Both infrared and Raman spectroscopy produce transitions between quantised vibrational levels although the mechanisms that induce these transitions are different.
The IR and Raman spectra of a compound provide great information about its internal properties (chemical composition, impurities, interaction between substituents, analysis of functional groups, etc.), and are therefore of great importance in qualitative analysis.
Among the multiple applications of these techniques we can highlight, for example, the analysis of polymers, addictives, forensic studies, identification of environmental pollutants, medicine, various areas of chemistry (organometallic, organic, inorganic, agricultural, industrial), heritage (pigment identification), etc.
Infrared spectroscopy is one of the most versatile and widely applied spectroscopic techniques. The possible applications of this technique are therefore innumerable. Some of the most important ones are listed below:
- Characterisation and identification of materials:
- Polymers and plastics.
- Inorganic solids (minerals, catalysts, composite materials...).
- Analysis of pharmaceutical and synthetic products.
- Analysis of contaminants.
- Forensic science (identification).
- Biomedicine (tissue analysis).
- Artistic conservation (analysis of pigments, materials used, etc.).
- Recycling industry (identification of polymeric materials).
- Monitoring of chemical processes, etc.
In Raman spectroscopy, the main feature of this technique is to exploit the Raman effect to identify and characterise the chemical structure of materials in a non-destructive and non-contact manner, so that it is possible to identify isolated particles of the order of a micron.
The field of application is very broad, with the following applications:
- Applications in gemology (characterisation of precious stones).
- Semiconductors.
- Pharmaceutical applications.
- Studies of interactions in aqueous and non-aqueous media.
- Studies of fibres and organic films.
- Applications in catalysis.
- Study of carbonaceous materials (fibres, carbonised, pitches, graphite, diamond...).
- Characterisation of pigments and paints in archaeology.
- Identification of drugs and explosives, etc.
Samples must be submitted to the Unit accompanied by a duly completed application form.
For IR
Types of samples: solid samples (powders, surfaces, fibres, foams, films, etc.) and liquid samples (pastes, gels, oily, etc.) can be registered.
In transmission work mode (KBr tablets, IC sales), solid samples must be completely free of water or solvent residues to be mixed with KBr and ground before forming into a tablet.
In ATR mode, the samples do not need any preparation, although some requirements must be met. The use of the diamond crystal ATR accessory allows ATR spectra to be obtained on materials that are unusual in this technique, such as powdery solids, foams, fibres, aqueous samples, etc. It is a non-destructive technique
Analysis of liquid and solid samples by both ATR and transmission is now possible.
For Dispersive Raman
This technique is non-destructive in nature. Limitations may be related, in some cases, to multicomponent samples and very dilute sample solutions, since the associated Raman spectrum may be intrinsically weak.
Type of samples
The nature of the samples can be solid (with different states of aggregation such as powders, pieces, particles, etc.) and liquid. As this is a micro-spectroscopy technique, the sample size can be microscopic, allowing different areas within the same sample to be analysed. An accessory is available that allows this type of recording to be carried out in the case of larger samples, or samples that cannot be adapted to the base of the microscope.
Preparation of the sample
The samples to be analysed do not require any pre-treatment. Preferably, the sample should be brought in slides or coverslips to better adapt to the base of the microscope, otherwise the preparation can be studied.
In infrared (IR) spectroscopy, the molecule absorbs energy and jumps from one vibrational level to another.
In Raman spectroscopy, a light scattering phenomenon occurs. When a highly contaminated, monochromatic bundle of light has an impact on a sample and the frequencies of the scattered light are channelled, we find that most of the light is scattered unchanged, a process known as the Rayleigh effect. However, a small part of the incident light is scattered with changes in its original frequency, so frequencies with higher and lower values will be found in relation to that original frequency. These frequencies are called Raman frequencies.
The difference in molecular excitation mechanisms results in different selection rules, so Raman spectroscopy does not replace infrared spectroscopy or vice versa. On the contrary, both spectroscopies are complementary. Thus, in both Raman and infrared spectroscopy, one can expect to observe the 3n-6 (or 3n-5) normal modes of vibration in the spectra, or a smaller number according to the selection rules imposed by molecular symmetry.
Indeed, in centro-symmetric molecules, the so-called ‘mutual exclusion rule’ operates and, therefore, no cadrancies will be observed in the Raman and Infrared spectra. However, in non-centrosymmetric structures, a variable number of cadrancies are observed and, therefore, Raman and infrared spectra will need to be obtained in the study of the molecular structure of the compound.
Nevertheless, Raman spectroscopy can have some advantages over infrared spectroscopy. For example:
- The complete Raman spectrum, i.e. between 4000-10 cm-1 can be obtained in a single experiment.
- Water has a very weak Raman spectrum, which facilitates the analysis of biological samples and those in which it is the solvent, whereas Infrared would require the use of special cells due to the high absorption of water in the infrared region.
- In quantitative analysis, Raman spectroscopy has the advantage that the intensity of a band is directly proportional to the concentration; in infrared spectroscopy, the relationship between the intensity of a band and the concentration is logarithmic.
- Raman spectroscopy does not require special preparation of the sample, which, after analysis, is perfectly recoverable, unlike infrared spectroscopy, which in some cases does require preparation of the sample for analysis, making it difficult to recover.
- Raman spectra are much simpler than infrared spectra, since in most cases the combination bands and overtones that are so characteristic of infrared are not observed.
- The use of calco lasers as excitation sources makes it possible to obtain the Raman spectrum in a few milliseconds. This is a very important technique in the study of fast chemical reactions or short-lived species.
However, both spectroscopies are applied as a quantitative and qualitative analytical technique in obtaining data on functional groups and other structural features. Qualitatively, they determine the presence or absence of certain functional groups according to the appearance or disappearance of bands characteristic of those groups.
IR-Raman unit
- Research Support Building. Santiago
- Rúa de Constantino Candeira, 1. Campus Vida , 15782Santiago de Compostela
- 881 816 255
- 881 816 203 (Fax)