Fabricating a Microfluidic Chip for Environmental Testing with Miniaturised Gas Chromatography

By AZoNano

Table of Contents

Introduction to Gas Chromatography
Microfabrication of a Glass GC Chip
Gas Chromatography Analysis
Conclusion
About Dolomite

Introduction to Gas Chromatography

Gas Chromatography (GC) is a highly sensitive chemical analysis method used in a myriad of applications. Existing commercial GC instruments are typically quite bulky and fragile. The GC column is the key component of a GC system to separate out chemical components when samples are passed through it. The separated components are then passed through a detector to create a chromatogram, which is utilized to determine the different chemical components.

A typical GC column features a capillary which is wound onto a spindle that is heated in a turbulent fan oven. The GC column is typically coated with a 'stationary phase' to facilitate separation. Portable, rugged, low power GC instruments are essential for environmental testing, especially for atmospheric monitoring. Microfluidics facilitates the GC column miniaturization and low power techniques for column heating.

Microfabrication of a Glass GC Chip

Dolomite fabricates GC chip by wet (isotropic) etching two glass wafers, creating open channels in each layer with semicircular cross section. Then one of the wafers is drilled to create holes for fluid access, followed by diffusion bonding of the wafers together without using any adhesive. After the diffusion bonding process, the base glass layer’s thickness was decreased to 300 µm, to optimize the heat transfer rate in the channels. The chip with a Dolomite Mitos edge connector to make a connection to PEEK fluid pipes is shown in Figure 1.

Figure 1. 100 mm x 100 mm glass Gas Chromatography chip with Mitos fluid connector

The chip design comprised of an injection zone in conjunction with 7.5 m and 1.4 m long channels with 320 µm internal diameter. These channels serve as a replacement for the wound capillary used in existing GC columns. Dolomite successfully surmounted the difficulty in the photolithography and etching processes, which require a chip footprint area of 100 x 100 mm to be produced without any imperfections between channels.

Previously, microfluidic GC chips were made from silicon with channels of square cross section. These channels faced issues with uneven application of the coating. The chip from Dolomite has a near circular cross section with accurately aligned two chip layers, as shown in Figure 2. This facilitates even application of the stationary phase to the interior surface of the channels.

Figure 2. Chip sectioned to show near-circular channel cross section (320µm diameter)

A constriction etched into the channel is shown in Figure 3. This constriction enables loading and retention of activated carbon particles in the area, thus creating a sample absorption column in the chip’s injection zone, as depicted in Figure 4.

Figure 3. 320 µm diameter channel with 50 µm constriction

Figure 4. Sample absorption column area

Gas Chromatography Analysis

Independent heating and cooling of the two on-chip columns and the injection zone is required for GC. Through the use of Peltier devices and resistive heaters, the columns are directly heated and cooled.

The low thermal conductivity of glass facilitates multiple homogeneous temperature zones within a single glass chip, achieving temperature control over the range 10-200 °C. Peak power demand is roughly 25 W, two orders of magnitude below a traditional turbulent fan oven. The temperature profile under the 300 µm thick chip layer is illustrated in Figure 5.

Figure 5. Measured temperature profile (on left) with the 7.5 m column heated to 100 °C, all other areas left at ambient. Measured temperature profile (on right) with the 7.5 m column cooled to 10 °C whilst holding the temperature of the 1.4 m column at 100 °C. The temperature remained consistent (±2 °C) over the 7.5 m column and the unheated area remained at ambient temperature.

The column is then connected with a standard flame ionization detector (FID) and a lightweight 100 mW photoionization detector (PID) for detecting the separated components. Column performance is then evaluated with gas mixtures of monoaromatic and monoterpene species at the ppm concentration level.

The low power GC-PID device demonstrated sub ng detection sensitivity to monoaromatics and superior performance for a small set of volatile organic compounds. A chromatogram of the separated components of the gas mixture is shown in Figure 6.

Figure 6. Chromatogram showing separation of the 10 ppm BTEX standard gas mixture. 0.5 mL sample loop in combination with the planar glass 7.5 m GC column programmed from 10 to 100 °C at 20 °C/min heated with Peltier and resistive elements. Detection is by low power photoionization detector.

Conclusion

The combination of the microfluidic glass GC device and a low power PID shows promise as a field portable GC device and is a valuable replacement to typically square channeled silicon devices of higher material cost and restricted physical size.

The larger glass wafer areas, which may be micro-fabricated at affordable costs when compared to silicon, hold potential for planar devices column sizes akin to those utilized in laboratory GC instruments.

Like other planar GC methods, the ability to directly cool this device with the Peltier effect may provide meaningful advantages in the study of very volatile species when compared to typical cryogenic cooling of drawn capillaries in standard GC ovens.

About Dolomite

Microfluidics, also known as “lab-on-a-chip”, enables small scale fluid control and analysis, and is an emerging technology that is changing the future of instrument design. Connecting microfluidic devices to macro-scale systems presents many challenges. To help ensure success, Dolomite provides several microfluidic solutions including chips, pumps, flow sensors and other microfluidic accessories.

In addition to the wide range of standard components, Dolomite also offers the design, development and manufacturing of bespoke solutions, including custom devices, turnkey solutions and fully automated systems.

By combining specialist glass, quartz and ceramic technologies with knowledge of high performance microfluidics, Dolomite is able to provide solutions for a broad range of industries enabling manufacturers to develop more compact, cost-effective and powerful instruments.

This information has been sourced, reviewed and adapted from materials provided by Dolomite.

For more information on this source, please visit Dolomite.

Date Added: Jun 3, 2013 | Updated: Jun 11, 2013
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