**4. Micro pre-concentrator**

After removing vapor and particulates in environmental samples by the micro dryer and purifier, the sample can be introduced into a pre-concentrator for further processing.

The principle of the gas concentrator was detailed as follows: Analytes were transported into the pre-concentrator through a sampling pump, and the analytes were absorbed by the adsorption material. After the adsorption capacity was saturated, the pre-concentrator was heated to release the absorbed components. Finally, the released components with high concentration were transported into the detector.

The pre-concentrator can concentrate trace gas, that is, makes its concentration reach or exceed detection limit of the detector and makes the detector yield a good response for trace envi‐ ronmental sample. Therefore, pre-concentration of sample prior to analysis as it enables detection of trace gas and also improves detection sensitivity of mini-instruments, and sample pretreatments occupied in an important position for on-site rapid detection.

The conventional pre-concentrator configurations consist of a trap metal tube that is cooled by a flow of cold gas during sample collection and successively heated in order to release the sample through a rapid thermal desorption mechanism. However, the typical pre-concentrator needed a high power due to its larger thermal mass. Moreover, the conventional pre-concen‐ trator is very difficult to be integrated into a portable GC system, resulting in an inconvenient in-field application.

Micro pre-concentrator with reduced thermal mass can raise temperature much faster at lower power compared with conventional desorption tubes, realizing a much higher concentration factor. Micro pre-concentrators with a small size and high concentration efficiency were easily integrated into micro/portable chromatographic systems, which make these systems very suitable for on-site or online analysis. Therefore, micro pre-concentrator received an unpre‐ cedented development with the development of MEMS technology.

Many efforts to improve concentration efficiency have been made in many works [5-13], and these efforts included the use of promising materials for high adsorption capacity, fabrication of high aspect ratio of the channels, improvement of sample capacity by fabricating multi‐ channels, and fabrication of micro channels with embedded micro-pillars which are able to fill more sorbent material and increase high aspect ratio of the channels. Although these efforts have yielded important progress, the released components during thermal desorption process should be controlled and the peak broadening also should be compressed.

#### **4.1. Consideration of the micro pre-concentrator**

As concentration of components in environmental gas is very low (ranged from several ppb to hundreds ppb) and the available analytical instruments are not sufficiently sensitive, the development of micro pre-concentrators for effectively concentrating components is very important and necessary.

According to theories of the pre-concentrator, there are several methods for improving concentration factor: (1) optimization of device structure for increasing the adsorption capacity; (2) the use of efficient adsorption material for improving specificity and adsorption capacity; (3) integration of cooler and heater on the pre-concentrator for improving adsorption capacity and desorption efficiency, respectively; and (4) integration of a micro valve behind the pre-concentrator for closing the released components during the desorption stage.

Therefore, in order to improve the concentration factor, in this work, we optimized the above considerations including measures as follows: the use of nanomaterials as adsorbent materials, integration of a heater and cooler on the pre-concentrator, and integration of a valve behind the pre-concentrator.

### **4.2. The adsorption material**

**4. Micro pre-concentrator**

132 Current Air Quality Issues

in-field application.

concentration were transported into the detector.

After removing vapor and particulates in environmental samples by the micro dryer and

The principle of the gas concentrator was detailed as follows: Analytes were transported into the pre-concentrator through a sampling pump, and the analytes were absorbed by the adsorption material. After the adsorption capacity was saturated, the pre-concentrator was heated to release the absorbed components. Finally, the released components with high

The pre-concentrator can concentrate trace gas, that is, makes its concentration reach or exceed detection limit of the detector and makes the detector yield a good response for trace envi‐ ronmental sample. Therefore, pre-concentration of sample prior to analysis as it enables detection of trace gas and also improves detection sensitivity of mini-instruments, and sample

The conventional pre-concentrator configurations consist of a trap metal tube that is cooled by a flow of cold gas during sample collection and successively heated in order to release the sample through a rapid thermal desorption mechanism. However, the typical pre-concentrator needed a high power due to its larger thermal mass. Moreover, the conventional pre-concen‐ trator is very difficult to be integrated into a portable GC system, resulting in an inconvenient

Micro pre-concentrator with reduced thermal mass can raise temperature much faster at lower power compared with conventional desorption tubes, realizing a much higher concentration factor. Micro pre-concentrators with a small size and high concentration efficiency were easily integrated into micro/portable chromatographic systems, which make these systems very suitable for on-site or online analysis. Therefore, micro pre-concentrator received an unpre‐

Many efforts to improve concentration efficiency have been made in many works [5-13], and these efforts included the use of promising materials for high adsorption capacity, fabrication of high aspect ratio of the channels, improvement of sample capacity by fabricating multi‐ channels, and fabrication of micro channels with embedded micro-pillars which are able to fill more sorbent material and increase high aspect ratio of the channels. Although these efforts have yielded important progress, the released components during thermal desorption process

As concentration of components in environmental gas is very low (ranged from several ppb to hundreds ppb) and the available analytical instruments are not sufficiently sensitive, the development of micro pre-concentrators for effectively concentrating components is very

pretreatments occupied in an important position for on-site rapid detection.

cedented development with the development of MEMS technology.

should be controlled and the peak broadening also should be compressed.

