Comparison of SPELEC RAMAN and standard Raman microscopes

Different aspects must be considered to compare the measurements performed with SPELEC RAMAN and a standard Raman microscope.

Although standard Raman microscopes are traditionally limited to spot sizes on the order of 0.5–10 µm, the laser spot diameter depends on the laser wavelength and the objective used. Raman spectra collected with confocal microscopes allow the characterization of very small areas due to the miniature spot diameter, but they provide limited information in bulk and microscopic analysis. For instance, the small spot size can cause misinterpretations in non-homogeneous samples when the Raman response of a specific area is considered representative of the whole sample.

Figure 2a shows the Raman spectra recorded with a standard Raman microscope at four different positions of a sample of single-walled carbon nanotubes (SWCNT). Not only the intensity, but also the ratio of D and G bands (I_D/I_G), located at 1350 cm^-1 and 1598 cm^-1, respectively, differ depending on the position: 0.679, 0.843, 0.837, and 0.448 (average I_D/I_G = 0.702, RSD = 26.44%).

SPELEC RAMAN’s larger spot size (190 µm practical diameter of the Raman probe) allows characterization of a broader area in a single experiment. With SPELEC RAMAN, one Raman spectrum is representative of the analyzed system. This avoids the need to perform additional measurements, saving time and costs. Here, the same SWCNT sample that was previously analyzed in Figure 2a was characterized with SPELEC RAMAN (Figure 2b). The ratio obtained with SPELEC RAMAN in a single spectrum, I_D/I_G = 0.701, exactly matches the average of the four spectra taken with the Raman microscope.

In order to assess the reproducibility of SPELEC RAMAN, three additional spectra were recorded in the same sample, and the following ratios were obtained: 0.733, 0.726, and 0.713. The average of the four measurements results in the following values: I_D/I_G = 0.718 and RSD = 1.97%. Then, the good reproducibility of measurements performed with 190 µm spot laser of SPELEC RAMAN is demonstrated. 

Furthermore, higher laser power can be applied to the sample with SPELEC RAMAN. This is because the large focal area (0.028 mm² ) on the sample allows the energy to be distributed.

The Raman probe can be easily adapted to a wide variety of different configurations, demonstrating the versatility of SPELEC RAMAN. The optimal focal distance of the Raman probe (8 mm) facilitates its combination with different cells—it is not limited to Metrohm DropSens cells (Figure 3) and allows the study of different electrochemical processes and the characterization of myriad samples. The Raman probe can be customized and adapted to specific laboratory and industrial applications. However, the universal connectors of SPELEC RAMAN instrument (FC/PC and SMA905 for excitation and collection fibers, respectively) allow coupling with a Raman microscope in case it is needed for further study.

Traditional Raman microscopes have a specific compartment for samples, and therefore cells must be designed according to the specifications of each instrument (sample compartment, objectives, focal distance, etc.).

Apart from the excitation laser source and the spectrometer, Raman microscopes require more components (e.g., the microscope module) which increase the price. SPELEC RAMAN is a fully integrated instrument with laser, spectrometer, and potentiostat/galvanostat combined in a single housing.

The small size (25 × 24 × 11 cm) of SPELEC RAMAN makes it an easily portable instrument; it does not require a fixed location nor specific furnishings to operate. Furthermore, the small dimensions of this Metrohm DropSens instrument allows its use in a glovebox without issues.