Infrared spectroscopy

Infrared Spectroscopy

For a table of IR spectroscopy data, see infrared spectroscopy correlation table

For more information, please visit wireless infrared interpretation system.

Measurement of infrared radiation's interaction with matter

Infrared spectroscopy (IR spectroscopy or vibrational spectroscopy) involves measuring the interaction of infrared radiation with matter through absorption, emission, or reflection. This technique is essential for identifying and studying chemical substances or functional groups in different states such as solid, liquid, or gaseous forms. It is valuable for characterizing new materials, and for identifying or verifying known and unknown samples. IR spectroscopy is performed using an instrument called an infrared spectrometer (or spectrophotometer) which generates an infrared spectrum. The IR spectrum is typically displayed as a graph with infrared light absorbance (or transmittance) on the vertical axis and frequency, wavenumber, or wavelength on the horizontal axis. Common units of measurement for the wavenumber in IR spectra are reciprocal centimeters (cm−1), and IR wavelengths are often given in micrometers (previously known as "microns"), symbolized as μm. One of the most commonly used laboratory instruments for this technique is the Fourier transform infrared (FTIR) spectrometer. There are also advanced techniques like two-dimensional IR, which will be discussed further below.

The infrared portion of the electromagnetic spectrum is typically divided into three regions: near-infrared, mid-infrared, and far-infrared. Near-infrared, with wavelengths roughly between 0.7–2.5 μm (14,000–4,000 cm−1), can excite overtone or combination modes of molecular vibrations due to its higher energy. Mid-infrared, approximately 2.5–25 μm (4,000‒400 cm−1), is commonly used to study fundamental vibrations and their rotational-vibrational structures. Far-infrared, with wavelengths between 25‒1,000 μm (400‒10 cm−1), possesses lower energy and is used for rotational spectroscopy and low-frequency vibrations. Additionally, the range from 2‒130 cm−1, which borders the microwave region, is known as the terahertz region and is useful for investigating intermolecular vibrations. These classifications are based on conventions and have loose associations with relative molecular or electromagnetic properties.

Uses and Applications

Infrared spectroscopy is a versatile and reliable technique widely used in both organic and inorganic chemistry, serving critical roles in research and industry. In catalysis research, it is invaluable for characterizing catalysts and detecting intermediates and products during catalytic reactions. Additionally, it finds applications in quality control, dynamic measurement, and monitoring processes, such as long-term CO2 monitoring in greenhouses using infrared gas analyzers.

The technique also has significant forensic applications, particularly in criminal and civil cases for identifying polymer degradation or determining blood alcohol levels in suspected drunk drivers. IR spectroscopy has proven successful in analyzing and identifying pigments in paintings and other art objects, including illuminated manuscripts.

In polymer manufacture, IR spectroscopy helps measure the degree of polymerization by assessing changes in the character or quantity of specific bonds over time. Modern research instruments can perform infrared measurements up to 32 times per second, enabling faster and more accurate observation of chemical reactions and processes.

Infrared spectroscopy is effectively utilized in semiconductor microelectronics, with applications to various materials like silicon, gallium arsenide, zinc selenide, and more. The food industry also benefits from IR spectroscopy for measuring compound concentrations in various food products. Portable instruments make field trials feasible.

Gas leak detection devices such as the DP-IR and EyeCGAs employ infrared spectroscopy to detect hydrocarbon leaks in natural gas and crude oil transportation. In 2014, NASA enhanced its database for tracking polycyclic aromatic hydrocarbons (PAHs) using IR spectroscopy, which are notable for their abundance in the universe and possible role in life formation.

IR spectroscopy plays a crucial role in recycling household waste plastics by providing an efficient method for sorting different polymers. Advances include miniature IR-spectrometers linked to cloud databases and NIR-spectroscopic chips for integration into everyday gadgets.

When combined with machine learning and artificial intelligence, infrared spectroscopy offers potential for rapid and accurate non-invasive bacteria sensing. Advanced statistical methods and deep-neural networks are used to extract essential features from complex datasets. This technology has shown promise for differentiating bacteria at various taxonomic levels and for antimicrobial susceptibility testing in clinical settings, though challenges remain in large-scale database construction. Bruker’s IR Biotyper attempts to address this in food microbiology.