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Infrared Spectroscopy Part 2, Analytical Procedures and General Directions eISBN: 9780841230460 Tom Tyner Chair, ACS Committee on Analytical Reagents James Francis Secretary, ACS Committee on Analytical Reagents
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ABSTRACT Infrared (IR) spectroscopy is an absorption method widely used in both qualitative and quantitative analyses. The infrared region of the spectrum includes electromagnetic radiation that can alter the vibrational and rotational states of covalent bonds in organic molecules. The IR spectrum of an organic compound is a unique physical property and can be used to identify unknowns by interpretation of characteristic absorbances and comparison with spectral libraries. IR spectroscopy is also used in quantitative techniques because of its sensitivity and selectivity. It can be used to quantitate analytes in complex mixtures and is used extensively in detection of industrial pollutants in the environment. The IR technique is discussed here primarily for application in identification of organic compounds and will focus on the mid-infrared region. Instrumental operating procedures are not given because they will vary depending on instrument design. A brief discussion of the theory will be followed by a discussion of instrumentation, sample handling techniques, and qualitative analysis.
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GENERAL BACKGROUND Unlike UV and visible spectroscopy, which use larger energy absorbances from electronic transitions, IR spectroscopy relies on the much smaller energy absorbances that occur between various vibrational and rotational states. Only molecules that undergo a net change in the dipole moment during vibrational and rotational motions can absorb IR radiation. Homonuclear molecules, such as O2, N2, or Cl2, are not IR-active because no net change in the dipole moment occurs. Molecular vibrations can be classified as either stretching or bending. Stretching is a result of continuous changing distances in a bond between two atoms. Bending refers to a change in the angle between two bonds. Bending motions include scissoring, rocking, wagging, and twisting. The various types of vibrations and rotations absorb at different frequencies within the infrared region, thus resulting in unique spectral properties for different molecular species.
INSTRUMENTATION Basic instrumentation for IR spectroscopy includes a radiation source, wavelength selector, sample container, transducer (detector), and signal processor. Sources, transducers, and beam splitters will vary according to whether near-, mid-, or far-IR
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DOI:10.1021/acsreagents.2008.20160601 ACS Reagent Chemicals, Part 2
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spectra are being measured. Most identification of organic compounds can be done in mid-IR. For mid-IR, the radiation source can be a Nernst glower, globar, or laser. The IR transducers (detectors) are temperature-sensitive devices that undergo large changes in certain properties (e.g., electrical resistance and temperature-dependent potential) upon small temperature variations. Examples include bolometers, thermocouples, and photoconducting transducers. Sources and transducers are the same for dispersive and Fourier transform infrared (FTIR) instruments. Dispersive IR instruments record energy absorption as the instrument scans through the IR energy spectrum. These instruments operate in the frequency domain. FTIR instruments work in the time domain. In spectra produced by Fourier transform infrared instruments, all wavelengths emitted by the source are present. The frequency data from the FTIR instrument is obtained using mirrors in a Michelson interferometer to produce a signal that is transformed to data with a much lower frequency while containing the same information as the original IR signal. The use of an interferometer offers an advantage when using background subtraction techniques. FTIR instruments utilize a Michelson interferometer to modulate the incoming optical radiation by using two mirrors that are perpendicular to each other (one fixed and one movable) and a beam splitter placed on the path of the incoming radiation. The beams first split and then recombine again thus undergoing interference that depends on the difference in the path lengths. The interferogram obtained is then decoded by Fourier transform thus providing the spectrum of the target radiation. Michelson interferometer FTIRs offer significant sensitivity improvement over other types of instruments.
