2081

Journal of Food Protection, Vol. 77, No. 12, 2014, Pages 2081–2087 doi:10.4315/0362-028X.JFP-14-138 Copyright G, International Association for Food Protection

Feasibility of Using Terahertz Spectroscopy To Detect Seven Different in Wheat Flour

INHEE MAENG,1,2 SEUNG HYUN BAEK,1 HWA YEON KIM,1 GYEONG-SIK OK,2 SUNG-WOOK CHOI,2 AND HYANG SOOK CHUN1*

1Department of Chemistry and School of Food Science and Technology, Chung-Ang University, Ansung, Gyounggi 456-756, Republic of Korea; and Downloaded from http://meridian.allenpress.com/jfp/article-pdf/77/12/2081/1684084/0362-028x_jfp-14-138.pdf by guest on 29 September 2021 2Food Safety Research Center, Korea Food Research Institute, Sungnam 463-746, Republic of Korea

MS 14-138: Received 22 March 2014/Accepted 31 July 2014

ABSTRACT This study investigated the feasibility of detecting pesticides using terahertz (THz) spectroscopy in high-density polyethylene and/or wheat flour mixtures. The absorption spectra of seven pesticides (dicofol, , chlorpyrifos-methyl, daminozide, , diethyldithiocarbamate, and dimethyldithiocarbamate) were measured in the frequency range 0.1 to 3 THz at room temperature. Five of the seven pesticides exhibited specific absorption peaks in the low-energy THz range. The two remaining pesticides had no specific absorption peaks in this frequency range, but they exhibited different frequency- dependent refractive indices. The absorption coefficients of imidacloprid increased with its increasing weight ratio in high-density polyethylene, and the fitted power absorptions and refractive indices using a Maxwell-Garnett effective medium model were comparable to the measured data. Imidacloprid was also identified from its characteristic absorption peaks in wheat flour mixtures, and a linear relationship between the absorption coefficient and the weight ratio was observed. Our results show the potential of detection of selected pesticides in foods, such as wheat flour, using THz spectroscopy.

