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76 Infrared Spectroscopy Reagent ChemicalsNinth Edition

ACS SpecificationsOfficial from January 1, 20002000 American Chemical Society

Resolution, R, can be specified to ensure separation of closely eluting compo-nents or to establish the general separation efficiency of the system. R can be cal-culated as follows:

where t2 and t1 are the retention times of the two component peaks, and W(h/2)1and W(h/2)2 are the half-height widths of the two peaks in units of time. An R valueof 1.0 means that the resolution is 98% complete. This condition is sufficient, inmost cases, for peak area calculations. An R value of 1.5 or greater representsbaseline, or complete, separation of the peaks.

INFRARED SPECTROSCOPY

Infrared (IR) spectroscopy is an absorption method widely used in both qualita-tive and quantitative analysis. The infrared region of the spectrum includes wavenumbers ranging from about 12,800 to 10 cm1. The IR range can be separatedinto three regions: the near-infrared (12,8004,000 cm1), the mid-infrared(4,000200 cm1), and the far-infrared (20010 cm1). Most analytical applica-tions fall in the mid-infrared region of the spectrum. IR spectroscopy has prima-rily been used to assist in identification of organic compounds. The IR spectrumof an organic compound is a unique physical property and can be used to identifyunknowns by interpretation of characteristic absorbances and comparison tospectral libraries. IR spectroscopy is also employed in quantitative techniques.Because of its sensitivity and selectivity, IR spectroscopy can be used to quantitateanalytes in complex mixtures. Quantitative analysis is used extensively in detec-tion of industrial pollutants in the environment. The near-infrared region hasbeen used for quantitative applications, such as determination of water in glyceroland determination of aromatic amines in complex mixtures. The IR technique isdiscussed here primarily for application in identification of organic compoundsand will focus on the mid-infrared region. Instrumental operating procedures arenot given because they will vary depending on instrument design. A brief discus-sion of the theory will be followed by a discussion of instrumentation, samplehandling techniques, and qualitative analysis.

General Background

Unlike UV and visible spectroscopy, which uses larger energy absorbances fromelectronic transitions, IR spectroscopy relies on the much smaller energy absor-bances that occur between various vibrational and rotational states. When molec-ular vibrational or rotational events occur that cause a net dipole moment, IRabsorption can occur. Molecular vibrations can be classified as either stretching or

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Infrared Spectroscopy 77 Reagent ChemicalsNinth Edition


bending. Stretching is a result of continuous changing distances in a bond betweentwo atoms. Bending refers to a change in the angle between two bonds. Bendingmotions include scissoring, rocking, wagging, and twisting. The various types ofvibrations 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, wave-length selector, sample container, detector, and signal processor. Continuoussources are used in the mid-infrared region and include the incandescent wiresource, the Nernst glower, and the Globar. The Nernst glower is hotter andbrighter than the incandescent wire. The Globar is a rod of silicon carbide that iselectrically heated and that typically requires a water cooling system. The Globarprovides greater output than the Nernst glower in the region below 5 mm. Other-wise, their spectral energies are similar. Newer technologies have also employedvarious lasers as sources for infrared applications.

The three general types of infrared detectors are thermal, pyroelectric, andphotoconducting detectors. Thermal detectors measure very minute temperaturechanges as a method of detection. They operate over a wide range of wavelengthsand can be operated at room temperature. The main disadvantages of thermaldetectors are slow response and less sensitivity when compared to other detectors.Pyroelectric detectors are specialized thermal detectors that provide fasterresponse and more sensitivity. Photoconducting detectors rely on interactionsbetween incident photons and a semiconductor material. These detectors alsoprovide increased sensitivity and faster response.

The two main types of instruments used for qualitative analysis are disper-sive-grating spectrophotometers and multiplex instruments that employ Fouriertransform. Wavelength selection in dispersive-grating instruments is accom-plished using filters, prisms, or reflection gratings. Traditional instruments haveconsisted of a filter-grating or prism-grating system that covers the range between4000650 cm1. The most common is a double-beam instrument that uses reflec-tion grating for dispersing radiation.

Fourier transform has been applied to infrared spectroscopy and is now avery common technique. Fourier transform infrared (FTIR) spectroscopy offersenhanced sensitivity compared to dispersive IR spectroscopy. Signal-to-noiseratios are often improved by an order of magnitude. Most commercial FTIRinstruments are based on the Michelson interferometer and require a computerinterface to perform the Fourier transform and process data. Using an interfer-ometer provides an increase in resolution and results in accurate and reproduc-ible frequency determinations. This offers an advantage when using backgroundsubtraction techniques. Most Fourier transform instruments are now benchtopsize and relatively easy to maintain. These instruments are largely replacing tradi-tional dispersive instruments.

