Design and Development of a Bimodal Optical Instrument for Simultaneous Vibrational Spectroscopy Measurements

Abstract

Vibrational spectroscopy techniques are widely used in analytical chemistry, physics and biology. The most prominent techniques are Raman and Fourier-transform infrared spectroscopy (FTIR). Combining both techniques delivers complementary information of the test sample. We present the design, construction, and calibration of a novel bimodal spectroscopy system featuring both Raman and infrared measurements simultaneously on the same sample without mutual interference. The optomechanical design provides a modular flexible system for solid and liquid samples and different configurations for Raman. As a novel feature, the Raman module can be operated off-axis for optical sectioning. The calibrated system demonstrates high sensitivity, precision, and resolution for simultaneous operation of both techniques and shows excellent calibration curves with coefficients of determination greater than 0.96. We demonstrate the ability to simultaneously measure Raman and infrared spectra of complex biological material using bovine serum albumin. The performance competes with commercial systems; moreover, it presents the additional advantage of simultaneously operating Raman and infrared techniques. To the best of our knowledge, it is the first demonstration of a combined Raman-infrared system that can analyze the same sample volume and obtain optically sectioned Raman signals. Additionally, quantitative comparison of confocality of backscattering micro-Raman and off-axis Raman was performed for the first time.

Keywords: FTIR spectroscopy; Raman depth profile; Raman spectroscopy; biochemical analysis; molecular fingerprint; optical design; optical sectioning.

Figure 1

Schematic of the combined FTIR–Raman setup. (a) Upright configuration for micro-Raman. (b) Inverted off-axis configuration for Raman. Subfigures (c,d) illustrate ray tracing within the ATR crystal for both techniques: (c) Raman collection arm and (d) FTIR with three internal reflections.

Figure 2

(a) Diagram of the Raman depth profile assembly. (b) Collection and excitation scheme with a microscope objective in the 180° geometry. (c) Collection and excitation scheme with different lenses in oblique configuration.

Figure 3

Comparison of Raman signal depth profiles for 519 cm⁻¹ band of silicon wafer, thickness of 550 µm, measured with (a) micro-Raman confocal back scattering configuration with fibers of different core sizes acting as pinholes and (b) off-axis configuration capable of optical sectioning with 45° illumination compared to a traditional 180° configuration. Both systems utilize the same collection optics; the excitation optics differ slightly (180°: NA = 0.13, off-axis: NA = 0.17).

Figure 4

Raman signal of 50 µm PMMA thin film on top of the diamond ATR crystal. The right part shows the full spectrum with complete dynamics of the diamond peak, whereas the left figure zooms into the ordinate, making the much weaker PMMA Raman signals visible.

Figure 5

(a) FTIR spectra of sodium lactate for concentrations in the range of 0.0–5.0 M. (b) Peak intensities at 1040 cm⁻¹ (black), 1120 cm⁻¹ (red), 1314 cm⁻¹ (blue), and 1416 cm⁻¹ (green) as a function of the molar concentration. The peak intensities follow a linear trend with coefficients of determination around 0.999 for all peaks.

Figure 6

(a) Raman spectra of sodium lactate for concentrations in the range 0.0–5.0 M. (b) Peak intensities for 856 cm⁻¹ (black), 1043 cm⁻¹ (red), and 1086 cm⁻¹ (blue) as a function of the molar concentration. The peak intensities follow a linear trend with coefficients of determination larger than 0.96 for all peaks.

Figure 7

Raman and FTIR absorbance spectra from BSA acquired using the novel combined Raman−FTIR system. Corresponding vibrational bands are indicated.

Figure 8

(a) Photograph of the combined FTIR–Raman setup with the Raman system in an inverted configuration. (b) ATR crystal with sample cavity from top and view from bottom.