How to Read FTIR Spectra

This guide walks you through infrared spectrum interpretation in five concrete steps. Whether you are a student encountering your first IR printout or a researcher confirming a synthesis product, the process is the same: examine the overall shape, then work through the spectrum region by region. By the end you will know how to interpret an infrared spectrum confidently and extract functional group information from any FTIR dataset.

The 5-step process

Check baseline → Identify X-H peaks (4000–2500) → Find triple/double bonds (2500–1500) → Examine fingerprint region (1500–400) → Confirm with corroborating peaks.

What You Need Before Starting

A plotted FTIR spectrum — either in transmittance (%T) or absorbance format.
An FTIR reference table listing functional group absorption bands and their wavenumber ranges.
Knowledge of what sample was analyzed (organic compound, polymer, mixture, etc.) — context narrows the possibilities considerably.

Step 1: Check the Baseline and Overall Shape

Before reading individual peaks, look at the spectrum as a whole. A good baseline should be relatively flat in regions with no absorption — near 100% T in transmittance mode or near 0 in absorbance mode. A sloping or noisy baseline can indicate sample preparation issues (e.g., too thick a film, moisture contamination, or poor background subtraction).

Note which format your spectrum uses. Transmittance plots show absorption peaks pointing downward (dips), while absorbance plots show peaks pointing upward. Most FTIR reference tables, including the absorption bands table on this site, describe peak positions and intensities that apply to either format.

Step 2: Identify Major Peaks in the X-H Region (4000–2500 cm⁻¹)

The high-wavenumber region is dominated by stretching vibrations of bonds to hydrogen. These are often the most visually prominent features in a spectrum.

O-H stretch — 3200–3550 cm⁻¹. Appears as a broad, rounded absorption when hydrogen-bonded (alcohols, carboxylic acids). A free O-H (dilute solution) gives a sharp peak near 3600 cm⁻¹.
N-H stretch — 3300–3500 cm⁻¹. Sharper than O-H. Primary amines show two peaks (symmetric and asymmetric stretching), secondary amines show one.
C-H stretch — 2850–3000 cm⁻¹ for sp³ C-H (alkanes), just above 3000 cm⁻¹ for sp² C-H (alkenes, aromatics), and near 3300 cm⁻¹ for sp C-H (terminal alkynes).

The broad-versus-sharp distinction is critical. A broad absorption centered around 3300 cm⁻¹ almost certainly involves O-H or N-H hydrogen bonding, while sharp peaks in the same region point to N-H or free O-H.

Step 3: Look for Triple and Double Bond Absorptions (2500–1500 cm⁻¹)

This region contains some of the most diagnostic peaks in FTIR spectroscopy.

C≡C and C≡N — 2100–2260 cm⁻¹. These appear as sharp, medium-intensity peaks. The nitrile (C≡N) stretch near 2220 cm⁻¹ is particularly easy to spot because few other bonds absorb in this region.
C=O stretch — 1650–1800 cm⁻¹. The carbonyl peak is the single most diagnostic absorption in IR spectroscopy: strong, sharp, and hard to miss. Its exact position reveals the carbonyl type — ketones near 1715 cm⁻¹, esters near 1735–1750 cm⁻¹, carboxylic acids near 1710 cm⁻¹, amides near 1630–1680 cm⁻¹.
C=C stretch — 1600–1680 cm⁻¹ for alkenes, and 1450–1600 cm⁻¹ for aromatic ring vibrations. These are generally weaker than C=O absorptions.

Step 4: Examine the Fingerprint Region (1500–400 cm⁻¹)

The fingerprint region gets its name because the complex pattern of peaks here is unique to each compound, much like a human fingerprint. It contains C-O, C-N, and C-C single-bond stretches along with various bending modes that overlap and interact.

Unlike the functional group region above 1500 cm⁻¹, individual peaks in the fingerprint region are difficult to assign to specific bonds. Instead, this region is most useful for compound identification — comparing your spectrum against a reference library to confirm (or rule out) a specific substance. A match in the fingerprint region is strong evidence that two samples are the same compound.

