Development of a Miniature Gas Chromatograph (µCAD) with Sample Enrichment, Programmed Temperature GC and Plasma Emission Detection (PED) - A miniature gas chromatograph incorporating a miniatur

Development of a Miniature Gas Chromatograph (µCAD) with Sample Enrichment, Programmed Temperature GC and Plasma Emission Detection (PED)

A miniature gas chromatograph incorporating a miniaturized chemical trap for enrichment, rapid thermal desorption of the trap, a resistively heated capillary column for programmed GC analysis and a

Mar 1, 2009
By: Bart Tienpont, Frank David, Pat Sandra, Wim Witdouck
LCGC Europe
Volume 22, Issue 3

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There are two main types of field portable instruments for monitoring chemicals in air samples. The first type includes (hand) portable systems based on sensors or selective detectors where no chromatographic separation is involved.¹ These instruments are generally used in cases where the target application is well defined (e.g., one compound or a class of compounds). The second type is basically a scaled-down bench top system and can incorporate sample enrichment, chromatographic separation and universal or selective detection. Some commercial devices are equipped with (mass) selective detection and this improves the versatility of these instruments.^2–4

These latter systems, however, often require additional parts (external power sources, gas supplies, etc.) limiting their portability. Scaling down a system to a portable size requires the implementation of new technologies in hardware components (column, pneumatics) and in micro-electronics for system control. Ultimately, chromatography is performed at the scale of microchips (micro-electro-mechanical systems, MEMS)^5–7 although this level of miniaturization has not been implemented in practice.

This paper describes the construction and optimization of a recently introduced micro-GC (micro-chemicals in air detector, µCAD) for the analysis of air and gaseous samples.⁸ The system consists of built-in air pump and a miniature programmable chemical trap, a programmable resistively heated capillary column and a novel micro-chip plasma emission detector (μPED). The components are based on established technologies in benchtop systems that are now combined in a portable standalone unit. The hardware configuration, the sampling conditions and the chromatographic settings were optimized for the analysis of volatile compounds in air.

Experimental

Figure 1

Micro-GC: The micro-GC (μCAD) was developed for the analysis of air samples and it contains a micro pump that aspirates the air sample over a miniature chemical trap. In a second step, helium is purged through the trap in back-flush mode and sent through the capillary column. The temperature of the capillary column can be programmed. Finally, the solutes are detected by a newly developed μPED. A detailed flow scheme can be found in reference 8. The system is built in a metal casing of 250 mm x 200 mm x 150 mm (width x depth x height) and weighs 5 kg. The system is powered by a 24 VDC power adapter (150 W). Figure 1 shows the system with the cover open and the different parts labelled.

Figure 2

All sampling and chromatographic parameters are programmed through the microcontroller via a keypad build in the top cover. A laptop is connected through a USB connection onto the spectrometer to collect the spectroscopic data (not shown). The system also includes a gas cylinder (Figure 2, 1) that is connected to a dual-stage pressure release valve (2). The gas cylinder is filled up with helium (N6.0 quality, Air Products, Vilvoorde, Belgium) up to 124 bar and provides 18 L of expanded gas (@ 5 mL/min = 60 h of operation). A manual three-way valve (3) allows connection onto an external gas supply. The helium pressure is finally controlled by a proportional valve in the manifold. Three additional solenoid valves are programmed to set the flow paths during sampling, thermal desorption and chromatographic analysis.

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