The presence of residues in finished and raw molecules. In contrast, the absorption features in the THz food products is a growing concern among consumers. To region are dominated by the intermolecular vibrations date, pesticide detection depends largely on the use of corresponding to motion associated with coherent, delocal- chromatographic methods, such as gas chromatography and ized movements of large numbers of atoms and molecules high-performance liquid chromatography (HPLC) coupled (6, 9). Because these properties are unique to a particular with different detectors (3, 8). Although these chromatog- molecule, it is possible to obtain a ‘‘THz fingerprint,’’ raphy-based methods have a high detection precision, they enabling the identification of a compound (14, 17, 19, 23, are labor intensive and time consuming (8). Spectroscopic 25, 28, 30). Consequently, THz spectroscopy could methods, including UV spectroscopy and near-infrared (IR) potentially be used to characterize the properties of spectroscopy, are used as alternative methods because their pesticides. procedures are faster and easier to use (1, 27). UV THz spectroscopy is a low-energy technique that allows spectroscopy and near-IR spectroscopy are suitable for nondestructive analysis of materials, because the energy of a qualitative analysis with the complex spectra obtained from THz pulse is less than that of the typical background intramolecular overtones and the combination bands of radiation present in the environment. THz frequencies can compounds, but quantitative analysis requires a lot of also penetrate a wide range of dielectric materials, such as information (2, 11). clothing, paper, cardboard, wood, plastic, and ceramics. Terahertz (THz) spectroscopy, which uses the frequen- This makes it possible to directly monitor packaging cy range between the far-IR and the millimeter or materials for quality control using THz spectroscopy or microwave regions, enables us to analyze samples both imaging (10, 16, 26). With the development of THz qualitatively and quantitatively (7, 13, 29). The THz region instrumentation, the applications of THz spectroscopy have (0.1 to 30 THz, 3.3 to 1,000 cm21) has a unique property: expanded to include the food industry. many materials are transparent or semitransparent to THz Recently, THz time-domain spectroscopy (THz-TDS) radiation, whereas many crystalline materials exhibit has been used as a noninvasive tool to detect pesticides in characteristic spectral features in the THz region (13, 20, sticky rice, sweet potato, and lotus roots. It was shown that 24). The absorption features within the mid-IR region are four pesticides (imidacloprid, carbendazim, tricyclazole, and dominated by intramolecular vibrations of the sample ) had specific absorbance peaks in the THz range (0.5 to 1.6 THz) (12). It has also been reported that THz- * Author for correspondence. Tel: z82-31-670-3290; Fax: z82-31-675- TDS can potentially detect antibiotic residues in feedstock. 4853; E-mail: [email protected]. Standard mixtures of antibiotics and powdered polyethylene 2082 MAENG ET AL. J. Food Prot., Vol. 77, No. 12 were prepared in a pelletized form, and transmission THz- any pellets. HDPE, which has a low refractive index and low TDS spectra were obtained in the frequency range from 0.1 absorption in the THz region, was used as a binder for the pellets. to 2 THz (21). Eight of the 11 antibiotics tested had specific The pesticide compound was mixed in an HDPE matrix in a signatures in the THz region, whereas two of them loading ratio of 10 wt%. Circular tablets (total weight, 360 mg; diameter, 13 mm; thickness, 2.8 to 3.1 mm) were formed using a (doxycycline and sulfapyridine) were examined when compressive load of 5 tons. In the next step, imidacloprid, which mixed with different food matrices (e.g., animal feed, milk, showed strong absorption features, was mixed with various weight and egg powder). However, substantial variation was fractions of HDPE and wheat flour. To prepare food mixture reported in absorption spectra recorded using a common samples, known amounts of pure imidacloprid were added to the compound; therefore, a wide range of food compounds and wheat flour used as a food matrix. All the samples were prepared physical conditions must be measured (10). In addition, and measured in triplicate. there have been relatively few feasibility studies for applications of THz-TDS in the food sector. Measurements. THz-TDS is a powerful technique; the pump-probe configuration allows direct measurement of the With this background, we analyzed seven different Downloaded from http://meridian.allenpress.com/jfp/article-pdf/77/12/2081/1684084/0362-028x_jfp-14-138.pdf by guest on 29 September 2021 transient electromagnetic field of the THz pulse. The THz spectrum pesticides (two , one organochlorine, two is then obtained from the time domain data by Fourier transform. dithiocarbamates, one aminozide, and one chloronicotinoid) Because the absorption coefficient and the refractive index of the to investigate the feasibility of detecting pesticides using material studied are directly related to the amplitude and the phase THz spectroscopy