Terahertz applications for the analysis of solid dosage forms

Since initial research on terahertz radiation began to gain momentum nearly 20 years ago, this technology has evolved from being a pure research tool to one that is used for pharmaceutical applications. More specifically, techniques using pulsed terahertz radiation such as terahertz pulsed spectroscopy (TPS) and terahertz pulsed imaging (TPI) are now increasingly employed to analyse solid dosage forms. So far, TPS has been predominantly used to acquire physical information of solid-state properties of active pharmaceutical ingredients (APIs), particularly in the field of polymorph recognition, characterization and quantification.

TPI can be used to nondestructively analyse tablet matrix physicochemical composition, coatings and buried interface properties in solid oral dosage forms because of its pulsed and coherent nature of the radiation together with its frequency-dependent unique penetration characteristics. In addition, both two- and three-dimensional (2D, 3D) imaging techniques are currently being developed.^1–3

Terahertz radiation

The term 'terahertz radiation' corresponds to the far-infrared region of the electromagnetic spectrum (3–333 cm^-1 or 100 GHz–10 THz), or parts thereof, residing between microwave and mid-infrared radiation.² Many modern TPS devices typically utilize the frequency range from 0.1–4 THz (3.3–130 cm^-1). Rather than representing information originating from intramolecular vibrations in the mid-infrared region of the spectrum, terahertz radiation induces intermolecular noncovalent bond vibrations and translations in solids. As terahertz radiation directly probes interactions between molecules, it is intrinsically sensitive to changes in crystalline structure (crystalline phonon vibrations).⁴ Raman and mid-infrared spectroscopy, however, are classic intramolecular spectroscopic techniques that are most sensitive to chemical information.²

Terahertz technology

Both TPS and TPI are built around the same core terahertz technology. The source of radiation is a photoconductive semiconductor antenna that generates broadband pulses of terahertz light by injecting charge carriers into the substrate with a femtosecond laser. After transmission through the sample material, the radiation is detected as a time domain waveform of the terahertz pulse in a reverse process using a time-gated probe beam for the detection process (Figure 1).

Figure 1 Setup of the TPS system (left) and the TPI system (right).

In contrast to conventional far-infrared spectroscopy, terahertz radiation is unique in that its generation by the photoconductive antenna is coherent and pulsed.⁵ Moreover, compared with most other spectroscopic techniques, in TPS and TPI the photo-induced electrical field is measured directly rather than just its signal intensity. Using the information on the amplitude and phase of the signal both the absorption coefficient and the refractive index of the sample can be measured directly.^1,5

After performing a Fourier transformation of both sample and reference waveform into the frequency domain, the terahertz absorbance spectrum is calculated. For the structural image generation in TPI, the measured time delay originating from reflections at interfaces within the sample in the time domain waveform is used to resolve the depth dimension. Spatial information (horizontal and vertical dimensions) is obtained by fully automated point mapping of the sample. This TPI waveform data for each pixel can also be Fourier transformed to yield a fourth dimension with spectral information of the sample. Using this approach, both structural information and chemical composition in the sample can be described using TPI.^5–7

TPS and polymorph recognition

Polymorphism is a solid-state phenomenon where, though possessing the same chemical structure, pharmaceutical solids may exhibit different arrangements of their molecules in the unit cell, leading to subsequent physicochemical differences for these modifications for properties such as bioavailability, processibility and stability. The prevalence of polymorphism in pharmaceutical solids and the paramount importance in its identification are consistently highlighted in the literature.^6–9 TPS has been used to distinguish amorphous, crystalline, hydrate, solvate and liquid crystalline solid states in a number of drug molecules. It is now commonly accepted in the literature that terahertz spectra provide sufficient information to distinguish subtle differences in condensed matter properties.

Examples of drugs studied by TPS include different polymorphic modifications of carbamazepine, enalapril maleate, ranitidine hydrochloride and sulfathiazole.^10–12 All compounds showed significant spectral peak differences between the polymorphs in the terahertz region (Figure 2). Amorphous samples of glucose, fructose, sucrose and indomethacin all exhibit no apparent peaks;^10,13 instead, they show smooth, featureless and increasing terahertz absorption spectra. This is largely because of diminished long-range order in the amorphous state.

