BY-NC-ND 3.0 license Open Access Published by De Gruyter June 13, 2018

Toxic chemical compound detection by terahertz spectroscopy: a review

Liu Yang, Tengxiao Guo, Xu Zhang, Shuya Cao and Xuequan Ding

Abstract

Terahertz (THz) spectroscopy is a rapidly emerging technology in the field of analytical chemistry. THz spectroscopy shows substantial scientific potential given that numerous absorption and emission molecular lines of interest in the chemical sciences belong to this spectral region. This article focuses on the current research progress on the detection of harmful gases, pesticides, antibiotics, toxic chemical compounds, and drugs of abuse by THz spectroscopy. The article also analyzes the key factors used for the detection of toxic chemical compounds by THz spectroscopy.

Introduction

Terahertz (THz) molecular spectroscopy provides considerable scientific potential because numerous absorption and emission molecular lines of interest in the chemical sciences belong to this spectral region, where numerous chemical species exhibit strong characteristic rotational and ro-vibrational transitions. Electromagnetic waves with frequencies between 0.3 and 10 THz or millimeter-wavelength (3–300 cm−1) are described as THz radiation. They are assigned in the electromagnetic spectrum between the microwave/millimeter and the far-infrared regions. Molecular rotations in the gas phase, crystalline phonon vibrations, low-frequency vibrations, and intermolecular vibrations in the solid state have been proven by spectroscopic procedures in the THz range (Figure 1) (Walther et al., 2010; Son, 2014). In this range, numerous biological and chemical compounds exhibit characteristic absorptions and dispersions because of vibrational transitions, mostly collective and intermolecular vibrational modes. These THz vibrational modes provide fingerprint information for THz detection and identification given that THz vibrational modes provide fingerprint information of organic molecules for identification. THz molecular spectroscopy has been widely used in chemistry and related fields. In this minireview, we will concentrate on the research progress on the detection of harmful gases, pesticides, antibiotics, toxic chemical compounds, and illicit drugs by THz systems. The article also analyzes the key factors used for the detection of toxic chemical compounds by THz spectroscopy.

Figure 1: Characteristic absorption frequencies of chemical compounds in the THz region.

Figure 1:

Characteristic absorption frequencies of chemical compounds in the THz region.

Characterization of THz spectroscopy

THz time-domain spectroscopy (THz-TDS) has been widely used for chemical detection and identification (Pereira & Shulika, 2013; Saeedkia, 2013; Song & Nagatsuma, 2015). A typical THz-TDS system is shown schematically in Figure 2 (Qin, Ying, and Xie 2013). The system is composed of an ultrafast pulsed laser, THz emitter, THz detector, and a time-delay stage. The ultrafast laser beam is split into pump and probe beams. The pump beam is incident on the THz emitter to generate THz pulses, and the THz pulses are collimated and focused on the sample by parabolic mirrors. After transmission through the sample, the THz pulses are collimated and refocused on the THz detector. The probe beam is used to gate the detector and measure the instantaneous THz electric field. A delay stage is used to offset the pump and probe beams, and allows the THz temporal profile to be iteratively sampled. Coming from the same source, the pump and probe pulses show a defined temporal relationship. Propagating along an optical delay line, the probe beam samples the THz pulses and records their electric field as a function of time delay. The THz pulses passing through a sample show amplitude attenuation and time delay. A Fourier transform is then used to convert this TDS to a frequency domain spectrum.

Figure 2: Schematic of the THz-TDS system.

Figure 2:

Schematic of the THz-TDS system.

Different pulse-wave THz radiation methods were used, such as THz-TDS. Continuous-wave (CW) THz radiation instrument was also used for THz experiment (Mittleman, 2003; Thompson, 2008). The simplicity and potential low cost of CW sources can be advantageous for certain applications, such as far-infrared region spectrometers.

Many polar gases show narrowband absorption features in the THz range caused by rotational transitions (Exter, Fattinger & Grischkowsky, 1989; Theuer et al., 2011). Generally, liquids do not have narrowband absorption features. Nonpolar liquids show a weak broadband absorption. Polar liquids have strong absorption in the entire THz range. Many solid crystalline molecules show vibrational phonon modes in the THz range. Crystalline solid samples have distinguishable absorption lines due to the low-frequency vibrational modes (Hangyo, Tani & Nagashima, 2005; Theuer et al., 2011).

Pollutant gas detection

The detection of gas-phase samples is a major application of THz-TDS and plays a role in diverse fields. The characterization is used for gas detection because numerous polar gases exhibit unique spectral signatures from probing transitions between rotational quantum levels.

