By Owais AliReviewed by Lexie CornerMar 21 2025Fourier Transform Infrared (FTIR) Spectroscopy is a widely used analytical technique for identifying chemical compounds and examining molecular structures. It measures how a sample absorbs infrared radiation, producing a spectrum that reflects its molecular composition.
Each spectrum acts as a unique chemical fingerprint, allowing for the accurate identification of compounds and the detection of structural or compositional changes.1
This article provides a comprehensive guide to interpreting FTIR spectra, helping users understand their results and apply them effectively in chemical analysis.
Image Credit: S. Singha/Shutterstock.com
Understanding the FTIR Spectrum
An FTIR spectrum graphically represents how a sample absorbs infrared light. The x-axis shows wavenumbers (cm⁻¹), which correspond to energy levels, while the y-axis displays absorbance or transmittance. Peaks in the spectrum reflect specific molecular vibrations.
X-axis: Wavenumber, cm⁻¹
The x-axis measures the frequency of absorbed infrared light in wavenumbers (cm⁻¹), which indicates the energy of molecular vibrations. In mid-infrared spectroscopy, the typical range spans from 4000 to 400 cm⁻¹. Higher wavenumbers correspond to higher energy vibrations, such as O–H and C–H stretching.
These regions are useful for identifying specific functional groups based on their characteristic absorption bands.
Y-axis: Transmittance or Absorbance
The y-axis indicates how much infrared radiation is absorbed or transmitted by the sample.
In an absorbance spectrum, peaks represent frequencies where particular bonds absorb infrared light. In a transmittance spectrum, these same absorption events appear as downward valleys.
Both formats provide insight into the molecular composition of the sample and help identify the types and relative quantities of functional groups present.
Peaks (Absorbance Bands)
Each peak, or absorbance band, corresponds to the vibration of a specific atomic group within the molecule. These vibrations occur at characteristic frequencies based on bond type, bond strength, and surrounding chemical environment. For example, carbonyl (C=O) stretching typically appears as a sharp, intense peak near 1700 cm⁻¹.
The position, intensity, and shape of peaks offer critical information for identifying compounds and functional groups.2
Example of an FTIR spectrum comparing two samples, with overlaid absorbance spectra shown in green and red. The x-axis represents wavenumber (cm⁻¹), while the y-axis indicates absorbance. Differences between the spectra highlight variations in molecular composition or structural changes. Peaks correspond to characteristic molecular vibrations, aiding in functional group identification. Image Credit: M. Schauer/Shutterstock.com
How to Interpret Peaks
There are five steps to interpret the FTIR spectrum:
Step 1: Determining the Number of Absorption Bands
The first step in FTIR interpretation is analyzing the number of absorption bands in the spectrum. A simple spectrum with fewer than five peaks typically corresponds to small organic molecules, inorganic compounds, or simple salts.
In contrast, a complex spectrum with more than five peaks suggests the presence of multiple functional groups, indicating a structurally diverse or high-molecular-weight compound.
Step 2: Identifying Key Regions and Functional Groups
FTIR spectroscopy divides molecular vibrations into specific regions within the infrared spectrum, each associated with characteristic bond vibrations. This allows for the identification of functional groups based on their absorption within defined wavenumber ranges.
These regions help detect functional groups, such as hydroxyl, carbonyl, amine, and alkene, which are important for structural and chemical analysis.3
The four key regions are:
- Single-Bond Region (4000-2500 cm⁻¹)
This region includes stretching vibrations of O-H (found in alcohols and acids), N-H (present in amines), and C-H (common in alkanes, alkenes, and aromatic compounds).
O-H and N-H stretching typically appear between 3300-3600 cm⁻¹. O-H bands are usually broad, while N-H and C-H bands are sharper. C-H stretching appears around 3000 cm⁻¹ and can help distinguish between aliphatic and aromatic compounds.
- Triple-Bond Region (2500-2000 cm⁻¹)
Absorptions in this range correspond to triple-bond stretching, specifically C≡C (alkynes) and C≡N (nitriles). The C≡C peak is often weak due to low polarity, whereas C≡N shows a stronger absorption.
Terminal alkynes, such as ethyne (acetylene) and 1-butyne, also display an additional sharp peak for the ≡C-H stretch near 3300 cm⁻¹.
- Double-Bond Region (2000-1500 cm⁻¹)
This region is dominated by C=O (carbonyl) and C=C (alkene and aromatic) stretching vibrations. Carbonyl groups in ketones, aldehydes, esters, and acids exhibit strong absorptions around 1680-1750 cm⁻¹, while C=C stretching in alkenes appears between 1600-1680 cm⁻¹.
Aromatic compounds, such as benzene and toluene, produce multiple peaks due to conjugation effects, aiding in structural identification.
- Fingerprint Region (1500-500 cm⁻¹)
This region contains complex absorption patterns unique to individual compounds. It includes vibrations from bonds such as C-C, C-O, C-N, and C-X (halogens). Due to overlapping peaks, reference spectra are often needed to interpret this region accurately.1,4
Step 3: Analyzing Peak Shape and Intensity
The shape and intensity of absorption peaks can reveal important information about molecular interactions and bond characteristics.
- Broad peaks between 3650 and 3250 cm⁻¹ are typically associated with hydrogen bonding. These broad absorptions are common in hydroxyl (–OH) and amine (–NH) groups, where extensive intermolecular interactions lead to a wider peak shape.
- Sharp peaks, such as the C≡N stretch near 2200 cm⁻¹, are characteristic of isolated or weakly interacting polar bonds, where minimal intermolecular forces maintain a distinct absorption pattern.