SAMPLE HANDLING Sample containers and handling can present a challenge in the infrared region. Materials used to produce cuvettes are not transparent and cannot be used. Cells prepared from alkali halides such as sodium chloride are widely used due to their transparent properties in the infrared region. A common problem with sodium chloride cells is that they absorb moisture and become fogged. Polishing is required to restore the cells to a more transparent state. Liquids may be analyzed in their neat form by placing a small amount of sample on a sodium chloride plate and then placing a second plate on top to form a sample film. The plates are then placed in an appropriate holder in the sample compartment of the instrument. This technique provides adequate spectra for qualitative use. Alternatively, the use of silver chloride or silver bromide “disposable” windows has gained widespread acceptance for use with liquids and Nujol (a heavy hydrocarbon oil) mulls. Solutions of liquid or solid materials can also be analyzed by IR spectroscopy. Solvents should be chosen that do not have absorbances in the region of interest. Unfortunately, no solvent is completely transparent in the mid-infrared region. With double-beam instruments, a reference cell containing blank solvent can be used. Common moderate absorbances will not be observed. Solvent transmission should always be above 10% when using a solvent reference cell. Influences of solvent on the absorbance of the solute should be considered. For example, hydrogen bonding of alcohols or amines with the solvent may affect the characteristic vibrational frequency of the functional group. When practical, it is desirable to analyze neat materials for qualitative analysis. A method commonly used for analysis of neat solid samples is the mull technique. The technique consists of grinding the material into a fine powder and then dispersing it into a liquid or solid matrix to form a mull. Liquid mulls have been formed by combining the powdered analyte with Nujol. The liquid mull is analyzed between salt plates as described above. The disadvantage of Nujol is that hydrocarbon bands may interfere with analyte absorbances. A second method of forming a mull involves grinding the powdered analyte with dry potassium bromide and forming a disk. The ratio of analyte to potassium bromide is usually about 1:100. The materials are ground together using a mortar and pestle or a small ball mill. The mixture is then pressed in a die at 10,000–15,000 psi to form a small transparent disk and analyzed. Care must be taken when preparing the disk to protect it from moisture. It is common to see absorbances for moisture when using potassium bromide disks. In some instances, vapor-phase analysis provides differences in absorbance frequency and intensity when compared with solid- or liquid-phase analysis.
QUALITATIVE ANALYSIS The most widely used application of IR spectroscopy is for qualitative analysis of organic compounds. Compounds have unique spectra that depend on molecular attributes. A common method of interpreting IR spectra is to consider two regions: the functional group frequency region (3600–1200 cm–1) and the “fingerprint” region (1200–600 cm–1). A combination of interpreting the functional group region and comparing the fingerprint region with those in spectral libraries provides, in many cases, sufficient evidence to positively identify a compound.
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DOI:10.1021/acsreagents.2008.20160601 ACS Reagent Chemicals, Part 2
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The functional group region provides evidence of functional groups in a molecule based on the absorbance frequency. Tables are available in standard spectra manuals that provide ranges of absorbances for specific functional groups. Common groups with characteristic absorbances include aldehydes, ketones, esters, alkenes, alkynes, alcohols, amines, amides, carboxylic acids, nitro groups, and nitriles. While most functional groups fall in the 3600–1200 cm–1 range, some can also fall in the fingerprint region. For example, C–O bonds can be around 1000 cm–1, and C–Cl absorbances are typically found in the range of 600–800 cm–1. The fingerprint region is often unique to the analyte. In this region, small differences in structure can lead to differences in absorbances. Most single bonds absorb in this area, and differences in the skeletal structure of molecules will result in frequency and intensity differences. The fingerprint region is most effectively used by comparison to existing spectra. Computer-based search systems offer a rapid method of comparing unknown samples to known spectra.
Procedure for IR Spectroscopy IR spectroscopy is used in selected standard-grade reference materials specifications as a technique for identity confirmation. The criteria under which standard-grade reference materials pass identity tests are based on observation of characteristic absorbances and by comparison to a spectral library, if one is available. Each entry of selected standardgrade reference materials to be analyzed by IR spectroscopy will contain a minimum of three absorbances that must be present to pass the test. A standard-grade reference material will pass the comparison test if all absorbances found in the NIST spectrum are present in the test spectrum. The NIST spectral library contains many of the standard-grade reference materials listed in this book. Comparison of test spectra to NIST spectra should be limited to the relative presence of absorbances and should not be an exact comparison of absorbance frequency or intensity. An alternative spectral library is provided by the National Institute of Advanced Industrial Science and Technology (AIST).
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DOI:10.1021/acsreagents.2008.20160601 ACS Reagent Chemicals, Part 2
