**5. Micro GC column**

With more and more demands for online monitoring of environmental sample, high sensitivity of portable GC instruments were very urgent to be used. However, the development of the portable GC systems was limited due to the large volume, high power consumption of traditional GC column. As MEMS technology matures, the micro GC columns [15-19] have a prospect future with some advantages of small size, rapid analysis, batch production, and less power consumption. The portable system integrated with micro GC columns can provide realtime monitoring for quantification and identification analysis of environmental sample. Therefore, these attributes make micro GC columns attractive for a number of applications.

In this work, a high-separation-efficiency micro-fabricated GC column embedded with micropillars was developed. These embedded pillars can significantly improve the overall surface area of the columns and reduce the effective width of the column, which can enhance the sample capacity and obtain an excellent separation performance.

#### **5.1. Column fabrication**

In this work, an (100) n-type silicon wafer and Pyrex 7740 glass wafer were used as the substrates. The proposed column was fabricated through a series of lithography, etching, and bonding process, where the channels and pillars were fabricated using a deep reactive-ion etching technology. For details on the fabrication process of the micro column, refer to this work [20]. Figure 9 shows a photograph of the fabricated GC column and SEM of the channel and pillars. The diameter of pillars is 50 µm; the depth and the width of the micro channels were 350 µm and 300 µm, respectively; and the length of the micro column is 2 m. Therefore, the overall surface area of the columns, which was able to support the stationary phase, is over three times larger than that of open rectangular columns with the same dimensions, leading to higher column efficiency.

In order to separate the sample, OV-101 was acted as the stationary phase, and the stationary phase was coated via a static coating procedure. The coating process was shown as follows:


**Figure 9.** (a) (b) The channels embedded with micro-pillars, (c) the heaters integrated on the GC column, and (d) pho‐ tograph of the fabricated GC column

#### **5.2. Column efficiency**

**5. Micro GC column**

138 Current Air Quality Issues

**5.1. Column fabrication**

to higher column efficiency.

With more and more demands for online monitoring of environmental sample, high sensitivity of portable GC instruments were very urgent to be used. However, the development of the portable GC systems was limited due to the large volume, high power consumption of traditional GC column. As MEMS technology matures, the micro GC columns [15-19] have a prospect future with some advantages of small size, rapid analysis, batch production, and less power consumption. The portable system integrated with micro GC columns can provide realtime monitoring for quantification and identification analysis of environmental sample. Therefore, these attributes make micro GC columns attractive for a number of applications. In this work, a high-separation-efficiency micro-fabricated GC column embedded with micropillars was developed. These embedded pillars can significantly improve the overall surface area of the columns and reduce the effective width of the column, which can enhance the

In this work, an (100) n-type silicon wafer and Pyrex 7740 glass wafer were used as the substrates. The proposed column was fabricated through a series of lithography, etching, and bonding process, where the channels and pillars were fabricated using a deep reactive-ion etching technology. For details on the fabrication process of the micro column, refer to this work [20]. Figure 9 shows a photograph of the fabricated GC column and SEM of the channel and pillars. The diameter of pillars is 50 µm; the depth and the width of the micro channels were 350 µm and 300 µm, respectively; and the length of the micro column is 2 m. Therefore, the overall surface area of the columns, which was able to support the stationary phase, is over three times larger than that of open rectangular columns with the same dimensions, leading

In order to separate the sample, OV-101 was acted as the stationary phase, and the stationary phase was coated via a static coating procedure. The coating process was shown as follows: **1.** Preparation of the coating solution. Stationary phase OV-101 with a mass of 10 mg was dissolved in n-pentane and dichloromethane solvent (the volume ratio of n-pentane and dichloromethane solvent is 1:1), and the coating solution concentration is 15 mg/ml. **2.** Coating of the column. The coating solution was transported into the column by a micropump, and the column was kept in a water bath at 40 °C. After the column was filled with coating solution, one end of the column was sealed by wax, and the excess solvent is slowly

**3.** Aging of the column. The fabricated column was put into a temperature-programmed oven which was protected by a nitrogen flow, and the temperature of the column was successively increased to 80 °C and 120 °C for 2 h to harden the stationary phase film; then the temperature of column was heated up to 220 °C for 4 h to cure the stationary phase film, leaving behind a layer of thin and uniform stationary phase film with a thickness of

sample capacity and obtain an excellent separation performance.

evaporated by the pump from the other end of the column.

about 0.20 µm on the column walls.

There are many factors to evaluate the performance of GC column, such as the separation resolution, theoretical plate number, and separation speed, but the theoretical plate number can basically determine the separation performance of GC column. According to theory of chromatography, theoretical plate number and height equivalent to a theoretical plate (*HETP*) have a reciprocal relationship. Therefore, in order to evaluate separation efficiency of the proposed GC columns, we carried out a detailed theoretical analysis on *HETP.* According to previous reports, the *HETP* can be expressed in Equation 1 [21].

$$\text{HETP} = 2\frac{D}{u}f\_1f\_2 + \left(\frac{1+9k+25.5k^2}{105\left(1+\mathbf{k}\right)^2}\frac{\text{w}^2}{D\_\text{g}}\frac{f\_1}{f\_2} + \frac{2\,\text{kd}\_f\left(\text{w}+\mathbf{h}\right)^2}{3D\_s\left(1+\mathbf{k}\right)^2h^2}\right)u\tag{1}$$

where Dg and Ds are the binary diffusion coefficients in the mobile and stationary phases, respectively; df is the stationary phase thickness; w and h are the channel width and height, respectively; and f1 (varies between 1 and 1.125) and f2 (varies between 0 and 1) are the Gidding-Golay and Martin-James gas compression coefficients, respectively.

The theoretical analysis of curve in Figure 10 shows height equivalent to a theoretical plate versus average carrier gas velocity from equation 5.1. The Dg and Ds were considered as 0.093 cm2/s and 6.4 x 10-6 cm2/s, respectively. k, h, and w were 3, 350 µm and 150 µm in the calculations, respectively. The minimum HETP value, Hmin, found at the optimal average carrier gas velocity, uopt, gives the maximum number of theoretical plates N. The column yielded a minimum HETP of 0.011 cm (9,100 plates/m) at a linear gas velocity of 18 cm/s.

**Figure 10.** Height equivalent to a theoretical plate versus average carrier gas velocity