78 Infrared Spectroscopy Reagent ChemicalsNinth Edition


Sample HandlingSample containers and handling can present a challenge in the infrared region.Materials used to produce cuvettes are not transparent and cannot be used. Cellsprepared from alkali halides such as sodium chloride are widely used due to theirtransparent properties in the infrared region. A common problem with sodiumchloride cells is that they absorb moisture and become fogged. Polishing isrequired to restore the cells to a more transparent state.

Liquids may be analyzed in their neat form by placing a small amount ofsample on a sodium chloride plate and then placing a second plate on top to forma sample film. The plates are then placed in an appropriate holder in the samplecompartment of the instrument. This technique provides adequate spectra forqualitative use.

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 beemployed. Common moderate absorbances will not be observed. Solvent trans-mission should always be above 10% when using a solvent reference cell. Influ-ences of solvent on the absorbance of the solute should be considered. For exam-ple, hydrogen bonding of alcohols or amines with the solvent may affectcharacteristic vibrational frequency of the functional group. When practical, it isdesirable to analyze neat materials for qualitative analysis.

A method commonly used for analysis of neat solid samples is the mulltechnique. The technique consists of grinding the material into a fine powderand then dispersing it into a liquid or solid matrix to form a mull. Liquid mullshave been formed by combining the powdered analyte with Nujol (a heavyhydrocarbon oil). The liquid mull is analyzed between salt plates as describedabove. The disadvantage of Nujol is that hydrocarbon bands may interfere withanalyte absorbances. A second method of forming a mull involves grinding thepowdered analyte with dry potassium bromide and forming a disk. The ratio ofanalyte to potassium bromide is usually about 1:100. The materials are groundtogether using a mortar and pestle or a small ball mill. The mixture is thenpressed in a die at 10,00015,000 psi to form a small transparent disk and ana-lyzed. Care must be taken when preparing the disk to protect it from moisture.It is very common to see absorbances for moisture when using potassium bro-mide disks.

Another technique for handling samples is the use of disposable samplecards. These commercially available cards contain an IR-transparent material onwhich a neat liquid or solution can be placed for analysis. An analyte can be dis-solved in a volatile solvent, placed on the card, and the solvent evaporated to forma thin coating of sample, which can then be analyzed. This technique may be usedwhen a limited amount of analyte material is available.

Infrared Spectroscopy 79 Reagent ChemicalsNinth Edition


Qualitative AnalysisThe most widely used application of IR spectroscopy is for qualitative analysis oforganic compounds. Compounds have unique spectra that depend on molecularattributes. A common method of interpreting IR spectra is to consider tworegions: the functional group frequency region (36001200 cm1) and the fin-gerprint region (1200600 cm1). A combination of interpreting the functionalgroup region and comparing the fingerprint region with those in spectral librar-ies provides, in many cases, sufficient evidence to positively identify a compound.

The functional group region provides evidence of functional groups in amolecule based on the absorbance frequency. Tables are available in standardspectra manuals that provide ranges of absorbances for specific functionalgroups. Common groups with characteristic absorbances include aldehydes,ketones, esters, alkenes, alkynes, alcohols, amines, amides, carboxylic acids, nitrogroups, and nitriles. While most functional groups fall in the 36001200 cm1

range, some can also fall in the fingerprint region. For example, CO bonds canbe around 1000 cm1, and CCl absorbances are typically found in the range of600800 cm1.

The fingerprint region is often unique to the analyte. In this region, verysmall differences in structure can lead to differences in absorbances. Most singlebonds absorb in this area, and differences in skeletal structure of molecules willresult in frequency and intensity differences. The fingerprint region is most effec-tively used by comparison to existing spectra. Many commercial suppliers if IRinstrumentation also offer searchable electronic spectral libraries. Computer-based search systems offer a very rapid method of comparing unknown samplesto known spectra. The systems usually yield a list of possible compounds rankedin order of best fit of the unknown spectrum to the library spectrum.

ProcedureIR spectroscopy is used in selected standard-grade reference materials specifica-tions as a technique for identity confirmation. The criteria under which standard-grade reference materials pass identity tests are based on observation of character-istic absorbances and by comparison to a spectral library, if one is available. Eachentry to be analyzed by IR spectroscopy will contain a minimum of three absor-bances that must be present to pass the test. In addition, NIST should be used as asource to obtain comparison spectra. A standard-grade reference material willpass the comparison test if all absorbances found in the NIST spectrum arepresent in the test spectrum. The NIST spectral library contains many of the stan-dard-grade reference materials listed in this book, and spectral comparisonsshould be made if NIST spectra are available. NIST spectra are available by con-tacting NIST or by accessing NISTs Web site, http://webbook.nist.gov/chemistry/.

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