- Strong peaks in the carbonyl region (1850–1650 cm⁻¹) suggest the presence of highly polar bonds. Functional groups like ketones, aldehydes, and esters fall into this range and produce strong absorptions due to the high dipole moment of the carbonyl bond.
- Weaker or moderate peaks in the fingerprint region (1500–500 cm⁻¹) are typically related to bending vibrations or less polar functional groups. Examples include C–H bending in alkanes or C–O stretching in ethers, which produce subtler spectral features.
Step 4: Comparing with Reference Spectra
Accurate interpretation of an FTIR spectrum often requires comparison with reference spectra from established databases. This helps confirm the presence of specific functional groups and ensures correct compound identification.
The fingerprint region (1500–500 cm⁻¹) is especially important for this step, as it contains complex but highly specific absorption patterns unique to individual compounds. Matching these patterns with reference data can distinguish between structurally similar molecules.
For example, simple aromatic compounds exhibit characteristic absorption bands at 1600 cm⁻¹ and 1500 cm⁻¹, along with additional weaker bands between 2000 and 1700 cm⁻¹. These features, when cross-referenced with known spectra, support confident identification.
Step 5: Database and Software Utilization
FTIR analysis is often complemented with additional techniques such as NMR spectroscopy or mass spectrometry, and software tools are used to match spectra and identify molecular structures.
These complementary methods provide a more comprehensive molecular characterization, ensuring precise identification of compounds, especially in complex mixtures.
This integrated approach minimizes misinterpretation and enhances the reliability of spectral analysis by cross-verifying functional group assignments and molecular structures.1,5
Common Mistakes and Troubleshooting
Sample Preparation Errors
Accurate FTIR results depend on proper sample preparation. Contamination from residues, environmental pollutants, or improper handling can introduce extraneous absorption bands.
Variations in sample thickness during transmission measurements may distort the baseline and affect peak intensities, highlighting the need for consistent sample preparation and uniform pressure during analysis.
Instrumental Factors
Instrumental issues, such as optical contamination or detector saturation, can reduce signal quality and distort peak shapes. Regular maintenance of optical components—like mirrors, beam splitters, and detectors—is essential to ensure reliable performance.
Over time, fluctuations in light source intensity can also lead to spectral drift, making routine calibration and the use of neutral density filters or appropriate gain settings important for maintaining accuracy.
Environmental Interferences
Water vapor and carbon dioxide in the atmosphere can introduce absorption bands near 3400 cm⁻¹ and 2300 cm⁻¹, potentially overlapping with sample peaks. Purging the instrument with dry air or nitrogen can help minimize these effects.
Temperature changes, humidity, and electromagnetic interference from nearby equipment can also influence results, so stable environmental conditions and proper shielding are recommended.
Data Processing Pitfalls
Errors during data processing—such as incorrect baseline correction—can create artificial features or obscure genuine peaks.
Choosing the wrong spectral resolution or applying inappropriate mathematical transformations (for example., using absorbance instead of Kubelka-Munk units in diffuse reflectance mode) can lead to peak overlap and reduced precision in analysis.
Misinterpretation of Spectral Features
Incorrectly assigning peaks or overlooking key spectral regions can lead to errors in compound identification. The fingerprint region (400–1500 cm⁻¹), in particular, contains unique patterns critical for differentiating similar compounds.
Because FTIR spectra represent a continuous function of absorbed energy, overlapping peaks, baseline noise, and artifacts must be carefully considered during interpretation. Accurate analysis depends on evaluating the full spectral context, comparing it with reference data, and accounting for experimental conditions.6
For a detailed walkthrough of Fourier Transform Infrared (FTIR) Spectroscopy, watch:
Back to Basics: Fourier Transform Infrared Spectroscopy
To learn more about the applications of FTIR, visit:
References and Further Reading
- Nandiyanto, ABD., Oktiani, R., Ragadhita, R. (2019). How to read and interpret FTIR spectroscope of organic material. Indonesian Journal of Science and Technology. http://dx.doi.org/10.17509/ijost.v4i1.15806
- Mathias, J. (2018). A Beginner’s Guide to FTIR Analysis: Interpreting & Analyzing Results. [Online] Innovatechlabs. Available at: https://www.innovatechlabs.com/newsroom/1882/interpreting-analyzing-ftir-results/
- RTI Laboratories. (2025). FTIR Analysis. [Online] RTI Laboratories. Available at: https://rtilab.com/techniques/ftir-analysis/
- LibreTexts. (nd). Interpreting Infrared Spectra. [Online]. LibreTexts. Available at: https://chem.libretexts.org/Bookshelves/Organic_Chemistry/Organic_Chemistry_(OpenStax)/12%3A_Structure_Determination_-_Mass_Spectrometry_and_Infrared_Spectroscopy/12.07%3A_Interpreting_Infrared_Spectra
- Nandiyanto, ABD., Ragadhita, R., Fiandini, M. (2023). Interpretation of Fourier transform infrared spectra (FTIR): A practical approach in the polymer/plastic thermal decomposition. Indonesian Journal of Science and Technology. https://doi.org/10.17509/ijost.v8i1.53297
- Miseo, EV., Briggs, J. (2021). Common Problems with FT-IR Instruments and How to Avoid Them. [Online] Spectroscopy Online. Available at: https://www.spectroscopyonline.com/view/common-problems-with-ft-ir-instruments-and-how-to-avoid-them
Disclaimer: The views expressed here are those of the author expressed in their private capacity and do not necessarily represent the views of AZoM.com Limited T/A AZoNetwork the owner and operator of this website. This disclaimer forms part of the Terms and conditions of use of this website.