qualitatively and quantitatively in high- of the transmitted electromagnetic field, respectively, complex density polyethylene (HDPE) and wheat flour. We first optical properties can be obtained with high accuracy. In contrast, sought to identify different pesticides based on their specific Fourier transform far-IR spectroscopy measures only the field absorption fingerprint in the frequency range 0.1 to 3 THz. intensity and, therefore, measures only the absorption coefficient. For our quantitative analysis, we analyzed imidacloprid with Although the refractive index can be calculated from Fourier different weight ratios in HDPE and calculated the expected transform far-IR spectroscopy data using Kramers-Kro¨nig rela- spectra using a Maxwell-Garnett (MG) effective medium tions, this calculation is not straightforward and has many potential sources of error. As a further consequence of its coherent pump- model. Then, the partial least squares (PLS) method was probe detection scheme, TDS has a very high signal-to-noise ratio, applied to assess our quantitative analysis of imidacloprid in typically .106 for the powder, compared with a signal-to-noise a matrix of wheat flour. ratio of approximately 300 for far-IR Fourier transform spectros- copy (19). MATERIALS AND METHODS A TPS spectra 3000 spectrometer (TeraView Ltd, Cambridge, Chemicals and samples. The seven pesticides were selected UK; http://www.teraview.com) (22), with a spectral resolution of from different chemical classes of organic compound: dicofol 0.036 THz over the range 0.1 to 3 THz, was used for the THz (2,2,2-trichloro-1,1-bis[4-chlorophenyl]ethanol, an organochlorine pulsed spectroscopy measurements. The samples were measured in ), chlorpyrifos (O,O-diethyl O-3,5,6-trichloropyridin-2- the very stable fast-scan mode, which allowed 30 measurements to yl phosphorothioate, an insecticide), chlorpyr- be performed per second. We obtained 1,800 signals and averaged ifos-methyl (O,O-dimethyl O-3,5,6-trichloropyridin-2-yl phos- them to analyze the spectrum. The sample chamber was filled with phorothioate, an organophosphate insecticide), daminozide (N- dry air to prevent any water absorption occurring. The signals, [dimethylamino]succinamic acid, an herbicide), imidacloprid ([E]- transmitted through pinholes with diameters of 3 and 10 mm 1-[6-chloro-3-pyridylmethyl]-N-nitroimidazolidin-2-ylideneamine, depending on the size of the samples, were used as reference a chloronicotinoid insecticide), diethyldithiocarbamate sodium signals. salt trihydrate (sodium N,N-diethylcarbamodithioate trihydrate, a Data analysis. A TPS spectra 3000 spectrometer acquired the dithiocarbamate herbicide), and dimethyldithiocarbamate sodium time-domain data using the time delay between generation and salt hydrate (N,N-dimethyldithiocarbamate sodium salt hydrate, a detection. The time-domain data were transformed into the dithiocarbamate herbicide). Chlorpyrifos (CAS no. 2921-88-2), frequency domain using a Fourier transformation (TeraView). chlorpyrifos-methyl (CAS no. 5598-13-0), daminozide (CAS The Black-Harris three-term apodization method was applied to the no. 1596-84-5), dicofol (CAS no. 115-32-2), N,N-diethyldithio- Fourier transformation (TeraView). The power absorption coeffi- sodium salt trihydrate (CAS no. 20624-25-3), and N,N- cient, a(n), is given by a ~ 4pk/l, where k is the imaginary dimethyldithiocarbamate sodium salt hydrate (CAS no. 207233- refractive index and l is the wavelength. The refractive index, n(n), 95-2) (Dr. Ehrenstorfer, Augsburg, Germany) had purities of 98, of the sample was calculated as a function of the applied THz 97.5, 97, 98.5, 99.5, and 99%, respectively. Imidacloprid (CAS frequency, n, using the following equations: no. 138261-41-3, Sigma Chemicals, St. Louis, MO) had a purity of 99.9%. a(n)~{ln½(T(n)Esample(n))=(Ereference(n))=d ð1Þ HDPE powder (particle size, ,10 mm) was purchased from ÂÃ ~ z { TeraView Ltd (Cambridge, UK). The wheat flour matrix used was nðÞn 1 c wsampleðÞn wreferenceðÞn =2pnd ð2Þ purchased at local stores and characterized using HPLC analysis to ~ { { 2 z 2 be free of imidacloprid residues (18). Sample tablets for qualitative TðÞn 1 ½nðÞn 1 =½nðÞn 1 ð3Þ detection were prepared using both pure pesticide powders and where E and w denote the THz amplitude and the phase, those mixed with HDPE powder. The pellets were loaded in a respectively. The term c is the velocity of light, d is the sample hydraulic press (Specac Ltd, Orpington, UK) at different pressures thickness, and T(n) is the amplitude transmission coefficient given depending on pellet diameter. The pure pesticide pellets contained by the Fresnel equation. The complex dielectric constants e˜(n) ~ 36 mg of pesticide powder using 2 tons of compressive loads. The e1(n) z ie2(n) are related to the refractive index, n˜(n) ~ n(n) z resulting sample tablets (5-mm diameter, ,1.4-mm thickness) ik(n), and to each other through n˜(n)2 ~ e˜(n). In the case of a were easily broken, and chlorpyrifos-methyl powder did not form mixed sample, they are characterized by the macroscopic J. Food Prot., Vol. 77, No. 12 DETECTION OF PESTICIDES BY TERAHERTZ SPECTROSCOPY 2083