**4.1. Consideration of the micro pre-concentrator**

important and necessary.

purifier, the sample can be introduced into a pre-concentrator for further processing.

The adsorption material is a key factor for affecting performance of a pre-concentrator device. Firstly, the material characteristics determine the types of concentrated gas. Secondly, material characteristics also determine the adsorption capacity of samples. Generally speaking, in order to improve the concentration factor, the first consideration is the selectivity and specificity of adsorption material, and then the adsorption capacity of the material is the next consideration.

With the development of technology, there are more adsorption materials that are available than before. In addition to the traditional adsorption material, such as Tenax-TA, Carbopack X, and activated carbon, recently some nanomaterials were developed and widely used as adsorption material, such as carbon nanotubes, molecularly imprinted polymers, nanoparti‐ cles, and so on.

Single-walled carbon nanotubes (SWCNTs) have a lot of advantages, such as large aspect ratio, high effectiveness of surface area, chemical and thermal stability, a large affinity to nonpolar compounds, and higher adsorption capacity than Carbopack X. Moreover, SWCNTs cannot be dissolved in water and organic solvents. Therefore, SWCNTs were the preferred material used as adsorbent material.

#### **4.3. Fabrication of micro pre-concentrator**

In this work, in order to improve the concentration factor of the pre-concentrator, a micro preconcentrator with four parallel channels filled with SWCNTs as adsorbent materials was developed for concentrating VOCs in environmental samples. The details of the fabrication process of the pre-concentrator are the following (refer to Figure 3). Firstly, configuration of the micro pre-concentrator was drawn by AutoCAD software. Subsequently, rectangular micro channels were etched on silicon and glass wafer by laser dicing saw according to the configuration, and the length, depth, and width of the channels were 20 mm, 400 µm, and 1000 µm, respectively. Then these channels on silicon were aligned and bonded to these channels on glass wafer.

In order to increase adsorption capacity as much as possible in adsorption process, a cooler was integrated upside of the pre-concentrator, and the temperature of the pre-concentrator

**Figure 3.** The details of the fabrication process

can be cooled down to 5 °C in 100 s. In the desorption stage, the faster the increase of the temperature, the higher concentration factor of the pre-concentrator. Therefore, a micro heater was integrated on the backside of the pre-concentrator, which can quickly release the analytes. The heater with resistance of 8 Ω was fabricated as a 20 nm/250 nm Cr/Pt stack deposited by a magnetron sputtering technology and patterned by a lift-off technology. The heater can rapidly heat the pre-concentrator at a speed of 10℃ per second, and the highest temperature can be raised up to 200℃ in less than 30 s, which the instantaneous concentration factor can be significantly improved. Furthermore, to compress peak broadening and further improve the concentration factor, a micro valve (purchased from Shenzhen Keyto Fluid Control Co., Ltd) was integrated behind the pre-concentrator (refer to Figure 4), which can close the released components during thermal desorption process, thus increasing the concentration factor as maximum as possible. In addition, four micro filters integrated in the end of the channels can prevent SWCNTs out of the pre-concentrator from the outlet. A photograph of the fabricated micro pre-concentrator is shown in Figure 5.

**Figure 4.** The diagram of the pre-concentrator

**Figure 5.** The channels on the glass wafer (a) and silicon wafer (b), the fabricated micro pre-concentrator (c)(d)

#### **4.4. Filling of the adsorbent material**

can be cooled down to 5 °C in 100 s. In the desorption stage, the faster the increase of the temperature, the higher concentration factor of the pre-concentrator. Therefore, a micro heater was integrated on the backside of the pre-concentrator, which can quickly release the analytes. The heater with resistance of 8 Ω was fabricated as a 20 nm/250 nm Cr/Pt stack deposited by a magnetron sputtering technology and patterned by a lift-off technology. The heater can rapidly heat the pre-concentrator at a speed of 10℃ per second, and the highest temperature can be raised up to 200℃ in less than 30 s, which the instantaneous concentration factor can be significantly improved. Furthermore, to compress peak broadening and further improve the concentration factor, a micro valve (purchased from Shenzhen Keyto Fluid Control Co., Ltd) was integrated behind the pre-concentrator (refer to Figure 4), which can close the released components during thermal desorption process, thus increasing the concentration factor as maximum as possible. In addition, four micro filters integrated in the end of the channels can prevent SWCNTs out of the pre-concentrator from the outlet. A photograph of the fabricated

micro pre-concentrator is shown in Figure 5.

