Raman Spectroscopy: Successes, Challenges, and Pitfalls

Raman spectroscopy is one of the strong analytical techniques, which are suited for applications in materials science, medicine, chemistry, earth science, and planetary science. C.V. Raman discovered the Raman effect, for which he was awarded the Nobel prize in the field of Physics in 1930. Raman shift happens due to the vibrational effect of molecules. 

Following the development of laser sources and photon detectors, Raman spectroscopy has been predominantly used in the fields of physics and chemistry. With the commercial availability of Raman microprobes, Raman spectroscopy had become popular. Similar to electron probe microanalysis and isotope analysis, Earth scientists are quick to follow new applications in Raman spectroscopy. This event opens up multiple research fields like fluid-inclusion-based studies, traces of life in extraterrestrial materials, and the oldest terrestrial rocks. Through in situ characterization in diamond-anvil cells, the mineralogy of deep earth is performed. Another prominent usage of Raman spectroscopy is in the field of gemology for the authentication of gems and mineral inclusions within them and verification of natural origin.

This article enlightens the readers about how Raman spectroscopy is used in minute applications and at the same time, it also discloses the challenges and pitfalls faced by Raman spectroscopy. As dos and don’ts of Raman spectroscopy are beyond the scope, cans, and can’ts of Raman spectroscopy have been discussed. This also tells what types of information are obtainable and what types of samples are most amenable to Raman analysis. 

CANS and CAN’TS of Raman Spectroscopy

The preparation of samples is very minimal. Polishing is not a requisite. No coating of vacuum chambers or samples is needed. Within seconds, a straightforward Raman spectrum and identification can be accomplished. 

Though crystals and their powders can be analyzed, they generally reveal differences in their respective background intensities and signals.

The surface state and overall optical absorption of the sample at excitation wavelength are principal parameters in the control of excitation volume during the analysis.

Destructive Heat

In micro-Raman spectroscopy via a confocal optical system, the beam of the exciting laser is generally focused to a spot diameter in the range of hundreds of nanometers to some micrometers, which is sufficient to offer compositional information. The technique is generally non-destructive, yet opaque and intensely colored samples heat up destructively. Hence, hydrated materials will dehydrate because of the absorption of incident light.

Raman spectroscopy is compatible with amorphous, crystalline, mineral, and organic solids, liquids, and gases. The analysis can be done on targets below the sample surface in transparent samples. Also, analysis can be performed in aqueous components and water, in the case of dissolved sulfur and minerals undergoing a reaction. In nanocomposites of inorganic and organic compounds like bone, the interwoven phases customarily possess Raman bands in various parts of the spectrum. This leads to the identification of the Raman bands without the destructive treatment of the sample.

Identification of Individual Components

Though identification of the compound itself reveals much about the chemistry, Raman spectroscopy does not recognize individual atoms. The sample is dominated by Mg, Si, Ca, and O if the Raman spectral identification is diopside. As vibrational frequencies of bonds are being monitored, whatever affects the bonds will affect the frequency and specific peak positions like site vacancies, defects, and isomorphous ion substitution. Also, the Raman spectrum is more sensitive to changes or differences between the compounds, than the particular molecular environment, which is causing the change and to the identity of the element.

Speciation and Analysis of Substituents

Diverse chemical aspects of solid phases, aqueous species, and gases can be scrutinized and characterized by Raman spectroscopy. The Raman spectrum reveals the speciation of the involved atoms, as the specific bonded elements interact with the incident photons. It shows that not only, sulfur is dissolved in water, but also says that the dominant sulfur species in water is (HSO₄)^-, with a lesser concentration of (SO₄)^2-

The effect on the vibrational frequency of substituting ions with isotopes of the same element, but different mass can be calibrated against independently analyzed reference materials. At the same time, mixtures of more than two substituents of different masses are tough to interpret. 

The relative state of carbonation, hydroxylation, and hydration of a solid-state can be determined by Raman spectroscopy. Though polymorphs possess the same chemical composition, they have contrasts in structure, which are revealed by the differences in the number and positions of Raman bands.

Not all compounds are Raman active and produce Raman spectrum and Raman lines. But some compounds have stronger Raman scattering than others. Diamond exhibits stronger Raman spectra, compared to nanocrystalline iron oxides, which exhibit a weaker Raman effect.

Impurities, Background Signal & Emission Lines

Most of the natural samples consist of impurities like organics, trace elements, and defects, which are capable of generating a luminescence signal when excited. The luminous signal can have different expressions like the inclusion of broad bands and continuous background. There are some discrete lines along with the Raman lines as displayed by the rare-earth elements in Raman active or Raman inactive materials. The expressions can be so intense that the Raman signal and Raman spectrum can be subsumed and indistinguishable from the luminescence band and background band. 

In addition, narrow and laser-induced emission lines can be wrongly observed as Raman bands, thereby creating confusion and possible errors in the identification of samples. There is also anxiety on sample alteration like chemical breakdown, melting and oxidation due to laser heating. There are also affairs of heat-induced downshifting of Raman bands.

Focussing Specifications

Aspects of the specific instrument and physics of Raman imaging and Raman effect put restrictions on the kind of information available and the specificity of the interpretation of the Raman spectra. The spectrum is capable of showing only the area which the focussed laser was able to sample, especially in highly confocal instruments. The spectrum is dominated by the surficial phases and species in opaque and strongly light-scattering samples like fine-grained powders. This case leads to difficulty in the determination of the quantitative ratio of solid phases in a mixture using a Raman microprobe. This happens because the scattering signal (Raman signal) rather than volume integrating absorption signal is evaluated. The relative intensities of Raman peaks are affected by the orientation of the non-isometric crystals against the plane of polarization of the incoming laser and inherent strain in the material.

Conclusion

A short introduction to Raman spectroscopy has been provided. The pros and capabilities of Raman spectroscopy have been detailed. At the same time, the cons and inabilities have also been discussed. Hence, it is essential to know the use of a specific variant of Raman spectroscopy after knowing what kind of information is needed from the experiment.