samples showed a different time delay that had a decrease in amplitude compared with the reference signal. The pesticide samples that had not been premixed had only their THz spectral properties measured below 2 THz because the transmitted THz signals were undetectable over 2 THz. The amplitude of transmitted THz signals depended on the optical properties and the thickness of the sample. The pure pesticide samples had a physical limit on their thickness, which is why we opted to use HDPE as a binder. The THz spectral properties of the mixed samples containing HDPE were also extracted, and the THz spectral properties of the pure pesticides were calculated using the MG effective

medium model. We assumed that the sample contained 3 to Downloaded from http://meridian.allenpress.com/jfp/article-pdf/77/12/2081/1684084/0362-028x_jfp-14-138.pdf by guest on 29 September 2021 5% air. The power absorption spectra and refractive indices of seven pesticide samples are shown in Figure 2a and 2b. The FIGURE 1. Transmitted THz time-domain signals of pesticide dotted lines represent the properties of the pure pesticide samples that were not premixed (imidacloprid, N,N-diethylcarba- samples, and the various lines represent the premixed modithioate trihydrate, daminozide, and N,N-dimethyldithiocar- samples. As shown in Figure 2, the properties of the pure bamate sodium salt hydrate) and sample holder as a reference. samples and those calculated using the MG effective properties of a medium based on each of the constituent properties medium model were in reasonable agreement and show and the relative fractions of each of its components. The MG many absorption peaks. The absorption peaks of the seven effective medium model, which is widely used to determine the pesticides are summarized in Table 1. The imidacloprid, optical properties of heterogeneous materials, is described by N,N-diethyldithiocarbamate sodium salt trihydrate, damino- equation 4 (15): zide, and N,N-dimethyldithiocarbamate sodium salt hydrate e~ {e~ e~ {e~ samples clearly showed more than three absorption peaks in eff h ~ i h ~ ~ fi ~ ~ ð4Þ this region. The molecular structures of the two DTC-based eeff z2eh eiz2eh pesticides, N,N-diethyldithiocarbamate sodium salt trihy- where e˜eff is the complex dielectric constant of the effective medium, e˜ is the complex dielectric constant of the host HDPE, e˜ drate and N,N-dimethyldithiocarbamate sodium salt hydrate, h i are similar, but the THz spectra of the two compounds were is the complex dielectric constant of pesticide, and fi is the volume- different. Dicofol showed much broader and weaker peaks filling factor of the pesticide. The dielectric constant of HDPE, e˜h, was measured and the filling factor, fi, was calculated using the than four other pesticides (imidacloprid, N,N-diethyldithio- density of the fabricated samples. We estimated the frequency- carbamate sodium salt trihydrate, daminozide, and N,N- dependent absorption coefficients and the refractive indices of the dimethyldithiocarbamate sodium salt hydrate). However, no pesticides from our experimental data using the MG effective specific THz absorption peaks were observed for the two medium model. organophosphate , chlorpyrifos and chlorpyri- A PLS regression analysis was performed using the fos-methyl. Unscrambler v.10.2 (Camo Software, Oslo, Norway) to analyze The specific absorption peaks of each pesticide the imidacloprid mixtures quantitatively (5). The PLS technique component (Table 1) allow them to easily be distinguished first extracts the orthogonal features from the original spectra from each other: for imidacloprid, peak positions occurred within a selected frequency range and then correlates these features at 1.36, 1.76, and 2.15 THz; for N,N-diethyldithiocarbamate to the target variable. Calibration models were developed using a test set validation by splitting the samples randomly into two sodium salt trihydrate, 0.63, 1.11, 1.41, 1.74, and 2.40 THz; groups: one group (six samples) was used for calibration purposes, for daminozide, 1.32, 2.40, and 2.79 THz; for N,N- and the other group (six samples) was used for validation purposes. dimethyldithiocarbamate sodium salt hydrate, 0.99, 1.41, The developed models were then tested using an independent set of and 1.50 THz; and for dicofol, 2.34 THz. imidacloprid mixtures (seven samples). The correlation coefficient Figure 2b shows the refractive index of the seven and the root mean square error (RMSE) of the calibration set pesticides in the range 0.1 to 3 THz, derived from the (RMSEC) were used to calculate the model precision to evaluate transmission amplitude of the THz field passing through the model performance, and t and R, the root mean square error of samples. The average refractive indices for chlorpyrifos, the prediction set (RMSEP) and the relative error of the prediction chlorpyrifos-methyl, daminozide, dicofol, N,N-diethyl- set (Er), were applied to test the precision of the model prediction. dithiocarbamate sodium salt trihydrate, N,N-dimethyldithio- carbamate sodium salt hydrate, and imidacloprid in the RESULTS AND DISCUSSION range 0.1 to 2.5 THz were 1.67, 1.58, 1.68, 1.70, 1.99, 2.08, THz spectra of the seven pesticides. Figure 1 shows and 1.87, respectively. Although there were no peaks in the the time-domain THz signals measured passing through the THz spectra of chlorpyrifos and chlorpyrifos-methyl, it is pesticide samples that had not been premixed, imidacloprid, obvious that the refractive indices of chlorpyrifos and N,N-diethylcarbamodithioate trihydrate, daminozide, and N, chlorpyrifos-methyl were different in this range. N-dimethyldithiocarbamate sodium salt hydrate, and also Hua and Zhang (12) reported that four pesticides the empty sample holder as a reference. The signals from the (imidacloprid, carbendazim, tricyclazole, and buprofezin) 2084 MAENG ET AL. J. Food Prot., Vol. 77, No. 12