Medvedev, Behnke, and Lucia (2005) applied a THz-TDS experiment system to acquire the spectra of CS2, CH2F2, CHF3, and CHI3. The mixture of these gas species was detected, and gas mixture analysis is possible using THz-TDS in combination with powerful signal processing algorithms. Assefzadeh, Jamali, and Aleksander (2016) introduced an electronic source with an on-chip antenna that can generate picosecond pulses to perform broadband THz gas spectroscopy for trace gas detection. The result showed NH3 exhibits the strongest absorption peak at 572 GHz, and SO2 and H2S can be detected as well. Hsieh et al. (2016a, 2016b) developed a method to decrease the influence of unwanted aerosols through gas detection using a fiber-based, asynchronous optical-sampling THz-TDS with the ability to obtain the CH3CN spectral fingerprints of rotational transition and a method to provide immunity against aerosol scattering CH3CN influence. Hsieh et al. (2016a, 2016b) developed a photomixing THz synthesizer phase-locked to dual optical frequency comb CW THz radiation THz-TDS experiment apparatus for gas detection. THz frequency-domain spectroscopy of CH3CN gas and its isotope CH313CN was tested by a THz-TDS apparatus in the frequency range of 0.600–0.720 THz. The result shows that the THz synthesizer is highly promising for high-precision THz-TDS of low-pressure molecular gases and will facilitate the qualitative and quantitative analyses of multiple gases. Smith and Arnold (2015) measured the THz spectra of water, ammonia, acetonitrile, methanol, ethanol, acetaldehyde, propionaldehyde, and propionitrile by commercial THz-TDS (see Figure 3). Selectivity coefficients for THz spectra are higher than those derived from the fingerprint region of the IR spectrum. Superior selectivity was further developed in THz spectroscopy for gas analytical measurements and identification. Kilcullen et al. (2015) applied a commercial THz-TDS system for the detection of carbon monoxide detection. The sample gas cell is 13.6 cm in length with high-resistivity Si windows on each end. RMS (root mean square) noise floor method was used for the apparatus. The limit of detection (LOD) for THz-TDS systems and methods was 40 ppm of carbon monoxide. Liang (2015) developed a THz technique for the identification and determination of the principal gas components, including methane (CH4), ethane (C2H6), carbon monoxide (CO), and carbon dioxide (CO2) distilled from oil shale. C2H6 represented the largest refraction property. The results indicated that THz technique combined with mathematical method is an effective tool for gas detection in the unconventional natural gas industry. Mouret et al. (2013) set up a THz sensor system based on a commercially available multiplier chain applied to the analysis of trace gases. The pollutant SO2 gas was generated in a 1.25-m-long stainless steel gas cell with Teflon windows, and the test frequency ranged from 630 to 930 GHz. The estimated LOD for SO2 was 1 ppm.

Figure 3: Expanded view of absorbance for THz spectra of acetaldehyde (black), acetonitrile (red), ethanol (green), water (yellow), methanol (blue), ammonia (magenta), propionaldehyde, propioaldehyde (cyan), and propanenitrile propionitrile (gray).

Figure 3:

Expanded view of absorbance for THz spectra of acetaldehyde (black), acetonitrile (red), ethanol (green), water (yellow), methanol (blue), ammonia (magenta), propionaldehyde, propioaldehyde (cyan), and propanenitrile propionitrile (gray).

Azzam et al. (2013) reported the pure rotational transition spectrum of hydrogen sulfide (H2S) detection by a THz instrument. A total of 320 pure rotational transitions of H232S in its first excited bending vibrational state are recorded and analyzed for the first time, and 86 transitions were analyzed for H234S. Neese et al. (2012) acquired the submillimeter/THz spectrum of a mixture of 14 gases, including HCN, ClCN, CO, NO, C2H2, NH3, HCl, and others, in the 210–270 GHz region. Different gases show complex redundancy of the rotational fingerprint. The gas cell was coupled with preconcentration to increase sensitivity. The detection limit of deuterated acetonitrile by THz apparatus was 69 ppt. Cai, Wang, and Shen (2013) collected the spectral characteristics of the air pollution gases SO2 and H2S. Spectra were measured by THz-TDS technique within a range of 0.2–2.6 THz. Shimizu et al. (2009, 2011) measured N2O gas by the THz-TDS instrument. They developed an experimental system for remotely detecting dangerous gases. The gas cell was 1 m in length, and detection experiments were conducted at long distances of 3.6 m and low N2O concentration of 25%. The result shows that gases can be remotely detected using active THz wave detection (Figure 4). Demers and Garet (2018) reported on a portable THz spectrometer mounted to a consumer drone UAV for gas monitor. Tanno et al. (2017) investigated on butane and ethane gases absorbed on ZIF-8 detection by THz spectrometer. Mihrin et al. (2018) reported that (HCN)2 was tested by high-resolution synchrotron THz.

Figure 4: Operating principle of remote gas-sensing system based on THz spectroscopy.

Figure 4:

Operating principle of remote gas-sensing system based on THz spectroscopy.

Lu, Karpowicz, and Zhang (2009) established a field-induced second-harmonic generation pulsed THz apparatus for gas detection. More than 10 gaseous samples were measured, and the selected gas samples included nitrogen, xenon, sulfur hexafluoride (SF6), carbon disulfide (CS2), cyclopentene (C5H8), and alkane gases (CH4, C2H6, C3H8, n-C4H10, n-C5H12, and C6H14). Fused quartz was used for gas cell windows. A figure of merit method was introduced to characterize the sensitivity of gases. Harmon and Cheville (2004) reported on THz-TDS based on a gas cell-detecting gas species in the low part-per-million range in near real time. Gas cell path length can be changed, and the number of passes (4, 8, or 12 single passes) was controlled by the angles of mirrors A and C, corresponding to path lengths of 2.5, 5.0, or 7.5 m, respectively. Methyl chloride vapor can be detected at low pressures to 1 Pa in gas cells. Lin et al. (2008) introduced a method that involves extracting line positions from gas species without a reference pulse and classifying the positions by means of the minimum Euclidean distance method using the THz-TDS spectral line catalog. Hindle et al. (2008) reported a monochromatic CW THz source THz system for gas detection. THz spectrum characterization was used for the collection and analysis of H2S pollution gas. Almoayed, Piyade, and Afsar (2007) reported the THz spectra of SO2 gas and CO gas by dispersive Fourier transform spectroscopy (DFTS). The frequency range of DFTS was 60 GHz to 6 THz. The THz spectra of CO and SO2 at different pressure were acquired, and the result showed that gas pressure is related to absorption spectra and refractive index. Medvedev, Behnke, and Lucia (2005) applied the THz-TDS instrument for gas monitoring and quantification. The spectral resolution approach can quantify the complex rotational spectrum of gases at a rate of 10 elements per second at high signal-to-noise ratio. The result shows that THz-TDS spectra provide “absolute” specificity and “zero” false-alarm rates even in complex mixtures.