Figure 2 Terahertz spectra of the five polymorphic forms of sulfathiazole.

As terahertz spectra are reflecting intermolecular vibrations, TPS is an excellent tool for differentiating amorphous systems from their respective crystalline counterparts.^10,13 Super-cooled liquid crystals are also solid-state systems with limited long-range order and it is speculated that this is the main reason for their featureless terahertz spectrum, which is quite similar to that of amorphous systems. Fenoprofen calcium in its quench cooled liquid crystalline state has been used to demonstrate the ability of TPS to differentiate liquid crystalline from the crystalline form. However, further work must be performed to confirm the main causes of the diffused terahertz spectrum.¹⁰

As in many other polymorphic characterization studies, powder samples of the above drugs were used for the studies, raising the question whether TPS is also able to identify polymorphs in commercial tablets. In 2002, Taday et al. used commercial tablets of ranitidine hydrochloride (Zantac and apo-ranitidine) to demonstrate the capability of TPS for polymorph recognition. Pronounced differences were found in the terahertz transmission spectra of the different tablet samples and it was clearly shown that Zantac contained polymorph II while apo-ranitidine consisted of polymorph I.¹¹

TPS and polymorph quantification

TPS is not only a very valuable tool in polymorph recognition, it is also effective for solid-state quantification. The clear differences generated between different crystalline forms in the terahertz spectra can be exploited for quantitative analysis. Binary mixtures of a number of different forms of polymorphic drug materials (carbamazepine and enalapril maleate), crystalline in amorphous form (indomethacin) and crystalline in liquid crystalline form (fenoprofen calcium) were quantified by Strachan et al.¹⁴ In combination with multivariate analysis using partial least squares (PLS) algorithm, a limit of detection of one polymorphic form in the other as low as 1.80% and crystallinity limit of detection as low as 1.05% were observed (Figure 3). Furthermore, paracetamol has been quantified in the presence of common excipients using the same technique and has shown promising results.² In addition to these proof-of-principle studies, work is in progress on an exhaustive validation study investigating the quantitative capabilities of TPS.

Figure 3 (a) Terahertz spectra of binary mixtures of form I and III of carbamazepine; (b) PLS regression used for the quantification of low levels of carbamazepine in binary mixtures of form I in form III.

TPS and practicality issues

As illustrated previously, TPS is a nondestructive technique that requires minimal sample preparation and operates at long wavelength and low power densities, minimizing any solid phase changes or thermal strains in the sample. Polymorphic recognition by TPS is very fast with acquisition times as low as 30 ms. Because most plastic packaging materials used for blistering pharmaceutical solid dosage forms are transparent within the terahertz range, this allows for possible applications as an in-line process analytical tool (PAT).^2,11

2D TPS state chemical mapping

Applications of terahertz imaging in pharmaceutical sciences have only been reported in the last year.^15,16 While Fourier transform infrared and Raman spectroscopy imaging techniques are extensively developed and are now widely employed for chemical imaging in the near- and mid-infrared regions, the far-infrared region has not been exploited for imaging. This is largely because of the difficult detection process. TPI, however, fills the gap of far-infrared imaging, capable of operating at room temperature without the need for cryogenic cooling.¹⁶

2D chemical maps are acquired from 3D data sets with two spatial dimensions (vertical and horizontal) and the spectral frequency as the third dimension. This imaging technique can either be performed in transmission or reflection mode depending on the sample investigated and the information needed.^16–19

When operating in reflection mode, rapid scans of the sample can be achieved (20 pixels/s) with a spectral resolution of 1 cm^-1. Pellets featuring spatially resolved domains of lactose, sucrose and mixtures of the two were successfully scanned. Each measurement could map out the spatial distribution of the respective chemical component.¹⁷ The spatial resolution for such mapping experiments is diffraction limited and in the order of a few hundred micrometers for a broadband TPI system.

