Raman or Infrared Spectroscopy----Can You Really Tell Them Apart?

I. Introduction

Infrared spectrometry (IR) and Raman spectrometry are powerful tools for studying molecular structures and chemical compositions. Due to their advantages of being fast, highly sensitive, and requiring small sample amounts, they are widely used in fields such as materials science, chemical engineering, environmental protection, and geology. From an analytical testing perspective, using both techniques together often provides better molecular structural information. Although both IR and Raman spectrometry are molecular vibrational spectroscopies, there are significant differences between them: IR spectrometry is an absorption spectroscopy, while Raman spectrometry is a scattering spectroscopy. Furthermore, the spectra of the same molecule often differ between the two methods, which is closely related to molecular symmetry and is strictly limited by molecular vibration rules. If one does not have a clear understanding of their mechanisms and applications, the convoluted spectral lines alone may not help in distinguishing their correlations and differences. The following will unveil the "veil" of these two spectroscopies through theoretical and practical examples, providing reference for readers.

II. Basic Introduction

1 Detection Principles

Infrared Spectrometry: When electromagnetic radiation interacts with molecular substances, and its energy matches the difference in vibrational or rotational energy levels of the molecules, it causes the molecules to transition from a lower to a higher energy level. As a result, certain wavelengths of electromagnetic radiation are absorbed by the molecules. Measuring the radiation intensity at different wavelengths yields the infrared absorption spectrum, reflecting the vibrational and rotational transitions of molecules. Thus, IR spectrometry is also known as molecular vibrational-rotational spectroscopy. (In short, IR spectra are generated due to the absorption of light energy, causing vibrations that change the dipole moment of the molecule.)

Raman Spectrometry: When light irradiates a substance, causing photons to collide with the electrons within the molecules, if the collision is inelastic, some of the photon's energy is transferred to the electrons, resulting in the scattered light's frequency differing from the incident light's frequency. This scattering is known as Raman scattering, and the resulting spectrum is called the Raman spectrum. (In short, Raman spectra are generated due to the monochromatic light irradiation, leading to a comprehensive scattering effect that causes vibrational changes in molecular polarizability.)

2 Activity Determination

Mutual Exclusion Rule: For molecules with a center of symmetry, if their molecular vibration is Raman active, then it is IR inactive, and vice versa.

Mutual Allowance Rule: For molecules without a center of symmetry, both their Raman and IR spectra are active (with few exceptions).

Mutual Forbidden Rule: For a few molecular vibrations, both Raman and IR are inactive (e.g., ethylene molecules).

3 Detection Instruments

Infrared Spectrometry:

Dispersive IR Spectrometers: Similar to UV-visible spectrophotometers, consisting of a light source, monochromator, absorption cell, detector, and recording system. Using prisms or gratings as dispersive elements, these instruments have limited energy due to the use of slits, leading to long scanning times, lower sensitivity, resolution, and accuracy.

Fourier Transform Infrared Spectrometers (FTIR): Without dispersive elements, mainly consisting of a light source, Michelson interferometer, detector, and computer. Compared to dispersive IR spectrometers, FTIR has high resolution, high wavenumber accuracy, fast scanning speed, wide spectral range, and high sensitivity.

Raman Spectrometry:

Dispersive Laser Raman Spectrometers: Mainly consist of a sample chamber, laser, monochromator, and detector.

Fourier Transform Near-Infrared Laser Raman Spectrometers: Mainly consist of a sample chamber, laser source, Michelson interferometer, filter group, and detector.

Laser Microscope Raman Spectrometers: Use incident laser focused on small sample areas through a microscope, employing devices like camera tubes and monitors to directly observe magnified images, allowing precise targeting of the laser spot to acquire Raman spectra without interference from surrounding substances.

4 Similarities and Differences

Similarities: For a given chemical bond, the IR absorption frequency and Raman shift are equal, representing the energy of the first vibrational level. Thus, for a given compound, certain IR absorption wavenumbers and Raman shifts are identical, reflecting molecular structure information. Both Raman and IR spectrometry detect molecular vibrational and rotational energy levels.

Differences:

Essential Difference: IR spectrometry is absorption spectroscopy, while Raman spectrometry is scattering spectroscopy.

Ease of Measurement: IR is easier to measure with stronger signals, while Raman signals are weaker. However, Raman spectra are generally clearer with fewer overlapping bands, making interpretation easier.

Light Source: IR spectrometry uses infrared light (especially mid-infrared), whereas Raman can use visible to near-infrared light.

Target Groups: IR is often used to study asymmetric vibrations of polar groups, while Raman studies symmetric vibrations of nonpolar groups and skeletons.

Sample Suitability: Raman spectrometry can measure aqueous solutions (weak Raman scattering by water), while IR is unsuitable for aqueous solutions.

Sample Preparation: Raman spectrometry requires no special sample preparation, while IR does.

Measurement Containers: Raman spectrometry can measure in glass containers or capillaries, but IR cannot.

Complementarity: Often, strong IR signals correspond to weak Raman signals and vice versa.

Optimal Use: IR is better for identifying organic substances, while Raman is more comprehensive for inorganic compounds.

Interpretation: IR spectrum interpretation involves three elements (absorption frequency, intensity, peak shape), while Raman also considers depolarization.

III. Case Studies

Literature Overview: This article reviews the current status and pros and cons of using Raman spectroscopy to detect microplastics (< 20 um) and compares it with other detection techniques (such as IR spectrometry).

Figure 5: Polypropylene(a) Raman image (left) and IR image (right); (b) Raman spectrum (left) and IR transmission spectrum (right)

Analysis: To verify the effectiveness of IR and Raman imaging, the authors purified and filtered marine microplastic samples smaller than 400 um, and selected a 1000 * 1000 um filter for Raman and FTIR transmission imaging analysis. They compared the results of the two spectroscopic measurements, including the detectable number, type, and size of microplastics, as well as spectral quality, processing, and measurement time. Figure 5b shows spectra of smaller (15-20 μm) polypropylene (PP) particles, with a clear Raman spectrum (left) contrasted with a weaker and noisier IR spectrum (right). For IR spectrometry, the particle size being close to the lateral resolution and diffraction limit of IR microspectroscopy results in a lower signal-to-noise ratio. Notably, the detected IR spectrum might be from a mixture of the target particles and adjacent particles. In contrast, the Raman spectrum of the same particle shows a better PP spectrum, with a significantly improved signal-to-noise ratio despite relatively low Raman signal intensity.

References

[1] Araujo C F, Nolasco M M, Ribeiro A M P, et al. Identification of microplastics using Raman spectroscopy: latest developments and future prospects[J]. Water research, 2018, 142: 426-440.