Over the years, the portability and sampling flexibility of dispersive Raman spectroscopy have increased the deployment of this technology for material identification. This article discusses selecting the appropriate laser wavelength for Raman material identification.
The wavelength of the laser coupled to the Raman spectrometer system is one of the major concerns in the selection of an appropriate Raman instrument. Each laser wavelength has specific strengths and weaknesses. Although different options are available, the most widely used laser wavelengths are 532nm, 785nm and 1064nm. The following table summarizes the performance parameters of these lasers:
The lasers exhibit different excitation efficiency as shown in the table. Raman scattering efficiency varies in proportion to λ-4 as expressed below:
Where λ is the laser wavelength.
Raman scattering efficiency at 532nm is higher by a factor of 4.7 than at 785nm and by a factor of 16 than at 1064nm. This means that scan time at 532nm is much lower than at higher wavelengths when all other conditions remain constant.
Another concern is the detector sensitivity. Stokes Raman is employed for most Raman instruments. Raman signals are distributed in the visible range when excited by a 532nm laser and most silicon-based CCD chips show the optimum response in the visible range.
Raman signals excited by a 785nm laser are distributed in the NIR range (750-1050nm) and the CCD response is comparatively good in this range. However, the CCD shows no response above 1100nm for 1064nm. This leads to the use of an IR sensor InGaAs detector, which has one-tenth of the efficiency of the CCD. In addition, most dispersive 1064nm Raman instruments feature an embedded 512 pixel sensor for cost control measures. However, this leads to shorter spectral range coverage and poorer resolution.
Fluorescence is an important phenomenon playing a decisive role in cases where it is possible to sacrifice excitation efficiency. The process of fluorescence emission is very similar to Raman scattering, but involves a different mechanism. The occurrence of Raman effect is possible for any frequency of incoming light without affecting the separation from the excitation frequency.
Conversely, fluorescence shift will occur due to difference in excitation laser because it is anchored at a particular wavelength or frequency. Additionally, fluorescence signals will be fluctuated across a range due to the fluorescence bleaching effect. Hence, longer wavelength laser excitation is employed to reduce fluorescence interference with a Raman spectrum (Figure 1).
Figure 1. Longer wavelength laser excitation is used to minimize interference of fluorescence with a Raman spectrum
Laser absorption cannot be ignored due to changes in the samples (boiling of liquid samples and burning or ignition of colored, dark, or black samples) caused by sample heating. Sample heating will be more in the case of using longer excitation wavelength due to absorption of more light. This can be avoided or reduced by lowering the laser power or rotating the sample.
However, these actions increase the complexity and the measuring time while affecting the signal-to-noise ratio (SNR). With certain incorrect measurement configurations, some amount of damage will be caused to some sample materials by Raman although it is described as a non-destructive technology. Factors such as chemical bond resonance must be considered in the selection of the laser wavelength.
Sample Spectra Results
The following figures present some sample spectra exhibiting different performance of various excitations. There are several materials which can be scanned with any laser wavelength without a problem. Toluene is one such material (Figure 2).
Figure 2. Raman spectrum of toluene using all three standard excitation lasers
The good sensitivity of the 532nm laser excitation makes it suitable for carbon nanotube analysis, where sample burning may take place at 785nm. Although laser power can be lowered as an option for the higher wavelength, a lower SNR will be an issue. It is also recommended to use the 532nm laser for metal oxides or minerals and inorganic materials. The 532nm instrument can handle the entire range covering 175cm-1 to 4000cm-1. This is a key decisive factor for some applications involving characteristic signals in the higher Raman shift region such as –NH and –OH functional groups (Figure 3).
Figure 3. The 532nm instrument is ideal for applications where there are distinct signals in the higher Raman shift region.
The 633nm single mode laser is employed for most biomedical applications, which need precision in terms of excitation power and region without causing sample damage or illuminating fluorescence. The most popular and widely used wavelength is the 785nm due to its superior performance with more than 90% of chemicals with limited fluorescence interference.
The acquisition time for a single scan may vary from one second to several minutes based on the sample and the corresponding Raman signal strength. With the balance of spectral resolution and fluorescence reduction, the 785nm laser is the most versatile option among the three standard wavelengths.
Figure 4 shows the spectra for Norcodeine scanned using 785nm and 1064nm excitation. The details are more and the acquisition time is less for the 785nm laser when compared to the 1064nm laser. However, in most cases, reducing fluorescence is the major reason behind the selection of the 1064nm laser.
Figure 4. Raman spectra for Norcodeine scanned using 785nm and 1064nm excitation
As shown in Figure 5, sesame seed oil works at 1064nm excitation. Conversely, the spectra acquired at 532nm and 785nm are masked by strong fluorescence. In the case of cellulose, both 785nm and 1064nm can provide a good spectrum, but fluorescence interference can be observed for 532nm (Figure 6).
Figure 5. sesame seed oil works at 1064nm excitation, but the spectra collected from 532nm and 785nm are masked by strong fluorescence.
Figure 6. Raman spectra for cellulose
The 532nm laser is suitable for inorganic materials as it has the maximum energy to bombard the sample structure, thus resulting in higher fluorescence. With lower excitation efficiency and fluorescence, the 785nm laser provides the best economic performance and is suitable for most chemicals.
Fluorescence is very minimal in the case of the 1064nm laser, but the scanning time is longer to acquire adequate levels of signal for analysis. In addition, there is more chance for sample overheating. Hence, the proper use of the 1064nm laser makes it ideal for materials such as polymer, oils and dyes.
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