use of a different sample handling system. Moreover, because of the wider frequency range used in our study, we observed an additional fingerprint peak for imidacloprid at 2.15 THz. In this study, no distinct peaks were seen in the THz spectra of the two structurally similar organophosphates, chlorpyrifos and chlorpyrifos-methyl; however, because their refractive indices were different in this range, they could be easily distinguished from each other. Overall, these results provide a bibliographic record of the spectral features and refractive indices of these pesticides in the THz range.

Spectra from imidacloprid samples with different Downloaded from http://meridian.allenpress.com/jfp/article-pdf/77/12/2081/1684084/0362-028x_jfp-14-138.pdf by guest on 29 September 2021 weight percentages. We fabricated imidacloprid-HDPE mixtures samples with seven different mixture ratios from 0.5 to 10 wt% for quantitative detection measurements. The absorption coefficient and refractive indices of each imidacloprid sample were extracted using the MG effective medium model (see Fig. 3). The amplitude of the refractive index and the power absorption increased linearly with increasing imidacloprid content. The calculated power absorption and refractive indices are indicated with dotted lines in Figure 3a and 3b. The inset of Figure 3a shows the amplitude of the power absorption for each absorption peak along with the fit from the MG effective medium model. The fitted power absorptions and refractive indices were comparable to the measured data. However, the sample with 5 wt% imidaclo- prid had lower amplitude than expected. This discrepancy is considered to be an error derived from the fabrication process (mostly the density of the sample). The amplitude of power absorption for samples with ,1wt% content appeared to be similar, and the spectra were difficult to distinguish from each other. However, the amplitude increased linearly (see inset, Fig. 3a) with increasing content. Samples with ,0.5 wt% insecticide content could not be distinguished from each other.

Spectra of the imidacloprid and wheat flour FIGURE 2. Experimental absorption spectra (a) and the refrac- mixtures. The main absorption features of imidacloprid tive indices (b) of seven pesticide compounds (imidacloprid, N,N- were still identifiable when it was mixed with wheat flour as diethyldithiocarbamate sodium salt trihydrate, daminozide, N,N- a food matrix. Figure 4 shows the spectra of mixtures of dimethyldithiocarbamate sodium salt hydrate, dicofol, chlorpyr- imidacloprid with wheat flour at different weight fractions ifos-methyl, and chlorpyrifos). The dotted lines represent optical (0, 11, 20, 33, 50, and 100%), which circumvent the effect properties from the pure pesticide samples, and the various lines of HDPE using the MG effective medium model. The represent the calculated optical properties using the Maxwell– absorption spectrum of wheat flour increased almost linearly Garnett effective medium model for mixed samples with a weight with increasing frequency because of its strong absorption, ratio of 10%. whereas the absorption spectrum of imidacloprid with 0% wheat flour did not increase. Nonetheless, as the weight had specific absorbance peaks in the region from 0.5 to ratio of imidacloprid to wheat flour increased from 0 to 1.6 THz. In their study, imidacloprid exhibited absorption 100%, the amplitude of the absorption increased linearly. peaks at 0.89, 1.13, 1.24, 1.46, and 1.57 THz (12). We Peaks occurring at 1.36, 1.76, and 2.15 THz were clearly compared the absorption peaks of imidacloprid, a pesticide identified in most of the spectra. The power absorption common to both Hua and Zhang’s study and our study, in spectrum of the mixture of wheat flour with 11 wt% the range from 0.1 to 1.6 THz. In contrast to Hua and imidacloprid was not distinguishable from that of wheat Zhang, we did not observe absorption peaks at 0.89, 1.13, or flour alone, but the refractive indices showed clear 1.24 THz, and we found a deviation in the reported peak differences. The low sensitivity observed may result from positions at 1.46 (1.37) and 1.57 (1.76) THz. This large physical variations in the sample (e.g., particle size) causing deviation (peak occurring at 1.57 THz) may result from the scattering and a low system signal. Scattering may adversely J. Food Prot., Vol. 77, No. 12 DETECTION OF PESTICIDES BY TERAHERTZ SPECTROSCOPY 2085

