The power of spectroscopy stems from its ability to provide detailed information by measuring the interaction of electromagnetic radiation with a sample. The absorption/transmission, emission and scattering of electromagnetic radiation provide a wealth of information for determining sample composition and concentration, molecular and atomic structure, molecular interactions, sample identification and much more. The information obtained from spectroscopy measurements depends on many factors including the frequency of electromagnetic radiation used to probe the sample. In this technical tip, we discuss mid-IR spectroscopy and the specific applications that benefit from measurements using radiation from the mid-infrared region (mid-IR).
Unlike radiation from the ultraviolet (UV) (~190-400 nm) and visible (~400-800 nm) regions of the electromagnetic spectrum, energy associated with radiation from the IR region is not high enough to excite electrons to higher energy levels. Instead, IR energy excites vibrational motions in the covalent bonds in molecules. Depending on the complexity of the molecule, a range of motions can occur including bending, stretching, scissoring, twisting and rocking in addition to rotational motion around the bonds of the molecule. The specific vibrational and/or rotational modes observed when IR radiation interacts with a molecule are specific to the chemical structure of the molecule being measured.
The infrared part of the electromagnetic spectrum is typically separated into three distinct regions based on the frequency of the infrared (IR) radiation relative to the visible spectrum (~400-800 nm).
- Near-IR region: 12500-4000 cm-1 (~800-2500 nm wavelength) – closest to the frequency of visible light
- Mid-IR region from 4000-400 cm-1 (~2.5-5 µm wavelength)
- Far-IR region from 400-10 cm-1(~25-1000 µm wavelength) – farthest from the frequency of visible light
The detection of molecular vibrations and rotations that occur when samples absorb IR light is called Vibrational Spectroscopy. Near-IR, mid-IR and Raman spectroscopy are commonly used Vibrational Spectroscopy techniques. The vibrational modes measured when IR energy is absorbed by a molecule depend on the energy of the radiation interacting with the molecule. Overtone/harmonic and combination bands are excited by the higher energy in the near-IR (and visible) regions while lower energy mid-IR radiation excites the fundamental vibration bands. These fundamental vibrations arise from the simplest vibrational modes of the molecule and are stronger than the overtone and combination bands resulting from the fundamental vibrations. For this reason, spectra arising from fundamental vibrations are much cleaner resulting in a unique fingerprint for the sample, which can be used for identification. In the case of the far-infrared, the low energy of the radiation in this region make it useful for rotational spectroscopy of inorganic molecules.
When is Mid-IR Spectroscopy Used?
The interaction of mid-IR radiation with a given sample provides a spectral fingerprint useful for identification of the sample. The mid-IR spectrum results from the absorption of specific frequencies of mid-IR radiation based on the chemical structure of the sample. For this reason, the peaks and troughs in a mid-IR spectrum are very specific to the sample measured. This makes mid-IR spectroscopy well suited for a wide range of applications involving materials identification and characterization for measurements ranging from the analysis of fuels to food safety and detection of counterfeit materials. These applications and many others benefit from the fundamental bands measured with mid-IR, which yield a higher intensity, less convoluted spectra than the overtone and combination bands measured with near-IR and visible radiation.
Mid-IR spectroscopy is widely used by researchers and educators for basic and applied research and for teaching labs in Physics, Chemistry and Biomedical courses. A review of some of the many applications and measurements using mid-IR spectroscopy is below.
As the world explores alternate energy sources and ways to reduce the environmental impact of fossil fuels, detailed fuel characterization and testing is required to ensure optimum performance and reduce environmental impact. Mid-IR spectroscopy can be used in oil and petroleum analysis, to analyze FAME (fatty acid methyl ester) content in biodiesel, to test octane levels in fuel, to evaluate oil and lubricant degradation and to confirm the presence of fuel additives like ethanol.
The specificity and non-destructive nature of mid-IR spectroscopy make it great for use in industrial settings including chemical and pharmaceutical production. In these industries, inbound raw materials must be tested to confirm identity. In addition, the product must be measured throughout the production process to ensure final product quality. In other industries, mid-IR is used for quality assurance to detect defects in finished products. With the many advances in mid-IR technology, measurements are now easier to make moving from the laboratory setting to the process line.
Farm to Table
With the advent of precision agriculture techniques to improve crop yields and decrease waste, mid-IR spectroscopy is used for soil characterization and to measure the content of important product constituents like antioxidants in agricultural products. Mid-IR is a great technique for feeding the consumer desire to know more about the food they are eating including safety and quality. Mid-IR is used for composition analysis of edible oils and detection of adulterants often used to provide the consumer with an inferior, sometimes dangerous lower quality product. Mid-IR measurements are also very useful for confirming critical parameters like alcohol and sugar content.
With the current emphasis on protecting the environment, mid-IR spectroscopy is a great option for detecting soil and water contaminants including fuels and fuel products. Mid-IR spectroscopy is also used for airborne analysis to detect contaminant levels in the air and to monitor high risk areas for environmental contamination.
In these and the many other applications using mid-IR spectroscopy, spectral analysis is based on two major regions in the mid-IR spectrum. Vibrations of the functional groups within the sample occur in the Functional Group Region from 4000-450 cm-1 with the much more spectrally complex Fingerprint Region from 1450-500 cm-1 providing a unique spectral region for the identification of samples.
Mid-IR versus Near-IR
Near-IR and mid-IR vibrational spectroscopy are widely used techniques with their own strengths and weaknesses. Both techniques share the advantages of requiring no sample preparation and non-destructive analysis but they both have attributes that make them better for certain applications.
Near-IR spectroscopy provides higher energy to the sample, but the spectra are composed of weaker overtone and combination bands, resulting in more complex spectra with overlapping absorption bands. Mid-IR spectroscopy measures fundamental vibrational bands related to the functional groups in the sample. These fundamental bands give clean spectra ideal for determining sample composition and for the identification of samples using their unique mid-IR fingerprint. Near-IR spectroscopy does offer an advantage in terms of sampling techniques with the ability to make reflection measurements in addition to the absorption and transmission measurements typically used for mid-IR spectroscopy.
In terms of sampling, near-IR spectroscopy enables more representative sampling of a larger volume of sample by rotating it throughout the measurement. This makes near-IR a good option for inhomogeneous samples and trace analysis. Mid-IR spectroscopy, on the other hand, works best with smaller volumes of homogeneous samples making trace detection much more difficult using mid-IR.
Ocean MZ5 ATR Mid-IR Spectrometer System
The Ocean MZ5 is a compact, fully integrated ATR-MIR spectrometer with measurement capabilities from 1818–909 cm-1 (5.5-11 μm). Ocean MZ5 comprises a sample interface, light source, detector and software, and provides a fast, convenient alternative to traditional FTIR spectroscopy.
With Ocean MZ5, the company has partnered with Pyreos Limited (Edinburgh, Scotland) to expand its capabilities into new mid-IR solutions. The unique combination of Pyreos’ pyroelectric line arrays and Ocean Optics’ applied spectral knowledge opens up new application challenges involving samples with distinct mid-IR absorption characteristics. This includes detection of food adulterants and contaminants, determination of oil grade and quality, and analysis of alcohol purity.
- NIR Versus Mid-IR: How to Choose Paul Wilks; Spectroscopy Volume 21, Issue 4; Apr 01, 2006
- Analytical Vibrational Spectroscopy - NIR, IR, and Raman Spectroscopy Volume 26, Issue 10; Oct 01, 2011
- Infrared Spectrometers: NIR and MIR Compared Future Learn University of East Anglia
- What is the Pyroelectric Effect? Pyreos Sensing Solutions