**Figure 3.** The details of the fabrication process

134 Current Air Quality Issues

**Figure 4.** The diagram of the pre-concentrator

The details of filling process were reported as follows: (1) SWCNTs were added into TNWDIS solvent for forming suspending agent; (2) one end of the column was connected to a capillary which was submerged into a SWCNT solution, and the other end of the pre-concentrator was connected to a micro-pump. (3) The SWCNT solution was transported into the channels after the pump was turned on, and then nitrogen gas was continuously delivered through the channel to completely evaporate the TNWDIS solvent. (4) Then the pre-concentrator was put into a temperature-programmed oven under a nitrogen flow inside, and the temperature was increased gradually by 5 °C/min until 200 °C and kept for at least 4 h.

#### **4.5. Characterization of the micro pre-concentrator**

To improve the concentration factor, a cooler and a heater were, respectively, integrated on the upside and backside of the micro pre-concentrator. The cooler and heater cover the whole pre-concentrator chip region (i.e., the channel region) which has a temperature distribution over the chip surface as uniform as possible. The low temperature can increase adsorption capacity as much as possible in adsorption state, and the high temperature can rapidly and completely release the analytes from the adsorbent materials.

To evaluate the temperature characteristics of the micro pre-concentrator, a measurement cycle of 200 s was considered, and the result is reported in Figure 6. As can be disclosed, temperature of the pre-concentration chip can be dropped down to 5 °C in 100 s in the state of cooling and then can be raised up to 180 °C after 150 s and successively settles to a steady value of 200 °C in the heating state.

**Figure 6.** Temperature characteristics of the micro pre-concentrator

In order to evaluate the concentration factor of the fabricated micro pre-concentrator, tests of adsorption and desorption of analytes were carried out (refer to Figure 7). Firstly, the analytes were introduced into the micro pre-concentrator using a sampling pump, and the analytes were absorbed by the SWCNTs at the temperature of 5 ºC (the cooler is at cooling state) until the adsorption saturation. Then, the micro pre-concentrator was heated, but the desorbed analytes from SWCNTs were closed in the channels by the control of the valve. When the analytes were completely desorbed, the analytes were consequently carried to a laboratorymade portable photoionization detector (PID) [14] by carrier gas after the valve was opened.

In order to evaluate capabilities of the micro pre-concentrator, the sample composed of benzene with concentration of 500 ppb was used to be concentrated. The setup of the experi‐ ment is shown in Figure 7, where the outlet of the pre-concentrator was connected with a micro valve. Firstly, an amount of sample was transported into the pre-concentrator by opening the sampling pump. In this adsorption state, the cooler was working and the pre-concentrator was in cooling state; the sample was adsorbed in the SWCNTs (acted as adsorbent material). After the sample was completely adsorbed, the micro valve was closed and the micro heater was in working state; the analytes were quickly released from the SWCNTs by rapidly increasing the temperature of the pre-concentrator. After the analytes were completely released, the micro valve was opened, and the released analytes were consequently transported into the PID by carrier gas. Compared with the response without the proposed pre-concentrator (refer to Figure 8), the response with the pre-concentrator was significantly improved over 20 times more. Moreover, the chromatography peak broadening was greatly compressed, and peak tailing was satisfactorily solved. Therefore, the micro-fabricated pre-concentrator can be easy to deploy and suitable for a number of applications involving on-site monitoring of environ‐ mental samples.

**Figure 7.** The setup for evaluating performance of the micro pre-concentrator

**Figure 6.** Temperature characteristics of the micro pre-concentrator

mental samples.

136 Current Air Quality Issues

In order to evaluate the concentration factor of the fabricated micro pre-concentrator, tests of adsorption and desorption of analytes were carried out (refer to Figure 7). Firstly, the analytes were introduced into the micro pre-concentrator using a sampling pump, and the analytes were absorbed by the SWCNTs at the temperature of 5 ºC (the cooler is at cooling state) until the adsorption saturation. Then, the micro pre-concentrator was heated, but the desorbed analytes from SWCNTs were closed in the channels by the control of the valve. When the analytes were completely desorbed, the analytes were consequently carried to a laboratorymade portable photoionization detector (PID) [14] by carrier gas after the valve was opened.

In order to evaluate capabilities of the micro pre-concentrator, the sample composed of benzene with concentration of 500 ppb was used to be concentrated. The setup of the experi‐ ment is shown in Figure 7, where the outlet of the pre-concentrator was connected with a micro valve. Firstly, an amount of sample was transported into the pre-concentrator by opening the sampling pump. In this adsorption state, the cooler was working and the pre-concentrator was in cooling state; the sample was adsorbed in the SWCNTs (acted as adsorbent material). After the sample was completely adsorbed, the micro valve was closed and the micro heater was in working state; the analytes were quickly released from the SWCNTs by rapidly increasing the temperature of the pre-concentrator. After the analytes were completely released, the micro valve was opened, and the released analytes were consequently transported into the PID by carrier gas. Compared with the response without the proposed pre-concentrator (refer to Figure 8), the response with the pre-concentrator was significantly improved over 20 times more. Moreover, the chromatography peak broadening was greatly compressed, and peak tailing was satisfactorily solved. Therefore, the micro-fabricated pre-concentrator can be easy to deploy and suitable for a number of applications involving on-site monitoring of environ‐

**Figure 8.** The performance of the pre-concentrator
