44 May / June 2017 Methodology
The system and its applicability for field-based analysis has been previously described [3,4,5]. However, to better understand its practical capabilities, it’s important to take a closer look at the instrumental components [6]. Although this technology was built for portability and speed, the gas chromatograph was designed to provide comparable chromatographic resolution and performance to a benchtop system. The instrument weighs just 14.5 Kg including the battery, with dimensions of 38 cm x 39 cm x 23 cm. Its compactness and miniature size is achieved by replacing a conventional capillary column oven with a low thermal mass (LTM) column bundle using direct- contact electrical resistive heating. LTM/ GC uses a small diameter, metal capillary column, which is bundled with resistive heating and temperature-sensing wires that are braided together with insulator strands. This design provides for more controlled heating, greater heating and cooling speeds and very low power consumption. And since column heating requires considerably less operating power than a conventional GC, longer battery-lifetime is experienced. With its combination of direct resistive heating and rapid temperature ramp rates, the system can separate multi-component analytes in typically less than three minutes. The LTM column is shown in Figure 1.
Improved Spectral Quality
It is well recognised that elevated temperature program methods are required for the determination of many volatile organic compounds. However, when using conventional LTM column technology at this temperature, it is typical to get poor peak shapes and resolution. This problem is mainly caused by the real temperature in the column not matching the values that were set in the method, due to cooler spots existing on the bunched column especially in areas that are located near to the conductive foil covering layer. In addition, there also may be cooler sites on the column, which are not close to where the thermal sensors are located.
This phenomenon is exemplified in a conventional LTM column in Figure 2, which shows a round cross section bundle structure of metal or fused silica column rings, resistive temperature detector (RTD)
sensors and insulating material, covered by a conductive aluminium foil in such a way that the heat transfer and distribution can achieve the highest efficiency in two working stages of heating and cooling. However, in reality, heat distribution and transfer of the conventional LTM column are not identical as expected for the entire cross sectional area. Depending on the arrangement of the column rings, heater and sensor there may be heat transfer discrimination or on-site temperature differences especially when the column is used at high temperature. This issue limits the ability of low volatility compound elution
Because of this limitation in conventional LTM technology, a new planar LTM column was designed to minimise the cool spots by using thin aluminium covers to wrap around the one-layer regular-arrangement column and insulated heating wire. In this novel planar design, column rings, which can be either metal or fused silica are coiled side by side within single layer on the surface of a swirled aluminium band, compared to the conventional design where the column was bundled in a round cross section. This planar column provides identical heat distribution, but virtually eliminates cooler spots, thus improving the chromatographic separation at high temperature GC runs required for higher boiling point compounds. The principles of this new technology are shown in Figure 3.
Figure 2: Cooler sites experienced with conventional LTM column technology
Figure 3: Principles of planar LTM column technology, with the cool spots eliminated
Figure 4: Chromatographic comparison between a conventional LTM column (red) and the planar LTM column (black) described in this study for the separation and analysis of a mix of 18 PAH compounds (labelled and identified 1-18).
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