As discussed above, high chemical selectivity for pollutant gas measurements is observed at THz frequencies. The application of THz gas measurement technology may perform important roles in online gas detection and remote gas sensing.

The list of toxic chemical compounds detected by THz spectroscopy are shown in Table 1.

Table 1:

List of toxic chemical compounds detected by THz spectroscopy.

Type of THz spectroscopyTarget compoundRange of THzReference
THz-TDSDifluoromethane, methenyl iodide, etc.254.6–256.2 GHzMedvedev, Behnke, and Lucia (2005)
THz-TDSSulfur dioxide, ammonia, etc.572 GHzAssefzadeh, Jamali, and Aleksander (2016)
CW THz-TDSAcetonitrile0.60–0.72 THzHsieh et al. (2016a)
CW THz-TDSCH313CN0.60–0.72 THzHsieh et al. (2016b)
THz-TDSAmmonia, propionaldehyde, propionitrile, etc.2–125 cm−1Smith and Arnold (2015)
THz-TDSCarbon monoxide0–3 THzKilcullen et al. (2015)
THz-TDSMethane, ethane 0.2–1.5 THzLiang (2015)
Sub-THzSulfur dioxide630–930 GHzMouret et al. (2013)
FTSHydrogen sulfide 1.4–10.5 THzAzzam et al. (2013)
SMM/THzHydrocyanic acid, acetonitrile, etc.210–270 GHzNeese et al. (2012)
THz-TDSSulfur dioxide, hydrogen sulfide 0.2–2.6 THzCai, Wang, and Shen (2013)
THz standoff detectionNitrogen monoxide440–490 GHzShimizu et al. (2009)
THz standoff detectionNitrogen monoxide440–490 GHzShimizu et al. (2011)
Pulsed THz wavesSulfur hexafluoride, carbon disulfide, etc.0.3–10 THZLu, Karpowicz, and Zhang (2009)
THz-TDSTrichloromethane0.1–1.8 THzHarmon and Cheville (2004)
THz-TDSCarbon monoxide, nitrogen monoxide, etc.0.1–2.0 THzLin et al. (2008)
CW THzH232S; H233S1016–1028 GHzHindle et al. (2008)
DFTSSulfur dioxide, carbon monoxide10–30 cm−1Almoayed, Piyade, and Afsar (2007)
THz-TDSFluoromethane, carbon disulfide, etc.246.8–261.2 GHzMedvedev, Behnke, and Lucia (2005)
THz-TDSCarbon dioxide2 THzDemers and Garet (2018)
THz-TDSButane, ethane, etc.0.5–6.2 THzTanno et al. (2017)
THz-TDS(HCN)2119.1–119.5 cm–1Mihrin et al. (2018)
Reflectance THz-TDSChlorpyrifos-methyl0.3–2.3 THzXu et al. (2017)
Reflectance THz-TDSmethomyl0.5–2 THzLee et al. (2016)
THz-TDSCarbendazim0.1–2 THzQin et al. (2017a) and Qin, Xie, and Ying (2017b)
THz-TDSMethomyl, carbamate, etc.0.1–3 THzBaek et al. (2016)
THz-TDSCarbaryl1.8–6.3 THzSun et al. (2016)
THz-TDSDifenoconazole0.66–1.57 THzZhou et al. (2015)
THz-TDSImidacloprid0.3–1.7 THzChen (2015)
THz-TDSDicofol, chlorpyrifos, daminozide, etc.0.3–3 THzMaeng et al. (2014)
THz-TDSSulfapyridine, sulfathiazole, and tetracycline0.5–6.0 THzMassaouti et al. (2013)
THz-TDSAMS0.6–1.3 THzWang, Ma, and Wang (2012)
THz-TDSα-Endosulfan1.68–1.91 THzHou et al. (2012)
THz-TDSQuintozene0.1-1.05 THzWang et al. (2011)
FTScis-Permethrin20–400 cm−1Suzuki (2011)
THz-TDSDimethylurea0.6–1.8 THzZhao et al. (2018)
THz-TDSTetracycline hydrochloride0.3–2.0 THzQin et al. (2017a) and Qin, Xie, and Ying (2017b)
THz-TDSGlyphosate0.15–3 THzWang et al. (2017)
THz-TDSBenzoyl peroxide0.2–2.0 THzZou et al. (2017)
THz-TDSPotassium sorbate0.2–2.0 THzLi, Zhang, and Ge (2017)
THz-TDSAldrin, dieldrin, and endrin0.2–1.8 THzSong and Li (2012)
THz-TDS

FTS
Dibutyl phthalate0.2–2.6 THz (THz-TDs), 1.8–10 THz (FTS)Zhao et al. (2015)
THz-TDSDBP, DINP, DEHP0.2–2.0 THzLiu et al. (2016)
THz-TDSMelamine1.99–2.99 THzCui et al. (2009)
THz-TDSCopper sulfate, zinc sulfate0.2–1.6 THzLi, Zhao, and Li (2009)
THz-TDSPb cation0.1–1.2 THzLi and Chen (2016)
THz-TDSPb cation0.1–1.2 THzLi and Zhao (2016)
THz-TDSSodium nitrate, aluminum nitrate0.2–2.5 THzLi et al. (2015)
THz-TDSCocaine free base, cocaine hydrochloride, ephedrine hydrochloride, etc.0.1–6 THzDavies, Burnett, and Fan (2008)
THz-TDSKetamine, methylephedrine, cocaine, ephedrine, etc.0.1–2.5 THzPan (2010)
THz-TDSEphedrine, hydrochloride, papaverine hydrochloride0.2–2.6 THzDeng et al. (2009)
THz-TDSAcetaldehyde, methanol, isopropyl alcohol, etc.238–252 GHzNeumaier et al. (2015)
THz-TDS; DFTS38 kinds of pure illicit drugs0.1–2.5 THzHe and Shen (2013)

