**2. Need for portable analytical systems**

Conventional GCs provide accurate analysis of complex mixtures but at the cost of using large, power-hungry, and relatively expensive table-top instruments. Usually, samples are collected and brought back to the laboratory for analysis. On-site analysis is becoming increasingly important, especially in the area of environmental monitoring. It reduces the risk of contamination, sample loss, and sample decomposition during transport. Furthermore, on-site monitoring also results in much shorter analysis turnaround times and thus allows for faster response to the analytical results. Lightweight devices with low maintenance are needed. In order to achieve these features, the miniaturization of the main components of GCs is performed.

separation channels, valves B, C, and F are closed (**Figure 1**) [5, 6]. Moreover, a customized

**Figure 1.** Left: Illustration of the MEMS injector in loading and injecting modes, adapted from [6], right: Optical image

MEMS Devices for Miniaturized Gas Chromatography http://dx.doi.org/10.5772/intechopen.74020 151

As a general definition, the preconcentrator relies on an adsorbing material deposited on the active area adjacent to the heating element [8]. Ideally, the sorptive material must adsorb selectively one or more chemical species of interest over a time period necessary to concentrate the chemical compound in the adsorptive material. Then, the sorptive layer must be heated with a pulse of temperature for providing narrow desorption peaks with relatively high concentration to the connecting sensor or detector. This process must allow the analytes present in a large air volume to be purified and concentrated, so increasing the efficiency of detection. Since the first micro-machined preconcentrator designed by the ChemLab at Sandia National Laboratories in 1999 [9], many works have been carried out. In literature, different preconcentrating microstructures are now available in different designs and are com-

The optimization of the device performance (adsorption and desorption duration and flow rates, heating rates) is rather important for achieving a high preconcentration factor. A compromise must be then established between a suitable adsorbent, low power consumption, and

The gas chromatographic column is considered the "heart" in a gas chromatograph. Over the last three decades, the nature and design of the column have changed considerably. Conventional GCs are equipped with conventional columns: a silica or stainless steel tube containing an immobilized or a cross-linked stationary phase bound to the inner surface. Terry et al. [2] were the first group to introduce "miniaturized GC" and "planar column" concepts by etching channels into a substrate rather by using capillaries of conventional GC technology (**Figure 1**). However, this groundbreaking work had not led to further develop-

Silicon is a very common substrate for microelectronics. The material is relatively inexpensive, is abundant in nature, and can be ordered with well-controlled crystal orientation, thickness, and

volume injector (0.5–15 μL) was designed by Holland et al. [7].

of a 3D preconcentrator with embedded pillars, adapted from [8].

bined with a wide range of adsorbents [10–12].

ments of related skills or technology until the early 1990s.

simple fabrication technology.

**4.1. Technology fabrication**

**4. Columns**

Miniaturization of GC is based both on theoretical and practical considerations [1]. Theory predicts that reducing the dimensions of flow channels enhances the analytical performances. In practice, miniaturization also enhances analysis of small-volume samples and increases analysis speed. A microfabricated GC system requires a number of components to function properly: preconcentrator, micro-valves for injecting the sample into the carrier gas, microfabricated columns well-functionalized for the specific use, heaters and temperature sensors for controlling column temperature, and detector(s) for detecting the arrival of different types of molecules. Temperature stability is also critical for GC operation, as the adsorption/desorption processes responsible for molecular separation in the column are very sensitive to temperature. The issues of microfluidic integration are therefore critical in GC microsystems.

Despite the fact that the first work on microchip-based chromatographic system was a miniaturized gas chromatograph in 1979 [2] using microelectromechanical systems (MEMS), this development was hardly pursued afterward, probably because the analytical community was not yet ready to embrace this new technology.

## **3. Injectors-preconcentrators**

The injector is a device used for introducing liquid or gas samples into the gas chromatograph. The sample is introduced directly into the carrier gas stream via a temperature-controlled chamber temporarily isolated from the system by gas sampling valves. Among all reported studies, several research teams have used commercial injectors (part of a convention GC) in split mode or gas sample valves to introduce samples into the micro-columns. Some other teams designed and fabricated a chip-based preconcentrator instead of an injector to increase sensitivity and selectivity when solute concentration is below detection limit of the detector [3, 4]. In both cases, the device must be capable of generating sharp injection plugs.

A six-valve MEMS-based injector with constant 250 nL of sample volume and suitable for harsh environment was introduced in 2010 emulating Valvo® six-valve injector. Each valve is made from sandwiching polyether ether ketone (PEEK) membranes between silicon substrate and glass. The six valves opened and closed by changing the pressure through their actuation holes. In sampling mode, valves A, D, and E are closed, while for injecting samples onto

**Figure 1.** Left: Illustration of the MEMS injector in loading and injecting modes, adapted from [6], right: Optical image of a 3D preconcentrator with embedded pillars, adapted from [8].

separation channels, valves B, C, and F are closed (**Figure 1**) [5, 6]. Moreover, a customized volume injector (0.5–15 μL) was designed by Holland et al. [7].

As a general definition, the preconcentrator relies on an adsorbing material deposited on the active area adjacent to the heating element [8]. Ideally, the sorptive material must adsorb selectively one or more chemical species of interest over a time period necessary to concentrate the chemical compound in the adsorptive material. Then, the sorptive layer must be heated with a pulse of temperature for providing narrow desorption peaks with relatively high concentration to the connecting sensor or detector. This process must allow the analytes present in a large air volume to be purified and concentrated, so increasing the efficiency of detection. Since the first micro-machined preconcentrator designed by the ChemLab at Sandia National Laboratories in 1999 [9], many works have been carried out. In literature, different preconcentrating microstructures are now available in different designs and are combined with a wide range of adsorbents [10–12].

The optimization of the device performance (adsorption and desorption duration and flow rates, heating rates) is rather important for achieving a high preconcentration factor. A compromise must be then established between a suitable adsorbent, low power consumption, and simple fabrication technology.
