Fourier Transform Infrared (FTIR) versus Raman Spectroscopy

A review of FTIR and Raman spectroscopy methods and discussion of the range of materials analysis applications

FTIR is able to capture rich absorbance and emission spectral data from a wide array of liquids, gases, and solids making it well-suited for manufacturing, quality control, and failure analysis, among other uses. Raman, when coupled with microscopy, is able to discern fine surface structure for chemical analysis and 3-D depth profiling of optically transparent substances, using non-destructive techniques.

Both platforms are also used in polymer analysis and are important in plastics identification and characterization. Hence, they are important tools for detection of microplastics in the environment as well as plastics accumulation in biological specimens and consumer goods such as drinking water.

Although the two techniques can be complementary, they each are based on distinct physical processes and measurements. Here we take a brief moment to dissect FTIR versus Raman spectroscopy.

FTIR Principle

FTIR is based around the infrared spectroscopy (IR) concept, or the absorbance (and emission) of long wavelength light directed at a sample. IR can be used to detect specific functional groups in chemical compounds, or to measure sample purity and the presence of specific impurities by way of unique absorbance spectra.

The FTIR spectrometer is based on a similar platform as continuous wave classic transmission spectrometers (e.g. UV/Vis), with several important differences thus lending to unique capabilities.

• The classic spectrometer incorporates a monochromator to split the light source radiation into different wavelengths. A slit then selects the wavelengths of light that subsequently shine on the sample. Although very accurate devices can be designed, there are important limitations inherent in this analysis principle.
• The monochromator and slit configuration limit the total amount of light that can reach the sample, thus limiting the overall sensitivity of the instrument.
• Resolution of the spectral data can be improved by narrowing the slits, but a major consequence is further loss of sensitivity.
• Multiple scans can not be readily run in order to effectively build up signal-to-noise ratios.

Net results include limited ability to detect and quantify low level compounds in samples and the inability to report high-resolution information for chemical identification.

FTIR Analysis

FTIR overcomes several of the critical limitations listed above through the use of an interferometer in the place of the classic monochromator/slit components.

• The interferometer directs all light radiation from the source through to the sample.
• Instead of restricting the wavelengths of light (and hence energy) that can pass to the sample, the interferometer tunes the pathlength of the light beam with respect to a stationary beam.
• The difference of the two beams, and their respective path lengths, results in constructive and destructive interferences, and a resulting interferogram.
• The combined beams - upon passing through the sample - are used to generate a broad-spectrum absorbance profile.
• The detector reports total variation in energy versus time for all wavelengths simultaneously – which can then be converted to intensity versus frequency through use of Fourier Transform function calculations using an on-board computer.

Major advantages include:

• All source energy reaches the sample, resulting in significant signal-to-noise ratio improvements.
• Spectral resolution is limited only by the design of the interferometer, and even the lowest resolution device is substantially higher than the vast majority of classic spectrophotometers.
• Multiple scans can be collected improving signal-to-noise and data quality.
• Modern software allows a variety of data conditioning and analysis, thus enhancing analytical power.

Attenuated Total Reflectance (ATR)

ATR is a sampling technique which provides higher resolving power for solid and liquid samples, without the need for extensive sample prep procedures. ATR essentially works by measuring the changes that occur in an internally reflected IR beam as it comes into contact with the sample.

The technique is well-suited and preferred when working with strongly adsorbing or thick samples that product intense peaks using transmission IR. Homogenous solid samples, the surface layer of a multi-layered solid, or the coating on a solid are ideal. ATR is an excellent technique for analysis of samples in their native state, and particularly dense or strongly absorbing solids and liquids.

FTIR Applications

FTIR can be implemented in a single purpose tool or a highly flexible analytical research instrument. FTIR spectroscopy can be coupled with microscopy, thermal analysis, gas chromatography, mass spectrometry, and other techniques to provide high-resolution chemical analysis from a wide-variety of sources. The technique can also be miniaturized into handheld FTIR devices for remote chemical detection.

Raman Spectroscopy

Raman is used to measure vibrational, rotational, and other low-frequency modulation based on inelastic (Raman) scattering of monochromatic light. The technique is commonly used in chemistry to provide compound information for chemical identity in a wide variety of samples.

Raman Shift

The concept of Raman involves use of a laser light source illuminated on the sample of interest. The laser interacts with molecular vibrations or other excitable groups in the systems, resulting in a shift in energy of the laser photons. This Raman shift provides structural information about the vibrational modes in a given chemical system and can provide chemical identity, characterization, and distribution data.

Raman Spectroscopy Applications

Raman is useful in the chemical analysis and the identification of discrete chemical bonding arrangements, which have unique vibrational activity and therefore distinct fingerprints. Chemical structures such as carbon nanotubes and active fibers, including polypropylene, can be characterized owing to intrinsic vibrational properties. In addition, Raman has a wide variety of applications in biology and medicine owing to multiple functional advantages.

• Raman is not susceptible to inference from permanent dipole substances such as water, making it well-suited for macromolecular analysis of proteins, DNA, RNA, and others.
• Raman is non-invasive and non-destructive, making it highly useful for applications such as biological tissue imaging using confocal Raman microscopy.

Handheld Raman Spectrometer

Raman spectroscopy can also be used in remote devices for biomineral or explosives detection. The technique may find important future applications in point-of-care facilities as well, for detection of pathological conditions such as cancer via analysis of liquid biopsies of patient urine, blood, or other substances.

FTIR and Raman Spectroscopy

To summarize, both approaches can provide high-resolution chemical information in biological, materials, and remote applications. The two techniques produce distinct yet complementary information, which can be used to determine chemical purity (FTIR) and structural distribution of chemical species (Raman microscopy), among many other uses.

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