This article explains the "Raman Concatenation", a measurement approach that can overcome several trade-offs.
In general, fluorescence impacts many Raman measurements, necessitating the adoption of longer excitation wavelength (lower photon energy) lasers to prevent the fluorescence signal from overpowering the Raman. However, this reduces the sensitivity of low-cost silicon CCD detectors at longer wavenumbers, making it difficult or impossible to view the "stretch" section of the Raman spectra (2000-4000 cm-1). This is especially true for excitation wavelengths above 760 nm.
Longer wavelength excitation lasers have a lower Raman excitation cross-section, making it harder to detect and conduct quantitative analysis in the "fingerprint" portion of the Raman spectra (i.e., 0–2000 cm-1).
Raman concatenation addresses these challenges by combining two lasers, a single-grating spectrometer, and a single probe to visualize the complete Raman spectra from 0 to 4000 cm−1.
As an additional benefit, since a shorter wavelength (higher photon energy) laser is used to collect the "stretch" section of the spectrum, an improved signal is recorded in this region, allowing for increased quantitative precision.
The first longer wavelength laser is chosen to prevent fluorescence in the sample while probing the "fingerprint" portion of the spectrum. In the case of a 785 nm excitation laser, 0-2000 cm-1 translates to a single-grating spectrometer detection wavelength of 785 nm to ~950 nm.
The second, shorter wavelength laser is chosen to investigate the "stretch" portion of the spectrum with the same spectrometer detection wavelengths. In this case, a 680 nm excitation laser translates to a Raman shift of 2000-4000 cm-1.
The probe filters are chosen to enable excitation and collection at both wavelengths. In operation, each segment of the Raman spectra is gathered sequentially, and the composite spectrum is "concatenated" or stitched together.
Figure 1 shows a concatenated spectrum obtained on cyclohexane with a 680 nm / 785 nm laser pair. Note the considerable increase and enhanced peak discrimination of Raman signal strength in the stretch section of the spectrum.
Raman concatenation, with its ~10X improved signal and signal-to-noise ratio in the stretch area of the spectrum [Fig 2], is suitable for quantitative process control of CH, OH, and NH molecules in many applications.
Lastly, larger wavelength laser pairs can be used if fluorescence remains an issue at 680 / 785 nm. For instance, a 735 / 830 nm laser pair can be used with a silicon-based spectrometer, but an 830 / 1064 nm laser pair can be used with an InGaAs-based spectrometer.
Innovative Photonic Solutions holds the US 10,359,313 patent for the Raman concatenation system, which is also patent pending in the EU.
Figure 1. Concatenated Raman spectra for cyclohexane obtained with a 680 nm / 785 nm laser pair. Image Credit: m-oem
Figure 2. Enhanced stretch-band signal and discrimination obtained with Raman concatenation. Image Credit: m-oem
This information has been sourced, reviewed and adapted from materials provided by m-oem.
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