Terahertz (1 THz = 1012 Hz) spectroscopic techniques provide the ability to noninvasively explore low-frequency motions in molecular systems in the far-infrared region of the electromagnetic spectrum (1.2 cm–1–120 cm–1). These motions, such as flexing of the individual molecules or intermolecular interactions via either strong hydrogen bonds or weaker van der Waals bonding between neighboring molecules, provide unique insight into the structure and phase transitions of drug polymorphs,1–4 hydrates,5,6 and solvates of solid crystalline materials,4 which are of direct importance to the pharmaceutical industry. This paper illustrates how the terahertz spectra of materials that have applications in the biotechnology, pharmaceutical, and food industries were studied with pulsed terahertz attenuated total reflectance (ATR) spectroscopy. Pulsed terahertz ATR measurements were performed noninvasively using small quantities of sample material with no need for sample preparation.
Terahertz gap
The “terahertz gap” is a phrase historically used to describe an elusive and for many years unexplored portion of the electromagnetic spectrum, occupying frequencies from just above the microwave to just below the infrared region (0.04 THz–4 THz; 1.33 cm–1–133 cm–1). The past years have witnessed the development of new and effective techniques for the generation and detection of these frequencies, and thus an opportunity to explore this scientifically rich region of the electromagnetic spectrum has opened up.
Research in the terahertz area has been triggered by unique properties of terahertz radiation. These include extremely low photon nonionizing energies and semitransparence of many common organic and inorganic materials to terahertz radiation (for example, some packaging and tablet coating materials). Many materials show distinctive spectral features in this region of the electromagnetic spectrum.
Terahertz technology
Terahertz pulsed spectroscopy utilizes an ultrashort laser pulse and semiconductor devices for the generation and detection7 of terahertz radiation and fast Fourier transformation to convert the detected time-domain waveform into a corresponding terahertz absorbance spectrum. Ultrashort laser pulses are split into a pump beam for terahertz generation and a probe beam for terahertz detection. The pump beam is used for the generation of terahertz pulses by photoexcitation of a biased semiconductor device. A laser pulse creates electron-hole pairs in the semiconductor, which are then accelerated in the electric field created by the bias applied to the electrodes deposited onto the semiconductor. Electromagnetic radiation in the terahertz frequency range is emitted during this process. Once the terahertz beam is transmitted through (transmission mode) or reflected off (reflection mode) the sample material, it is directed to the receiver. The time evolution of the terahertz pulse is measured by systematically varying the time delay between pump beam and probe beam at the receiver.
The core technology employed by TeraView Ltd. (Cambridge, U.K.) in all of its instruments is based on this principle, and it provides information on both terahertz electric field intensity and phase, and thus enables one to determine quantities such as absorption coefficient and refractive index.
ATR spectroscopy using terahertz pulses
Pulsed terahertz attenuated total reflectance spectroscopy is an established technique that was used to acquire terahertz spectra of pharmaceutical materials. The advantage of this technique is that it is nondestructive, allows transmission measurements of highly absorbing materials to be performed, and uses only small amounts of sample—typically 1 mg for solids and 1 mL for liquids. Furthermore, pulsed terahertz ATR spectroscopy, unlike most pulsed terahertz spectroscopic measurements in transmission mode, requires no further sample preparation. Therefore, no dilution of a sample under study with a PTFE or polyethylene matrix (in order to reduce absorption in the sample itself) and compression of the mixture into pellets, which can induce polymorphic or physical changes, are required. This technique therefore allows for analyzed material to be recovered afterward, but also enables spectral reproducibility.
Pulsed terahertz ATR measurements permit the analysis of optical constants of the material under study. Penetration depths into the sample material that can be accessed by using the terahertz ATR are greater than those in the midinfrared region due to the longer wavelengths of the terahertz radiation (penetration depth is proportional to the wavelength; see Eq. [1]).
Figure 1 - Schematic illustration of the principle of the terahertz ATR spectroscopic technique: n1 and n2 are refractive indices of the ATR crystal and sample material, respectively. Note that condition n1 > n2 is required for the total internal reflection to occur at the ATR crystal–sample interface. Evanescent wave is represented by the pink dotted line.
The main principle of the terahertz ATR sampling technique is based on measuring the changes that occur in a totally internally reflected terahertz beam when it encounters an interface between an ATR crystal and a sample under study8 (Figure 1). The terahertz pulsed beam is directed onto the high-refractive-index silicon ATR crystal (3.42 in the terahertz range), brought to a focus at the horizontal surface of the ATR crystal using a z-cut quartz condensing lens, and then passed through the crystal at an angle θ. With the experimental arrangement employed in this work, the THz beam only gets reflected off the interface ATR crystal/sample interface once. This reflection sets up a standing wave–evanescent wave between incoming and outgoing terahertz beams, which penetrates into and interacts with the sample within a penetration depth. The penetration depth, defined as the distance required for the intensity of electromagnetic radiation to fall to 1/e of its value at the surface, is given by8:
where λ1 is the wavelength in the denser medium, and n21 is the ratio of the refractive index of the rarer medium over that of the denser medium; θ is the angle of incidence. The penetration depths achieved in this experiment with the frequencies in the 10 cm–1–120 cm–1 range contained in the naturally broadband terahertz pulse are approximately in the 140–10 μm range. The terahertz beam reflected off the ATR crystal/sample interface is directed toward the detector. It presents a measure of the interaction of the evanescent wave with the sample and, therefore, the obtained terahertz spectrum is a characteristic of the sample itself.
