**6. High-sensitivity PID**

In the developed portable GC system, the PID is the core elements, which can provide qualitative and quantitative analysis for air pollution. Therefore, it is very important for developing a high-sensitivity mini PID.

#### **6.1. Performance optimization of PID**

In general, there are several factors affecting its sensitivity, such as background noise, volume of the ionization chamber, and electron collection efficiency.

Firstly, it is common knowledge that background noise will affect the detection limit and detection sensitivity of the sensor; even large noise will cause failure of the detector. Therefore, reduction of background noises is the primary consideration in PID configuration.

In terms of ionization chamber, the large ionization chamber cell will increase the dead volume of the detector, which will greatly reduce the response sensitivity of the detector. In addition, larger ionization chamber will consume more carrier gas, which will greatly reduce the working time of portable GC system in field use.

Electron collection efficiency directly influences the response sensitivity of the detector, and the low collection rate will greatly reduce the sensitivity and detection limit of detector. Therefore, maximum collection rate is an important consideration for designing PID.

**Figure 11.** Schematic of the proposed PID

respectively; and f1 (varies between 1 and 1.125) and f2 (varies between 0 and 1) are the Gidding-

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.

Golay and Martin-James gas compression coefficients, respectively.

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

of the ionization chamber, and electron collection efficiency.

In the developed portable GC system, the PID is the core elements, which can provide qualitative and quantitative analysis for air pollution. Therefore, it is very important for

In general, there are several factors affecting its sensitivity, such as background noise, volume

Firstly, it is common knowledge that background noise will affect the detection limit and detection sensitivity of the sensor; even large noise will cause failure of the detector. Therefore,

reduction of background noises is the primary consideration in PID configuration.

**6. High-sensitivity PID**

140 Current Air Quality Issues

developing a high-sensitivity mini PID.

**6.1. Performance optimization of PID**

In order to develop a high-sensitivity PID, in this work, a mini PID with a lower background noise and a shorter response time was proposed. These important factors mentioned above were optimized for fabricating PID sensors. Figure 11 shows a side view of the PID proposed in this work. In this setup, a nozzle with a volume slightly less than the ionization chamber is parallel to the VUV lamp and positioned in the center of the ionization chamber, and thus the volume of the ionization chamber is substantially reduced to 10 µl, decreasing the response time and improving the response sensitivity.

In order to shield from the photoelectric effects produced by the VUV light radiation on the electrodes, an annular ion collection electrode that is perpendicular to the direction of energy radiated by the VUV lamp was positioned close to the inner wall of the ionization chamber, and an annular accelerating electrode was embedded in the groove of the nozzle. In addition, the ionization chamber was surrounded by an electromagnetic shield, which could prevent the external electromagnetic noises including the electromagnetic radiation of the VUV lamp from penetrating the ionization chamber. Therefore, these designs enable the PID to demon‐ strate a very low background noise and a small baseline drift.

In order to reduce the recombination of ions and electrons before collection, the collection electrode was closely positioned in the lamp window, in which the ions and electrons can be immediately captured by the collection electrode after gas molecule ionization. Moreover, a high voltage (which ranged from 100 V to 300 V) was applied between the collection and accelerating electrodes to capture all the ionized species resulting from exposure to VUV radiation. In the end, the output signal of the PID was amplified by an electrometer and monitored by a personal computer.

#### **6.2. Background noise level**

In this paper, a few key steps (e.g., shielding the electrodes from the VUV light radiation, surrounding the ionization chamber with an electromagnetic shield) have been used to reduce the background noise level, producing a one- to two-order decrease in the noise magnitude.

After a series of experiments, the results recorded a background current of 2×10-14 A; the output of signal acquisition system was 7.98 pA when the VUV lamp was off (see Figure 12 (a)). However, the output was increased to an upper limit of 8.00 pA when the VUV lamp was turned on (see Figure 12 (b)), which was still the lowest value reported in publications [22]. Moreover, the background noise of the PID was less than 1×10-14 A, lower than its commercial PID counterparts [23]. In addition, the baseline drift was extremely small, which was negligible in the test period.

**Figure 12.** The background noise and current of the PID when the VUV lamp was off (a) and on (b)

#### **6.3. The response time of the PID**

In this setup, the ionization chamber of the proposed mini PID with a cell volume of 10 µl was much smaller than that of its commercial PID counterparts (from 40 µl to 200 µl [22,23]). The relatively large volumes of commercial PIDs lead to a large dead volume which can distort GC elution profiles and compromise the device performance. With reduction of the ionization chamber, the response time can be dramatically reduced.

