Raman spectroscopy is a widely utilized quantitative and qualitative spectroscopic technique that can be applied to molecular and material samples. The technique measures the frequencies of chemical bond oscillations, and the specific position and intensity of these frequencies provide a sample with a unique Raman "fingerprint" that can be used for identification.
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As the local chemical environment around an atom significantly impacts the exact frequency at which a bond vibrates, Raman spectroscopy is an incredibly sensitive method for determining the functional groups present in a sample. It is commonly used in various fields, such as pharmaceutical development, quality control, materials analysis, and biomedical imaging.1
In Raman spectroscopy, balancing the weak Raman effect against the background and competing for fluorescent processes is a significant challenge. The intensity of a Raman signal is inversely proportional to the fourth power of the excitation wavelength (λ), and shorter excitation wavelengths increase the likelihood of observing fluorescence from a sample.
To address this challenge, filters play a vital role in preparing Raman spectrometers. Different filter types eliminate unwanted scattering signals, enhance signal-to-noise ratios in measurements, and suppress artifacts.
Some commonly used filter types in Raman spectroscopy include laser line, notch, long-wave pass, and short-wave pass filters.
Laser Line Filters
Raman spectroscopy relies on intense laser sources to generate adequate signal levels. The Raman scattered light is not specific to a particular wavelength (unlike fluoescence excitation), making it crucial to ensure that only a narrow and specific excitation wavelength reaches the sample under examination.
To achieve this, a laser line filter is used in Raman spectroscopy. It is chosen with a central wavelength that matches the excitation source and eliminates any unwanted "off-center wavelength" side band contributions from the laser.
Notch filters, also known as band-stop filters, are used in the detection path of Raman spectroscopy to block or reduce a specific wavelength region corresponding to the laser line wavelength. The goal is to only transmit the wavelengths that fall outside of the blocked region, as these correspond to the Raman scattered signal.
Long and Short-Wave Pass Filters
Long pass edge filters are employed in Raman spectroscopy to reduce unwanted Rayleigh scattered signal laser light and transmit Raman scattered light from samples at lower energy or longer wavelengths. This is accomplished by sending only wavelengths above a certain blocking wavelength.
To achieve this, an ultra-steep cut-on is desirable, and a well-designed filter can enable the recovery of more of the crucial fingerprint region in Raman spectroscopy, which is considered most beneficial for sample identification.
Short-wave pass filters are the inverse of long-pass edge filters and are used when it is desirable to recover the anti-Stokes scattering rather than the longer wavelength Stokes scattering.
Many of the same qualities apply to an ideal short-pass edge filter as they do to a long pass, but the primary distinction is that a short pass will block all frequencies above the set cut-on wavelength. To achieve strong contrast, both filters require high optical densities (ODs) in the blocked zones.
References and Further Reading
- Jones, R. R., Hooper, D. C., Zhang, L., Wolverson, D., & Valev, V. K. (2019). Raman Techniques: Fundamentals and Frontiers. Nanoscale Research Lett, 14, 231. https://doi.org/10.1186/s11671-019-3039-2
This information has been sourced, reviewed and adapted from materials provided by Iridian Spectral Technologies.
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