低频拉曼光谱技术
AN-RS-055
2026-04
zh
低频拉曼光谱技术可捕获低至 65 cm⁻¹ 的振动模式，拓展了常规拉曼分析的应用范围，助力更深入地探究分子结构、完成蛋白质表征、鉴别多晶型以及分析物相变化。
Raman spectroscopy is an analytical tool that enables the measurement of molecular structure and the identification of materials based on the rotational and vibrational modes of a molecule. Most laboratory Raman systems cover the fingerprint spectral region ranging from 200–3400 cm-1. However, access to the lower-frequency areas provides additional information for applications in protein characterization [1], polymorph detection and identification [2], along with material phase and structure determination.
The lab-grade fiber optic sampling probe for laboratory spectrometers from Metrohm can access lower-frequency modes down to 65 cm-1, providing a cost-effective solution for wider range measurements. This Application Note describes the use of low-frequency Raman spectroscopy for studying amino acids, polymorph detection, and phase change monitoring.
The low-frequency region augments the information content from the fingerprint region of the Raman spectrum and broadens possible applications. Detection of hydrogen bonds and other structural features, such as polymorphism, can be differentiated using this region of the spectrum. The result is increased specificity for very similar materials.
Metrohm's i-Raman Plus 785S laboratory Raman spectrometer with a 785 nm, 300 mW (maximum power output) laser and a sensitive, thermoelectrically cooled back-thinned CCD (charge-coupled detector) is used to collect Raman spectra.
The i-Raman Plus is equipped with a unique fiber optic probe that supports data collection over a full spectral range of 65–3500 cm-1 with a spectral resolution of 4.5 cm-1.
Raman spectra are collected at room temperature using SpecSuite software with integration ranging from 100 ms to 10 s.
Raman spectroscopy is used to study the structure and conformation of amino acids – the building blocks of proteins. 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 65–3200 cm-1 region of the Raman spectrum of L-asparagine in Figure 1. Specifically, the vibrational information in a Raman spectrum can help interpret molecular interactions and biological processes [3].
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.
Determining the structural form of active pharmaceutical ingredients (APIs) is of great concern for the pharmaceutical industry. This is especially true during drug development, manufacturing, and final product quality control.
APIs exhibit polymorphism – identical chemical composition but different solid-state structures. Polymorphs may affect bioavailability and therapeutic index, so 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 provides an example of the pseudo-polymorph D-glucose, demonstrating the ability of the i-Raman Plus probe to detect differences between monohydrate and anhydrous forms at frequencies below 200 cm-1.
A change from one polymorph to another as a result of melting followed by recrystallization can be observed in the low-frequency region. Exceptional specificity is required to monitor such phase changes, and the probe of the i-Raman Plus is well-suited, as demonstrated for sulfur (Figure 3).
Solid α-sulfur is deposited onto an aluminum tray and heated with a hot plate while Raman spectra are collected with an i-Raman Plus and fiber probe, using a laser power of 300 mW and 0.1 s integration time to measure both the solid and liquid phase.
After the sample is 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).
The i-Raman Plus 785S Raman spectrometer is a valuable tool for applications requiring low-frequency Raman detection down to 65 cm-1. The ability to characterize polymorphs and solvated forms supports manufacturing and formulation processes in pharmaceutical and biological industries.
Along with protein, polymorph, and phase characterization, low-frequency Raman spectroscopy can also be used to study semiconductor lattices [4], carbon nanotubes [5], solar cells, and an assortment of minerals, pigments and gemstones.
- Teixeira, A. M. R.; Freire, P. T. C.; Moreno, A. J. D.; et al. High-Pressure Raman Study of l-Alanine Crystal. Solid State Communications 2000, 116 (7), 405–409. https://doi.org/10.1016/S0038-1098(00)00342-2.
- Larkin, P. J.; Dabros, M.; Sarsfield, B.; et al. Polymorph Characterization of Active Pharmaceutical Ingredients (APIs) Using Low-Frequency Raman Spectroscopy. Appl Spectrosc 2014, 68 (7), 758–776. https://doi.org/10.1366/13-07329.
- Golichenko, B. O.; Naseka, V. M.; Strelchuk, V. V.; et al. Raman Study of L-Asparagine and L-Glutamine Molecules Adsorbed on Aluminum Films in a Wide Frequency Range. Semicond. Phys. Quantum Electron. Optoelectron. 2017, 20 (3), 297–304. https://doi.org/10.15407/spqeo20.03.297.
- Smith, E.; Dent, G. Modern Raman Spectroscopy: A Practical Approach, 2nd ed.; John Wiley & Sons, 2019.
- Pelletier, M. J. Analytical Applications of Raman Spectroscopy, 1st ed.; Blackwell Science: Oxford, 1999.
分享应用
下载PDF