TABLE 1. List of the pesticides used, along with their molecular formula and the positions of the power absorption peaks Substance Formula Power absorption peak (THz)

Imidacloprid C9H10ClN5O2 1.36, 1.76, 2.15 N,N-Diethyldithiocarbamate sodium salt trihydrate C5H10NNaS2 | 3H2O 0.63, 1.11, 1.41, 1.74, 2.40 Daminozide C6H12N2O3 1.32, 2.40, 2.79 N,N-Dimethyldithiocarbamate sodium salt hydrate C3H6NNaS2 | H2O 0.99, 1.41, 1.50 Dicofol C14H9Cl5O 2.34 Chlorpyrifos-methyl C7H7Cl3NO3PS — Chlorpyrifos C9H11Cl3NO3PS — affect measurements of the THz absorption of some samples (listed in Table 2); the RMSEP and RSEP values materials. When the grain size of a solid is comparable to were acceptable. The lowest detectable weight ratio of Downloaded from http://meridian.allenpress.com/jfp/article-pdf/77/12/2081/1684084/0362-028x_jfp-14-138.pdf by guest on 29 September 2021 the THz wavelength, then extinction spectra are markedly imidacloprid in wheat flour was estimated to be about influenced by scattering losses. Bandyopadhyay et al. #11%. Considering that foods and agricultural products are investigated the effect of scattering on THz-TDS spectra contaminated with pesticides in trace amounts, further in the range 0.2 to 1.2 THz for granular solids (ammonium research will be required to improve the sensitivity and nitrate, flour, and salt) as a function of grain size (4). increase the resolution. We used PLS to explore the potential of performing In conclusion, we have shown the potential of using quantitative analysis of imidacloprid in wheat flour THz-TDS for qualitative and quantitative analysis of mixtures. The PLS algorithm was applied to six synthetic pesticide components. We confirm that THz-TDS has the

FIGURE 3. Absorption spectra (a) and refractive indices (b) of imidacloprid- HDPE composites as a function of the weight fraction, with fitted lines using the Maxwell-Garnett effective medium model. The absorption peak of the amplitude and its fitted line are shown in the inset. 2086 MAENG ET AL. J. Food Prot., Vol. 77, No. 12

FIGURE 4. Absorption spectra (a) and refractive indices (b) of imidacloprid– wheat flour composites as a function of the weight fraction, with fitted lines using the Maxwell-Garnett model. The absorp- tion peak of the amplitude and its fitted line are shown in the inset. Absorption features occurring around 1.36, 1.76, and 2.15 THz were observed. Downloaded from http://meridian.allenpress.com/jfp/article-pdf/77/12/2081/1684084/0362-028x_jfp-14-138.pdf by guest on 29 September 2021

TABLE 2. Predicted values for imidacloprid and statistical results using the PLS method Spectral range (THz):

0.1–3.0 0.8–2.2 1.3–2.2

Added (mg) Determined (PLS)a Recovery (%) Determined (PLS)b Recovery (%) Determined (PLS)c Recovery (%)

