permitted a systematic investigation of peak shape in fluidic modulation. In the case of a non-focusing modulator for comprehensive two-dimensional gas chromatography, the systematic distortions induced when the modulator loads the second-dimension column give rise to a characteristic peak shape. Depending on the operating conditions this systematic distortion can be the dominant component of the second-dimension elution profiles. Understanding the factors that cause different peak shape observations provides a rugged approach to method development. It is shown that low flow ratio can lead to significant peak skewing and increasing the flow ratio reduces the magnitude of peak skewing. Validation of the peak shape model is made by comparison with experimental data. Finally GCxGC methodology was developed and applied to analysis of PHC contaminated soil. GCxGC results met or exceeded, the standards set by regulators and environmental scientists. Fluidic modulation approaches provided excellent sensitivity and permitted detailed monitoring of key PHC transport and degradation pathways, including evaporation, dissolution, and biodegradation.