Infrared Spectroscopy Identification
Also known as: IR spectroscopy, FTIR, infrared spectroscopy
Infrared (IR) spectroscopy measures the absorption of infrared radiation by chemical bonds, creating a spectrum unique to each compound. Discovered by William Herschel in 1800 and developed into a practical analytical tool in the mid-20th century, IR spectroscopy is indispensable for rapidly identifying functional groups and confirming compound structure in organic and inorganic chemistry.
Key highlights
- Fast and simple: results obtained in minutes with small sample quantities
- Non-destructive to the sample (for most sample preparations)
- Functional group identification is direct and unambiguous for common groups
- Inexpensive to operate once the instrument is acquired
- Modern FTIR instruments offer excellent sensitivity and spectral quality
Intuition
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How it works
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When to use it
IR spectroscopy is used to quickly identify functional groups in newly synthesized compounds, verify the structure of reaction products, detect impurities, and confirm compound identity. It is ideal when sample quantity is limited, rapid turnaround is needed, and the presence or absence of specific functional groups is the primary question. IR spectroscopy is less suitable as the sole technique for determining complete molecular structure, especially for isomers with the same functional groups; complementary methods like NMR or GC-MS are often required.
Strengths & limitations
- Fast and simple: results obtained in minutes with small sample quantities
- Non-destructive to the sample (for most sample preparations)
- Functional group identification is direct and unambiguous for common groups
- Inexpensive to operate once the instrument is acquired
- Modern FTIR instruments offer excellent sensitivity and spectral quality
- Not suitable for determining exact molecular structure; provides functional group information only
- Overlapping peak regions can make interpretation ambiguous for complex molecules
- Band intensities depend on sample concentration and path length, complicating quantification
- Some functional groups produce weak or overlapping absorptions, requiring expert interpretation
Common pitfalls
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Applications
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Frequently asked
What does wavenumber mean and why is it used instead of wavelength?
Wavenumber is the reciprocal of wavelength (cm⁻¹). It is used in IR spectroscopy because it is directly proportional to energy and vibrational frequency, making interpretation and comparison easier. The fingerprint region (below 1500 cm⁻¹) contains complex, compound-specific patterns useful for identification.
How do I prepare a sample for IR spectroscopy?
Common methods include making a thin film (for liquids), dissolving in a non-absorbing solvent such as CCl₄ (for solution IR), preparing a KBr pellet (for solids), or using Attenuated Total Reflection (ATR), which requires only a small sample on an optical crystal.
Why are some peaks sharp and others broad?
Sharp peaks typically arise from stretching vibrations (e.g., C=O, N–H). Broad peaks, especially in the 2500–3300 cm⁻¹ region, often indicate hydrogen bonding (as in carboxylic acids). Peak width also depends on sample purity and the specific functional group.
Can IR spectroscopy distinguish between isomers?
Only if the isomers have different functional groups. For constitutional isomers differing only in skeletal structure (same functional groups), IR spectra are very similar; NMR or mass spectrometry would be needed for definitive distinction.
Sources
- 1.Pavia, D. L., Lampman, G. M., Kriz, G. S., & Engel, R. G. (2014). A Small-Scale Approach to Organic Laboratory Techniques (4th ed.). Cengage Learning.ISBN 978-1285749297
- 2.Smith, B. C. (2018). Fundamentals of Fourier Transform Infrared Spectroscopy (3rd ed.). CRC Press.ISBN 978-1498761581
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ScholarGate. (2026, June 3). Infrared Spectroscopy Identification. ScholarGate. https://scholargate.app/chemistry/infrared-spectroscopy-id