6.1. Introduction

Global warming is a serious problem that may lead to natural disasters, destroys the biological chain, and thus threats the existence and development of human beings. As one of the most important greenhouse gases, releasing of carbon dioxide must be controlled. Measuring and analyzing stable isotopes of atmospheric carbon dioxide are very useful to search sources and sinks of carbon dioxide in this area and seek the processes which are caused by human's activities. Moreover, human enzyme activities assessment, organ functions, and transport processes in the medical area could be achieved by noninvasive 13C-breath analysis. For example, possible Helicobacter pylori infection of the stomach or the duodenum can be detected via 13C-breath analysis. Thus 13C-breath test can be easily performed and have a high patient acceptance [57].

6.3. Absorption line selection

Figure 22. Three-dimensional view of the experimental setup.

obtain an optimal SNR [58, 59].

For high-precision isotopic-ratio determination, it is necessary to select absorption lines which simultaneously fulfill the following conditions: (1) they should be located within the scanning range of the laser; (2) there should be no interferences from other atmospheric species, primarily water vapor; (3) the isotopologues of interest should have similar absorption strength to

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Using the above requirements, it is rather straightforward to identify spectral regions that may contain suitable sets of isotopic absorption lines. Spectra simulation of 5% H2O and 500 ppm CO2 based on line positions and line strengths reported in the HITRAN 2008 database in the spectral range of the DFB laser scanned is displayed in Figure 23. The two absorption lines of

Figure 23. (a) Simulated absorption spectrum of 5% H2O and 500 ppm CO2 in the spectral range of 3639–3645 cm<sup>1</sup>

of 20 mbar. 12CO2, 13CO2, and H2O absorption lines are shown in black, red, and gray, respectively.

Signal simulation of 500 ppm 12CO2 and 13CO2 based on HITRAN 2008 database with a path length of 107 m at a pressure

. (b)

The primary technology for determination isotopic ratio is isotope ratio mass spectrometry (IRMS) with a measurement precision from 0.01 to 0.1‰ by testing the mass of each isotope of samples. Although this method has high precision, the disadvantages of IRMS are obvious. For example, the instrument of IRMS is too large to move easily, and the sample must be pretreated in the case of the influence of other substances whose numbers of molecules are same with those need to be tested. These drawbacks make it impossible to measure the isotopic ratio in situ or online. TDLAS is a popular way to measure concentrations of gases. According to direct absorption, concentration and isotope ratio can be easily calculated when temperature, pressure, optical path length, and absorption line strength of gases are certain.

#### 6.2. Experimental setup

The experimental setup is depicted in Figure 22. The laser source is a room temperature operated DFB laser (nanoplus GmbH) with a center wavelength of 2.74 μm and a tuning range of 5 cm<sup>1</sup> . A visible He-Ne laser beam was used to do coalignment of the optical path since the mid-infrared light is not visible to human eyes. Positions of water vapor absorption lines from the HITRAN 2008 database provided an absolute frequency reference for frequency calibration. The laser beam was directed to a homemade multi-pass absorption cell with an optical path length of 107 m. In order to avoid the absorption line intensity fluctuation caused by the absorption cell temperature variation, the temperature of the multi-pass absorption cell was maintained at 30C by the use of a heater band and a temperature controller. The emerging absorption signal from the cell was focused onto a thermoelectrically cooled (TEC) photovoltaic VIGO detector (PVI-4TE-3). The detector output was sampled with a fast data acquisition card and then transferred to a personal computer for further data processing.

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy http://dx.doi.org/10.5772/intechopen.72948 227

Figure 22. Three-dimensional view of the experimental setup.

#### 6.3. Absorption line selection

The experimental results indicate that the system has good linearity, stability, and repeatability, combined with a quick response time and a low detection limit. The H2S detection system

Global warming is a serious problem that may lead to natural disasters, destroys the biological chain, and thus threats the existence and development of human beings. As one of the most important greenhouse gases, releasing of carbon dioxide must be controlled. Measuring and analyzing stable isotopes of atmospheric carbon dioxide are very useful to search sources and sinks of carbon dioxide in this area and seek the processes which are caused by human's activities. Moreover, human enzyme activities assessment, organ functions, and transport processes in the medical area could be achieved by noninvasive 13C-breath analysis. For example, possible Helicobacter pylori infection of the stomach or the duodenum can be detected via 13C-breath analysis. Thus 13C-breath test can be easily performed and have a high patient

The primary technology for determination isotopic ratio is isotope ratio mass spectrometry (IRMS) with a measurement precision from 0.01 to 0.1‰ by testing the mass of each isotope of samples. Although this method has high precision, the disadvantages of IRMS are obvious. For example, the instrument of IRMS is too large to move easily, and the sample must be pretreated in the case of the influence of other substances whose numbers of molecules are same with those need to be tested. These drawbacks make it impossible to measure the isotopic ratio in situ or online. TDLAS is a popular way to measure concentrations of gases. According to direct absorption, concentration and isotope ratio can be easily calculated when temperature,

The experimental setup is depicted in Figure 22. The laser source is a room temperature operated DFB laser (nanoplus GmbH) with a center wavelength of 2.74 μm and a tuning range

mid-infrared light is not visible to human eyes. Positions of water vapor absorption lines from the HITRAN 2008 database provided an absolute frequency reference for frequency calibration. The laser beam was directed to a homemade multi-pass absorption cell with an optical path length of 107 m. In order to avoid the absorption line intensity fluctuation caused by the absorption cell temperature variation, the temperature of the multi-pass absorption cell was maintained at 30C by the use of a heater band and a temperature controller. The emerging absorption signal from the cell was focused onto a thermoelectrically cooled (TEC) photovoltaic VIGO detector (PVI-4TE-3). The detector output was sampled with a fast data acquisition

. A visible He-Ne laser beam was used to do coalignment of the optical path since the

pressure, optical path length, and absorption line strength of gases are certain.

card and then transferred to a personal computer for further data processing.

based on TDLAS has the feasibility of online monitoring in many applications.