Materials and methods
Figure 2 - a) Terahertz pulsed spectrometer (TPS spectra 2000) with the ATR module and stainless steel clamp. b) Top view of the ATR crystal with the powdered solid material under study (top right) and reference polyethylene disk on ATR module (bottom right).
A terahertz pulsed spectrometer, TPS spectra 2000 (TeraView Ltd.) (Figure 2) equipped with 35° silicon ATR module, was used to collect all ATR spectra in the frequency range 0.3 THz–3.6 THz (10 cm–1–120 cm–1) with a spectral resolution of 0.04 THz (1.2 cm–1). In order to eliminate absorption by water vapor in the THz beam path, the system was purged with dry nitrogen.
To ensure a good optical contact of the sample with the ATR crystal and, hence, that the air is not a medium through which the evanescent wave traverses, the solid samples were pressed against the silicon ATR crystal. This also applies to a polyethylene disk used to record a reference measurement. Sample and reference waveforms were recorded at room temperature by coadding 1800 scans in less than 1 min.
Results and discussion
In order to be able to compare band intensities recorded via total internal reflection to those observed in transmission mode, the frequency-dependence of penetration depth has to be accounted for by applying Eq. (2) to the measured terahertz ATRsignal, (~ν ):
where~νc is the center wavenumber for normalizing the absorbance.
Figure 3 - Terahertz ATR spectrum of organic acid. Terahertz band positions are indicated by the arrows.
Figure 4 - Terahertz ATR spectrum of vitamin. Terahertz band positions are indicated by the arrows.
Figure 5 - Terahertz ATR spectrum of sugar alcohol. Terahertz band positions are indicated by the arrows.
Terahertz ATR spectra of solid materials that have applications in the biotechnology, pharmaceutical, and food industries are shown in Figures 3–5. Distinct peaks of absorption bands arising from low- frequency vibrations and phonon modes in the hydrogen-bonded crystalline lattices are observed in the 10 cm–1–120 cm–1 (0.3 THz–3.6 THz) range.
Conclusion
Pulsed terahertz ATR spectroscopy is a simple, fast, and accurate method for acquiring far-infrared spectra of various materials. It was successfully applied to record terahertz ATR spectra of solid materials and liquids covering the 10 cm–1–120 cm–1 range. This technique is not geometrically constrained, and is readily applied to various sample shapes. Pulsed terahertz ATR requires no sample preparation and is completely nondestructive; thus, sample remains intact after the analysis. It offers deeper beam penetration into samples than the same technique using the near-infrared radiation.
References
- Taday, P.F.; Bradley, I.V.; Arnone, D.D.; Pepper, M. Using terahertz pulse spectroscopy to study the crystalline structure of a drug: a case study of the polymorphs of ranitidine hydrochloride. J. Pharm. Sci.2003, 92, 831–8.
- Strachan, C.J.; Rades, T.; Newnham, D.A.; Gordon, K.C.; Pepper, M.; Taday, P.F. Using terahertz pulsed spectroscopy to study crystallinity of pharmaceutical materials. Chem. Phys. Lett. 2004, 390, 20–4.
- Strachan, C.J.; Taday, P.F.; Newnham, D.A.; Gordon, K.C. ; Zeitler, J.A.; Pepper, M.; Rades, T. Using terahertz pulsed spectroscopy to quantify pharmaceutical polymorphism and crystallinity. J. Pharm. Sci.2005, 94, 837–46.
- Zeitler, J.A.; Newnham, D.A.; Taday, P.F.; Threlfall, T.L.; Lancaster, R.W.; Berg, R.W.; Strachan, C.J.; Pepper, M.; Gordon, K.C.; Rades, T. Characterization of temperature induced phase transitions in the five polymorphic forms of sulfathiazole by terahertz pulsed spectroscopy and differential scanning calorimetry. J. Pharm. Sci. 2006, 95, 2486–98.
- Zeitler, J.A.; Kogermann, K.; Rantanen, J.; Rades, T.; Taday, P.F.; Pepper, M.; Aaltonen, J.; Strachan, C.J. Drug hydrate systems and dehydration processes studied by terahertz pulsed spectroscopy. Int.J. Pharm. 2007,334, 78–84.
- Liu, H.B.; Zhang, X.C. Dehydration kinetics of D-glucose monohydrate studied using THz time-domain spectroscopy. Chem. Phys. Lett.2006, 429, 229–33.
- Auston, D.H.; Cheung, K.P.; Smith, P.R. Picosecond photoconducting Hertzian dipoles. Appl. Phys. Lett. 1984, 45, 284–6.
- Internal Reflection Spectroscopy; Harrick, N.J., Ed.; Harrick Scientific Corp.: New York, NY, 1987; pp 1–65.
The authors are with TeraView Ltd., Platinum Bldg., St. John’s Innovation Park, Cambridge CB4 0WS, U.K.; tel.: + 44 1223 435 507; fax: + 44 1223 435 382; e-mail: [email protected].