BROCHURE

Extending Raman Into the
Thz Domain

Delivering Both Chemical Composition and Structural Information in a Single Measurement

Simultaneous Measurement of Both Chemical and Structural Properties

Coherent-patented1 THz-Raman™ systems boost both the efficiency and reliability of materials characterization, in a single, real-time, non-destructive measurement. By unveiling the low-frequency (low wavenumber) range of the Raman spectrum, often referred to as a secondary “structural fingerprint,” it is possible to directly observe and differentiate key structural properties of materials, while preserving the complete chemical fingerprint.

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The THz-Raman region (±5 cm-1 to 200 cm-1) corresponds to the THz-energy vibrations (150 GHz to 6 THz) of inter-molecular/intra-molecular vibrations, including phonon modes, lattice modes, and rotational modes. These are often 5 to 10 times stronger than normal vibrational modes, significantly boosting signal strength. By capturing both Stokes and anti-Stokes signals, spectral features can be validated and the excitation wavelength (0 cm-1) can be precisely determined due to signal symmetry, eliminating the need for system re-calibration.

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Figure 1: Benchtop THz-Raman Module

THz-Raman Spectroscopy Modules are designed as integrated, ultra-compact, plug-and-play solutions to upgrade your existing Raman spectrometer. Comprising an ultra-narrowband ASE-free laser source, NoiseBlock™ 90/10 beamsplitter, and dual-stage SureBlock™ notch filters, the system delivers >OD9 Rayleigh attenuation and signal capture of both Stokes and anti-Stokes signals down to 5 cm-1. (Figure 1)

Figure 1

Figure 2: THz-Raman spectra of Carbamazepine shows clearly differentiated polymorphic and hydrated forms.²

System Configurations

All TR-Series THz-Raman modules are ultra-compact and simple to connect via fiber to almost any spectrometer or Raman system. A high-power, wavelength-stabilized, single-frequency laser source is precisely matched to the ultra narrow-band ASE, beamsplitter and notch filters to assure maximum throughput and exceptional attenuation (>OD 9) of the excitation source. Systems are available in 532 nm, 633 nm, 785 nm, and 808 nm excitation wavelengths.

The TR-PROBE is a compact, robust THz-Raman probe that enables in-situ reaction or process monitoring, and can also be flexibly configured with a variety of sample interface accessories, including immersion or contact probe tips, a convenient vial/tablet holder, a Transmission Raman adapter, a microscope mount, or a steerable non-contact optic (see options below). A separate CleanLine™ laser provides ASE-free excitation via a multimode fiber, enabling the probe to operate in harsher environments where electrical connections are not permitted. Options include circular polarization or a dual-port/dual polarization output for simultaneous measurement of both S and P polarizations.

The TR-BENCH is configured for benchtop use and offers a similar range of interchangeable sample interface accessories holder for fast, easy measurements. The system also comes with a standard cage mounting plate (centered on the collimated output beam) to allow for customized collection optics or easy integration into a customized system. Options include circular polarization or a dual-port/dual polarization output for simultaneous measurement of both S and P polarizations.

The TR-MICRO mounts directly to a broad range of popular microscope platforms and micro-Raman systems, and can be easily switched in and out of the optical path. Linear polarization is standard, circular polarization is optional.

 

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THz-Raman modules are compatible with virtually any commercial Raman system or spectrometer, and Coherent can recommend or integrate an appropriate spectrometer and deliver as a complete turnkey system.


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A variety of sample interface accessories enable the TR-PROBE and TR-BENCH to be easily configured to match a broad range of applications. Immersion or contact probe tips may be mounted with either a fixed SwageLok mount, or for longer probes that may need alignment, an adjustable tip/tilt probe mount. The Vial/Tablet Sample Holder incorporates an adjustable steering mirror, interchangeable focusing lens, and safety shutter, and the Steerable Non-contact Optic Mount allows for projection and steering of the output beam with precision alignment and interchangeable focusing optics, for applications requiring long-range collection paths. New accessories include a Transmission Raman adapter (Probe only) which is ideal for bulk sampling of tablets or vials, and a Microscope mount with in/out optical switching and beam steering adjustments.

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THz-Raman Applications Examples

Crystal Monitoring and Analysis Identifying and monitoring the formation of cocrystals is also improved using THz-Raman spectra. The figure above shows the clearly recognizable peak shifts that occur when cocrystals are formed in a mixture of Caffeine and 2-Benzoic acid.

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Gas Sensing Rotational modes of many gases can be clearly seen in the THz-Raman region. Signal intensities can be up to 10x those in the fingerprint region, opening up the possibility of using Raman for extremely sensitive gas sensing applications. The Stokes/anti-Stokes ratios can also be used for in-situ sensing of temperature.

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Polymorph Identification Polymorphic forms and hydrates of pharmaceuticals can easily be distinguished in raw materials analysis, finished goods, process monitoring, and QC applications.

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Phase Monitoring Phase changes of Sulfur observed when heated from room temp.(α) to 95.2°C (β) and then to the melting point of 115.21°C (λ). Note the clearly recognizable changes in peak location, shape and magnitude in the THz-Raman region. Crystalline phases result in sharp peaks, which broaden and dissipate as the sulfur liquifies.

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Phase Monitoring Low frequency spectra can be used to monitor transformation of polymorphs. The waterfall plot above shows anhydrous theophylline before and after its transformation into a flocculated slurry, over a period of approximately 100 seconds. (data courtesy Clairet Scientific Ltd.)

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Synthetic Pathway Analysis: Explosives Forensics Multiple samples of ETN (Erythritol Tetranitrate), representing systematic variations of ingredients and preparation routes, show distinctive differences.

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1 US Patents 7,986,407 and 8,184,285.

2 Data taken using SureBlock™ notch filters and a single-stage spectrometer at 785 nm.