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The spectral absorption features of methamphetamine (MA), one of the most widely consumed illicit drugs in the world, are studied experimentally by Terahertz (THz) time-domain spectroscopy (THz-TDS), and the characteristic absorption spectra are obtained in the range of 0.2 to 2.6 THz. The vibrational frequencies are calculated using the density functional theory (DFT). Theoretical results show significant agreement with experimental results, and identification of vibrational modes are given. The calculated results further confirm that the characteristic frequencies come from the collective vibrational modes. The results suggest that use of the THz-TDS technique can be an effective way to inspect for illicit drugs.
©2005 Optical Society of America
1. Introduction
The absence of a nondestructive inspection technique for identifying illicit drugs hidden in envelopes has resulted in such drugs being easily smuggled by mail. There are several inspection techniques in use, such as x-ray scanning, canine detection, and trace detection [1]. However, x-ray scanners are limited to identifying the shape of drugs in the mail but not the type of drugs, while canine detection and trace detection are not effective if there are no detectable signs outside of the envelope. Different countries have specific penalties corresponding to different types and quantities of drugs. Identification of drugs is greatly important and necessary for the determination of crimes being committed; therefore a fast and effective technique is needed for the inspection and identification of illicit drugs.
Terahertz (THz), categorized between the microwave and infrared bands, is a burgeoning electromagnetic wave band. Since the vibration and rotation states of many biological molecules and the low-frequency vibration of crystal lattice lie in this band, their identification can be made by a THz characteristic absorption spectra fingerprint. THz time-domain spectroscopy (THz-TDS) has proved to be an effective technique to study biological molecules and crystal lattice. In biological studies, the far-infrared vibrational modes of DNA components, label-free bioaffinity detection, label-free probing of genes, and the binding state of DNA by time-domain THz sensing has been reported [2–5]. The conformational and collective vibrational modes of biomolecular were investigated as well [6, 7]. In the study of illicit drugs, Kawase et al. have made THz imaging of 3,4-methylenedioxymethamphetamine (MDMA) and methamphetamine (MA) using a THz wave parametric oscillator [1, 8]. However, to our knowledge there is little data on the experimental and theoretical results of the THz spectra of illicit drugs.
In this paper, the spectral features of MA are studied using the THz-TDS technique and its characteristic absorption spectra, ranging from 0.2 to 2.6 THz, are obtained. The calculations are made with the help of the packages of Gaussion03. Calculated results are compared with those obtained experimentally, and the identifications of methamphetamine’s vibrational modes are presented.
2. Experiment
2.1 Sample preparation
MA, also known as the drug “ice,” is one of the most widely consumed illicit drugs in the world today. Its molecular formula is C10H15N, and its molecular weight is 149.2. In our experiment, the sample is provided by the First Research Institute of the Ministry of Public Security of China, so its purity is above 90%. The sample is prepared without further purification by mixing the dry powder with a polyethylene powder, which has been proved to be transparent in the THz band at a mass ratio of about 1:5 and by pressing the mixture to a thin circular slice applying a pressure of 5 tons. The thickness of the slice is about 1.0 mm and the diameter is 13.0 mm.
2.2 Experimental setup
The reflecting-emitting THz system was used as shown in Fig. 1 where BS is a beam splitter, HWP is a half-wave polarizer, QWP is a quarter-wave Polarizer, M1 to M11 are reflecting mirrors, PM1 to PM4 are four parabolic mirrors, L1 to L3 are focusing lenses, P is a polarizer, D1 and D2 are two diaphragms, and PBS is a wollaston prism.
A Mai-tai laser (Spectra-physics) was applied to pump and detect the THz wave, with the central wavelength of 810 nm and pulse duration of 100 fs. The laser pulse was split into pump and probe beams. The pump beam was focused onto the InAs crystal to generate THz pulses. The generated THz pulses in our system ranged from 0.2 THz to 2.6 THz, which transmitted through four off-axis parabolic mirrors (PM) and were focused on a ZnTe crystal with a thickness of 2 mm. The probe beam propagated after a series of mirrors and was focused on the same spot as the THz pulses on the ZnTe crystal. The ZnTe crystal was used to detect THz pulses by applying an electro-optic sampling technique. The sample MA was placed on the focus of PM2 locating between PM2 and PM3. The setup was placed in a chamber full of N2 in order to eliminate the influence of the vapor in the air. The relative humidity was less than 4.7% in the experiment.