**Figure 13.** The response time of the proposed PID

the external electromagnetic noises including the electromagnetic radiation of the VUV lamp from penetrating the ionization chamber. Therefore, these designs enable the PID to demon‐

In order to reduce the recombination of ions and electrons before collection, the collection electrode was closely positioned in the lamp window, in which the ions and electrons can be immediately captured by the collection electrode after gas molecule ionization. Moreover, a high voltage (which ranged from 100 V to 300 V) was applied between the collection and accelerating electrodes to capture all the ionized species resulting from exposure to VUV radiation. In the end, the output signal of the PID was amplified by an electrometer and

In this paper, a few key steps (e.g., shielding the electrodes from the VUV light radiation, surrounding the ionization chamber with an electromagnetic shield) have been used to reduce the background noise level, producing a one- to two-order decrease in the noise magnitude.

After a series of experiments, the results recorded a background current of 2×10-14 A; the output of signal acquisition system was 7.98 pA when the VUV lamp was off (see Figure 12 (a)). However, the output was increased to an upper limit of 8.00 pA when the VUV lamp was turned on (see Figure 12 (b)), which was still the lowest value reported in publications [22]. Moreover, the background noise of the PID was less than 1×10-14 A, lower than its commercial PID counterparts [23]. In addition, the baseline drift was extremely small, which was negligible

**Figure 12.** The background noise and current of the PID when the VUV lamp was off (a) and on (b)

In this setup, the ionization chamber of the proposed mini PID with a cell volume of 10 µl was much smaller than that of its commercial PID counterparts (from 40 µl to 200 µl [22,23]). The relatively large volumes of commercial PIDs lead to a large dead volume which can distort GC

strate a very low background noise and a small baseline drift.

monitored by a personal computer.

**6.2. Background noise level**

142 Current Air Quality Issues

in the test period.

**6.3. The response time of the PID**

In order to evaluate the response time of the mini PID, a capillary (20 cm×0.32 mm) instead of the GC column was used to directly connect the sample injector to the ionization chamber of the mini PID. The flow rate was set to 30 sccm so that the elapsed time in the capillary can be ignored. Figure 13 indicates the response time of the proposed mini PID. The response time was less than 30 ms from the sample entering into the capillary to 90 % response of the sample. To the best of the authors' knowledge, the response time reported in this study was the shortest among all the reported commercial counterparts.

#### **7. Detection of harmful gases**

Because of a large number of emissions of industrial waste gas and a substantial increase in automobile, air pollution is more and more serious. At present, PM 2.5, heavy haze, and choking smog have seriously affected people's health. In order to solve the problem of air pollution, real-time monitoring of air quality is very urgent and important. Volatile organic compounds (VOCs) are the premise of pollutants for forming the PM2.5 or heavy haze and choking smog, so monitoring VOCs is a very important measure. Therefore, we use the developed portable PID system for detecting these target pollutants released into the atmos‐ phere, and these analytes mainly include benzene, toluene, styrene, phenol, etc.

Due to the lack of judgment on the qualitative components, the standard chromatogram of these main pollutants in the environmental air must be formulated. In this work, the standard sample with concentrations close to these components in environment was used to develop the standard curve, which can provide a scientific basis for environmental analysis (such as the city mobile monitoring air quality).

The experiments were performed under isothermal conditions at 40 °C with a carrier gas linear velocity of 18 cm/s. The pure He was used as carrier gas and the standard sample was composed of four components, namely, benzene, toluene, styrene, and phenol, with the concentrations of 5 ppm, 10 ppm, 8 ppm, and 10 ppm, respectively. The sample was diluted ten times and injected by a micro-pump and a six-port external sample injector, and carrier gas velocity was controlled by a gas flow controller. Figure 14 shows the chromatogram of the standard sample.

**Figure 14.** The chromatogram of standard sample

The result shows that the proposed system demonstrated a good separation and detection of these volatile organic compounds. The smallest response amplitude of the chromatographic peak (benzene) is also over 10 pA. Moreover, the minimum resolution of the closest two components is over 1.4. These experimental data are sufficient to show that the developed portable PID system can be widely applied to the trace detection of environment analysis.

#### **8. Conclusion**

The work here demonstrates that it is possible to fabricate a mini GC system integrated with a micro dryer and purifier, a micro pre-concentrator, a micro GC column, and a mini PID. The micro dryer and purifier can remove vapor and particulates from environmental samples, which makes the mini GC system suitable for field use. The micro pre-concentrator can concentrate the trace gas, which enables the mini GC system to detect environmental samples and also improves detection sensitivity of mini-instruments. In additional, the micro GC column can reduce the volume of the system and overcome low-resolution and poor antiinterference ability of other instruments. Based on the above experimental results, the mini GC system can effectively separate and detect the air pollutants. Therefore, the developed portable PID system can be widely applied to the trace detection of environment analysis. However, the standard sample instead of the actual sample was used in performing the experiments. In the following work, we will carry out the city mobile monitoring air quality based on this experimental basis, and the results will be reported in the next works.