  1. Submillimeter (SMM/THz).

Pesticides and antibiotics

The THz technology offers extensive applications in the research fields of pesticide and antibiotic identification and residual pesticide detection. Xu et al. (2017) set up a THz-TDS combined with metamaterials experiment apparatus used to detect the organophosphorus pesticide chlorpyrifos-methyl. Metamaterials were composed of square metal patch arrays (see Figure 5). The reflectance of the metamaterials was measured by a commercial THz-TDS and converted from time domain data to frequency domain data through fast Fourier transform method. The LOD of chlorpyrifos-methyl was 0.204 mg L−1. Lee et al. (2016) developed a THz-TDS system with nanoslot antenna array apparatus to detect residual pesticide methomyl. A THz spectrum in a reflection configuration was used to detect low-concentration methomyl absorbed at the fruit surface only with sample contact (see Figure 6). Qin et al. (2017a) and Qin, Xie, and Ying (2017b) combined THz-TDS with principal component analysis (PCA) and clustering by fast search and finding of density peaks (CFSFDP) method for pesticide detection. PCA-CFSFDP is a potential identification method for pesticide identification and detection. Baek et al. (2016) tested methomyl and carbamate insecticide by using THz-TDS. The characteristic THz absorption peaks of methomyl were 1, 1.64, and 1.89 THz. The test sample matrices were wheat and rice flour. Sun et al. (2016) determined carbaryl in rice powder by using THz-TDS. The characteristic THz absorption peak of carbaryl was in the frequency range of 1.8–6.3 THz. The partial least squares regression chemometric method was used to establish two quantitative analysis models. Wang et al. (2017) reported that glyphosate pollution in soil was detected by THz spectrometer.

Figure 5: (A) Schematic diagram of THz metamaterials detection of CM. (B) A scanning electron microscope (SEM) image of the THz metamaterials; the metal patch length is 105 μm and the period of the metamaterials is 140 μm. (C) Reflectance curves of simulation and experimental results of THz metamaterials detection.

Figure 5:

(A) Schematic diagram of THz metamaterials detection of CM. (B) A scanning electron microscope (SEM) image of the THz metamaterials; the metal patch length is 105 μm and the period of the metamaterials is 140 μm. (C) Reflectance curves of simulation and experimental results of THz metamaterials detection.

Figure 6: (A) A schematic image of a sensing chip contacting to the apple peel sample that was partially contaminated by a dropping of pesticide. (B) A schematic of THZ reflection configuration for multilayered samples and measured reflected.

Figure 6:

(A) A schematic image of a sensing chip contacting to the apple peel sample that was partially contaminated by a dropping of pesticide. (B) A schematic of THZ reflection configuration for multilayered samples and measured reflected.

Zhou et al. (2015) studied the THz spectra of the solid pesticide difenoconazole. The absorption spectra of difenoconazole were in the frequencies of 0.66, 0.80, 1.03, 1.31, and 1.57 THz, and the experiment results fit well with the simulated results by density functional theory (DFT) method. Chen (2015) reported the quantitative analysis of imidacloprid in rice powder samples by using a THz-TDS instrument. The absorption coefficient spectra of imidacloprid in the frequency range of 0.3–1.7 THz were obtained. Different chemometric methods were compared and used to quantitatively determine imidacloprid. Maeng et al. (2014) tested the THz spectra of dicofol, chlorpyrifos, chlorpyrifos-methyl, daminozide, imidacloprid, diethyldithiocarbamate, and dimethyldithiocarbamate in wheat flour mixtures. The THz absorption peaks were tested in the frequency range of 0.1–3 THz. Seven pesticides exhibited different frequency-dependent refractive indices, although five of the seven pesticides showed no specific absorption peaks in this frequency range. Massaouti et al. (2013) characterized the THz spectra of three antibiotics (sulfapyridine, sulfathiazole, and tetracycline), and two acaricides (coumaphos and amitraz). Multiple antibiotics were identified in their mixture with honey THz fingerprints. Wang, Ma, and Wang (2012) quantitatively detected the components of mixtures of the herbicide ammonium sulfamate (AMS) in agricultural products by using the THz instrument. The method was established by collection of the absorption coefficient spectra of pure AMS and quantitative analysis by partial least squares (PLS) technology.

Hou et al. (2012) investigated the absorption coefficient spectrum and refractive index spectrum of the pesticide α-endosulfan by THz instrument. The absorption peaks of α-endosulfan were at 1.7 and 1.88 THz in the experiment, partially matching with 1.68 and 1.91 THz obtained in the DFT calculation. The matching peaks are attributed to the intramolecular vibrational modes of α-endosulfan. Wang et al. (2011) studied the absorbance spectrum of bactericide quintozene in the THz range. Quantitative analysis was carried out on quintozene MtBE solutions in the concentration range of 30–100 μg/mL. Suzuki (2011) tested the THz spectra of cis-permethrin by FTIR at the THz region within an experimental frequency range of 20–400 cm−1. The absorption spectra interfered with the clay and emulsifier experiments that were also being conducted.

Toxic chemical compounds

Li, Zhang, and Ge (2017) applied the THz-TDS system for the detection of potassium sorbate in milk powder. The experiment results show that potassium sorbate exhibits a characteristic absorption peak at 0.98 THz. Song and Li (2012) measured the THz transmission spectra of three different persistent organic pollutants (POPs) (aldrin, dieldrin, and endrin) by using THz-TDS. Test frequencies were in the range of 0.2–1.8 THz. The absorption peak is in reasonably good agreement with the theoretical simulation by B3LYP DFT and experiment. The THz detection method may be used to study POPs in soil quality evaluation or safety inspection further. Zhao et al. (2015) proposed a method for detection of the known industrial plasticizer dibutyl phthalate by THz-TDS system. Liu et al. (2016) studied the THz transmission spectrum of phthalic acid esters. Test frequencies were in the range of 0.2–2.0 THz for three PAEs: di-n-butyl phthalate (DBP), di-isononyl phthalate (DINP), and di-2-ethylhexyl phthalate ester (DEHP). The refractive indices were 1.524, 1.535, and 1.563 for DINP, DEHP, and DBP, respectively.