4.5 4.2 93.3 3.8 84.4 3.7 82.2 9.0 8.6 95.6 8.1 90.0 8.1 90.0 18.0 17.2 95.6 16.9 93.9 16.8 93.3 36.0 36.3 100.8 36.4 101.1 36.5 101.4 7.2 7.8 108.3 8.2 113.9 8.3 115.3 14.4 13.8 95.8 13.1 91.0 13.0 90.3 21.6 21.9 101.4 22.5 104.2 22.4 103.7 a The root mean square error of prediction (RMSEP) and regression coefficient of the calibration model (R2) were 0.54 and 0.997, respectively. b The RMSEP and regression coefficient of the calibration model (R2) were 1.07 and 0.990, respectively. c The RMSEP and regression coefficient of the calibration model (R2) were 1.11 and 0.989, respectively. J. Food Prot., Vol. 77, No. 12 DETECTION OF PESTICIDES BY TERAHERTZ SPECTROSCOPY 2087 potential to be developed as an identification and quanti- 12. Hua, Y., and H. Zhang. 2010. Qualitative and quantitative detection fication technique in the food industry. Despite their low of pesticides with terahertz time-domain spectroscopy. IEEE Trans. Microw. Theory Tech. 58:2064–2070. sensitivity, the ability of THz waves to pass through a wide 13. Jansen, C., S. Wietzke, O. Peters, M. Scheller, N. Vieweg, M. Salhi, variety of packaging materials and to characterize the N. Krumbholz, C. Jo¨rdens, T. Hochrein, and M. Koch. 2010. molecular structure of many substances makes this tech- Terahertz imaging: applications and perspectives. Appl. Opt. 49:E48– nique an attractive detection tool for enhanced monitoring E57. of food. For the practical application of THz spectroscopy, 14. Jepsen, P. U., D. G. Cooke, and M. Koch. 2011. Terahertz much more work is required, including the development spectroscopy and imaging—modern techniques and applications, p. 124–166. In G. W. Fuchs (ed.), Laser and photonics reviews, vol. 5. of a cost-effective THz spectrometer with high sensitivity, Wiley-VCH, Weinheim, Germany. THz spectral library databases for a wide group of food 15. Landauer, R. 1978. Electrical conductivity in inhomogeneous media, chemicals, and dedicated software for the chemometric p. 2–45. In J. C. Garland and D. B. Tanner (ed.), Electrical transport treatment of spectral information. and optical properties of inhomogeneous media, vol. 40, AIP conference proceedings. American Institute of Physics, New York. Downloaded from http://meridian.allenpress.com/jfp/article-pdf/77/12/2081/1684084/0362-028x_jfp-14-138.pdf by guest on 29 September 2021 ACKNOWLEDGMENTS 16. Lee, Y. K., S. W. Choi, S. T. Han, D. H. Woo, and H. S. Chun. 2012. Detection of foreign bodies in foods using continuous wave terahertz This work was financially supported by Public Welfare & Safety, a imaging. J. Food Prot. 75:179–183. research program through the National Research Foundation of Korea 17. Li, J. S., and X. J. Li. 2009. Determination principal component (NRF), by the Ministry of Education, Science and Technology grant 2011- content of seed oils by THz-TDS. Chem. Phys. Lett. 476:92–96. 0020819, South Korea. 18. Liu, H., J. Song, S. Zhang, L. Qu, Y. Zhao, Y. Wu, and H. Liu. 2005. Analysis of residues of imidacloprid in tobacco by high-performance REFERENCES liquid chromatography with liquid–liquid partition cleanup. Pest Manag. Sci. 61:511–514. 1. Armenta, S., S. Garrigues, and M. de la Guardia. 2007. Partial least 19. Naftaly, M., and R. E. Miles. 2007. Terahertz time-domain squares-near infrared determination of pesticides in commercial spectroscopy for material characterization. Proc. IEEE 95:1658– formulations. Vib. Spectrosc. 44:273–278. 1665. 2. Auer, M. E., U. J. Griesser, and J. Sawatzki. 