How to Read and Interpret FTIR Spectra

A systematic guide to reading and interpreting FTIR spectra, from understanding the axes to identifying functional groups and confirming assignments.

Reading an FTIR Spectrum

An FTIR spectrum plots the wavenumber (in cm⁻¹) on the x-axis against either transmittance (%) or absorbance on the y-axis. The x-axis runs from 4000 cm⁻¹ on the left to 400 cm⁻¹ on the right (high to low wavenumber).

In transmittance mode (the most common display), absorption peaks point downward — a dip in the baseline indicates that the sample absorbs light at that wavenumber. In absorbance mode, peaks point upward.

Why wavenumber instead of wavelength?

Wavenumber (ν̃ = 1/λ) is directly proportional to frequency and energy. This means evenly spaced points on the x-axis correspond to evenly spaced energies — making spectral interpretation more intuitive.

Spectra measured by ATR-FTIR show the same band positions as transmission spectra (strong bands can sit a few cm⁻¹ lower), but bands at low wavenumber appear relatively stronger. Keep this in mind when comparing an ATR spectrum with a transmission reference.

Systematic Interpretation Workflow

Follow these six steps to systematically interpret any FTIR spectrum. Steps 2 to 5 work through the four regions shown below.

  1. X–H stretches 4000–2500 cm⁻¹
    O-H, N-H, C-H
  2. Triple and cumulated bonds 2500–2000 cm⁻¹
    C≡N, C≡C, N=C=O, atmospheric CO₂
  3. Double bonds 2000–1500 cm⁻¹
    C=O, C=C, C=N, aromatic ring
  4. Fingerprint region 1500–400 cm⁻¹
    C-O, C-N, C-C stretches; bending modes
Regions of the mid-infrared spectrum, drawn to scale. Boundaries are approximate; some bands fall outside their usual region.

Step 1: Broad picture

Full spectrum

Start by looking at the overall spectrum. How many peaks are there? Is the spectrum clean with well-resolved peaks, or complex with many overlapping features? A simple spectrum with few peaks often indicates a pure, simple compound.

Step 2: X-H stretches

4000–2500 cm⁻¹

This region contains O-H, N-H, and C-H stretching vibrations. Broad absorption around 3300 cm⁻¹ suggests O-H or N-H. Sharp peaks near 2960 and 2870 cm⁻¹ indicate sp³ C-H stretches (CH₃; CH₂ appears near 2925 and 2855 cm⁻¹). Bands just above 3000 cm⁻¹ come from alkene or aromatic C-H.

Step 3: Triple bonds

2500–2000 cm⁻¹

Look for C≡C (2100–2260 cm⁻¹; weak, and often absent in symmetric internal alkynes) or C≡N (2210–2260 cm⁻¹; sharp, medium intensity). A CO₂ artifact may appear as a doublet near 2350 cm⁻¹ — this is not from the sample.

Step 4: Double bonds

2000–1500 cm⁻¹

The carbonyl (C=O) region (about 1650–1850 cm⁻¹) is the strongest and most diagnostic peak in this region. C=C stretches appear near 1650 cm⁻¹. Aromatic C=C gives multiple bands around 1600 and 1500 cm⁻¹.

Step 5: Fingerprint region

1500–400 cm⁻¹

This complex region contains C-O, C-N, C-C stretches and many bending modes. While individual assignments are difficult, the overall pattern is unique to each compound — like a molecular fingerprint for confirmation.

Step 6: Consistency check

Full spectrum

Consider whether all observed peaks are consistent with a single compound. Do corroborating peaks support your assignments? For example, if you assigned a carbonyl peak to an ester, do you also see C-O stretches near 1250 and 1050 cm⁻¹?

Functional Groups Reference

Search by functional group name, category, or enter a wavenumber to find which groups absorb at that position. Click any row to expand its description. For quick lookups, you can also use our FTIR peak identifier tool.

Showing all 132 entries

Showing 30 of 132 results.

Annotated Example Spectrum

Polystyrene is the classic test material for checking an FTIR spectrometer's wavenumber scale, and its spectrum shows how the workflow above plays out: aromatic C-H stretches just above 3000 cm⁻¹, aliphatic CH₂ stretches just below it, ring stretches near 1600, 1490 and 1450 cm⁻¹, and strong out-of-plane bends at 756 and 698 cm⁻¹ typical of a monosubstituted benzene ring.

The curve below is the absorption index k (the imaginary part of the refractive index) of polystyrene, derived from FTIR measurements at the Pacific Northwest National Laboratory, rather than a raw absorbance spectrum. Its band maxima sit at the same positions as in an absorbance spectrum. The relative heights differ: the absorption coefficient is proportional to k multiplied by the wavenumber, so the C-H stretches near 3000 cm⁻¹ look weaker here than they would in an absorbance spectrum of a film.

Polystyrene

50010001500200025003000350000.10.20.30.4Wavenumber (cm⁻¹)Absorption index (k)
  1. 3026 cm⁻¹ — Aromatic C-H stretch
  2. 2924 cm⁻¹ — CH₂ stretch
  3. 1602 cm⁻¹ — Ring C=C stretch
  4. 1494 cm⁻¹ — Ring C=C stretch
  5. 1452 cm⁻¹ — CH₂ bend / ring
  6. 756 cm⁻¹ — C-H out-of-plane bend
  7. 698 cm⁻¹ — Ring out-of-plane bend

Scroll to explore full spectrum →

Frequently Asked Questions

What is the fingerprint region in FTIR?

The fingerprint region spans 1500–400 cm⁻¹. It contains complex overlapping peaks from C-O, C-N, C-C stretches and various bending modes. While individual peak assignments are difficult here, the overall pattern is unique to each compound — making it invaluable for confirming identity by comparison to reference spectra.

How do you identify functional groups from an FTIR spectrum?

Start with the diagnostic region (4000–1500 cm⁻¹) where key functional groups absorb: O-H and N-H stretches (3200–3600 cm⁻¹), C-H stretches (2850–3000 cm⁻¹ for sp³ C-H; 3000–3100 cm⁻¹ for alkene and aromatic C-H; about 3300 cm⁻¹ for terminal alkyne C-H), triple bonds (2000–2500 cm⁻¹), and C=O stretches (about 1650–1850 cm⁻¹). Use peak position, intensity, and shape together, then check for corroborating peaks in the fingerprint region.

What does a broad peak mean in FTIR?

A broad peak typically indicates hydrogen bonding. The most common examples are the O-H stretch of alcohols, a broad band centered around 3200–3400 cm⁻¹, and the O-H stretch of carboxylic acids, an even broader band spanning roughly 3300–2500 cm⁻¹ that overlaps the C-H stretches. Hydrogen bonding creates a distribution of bond strengths, spreading the absorption over a wide wavenumber range.

References

  1. Silverstein, R. M.; Webster, F. X.; Kiemle, D. J.; Bryce, D. L. Spectrometric Identification of Organic Compounds, 8th ed. Wiley, 2014.
  2. Socrates, G. Infrared and Raman Characteristic Group Frequencies: Tables and Charts, 3rd ed. Wiley, 2001.
  3. Griffiths, P. R.; de Haseth, J. A. Fourier Transform Infrared Spectrometry, 2nd ed. Wiley, 2007.