Cui et al. (2009) applied the THz-TDS system to detect melamine. The THz transmission spectra of pure melamine and two kinds of its mixtures with polyethylene and milk powder were collected. The THz transmission spectrum of melamine present two absorption peaks at 1.99 and 2.29 THz. Li, Zhao, and Li (2009) tested the THz spectrum of copper sulfate and zinc sulfate in soil by THz-TDS. The absorption coefficient and refractive index were tested in the frequency range of 0.2–1.6 THz. The methods can be used to measure the metal residues in the soil. Li and Chen (2016) and Li and Zhao (2016) studied the quantitative analyses of the Pb cation in soil by THz-TDS. Li et al. (2015) investigated the absorption coefficient and the refractive index of four types of nitrate solution (sodium nitrate, aluminum nitrate, calcium nitrate, and magnesium nitrate) using the THz-TDS system. A PLS model was used to establish quantitative analysis methods. Zhao et al. (2018) investigated the dimethylurea isomer spectrum detection by THz spectrometer. Qin et al. (2017a) and Qin, Xie, and Ying (2017b) developed a method to analyze tetracycline hydrochloride solution by attenuated total reflection THz-TDS. Zhou et al. (2015) investigated benzoyl peroxide in flour analyzed by THz spectrometer.

Drugs of abuse

Numerous studies have used THz spectroscopy in the detection of illicit drugs. THz waves are transparent to most dry dielectric materials, such as cloth, paper, wood, and plastic. Based on this property, THz waves can be used for the qualitative valuation of items in packaging or bags for security checking.

Davies, Burnett, and Fan (2008) reported the application of THz-TDS system for seven illicit drugs or drug precursor detection. The THz transmission spectrum of cocaine free base, cocaine hydrochloride, 3,4-methylenedioxy-N-methamphetamine (MDMA), ephedrine hydrochloride, amphetamine sulfate, diamorphine (heroin), and morphine sulfate pentahydrate (morphine) was collected by polytetrafluoroethylene(PTFE) compressed into pellet sample . The THz-TDS spectra of cocaine free base and cocaine hydrochloride have absorption peak at 1.54 THz. MDMA and 4,5-methylenedioxy amphetamine have absorption peak at 1.24, 1.71, and 1.90 THz. THz transmission spectrum of different packages were also collected in that study.

Pan (2010) collected THz spectra for seven pure illicit drugs. Support vector machine (SVM) was used to classify the THz absorption spectra and determine the main contents proportion of mixtures. Deng et al. (2009) measured and reported the THz spectra of ephedrine hydrochloride and papaverine hydrochloride. The experiment result matched more with theoretical calculation in gas-phase simulation by semi-empirical theory than by DFT for some chemical compounds. Neumaier et al. (2015) applied the THz spectra in illicit drugs and security detection. He and Shen (2013) established a THz database containing 38 kinds of pure illicit drugs. The database includes absorption coefficient and refractive index spectra of illicit drugs. SVM, back propagation (BP), radial basis function (RBF) artificial neural network algorithm methods were suited for identification analysis. Self-organizing feature map (SOM) algorithm methods were suited for quantitative analysis.

Conclusions and outlook

During the last decade, various THz technologies have undergone rapid development and there has been an increasing interest in their use for chemical sensing. THz technologies had been widely used for toxic chemical compound detection for its many merits such as fingerprint character, transmission property, and other uses. With continuous progress, THz spectroscopy will play important role in toxic chemical compound on-site detection and nondestructive examination.

Acknowledgments

This project was supported by the State Key Laboratory of NBC Protection for Civilian (SKLNBC2012-08).

References

Almoayed, N. N.; Piyade, B. C.; Afsar, M. N. High-Resolution Absorption Coefficient and Refractive Index Spectra of Common Pollutant Gases at Millimeter and THz Wavelengths. Proc. SPIE 6772, Terahertz Physics, Devices, and Systems II, 67720G, 2007. doi: 10.1117/12.737143. Search in Google Scholar

Assefzadeh, M. M.; Jamali, B.; Aleksander, K. Terahertz Trace Gas Spectroscopy Based on a Fully-Electronic Frequency-Comb Radiating Array in Silicon. In Conference on Lasers and Electro-Optics paper SM2L.7, 2016. Search in Google Scholar

Azzam, A. A. A.; Yurchenko, S. N.; Tennyson, J.; Martin-Drumel, M. A.; Pirali, O. Terahertz Spectroscopy of Hydrogen Sulfide. J. Quant. Spectrosc. Radiat. Transf.2013, 130, 341–351.10.1016/j.jqsrt.2013.05.035 Search in Google Scholar

Baek, S. H.; Kang, J. H.; Hwang, Y. H.; Ok, K. M.; Kwak, K.; Chun, H. S. Detection of Methomyl, a Carbamate Insecticide in Food Matrices Using Terahertz Time-Domain Spectroscopy. J. Infrared Millim. Terahertz Waves2016, 37, 486–497.10.1007/s10762-015-0234-9 Search in Google Scholar

Cai, H.; Wang, D.; Shen, J. L. Study on Terahertz Spectra of SO2 and H2S. Sci. China Phys. Mech. Astronomy2013, 56, 685–690.10.1007/s11433-013-5042-4 Search in Google Scholar