2003. Qualitative and 20. Rahman, A. 2011. Dendrimer based terahertz time-domain spectros- quantitative study of polymorphic forms in drug formulations by near copy and applications in molecular characterization. J. Mol. Struct. infrared FT-Raman spectroscopy. J. Mol. Struct. 661–662:307–317. 1006:59–65. 3. Bache, C., and D. Lisk. 1965. Determination of organophosphorus insecticide residues using the emission spectrometric detector. Anal. 21. Redo-Sanchez, A., G. Salvatella, R. Galceran, E. Roldo´s,J.A. ´ Chem. 37:1477–1480. Garcıa-Reguero, M. Castellari, and J. Tejada. 2011. Assessment of 4. Bandyopadhyay, A., A. Sengupta, R. B. Barat, D. E. Gary, J. F. terahertz spectroscopy to detect antibiotic residues in food and feed Federici, M. Chen, and D. B. Tanner. 2007. Effects of scattering on THz matrices. Analyst 136:1733–1738. spectra of granular solids. Int. J. Infrared Milli. Waves 28:969–978. 22. Shen, Y., P. F. Taday, and M. C. Kemp. 2004. Terahertz 5. CAMO Process AS. The Unscrambler tutorial manual v.10.2. Available spectroscopy of explosive materials. Proc. SPIE 5619:82–89. at: http://www.camo.com/downloads/U9.6%20pdf%20manual/The% 23. Stoik, C., M. Bohn, and J. Blackshire. 2010. Nondestructive 20Unscrambler%20Tutorials.pdf. Accessed August 2013. evaluation of aircraft composites using reflective terahertz time 6. Day, G. M., J. A. Zeitler, W. Jones, T. Rades, and P. F. Taday. 2006. domain spectroscopy. NDT&E Int. 43:106–115. Understanding the influence of polymorphism on phonon spectra: 24. Tonouchi, M. 2007. Cutting-edge terahertz technology. Nature 1:97– lattice dynamics calculations and terahertz spectroscopy of carba- 105. mazepine. J. Phys. Chem. 110:447–456. 25. Walther, M., B. M. Fischer, A. Ortner, A. Bitzer, A. Thoman, and H. 7. Ferguson, B., and X. C. Zhang. 2002. Materials for terahertz science Helm. 2010. Chemical sensing and imaging with pulsed terahertz and technology. Nat. Mater. 1:26–33. radiation. Anal. Bioanal. Chem. 397:1009–1017. 8. Fillion, J., F. Sauve, and J. Selwyn. 2000. Multiresidue method for the 26. Walther, M., P. Plochocka, B. Fischer, H. Helm, and P. U. Jepsen. determination of residues of 251 pesticides in fruits and vegetables by 2002. Collective vibrational modes in biological molecules investi- gas chromatography/mass spectrometry and liquid chromatography gated by terahertz time-domain spectroscopy. Biopolymers 67:310– with fluorescence detection. J. AOAC Int. 83:698–713. 313. 9. Fischer, B. M., M. Walther, and P. U. Jepsen. 2003. Noncovalent 27. Wilkowska, A., and M. Biziuk. 2011. Determination of pesticide intermolecular forces in polycrystalline and amorphous saccharides in residues in food matrices using the QuEChERS methodology. Food the far infrared. Chem. Phys. 288:261–268. Chem. 125:803–812. 10. Gowen, A. A., C. O’Sullivan, and C. P. O’Donnell. 2012. Terahertz 28. Yang, Y., S. S. Harsha, A. J. Shutler, and D. R. Grischkowsky. 2012. time domain spectroscopy and imaging: emerging techniques for food Identification of genistein and biochanin A by THz (far-infrared) process monitoring and quality control. Trends Food Sci. Technol. vibrational spectra. J. Pharm. Biomed. Anal. 62:177–181. 25:40–46. 29. Zhang, X. C., and J. Xu. 2010. Introduction to THz wave photonics. 11. Han, P. Y., M. Tani, M. Usami, S. Kono, R. Kersting, and X. C. Springer, Germany. Zhang. 2001. A direct comparison between terahertz time-domain 30. Zhao, X. L., and J. S. Li. 2011. Detection of pesticides residues in spectroscopy and far-infrared Fourier transform spectroscopy. J. rapeseed oil by terahertz time domain spectroscopy. J. Phys. Conf. Appl. Phys. 89:2357–2359. Ser. 276:012231.