6. CO2 isotope measurements

6.1. Introduction

226 Green Electronics

acceptance [57].

6.2. Experimental setup

of 5 cm<sup>1</sup>

For high-precision isotopic-ratio determination, it is necessary to select absorption lines which simultaneously fulfill the following conditions: (1) they should be located within the scanning range of the laser; (2) there should be no interferences from other atmospheric species, primarily water vapor; (3) the isotopologues of interest should have similar absorption strength to obtain an optimal SNR [58, 59].

Using the above requirements, it is rather straightforward to identify spectral regions that may contain suitable sets of isotopic absorption lines. Spectra simulation of 5% H2O and 500 ppm CO2 based on line positions and line strengths reported in the HITRAN 2008 database in the spectral range of the DFB laser scanned is displayed in Figure 23. The two absorption lines of

Figure 23. (a) Simulated absorption spectrum of 5% H2O and 500 ppm CO2 in the spectral range of 3639–3645 cm<sup>1</sup> . (b) Signal simulation of 500 ppm 12CO2 and 13CO2 based on HITRAN 2008 database with a path length of 107 m at a pressure of 20 mbar. 12CO2, 13CO2, and H2O absorption lines are shown in black, red, and gray, respectively.

3641.0311 cm<sup>1</sup> for 13CO2 and 3641.1338 cm<sup>1</sup> for 12CO2 were selected for isotope analysis of CO2 and free of interferences of water vapor absorption lines.

### 6.4. Results and discussion

Figure 24 shows an experimental spectrum of 12CO2 and 13CO2 in ambient air at 20 mbar with an optical path length of 107 m within a narrow scanned range of 0.1 cm<sup>1</sup> . Spectroscopic parameters of the selected absorption lines are provided in Table 3.

The instrument performance in terms of detection limit and long-term stability was tested using the Allan variance. The mixing ratios of CO2 were measured with 1 s collection time from a standard gas cylinder with 197 ppm CO2. Time series of this data is shown in Figure 25. From the associated Allan variance plot, an optimum averaging time of 130 s can be derived.

This instrument was used to measure the isotope ratios of CO2 in the ambient air. Time series of CO2 mixing ratio profiles and the derived δ13C values with 1 s average time are shown in Figure 26. The measured mean value of CO2 mixing ratios and <sup>δ</sup>13C is 454 ppm and 98.75‰, respectively. The 1σ standard deviation of δ13C is 1.8‰. According to the Allan variance, the optimum integration time is 130 s; the corresponding measurement precision can reach to

Figure 25. Time series and Allan plot of CO2 from a standard gas cylinder.

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Figure 26. Time series of CO2 mixing ratios and δ13C measured by the DFB spectrometer.

Figure 24. Direct absorption signals of 12CO2 and 13CO2 in ambient air at 20 mbar with an optical path length of 107 m.


Table 3. Spectroscopic parameters of the selected absorption lines for this work.

Environmental Application of High Sensitive Gas Sensors with Tunable Diode Laser Absorption Spectroscopy http://dx.doi.org/10.5772/intechopen.72948 229

Figure 25. Time series and Allan plot of CO2 from a standard gas cylinder.

3641.0311 cm<sup>1</sup> for 13CO2 and 3641.1338 cm<sup>1</sup> for 12CO2 were selected for isotope analysis of

Figure 24 shows an experimental spectrum of 12CO2 and 13CO2 in ambient air at 20 mbar with

The instrument performance in terms of detection limit and long-term stability was tested using the Allan variance. The mixing ratios of CO2 were measured with 1 s collection time from a standard gas cylinder with 197 ppm CO2. Time series of this data is shown in Figure 25. From the associated Allan variance plot, an optimum averaging time of 130 s can be derived.

This instrument was used to measure the isotope ratios of CO2 in the ambient air. Time series of CO2 mixing ratio profiles and the derived δ13C values with 1 s average time are shown in Figure 26. The measured mean value of CO2 mixing ratios and <sup>δ</sup>13C is 454 ppm and 98.75‰, respectively. The 1σ standard deviation of δ13C is 1.8‰. According to the Allan variance, the optimum integration time is 130 s; the corresponding measurement precision can reach to

Figure 24. Direct absorption signals of 12CO2 and 13CO2 in ambient air at 20 mbar with an optical path length of 107 m.

) Line strength (10<sup>21</sup> cm<sup>1</sup> cm<sup>2</sup>

Isotopologue Wavenumber (cm<sup>1</sup>

16O12C16O 3641.1338 5.637 16O13C16O 3641.0311 0.641

Table 3. Spectroscopic parameters of the selected absorption lines for this work.

. Spectroscopic

/molecule)

an optical path length of 107 m within a narrow scanned range of 0.1 cm<sup>1</sup>

parameters of the selected absorption lines are provided in Table 3.

CO2 and free of interferences of water vapor absorption lines.

6.4. Results and discussion

228 Green Electronics

Figure 26. Time series of CO2 mixing ratios and δ13C measured by the DFB spectrometer.

0.2‰. For our CO2 isotopologue measurement system based on TDLAS, high measurement precision has been obtained; the next step is to further improve the long-term stability of the system and perform calibration to get the correct isotope ratios and after that apply it to the medical area.