Chen, Z. W. Application of Terahertz Time-Domain Spectroscopy Combined with Chemometrics to Quantitative Analysis of Imidacloprid in Rice Samples. J. Quant. Spectrosc. Radiat. Transf.2015, 167, 1–9.10.1016/j.jqsrt.2015.07.018 Search in Google Scholar

Cui, Y.; Mu, K. J.; Wang, X. K.; Zhang, Y.; Zhang, C. L. Measurement of Mixtures of Melamine Using THz Ray. In Proc. SPIE 7385, International Symposium on Photoelectronic Detection and Imaging 2009: Terahertz and High Energy Radiation Detection Technologies and Applications, 73851E, 2009, 1–9. doi: 10.1117/12.835293. Search in Google Scholar

Davies, A. G.; Burnett, A. D.; Fan, W. H. Terahertz Spectroscopy of Explosives and Drugs. Mater. Today2008, 3, 18–26. Search in Google Scholar

Demers, J. R.; Garet, F. A UAV-Mounted THz Spectrometer for Real-Time Gas Analysis. In Proc. SPIE 10531, Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications XI, 2018. doi: 10.1117/12.2292783. Search in Google Scholar

Deng, F. S.; Shen, J. L.; Wang, G. Q.; Liang, M. Y. Spectroscopy Study of Ephedrine Hydrochloride and Papaverine Hydrochloride in Terahertz Range. In Proc. SPIE 7158, 2008 International Conference on Optical Instruments and Technology: Microelectronic and Optoelectronic Devices and Integration, 71580S, 2009. doi: 10.1117/12.805630. Search in Google Scholar

Exter, M.; Fattinger, C.; Grischkowsky, D. Terahertz Time-Domain Spectroscopy of Water Vapor. Opt. Lett.1989, 14, 1128–1130.10.1364/OL.14.00112819753077 Search in Google Scholar

Harmon, S. A.; Cheville, R. A. Part-per-Million Gas Detection from Long-Baseline THz Spectroscopy. Appl. Phys. Lett.2004, 85, 2128–2130.10.1063/1.1788896 Search in Google Scholar

Hangyo, M.; Tani, M.; Nagashima, T. Terahertz Time-Domain Spectrometry of Solid: A Review. Int. J. Infrared Millim. Terahertz Waves2005, 26, 1661–1689.10.1007/s10762-005-0288-1 Search in Google Scholar

He, T.; Shen, J. L. Applications of Terahertz Spectroscopy in Illicit Drugs Detection. Spectrosc. Spect. Anal.2013, 33, 2348–2353. Search in Google Scholar

Hindle, F.; Cuisset, A.; Bocquet, R.; Mouret, G. Continuous-Wave Terahertz by Photomixing: Applications to Gas Phase Pollutant Detection and Quantification. C. R. Phys.2008, 9, 262–275.10.1016/j.crhy.2007.07.009 Search in Google Scholar

Hou, D. B.; Yue, F. H.; Kang, X. S.; Huang, P. J.; Zhang, G. X. Terahertz Time-Domain Spectroscopy of Alpha Endosulfan Persistent Organic Pollutant. Guang Pu Xue Yu Guang Pu Fen Xi.2012, 32, 1170–1174.22827047 Search in Google Scholar

Hsieh, Y. D.; Nakamura, S.; Abdelsalam, D. G.; Minamikawa, T.; Mizutani, Y.; Yamamoto, H. Dynamic Terahertz Spectroscopy of Gas Molecules Mixed with Unwanted Aerosol Under Atmospheric Pressure Using Fibre-Based Asynchronous-Optical-Sampling. Sci. Rep.2016a, 6, 28114.10.1038/srep28114 Search in Google Scholar

Hsieh, Y. D.; Kimura, H.; Hayashi, K.; Minamikawa, T.; Mizutani, Y.; Yamamoto, H. Terahertz Frequency-Domain Spectroscopy of Low-Pressure Acetonitrile Gas by a Photomixing Terahertz Synthesizer Referenced to Dual Optical Frequency Combs. J. Infrared Millim. Terahertz Waves2016b, 37, 903–915.10.1007/s10762-016-0277-6 Search in Google Scholar

Kilcullen, P.; Hartley, I. D.; Jensen, E. T.; Reid, M. Terahertz Time Domain Gas-Phase Spectroscopy of Carbon Monoxide. J. Infrared Millim. Terahertz Waves2015, 36, 380–389.10.1007/s10762-014-0139-z Search in Google Scholar

Lee, D. K.; Kim, G. Y.; Son, J. H.; Seo, M. Highly Sensitive Terahertz Spectroscopy of Residual Pesticide Using Nano-antenna. In Proc. SPIE 9747, Terahertz, RF, Millimeter, and Submillimeter-Wave Technology and Applications IX, 97470S, 2016. doi: 10.1117/12.2212055. 1–6. Search in Google Scholar

Li, B.; Chen L. P. Exploration on Precision Farming Pollution Detection Using THz Technology. Infrared Laser Eng.2016, 45, 0425003.1–7. Search in Google Scholar

Li, B.; Zhao, C. J. Preliminary Research on Heavy Metal Pb Detection in Soil Based on Terahertz Spectroscopy. Trans. Chin. Soc. Agric. Mach.2016, 47, 291–296. Search in Google Scholar

Li, J. S.; Zhao, X. L.; Li, J. R. Study on the THz Spectra of Metallic Ion in Soil. In Proc. SPIE 7385, International Symposium on Photoelectronic Detection and Imaging 2009: Terahertz and High Energy Radiation Detection Technologies and Applications, 738500, 2009, pp. 1–6. doi: 10.1117/12.835028. Search in Google Scholar

Li, Q.; Zhan, H. L.; Qin, F. L.; Jin, W. J.; Liu, H. L.; Zhao, K. Detecting NO3 Concentration in Nitrate Solutions Using Terahertz Time-Domain Spectroscopy. Front. Optoelectron.2015, 8, 62–67.10.1007/s12200-014-0437-z Search in Google Scholar