That said, a few fingerprint-region peaks are diagnostic: strong C-O stretches near 1000–1260 cm⁻¹ (alcohols, ethers, esters) and out-of-plane C-H bends near 700–900 cm⁻¹ that reveal aromatic substitution patterns.

Step 5: Confirm with Corroborating Peaks

No single peak should be interpreted in isolation. Functional groups produce multiple absorptions, and checking for corroborating peaks dramatically reduces misidentification. Use the FTIR spectrum table to look up expected companion peaks for each assignment.

C=O + broad O-H (2500–3300 cm⁻¹) → carboxylic acid.
C=O + C-H at 2720 cm⁻¹ (Fermi resonance doublet) → aldehyde.
C=O + strong C-O near 1200 cm⁻¹ → ester.
Broad O-H + strong C-O (1000–1260 cm⁻¹) with no carbonyl → alcohol.

Common Mistakes in FTIR Interpretation

Assigning a peak from a single absorption. Always look for corroborating peaks. A lone peak near 1715 cm⁻¹ could be a ketone, aldehyde, carboxylic acid, or ester — companion absorptions tell you which.
Confusing moisture artifacts with real O-H peaks. Residual water vapor produces a distinctive spiky pattern near 1630 cm⁻¹ and 3400–3900 cm⁻¹. Compare against a background spectrum before concluding O-H is present.
Ignoring peak shape and width. A broad absorption and a sharp absorption at the same wavenumber mean different things. Peak shape carries as much information as peak position.
Over-interpreting the fingerprint region. Attempting to assign every peak below 1500 cm⁻¹ to a specific bond is rarely productive. Focus on the functional group region for structural assignments and use the fingerprint region for library matching.

Practice: Identify This Spectrum

Apply the five steps to a hypothetical spectrum with these features:

Strong, broad absorption centered at 3300 cm⁻¹ extending from 2500–3500 cm⁻¹
Strong, sharp peak at 1715 cm⁻¹
Strong absorption near 1200 cm⁻¹
C-H stretches at 2950 cm⁻¹ and 2870 cm⁻¹

Walkthrough

Step 1: The baseline is flat outside the absorption regions — no sample prep issues.

Step 2: The very broad absorption spanning 2500–3500 cm⁻¹ is characteristic of hydrogen-bonded O-H. This is not an alcohol O-H (which sits at 3200–3550 cm⁻¹) — the extension down to 2500 cm⁻¹ is typical of a carboxylic acid O-H, where strong hydrogen bonding broadens the peak enormously. The C-H peaks confirm an organic compound.

Step 3: The sharp peak at 1715 cm⁻¹ is a carbonyl C=O stretch. The position is consistent with a carboxylic acid (~1710 cm⁻¹).

Step 4: The strong absorption near 1200 cm⁻¹ is a C-O stretch, consistent with the C-O bond in a carboxylic acid group.

Step 5: Three corroborating features — very broad O-H, C=O at 1715 cm⁻¹, and C-O near 1200 cm⁻¹ — all point to a carboxylic acid. The aliphatic C-H stretches suggest a saturated (non-aromatic) acid. This spectrum is consistent with a simple compound like propanoic acid or butanoic acid. For a visual overview of all major peak regions, see the FTIR chart.

Frequently Asked Questions

What is the first step in reading an FTIR spectrum?

Check the baseline and overall shape of the spectrum before examining individual peaks. A flat baseline near 100% T (or 0 absorbance) in non-absorbing regions indicates good data quality. Note whether the spectrum is plotted in transmittance or absorbance mode, as peaks point in opposite directions.

How do I identify functional groups from FTIR peaks?

Match each observed peak position (in cm⁻¹) to known absorption ranges in an FTIR reference table. Confirm assignments by checking peak shape (broad vs. sharp), intensity (strong, medium, weak), and the presence of corroborating peaks from the same functional group.

What is the difference between absorbance and transmittance in FTIR?

Transmittance (%T) shows absorption peaks as downward dips — the more a sample absorbs, the lower the transmittance. Absorbance plots show peaks pointing upward and are directly proportional to sample concentration (per the Beer-Lambert law). Both formats contain the same information; transmittance is more common in routine analysis while absorbance is preferred for quantitative work.