While operated in reflection mode, TPI lends itself towards on-line analytical applications because of its precise, rapid scan capability; it is also possible to operate the TPI in transmission mode. Pharmaceutical packaging material such as plastic or paper are transparent at terahertz frequencies, thus a terahertz wave is able to detect and subsequently construct chemical images after penetrating through such materials.^17,18 Terahertz 2D images of lactose, aspirin, sucrose and tartaric acid pellets were acquired through layers of paper cover,¹⁷ while illicit drugs were identified through polyethylene bags.¹⁸

3D TPI and tablet coating investigation

Unlike Raman and infrared spectroscopy, where chemical imaging is restricted to spectral information from just below or on the sample surface, typical pulses of terahertz radiation used in TPI have a solid-state penetration depth of up to 3 mm depending on the material. Figure 4 demonstrates the capability of TPI in 3D imaging; here, a trilayered tablet 3D model is reconstructed from the terahertz signal. Various spectroscopic and imaging techniques have been employed to investigate tablet coating thickness and uniformity. Techniques such as confocal laser scanning microscopy, infrared spectroscopy, X-ray microtomography and electron paramagnetic resonance all show promising results.²⁰ However, at least one of the following analytical constraints is limiting their wider application: long data acquisition time, destructive measuring method or the fact that the generated images are limited to information from the surface of the tablet coating.

Figure 4 (a) and (c) 3D models reconstructed from the terahertz signal of a trilayered tablet (b) highlighting the interfaces between the three layers.

TPI is a nondestructive, high throughput (the acquisition of a single waveform takes less than 20 ms), 3D imaging technique.²¹ It can be used to acquire single point measurements for very fast tablet coating thickness measurements at one point, or using fully automated scans over the whole surface of a solid dosage form to produce coating uniformity maps. For measurements of solid oral dosage forms terahertz images are recorded in reflection mode. Here, the difference in refractive indices at the air/coating interface and the coating/core interface as a function of time is recorded. The time delay between the signals from the different interfaces is used to calculate the thickness of the coating layer(s). Using two different sugar-coated brands of ibuprofen tablets as illustrations, Fitzgerald et al. were able to convey the potential of TPI for investigating coating thickness, using changes in the refractive indices at interfaces within the tablet matrix. Furthermore, the coating uniformity can be visualized by virtual cross sections through the tablet. These scans revealed that the two different brands of ibuprofen exhibit very different coating structures and coating thickness (Figure 5).²¹

Figure 5 Coating layer thickness (a) and quality (b) of an enteric coated tablet.

Detecting formulation impurities

In addition to the structural images obtained by sample mapping, a spectral dimension can be introduced to the existing dataset. Thus a 3D terahertz image is constructed from a 4D (vertical, horizontal, axial and spectral frequency) dataset. Consequently, not only chemical information, but also the precise spatial location of each of the chemicals, in a heterogeneous sample matrix such as most pharmaceutical solid dosage forms is obtained. For example, a polyethylene tablet containing lactose and tartaric acid 'impurities' was imaged with TPI, employing the 3D imaging technique. As a result, the exact location and the chemical composition of these 'impurities' were mapped out.⁷ However, measurements such as this are still far from routine and more research is necessary before such images are readily available.

Conclusions

Compared with Raman and infrared spectroscopy, terahertz technology is an exciting and novel technique with a remarkable capability for the physical characterization of solid dosage forms. Spectroscopy and both chemical and structural imaging are possible. With high throughput and nondestructive characteristics, terahertz technology holds potential for application as a solid-state PAT tool. Current terahertz research in the area of solid-state applications is very active and new applications of the technology are still being explored. Emerging results of current applications will pave the way for its future use in pharmaceuticals.

Clare J. Strachan is chief research scientist in the Physical Pharmacy, Drug Discovery and Development Technology Centre, Faculty of Pharmacy, University of Helsinki (Finland).

Louise Ho and J. Axel Zeitler are PhD students at the School of Pharmacy, University of Otago (New Zealand) and Cavedish Laboratory, University of Cambridge (UK).

Jukka Rantanen is a professor in the department of pharmaceutics and analytical chemistry at the Danish University of Pharmaceutical Sciences (Denmark).

Thomas Rades holds a chair of pharmaceutical sciences at New Zealand's National School of Pharmacy and Keith C. Gordon is associate professor at the department of chemistry, both at the University of Otago (New Zealand).

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