Li, P. P.; Zhang, Y.; Ge, H. Y. Terahertz Spectral Detection of Potassium Sorbate in Milk Powder. In Proc. SPIE 10256, Second International Conference on Photonics and Optical Engineering, 102560A, 2017. doi: 10.1117/12.2268784.8. Search in Google Scholar

Liang, W. X. Rapidly Determinating the Principal Components of Natural Gas Distilled from Shale with Terahertz Spectroscopy. Fuel2015, 159, 84–88.10.1016/j.fuel.2015.06.072 Search in Google Scholar

Lin, H.; Withayachumnankul, W.; Fischer, B. M.; Mickan S. P.; Abbott, D. Gas Recognition with Terahertz Time-Domain Spectroscopy and Spectral Catalog: A Preliminary Study. In Proc. SPIE 6840, Terahertz Photonics, 2008, 68400X. doi: 10.1117/12.760558. Search in Google Scholar

Liu, L.; Shen, L.; Yang, F.; Han, F.; Hu, P.; Song, M. Determining Phthalic Acid Esters Using Terahertz Time Domain Spectroscopy. J. Appl. Spectrosc.2016, 83, 1–7. Search in Google Scholar

Lu, X.; Karpowicz, N.; Zhang, X. C. Broadband Terahertz Detection with Selected Gases. J. Opt. Soc. Am. B2009, 26, A66–A73.10.1364/JOSAB.26.000A66 Search in Google Scholar

Maeng, I.; Baek, S. H.; Kim, H. Y.; Ok, G. S.; Choi, S. W.; Chun, H. S. Feasibility of Using Terahertz Spectroscopy to Detect Seven Different Pesticides in Wheat Flour. J Food Prot.2014, 77, 2081–2087.10.4315/0362-028X.JFP-14-13825474054 Search in Google Scholar

Massaouti, M.; Daskalaki, C.; Gorodetsky, A.; Koulouklidis, A. D.; Tzortzakis, S. Detection of Harmful Residues in Honey Using Terahertz Time-Domain Spectroscopy. Appl Spectrosc.2013, 67, 1264–1269.2416087710.1366/13-07111 Search in Google Scholar

Medvedev, I. R.; Behnke, M.; Lucia, F. C. Fast Analysis of Gases in the Submillimeter/Terahertz with “absolute” Specificity. Appl. Phys. Lett.2005, 86, 1128. Search in Google Scholar

Mihrin, D.; Jakobsen, P. W.; Voute, A.; Manceronbc, L.; Wugt Larsen, R. High-Resolution Synchrotron Terahertz Investigation of the Large-Amplitude Hydrogen Bond Librational Band of (HCN)2. Phys. Chem. Chem. Phys.2018, 20, 8241–8246.10.1039/C7CP08412A29528052 Search in Google Scholar

Mittleman, D. Sensing with Terahertz Radiation; Springer: New York, 2003. Search in Google Scholar

Mouret, G.; Guinet, M.; Cuisset, A,; Croize, L.; Eliet, S. Versatile Sub-THz Spectrometer for Trace Gas Analysis. IEEE Sens. J.2013, 13, 133–138.10.1109/JSEN.2012.2227055 Search in Google Scholar

Neese, C. F.; Medvedev, I. R.; Plummer, G. M.; Frank, A. J.; Ball, C. D. Compact Submillimeter/Terahertz Gas Sensor with Efficient Gas Collection, Preconcentration, and ppt Sensitivity. IEEE Sens. J.2012, 12, 2565–2574.10.1109/JSEN.2012.2195487 Search in Google Scholar

Neumaier, P. F.; Schmalz, K.; Borngräber, J.; Wylde, R.; Hübers, H. W. Terahertz Gas-Phase Spectroscopy: Chemometrics for Security and Medical Applications. Analyst2015, 140, 213–222.10.1039/C4AN01570C25406969 Search in Google Scholar

Pan, R. Terahertz Spectra Applications in Identification of Illicit Drugs Using Support Vector Machines. Procedia Eng.2010, 7, 15–21.10.1016/j.proeng.2010.11.003 Search in Google Scholar

Pereira, M. F.; Shulika, O. Terahertz and Mid Infrared Radiation: Detection of Explosives and CBRN (Using Terahertz); IOS Press: Amsterdam, 2013. Search in Google Scholar

Qin, J.; Ying, Y. b.; Xie, L. J. The Detection of Agricultural Products and Food Using Terahertz Spectroscopy: A Review. Appl. Spectrosc. Rev.2013, 48, 439–457.10.1080/05704928.2012.745418 Search in Google Scholar

Qin, B. Y.; Zhi, L.; Luo, Z.; Li, Y.; Zhang, H. Terahertz Time-Domain Spectroscopy Combined with PCA-CFSFDP Applied for Pesticide Detection. Opt. Quantum Electron.2017a, 49, 244.10.1007/s11082-017-1080-x Search in Google Scholar

Qin, J. y.; Xie, L. j.; Ying, Y. b. Rapid Analysis of Tetracycline Hydrochloride Solution by Attenuated Total Reflection Terahertz Time-Domain Spectroscopy. Food Chem.2017b, 224, 262–269.10.1016/j.foodchem.2016.12.064 Search in Google Scholar

Saeedkia, D. Handbook of Terahertz Technology for Imaging, Sensing and Communications; Woodhead Publishing: London, UK, 2013. Search in Google Scholar

Son, J. H. Terahertz Biomedical Science & Technology; CRC Press, 2014. Search in Google Scholar

Song, H. J.; Nagatsuma, T. Handbook of Terahertz Technologies Device and Application; Taylor & Francis Group, LLC. Press: Florida, USA. 2015. Search in Google Scholar

