The low-frequency region augments the information content from the fingerprint region of the Raman spectrum and broadens possible molecular applications, such as detection of hydrogen bonds. The result is increased detection sensitivity and differentiation of very similar materials.

Amino acids

Raman spectroscopy is a modern way to study the structure and conformation of the building blocks of proteins—amino acids. Specifically, the vibrational information content in a Raman spectrum can help with interpretation of molecular interactions and biological processes [3].

Unlike many substances that do not exhibit peaks below ~400 cm^-1, the low-frequency portion of the Raman spectrum is a necessary source of information for a comprehensive study of amino acids. This is obvious in the full Raman spectrum of L-asparagine, from 65–3200 cm^-1 (Figure 1).

Figure 1 shows both the fingerprint region (blue) as well as the low-frequency Raman region (red) for L-asparagine; note the three dominant bands below 200 cm^-1.

i-Raman Plus 785S, B&W Tek’s laboratory Raman spectrometer that utilizes patented CleanLaze®, was used in this application. This instrument features
785 nm laser excitation with a linewidth of less than 0.2 nm and maximum power output of 300 mW.

i-Raman Plus is equipped with a sensitive, TE-cooled, back-thinned CCD. A BAC102 E-grade probe, utilizing proprietary technology, supports data collection within a full spectral range of 65–33500 cm^-1 with a spectral resolution of 4.5 cm^-1.

Raman spectra were collected at room temperature using 300 mW laser power with integration times that ranged from 100 milliseconds to 10 seconds (Table 1).

Determining the structural form of active pharmaceutical ingredients (APIs) is a primary concern for the pharmaceutical industry. This is especially true during drug development, manufacturing, and quality control.

APIs exhibit polymorphism—identical chemical composition but different solid-state structures. Polymorphs may affect bioavailability and therapeutic index. The efficacy of a drug can be compromised if the wrong form is used [2]. Pseudo-polymorphs include solvents suspended in a lattice structure.

Figure 2 is an example of the pseudo-polymorph D-glucose, demonstrating the ability of the E-grade probe to detect differences between the monohydrate and anhydrous forms at frequencies below 200 cm^-1.

Exceptional specificity is required to monitor phase changes like crystallization in chemical processes. The low-frequency E-grade probe can monitor such phase changes, as demonstrated for sulfur (Figure 3).

Solid α-sulfur was deposited onto an aluminum tray and heated with a hot plate while the Raman spectra were collected with an E-grade probe and i-Raman Plus, using 100% laser power (~300 mW) and 0.1 s integration time for both the solid and liquid phase.

After the sample was heated above its melting point at 115.2 °C, the low-frequency peak at 83.6 cm^-1 broadened and shifted, indicating the change from the α- to the λ-form. Note that there are no observable changes within the fingerprint region (Figure 3).