2.3 Experimental results
The electric field of the THz pulse transmitting through the sample is modified by the dispersion and absorption of the sample. Figure 2 shows the time-domain waveforms of the THz electric field without the sample (reference) and with the sample. The measured pulse is delayed because the refractive index of the sample is different from that in air. The amplitude of the measured pulse is decreased because of the absorption of the sample. By using fast Fourier transformation (FFT), the frequency domain spectra of both the reference and the sample can be obtained as shown in Fig. 3. The absorbance of the sample can be acquired as shown in Fig. 4(a) using the formula
where Is is the intensity of the reference and Ir is the intensity of the sample. This graph indicates that MA has a high absorbance in the THz waveband at 1.23, 1.67, 1.84, and 2.43 THz, respectively. The reason for these absorption peaks are explained by DFT calculations.
3. DFT Calculations
Using the packages of Gaussion03 [9], we applied the DFT method, chose the 6–31G basis set at the Becke-3-Lee-Yang-Parr (B3LYP) level [10,11], made the optimization of the spatial structure, and calculated the vibrational frequency of MA with the same basis. No negative frequencies were found. Figure 5 shows the predicted molecular structure of MA after the geometry optimization. The phenyl ring is deformed, due to the lack of symmetry in the ground state.
The calculation results are shown in Fig. 4(b), where all the calculated frequencies were not scaled with the scale factor. The graph indicates that the calculated absorption peaks at 1.22, 1.67, and 2.49 THz are corresponding to the absorption peaks at 1.23, 1.67, and 2.43 THz in the experimental results. The vibrational energy level interval between the first two peaks is calculated to be about 1.86 meV. The identification of molecular vibration modes is shown in Table 1. The peak 1.22 THz is caused by relative wagging of the phenyl ring and the whole chain out of plane; the peak 1.67 THz is caused by relative wagging of the phenyl ring and the whole chain in plane; and the peak 2.49 THz is caused by relative bending of the phenyl ring and the whole chain out of plane and the respective wagging of the C-CH3 and N-CH3 groups.
Table. 1. Assignments of observed vibrational frequencies (THz) for MA
These results have further proved the theory of collective vibrational modes to be right. A pair of absorption peaks at 1642 cm-1 and 1664 cm-1 near the characteristic absorption of benzene lying at 1596 cm-1 are calculated at the same time, and they are generated by the flexibility of the benzene ring and the extension between carbon and hydrogen of the benzene ring. The appearance of the two peaks largely confirms the existence of the aromatic ring and the validity of the adoptive molecular configuration. However, the absorption peak at 1.84 THz is not figured out in the calculation. In further analysis, there are two isomers at a ratio of half to half in MA sample, since in the molecule the number 15 carbon is a chiral atom. The calculation only referred to one of them, L-methamphetamine, which is the lowest energy state by geometry optimizing. The peak at 1.84 THz is probably caused by intramolecular collective vibrational modes of the other isomer, D-methamphetamine. Further improvements by applying high-level basis sets will be needed in calculation and will be done in future.
As the measurements were made at room temperature, the fine structure of the spectrum cannot be seen because room temperature spectrum is a superposition of transition from the excited vibrational states. M. Walther et al. discussed the THz spectrum of biological molecules at very low temperatures. More absorption peaks appeared at very low temperatures, and their intensities became stronger with the decrease of temperature [7]. We will re-do the experiment in the future when the low-temperature condition is available.
4. Conclusions
In this paper, THz absorption spectra of methamphetamine (MA) were experimentally studied using THz-TDS in the 0.2–2.6 THz range. DFT calculations have been successfully applied to study the geometric structure and vibrational frequencies of MA. Calculated vibrational frequencies with B3LYP/6–31G are in agreement with the experimental results. The assignment of vibrational frequencies is proposed, based on DFT computational results
The results prove that it is credible to use the THz-TDS technique for drug inspection and detection, since drugs can be identified by their characteristic absorption. Furthermore, realtime measurements can be realized with the development of THz sources and detectors. It is believed that one day THz-TDS will be a fast and effective technique for use in drug inspection and detection.
Acknowledgments
We gratefully thank Haibo Liu for his help in the experiment and Lantao Guo for the useful discussions. This project was supported by grants from the National Nature Science Foundation of China (No. 10390160) and the Beijing Nature Science Foundation (No. 6032006).
References and links
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