Shimizu, N.; Song, H. J.; Kado, Y. C.; Furuta, T.; Wakatsuki, A.; Muramoto, Y. Gas Detection Using Terahertz Waves. NTT Tech. Rev.2009, 7, 1–6. Search in Google Scholar

Shimizu, N.; Ikari, T.; Kikuchi, K.; Matsuyama, K.; Wakatsuki, A. Remote Gas Sensing in Full-Scale Fire with Sub-terahertz Waves. In Microwave Symposium Digest (MTT), 2011 IEEE MTT, 2011, 1–4. Search in Google Scholar

Smith, R. M.; Arnold, M. A. Selectivity of Terahertz Gas-Phase Spectroscopy. Anal. Chem.2015, 87, 10679–10683.10.1021/acs.analchem.5b0302826436438 Search in Google Scholar

Sun, T.; Zhang, Z. Y.; Xiang, Y. H.; Zhu, R. H. Determination of Carbaryl in Rice by Using FT Far-IR and THz-TDS Techniques. Guang Pu Xue Yu Guang Pu Fen Xi.2016, 36, 541–544.27209765 Search in Google Scholar

Suzuki, T. Characterization of Pesticide Residue, cis-Permethrin by Terahertz Spectroscopy. Eng. Agric. Environ. Food 2011, 4, 90–94.10.1016/S1881-8366(11)80007-8 Search in Google Scholar

Song, M. J.; Li, J. S. Detection of POPs in Soil by Using Terahertz Time-Domain Spectroscopy. In Proc. SPIE 8330, Photonics and Optoelectronics Meetings (POEM) 2011: Laser and Terahertz Science and Technology, 2012, 833017. doi: 10.1117/12.919970. Search in Google Scholar

Tanno, T.; Watanabe, Y.; Umeno, K.; Matsuoka, A.; Matsumura, H.; Odaka, M.; Ogawa, N. In Situ Observation of Gas Adsorption onto ZIF-8 Using Terahertz Waves. J. Phys. Chem. C2017, 121, 17921–17924.10.1021/acs.jpcc.7b04833 Search in Google Scholar

Theuer, M.; Harsha, S. S.; Molter, D.; Torosyan, G.; Beigang, R. Terahertz Time-Domain Spectroscopy of Gases, Liquids, and Solids. ChemPhysChem2011, 12, 2695–2705.2173551010.1002/cphc.201100158 Search in Google Scholar

Thompson, B. J. Terahertz Spectroscopy Principles and Applications; CRC Press: New York, 2008. Search in Google Scholar

Walther, M.; Bernd, M.; Ortner, F. A.; Bitzer, A.; Thoman, A.; Helm, H. Chemical Sensing and Imaging with Pulsed Terahertz Radiation. Anal. Bioanal. Chem.2010, 397, 1009–1017.10.1007/s00216-010-3672-1 Search in Google Scholar

Wang, J.; Zhang, B.; Lv, C.; Zhang, J. Detection of Glyphosate Pollution in Tealand Soil Using Terahertz Spectroscopy. Xiandai nongye keji2017, 8, 116–117. Search in Google Scholar

Wang, Q.; Ma, Y.; Wang, X. W. Quantitative Measurement of AMS and Orange Mixtures by Terahertz Time-Domain Spectroscopy. In Proc. SPIE 8366, Advanced Environmental, Chemical, and Biological Sensing Technologies IX, 83660U, 2012. doi: 10.1117/12.924299. Search in Google Scholar

Wang, X. W.; Wang, Q.; Ma, Y. H.; Wang, H. L. Qualitative and Quantitative Study of Quintozene by Terahertz Time-Domain Spectroscopy. In Proc. SPIE 8201, 2011 International Conference on Optical Instruments and Technology: Optoelectronic Measurement Technology and Systems, 82010W, 2011. doi: 10.1117/12.906502. Search in Google Scholar

Xu, W. D.; Xiea, L. J.; Zhu, J. F.; Wang, W. C.; Yea, Z. Z. Terahertz Sensing of Chlorpyrifos-Methyl Using Metamaterials. Food Chem.2017, 218, 330–334.2771991710.1016/j.foodchem.2016.09.032 Search in Google Scholar

Zhao, L. J.; Gao, L.; Jiang, C.; Yi, L. X.; Yu, X. S. Determination Of Dibutyl Phthalate With Thz Time-Domain Spectroscopy. Acta Opt. Sin.2015, 35, s130001.10.3788/AOS201535.s130001 Search in Google Scholar

Zhao, Y. h.; Li, Z.; Liu, J.; Chen, T.; Zhang, H.; Qin, B.; Wu, Y. Application of Terahertz Spectroscopy and Theoretical Calculation in Dimethylurea Isomers Investigation. Spectrochim. Acta Part A Mol. Biomol. Spectrosc.2018, 192, 336–342.10.1016/j.saa.2017.11.040 Search in Google Scholar

Zhou, Q.; Liu, Y.; Kou, K.; Zhao, G. Z. Study on Spectral Properties of Difenoconazole by Terahertz Time-Domain Spectroscopy. In Proc. SPIE 9625, 2015 International Conference on Optical Instruments and Technology: Terahertz Technologies and Applications, 962504, 2015. doi: 10.1117/12.21932697. Search in Google Scholar

Zou, D.; Dan, L.; Qing, Z.; Kunhong, Y.; Dianyu, Y.; Liqi, W. Determination of the Content of Benzoyl Peroxide in Flour Based on Terahertz Spectroscopy. Shi pin ke xue.2017, 38, 298–302. Search in Google Scholar

Received: 2017-09-22
Accepted: 2018-04-07
Published Online: 2018-06-13

©2018 Walter de Gruyter GmbH, Berlin/Boston

This article is distributed under the terms of the Creative Commons Attribution Non-Commercial License, which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.