FTIR spectra analysis is a powerful and widely used technique in material science, chemistry, and engineering for identifying functional groups, verifying molecular structures, and studying chemical reactions. Whether analyzing polymers, catalysts, films, or composites, a systematic approach to FTIR spectra analysis ensures accuracy and efficiency. This guide breaks down the entire process—from pre-test preparation to detailed peak interpretation—helping both beginners and practitioners navigate the complexities of infrared spectroscopy.
1. Critical Pre-Analysis Preparation
Successful FTIR spectra analysis starts with thorough preparation, which lays the foundation for reliable results.
Clarify Sample Background and Test Conditions
- Sample type: Determine if the sample is organic, inorganic, or composite (e.g., polymers, catalysts, thin films). Note its purity, potential impurities, or solvent residues, as well as preparation processes (e.g., heat treatment, pre- vs. post-reaction samples) that may affect spectral features.
- Test mode: Choose the appropriate mode—Transmission IR, Attenuated Total Reflection (ATR-IR), or Diffuse Reflectance IR. ATR-IR provides surface information (1–10 nm depth) with sharper peaks, while Transmission IR peak intensity depends on sample thickness. Diffuse Reflectance IR is ideal for powder samples.
- Sample state: Solid (powder/film), liquid, or gas—each state influences peak shape. For example, liquid samples often exhibit broader hydrogen-bonded peaks.
Background Subtraction, Baseline Correction, and Data Smoothing
Raw FTIR spectra require processing to eliminate interference and enhance feature clarity:
- Background subtraction: Subtract blank backgrounds (e.g., empty KBr pellets, pure solvents, or clean ATR crystals) to remove signals from air (e.g., CO₂ doublet at 2340 cm⁻¹; H₂O broad peak at 3400 cm⁻¹ + bending peak at 1640 cm⁻¹) or sample carriers. KBr, a common carrier, is infrared-inactive but may show a water peak at 3400 cm⁻¹ if moisture-absorbed.
- Baseline correction: Adjust for baseline tilt caused by uneven sample thickness, ensuring a flat baseline for accurate peak intensity calculation and preventing weak peaks from being obscured.
- Data smoothing: Apply mild smoothing to reduce noise (e.g., high-frequency fluctuations). Avoid over-smoothing, as it can distort peak shapes (e.g., narrowing sharp peaks or eliminating small peaks).
2. FTIR Spectral Regions: Functional Group Mapping
FTIR spectra are divided into distinct wavenumber regions, each corresponding to specific vibration types. Focus on high-priority regions first to quickly identify major functional groups, then use low-priority regions for verification.
| Wavenumber Range (cm⁻¹) | Region Name | Core Vibration Types | Key Functional Groups to Identify |
| 4000~2500 | Hydrogen-Containing Groups | O-H, N-H, C-H stretching | Alcohol/phenol/carboxylic acid O-H; amine N-H; saturated/unsaturated C-H |
| 2500~1900 | Triple Bonds/Cumulated Double Bonds | C≡C, C≡N, C=C=C stretching | Alkynes, nitriles, cumulated alkenes |
| 1900~1600 | Carbonyl Region | C=O stretching | Ketone/aldehyde/ester/carboxylic acid/amide C=O |
| 1600~1300 | Double Bonds/Skeleton Vibration | C=C, benzene ring skeleton, C-N stretching | Alkenes, benzene rings, amine/amide C-N |
| 1300~600 | Fingerprint Region | Single bond stretching + bending vibrations | C-O, C-X (halogens), benzene ring substitution, C-H bending |
| 600~100 | Far-Infrared Region | Metal-nonmetal bond stretching | M-O (e.g., Mo-O, Zn-O), M-S |
3. Step-by-Step FTIR Spectra Analysis Process
Follow this systematic workflow to interpret FTIR spectra accurately:
Step 1: Eliminate Interference Peaks
First, rule out peaks caused by sample residues, test environment, or carriers to avoid misidentifying functional groups:
○ Water peaks:
A broad peak at 3400±200 cm⁻¹ (O-H stretching) and a weak peak at 1640 cm⁻¹ (O-H bending) indicate moisture. Vacuum-dry the sample and retest, or weaken the peak via background subtraction.
○ CO₂ peaks:
A doublet at 2340 cm⁻¹ and 2320 cm⁻¹ (C=O stretching) comes from air. Purge the sample chamber with nitrogen during testing or subtract the air background.
○ Solvent residue peaks:
For example, ethanol residue shows O-H at 3300 cm⁻¹, C-H at 2950 cm⁻¹, and C-O at 1050 cm⁻¹. Thoroughly clean and dry the sample before testing.
- Carrier peaks: Moist KBr pellets show water peaks, while contaminated ATR crystals produce irregular miscellaneous peaks. Replace the carrier or clean the crystal and retest.
Step 2: Analyze the Hydrogen-Containing Groups Region (4000~2500 cm⁻¹)
Distinguish O-H/N-H from C-H groups using peak position, shape, and intensity:
- O-H/N-H identification:Broad, strong peaks at 3200~3600 cm⁻¹ indicate associated O-H (alcohols/phenols) with hydrogen bonding (e.g., ethanol at 3300 cm⁻¹, phenol at 3250 cm⁻¹).
- Sharp, medium-strong peaks at 3600~3650 cm⁻¹ correspond to free O-H (dilute phenol solutions) or primary amine N-H (doublet at 3300 cm⁻¹ and 3400 cm⁻¹).
- A single sharp, weak peak at 3350±50 cm⁻¹ signals secondary amine N-H. Tertiary amines show no peaks here (no N-H bonds).
- An extremely broad, strong peak spanning 2500~3300 cm⁻¹ indicates carboxylic acid O-H (dimeric hydrogen bonding), confirmed by a carbonyl peak at 1680~1720 cm⁻¹ (e.g., acetic acid).
- C-H identification (use 3000 cm⁻¹ as the dividing line):Strong doublets below 3000 cm⁻¹ correspond to saturated alkyl C-H (sp³ hybridization): methyl (asymmetric at 2960 cm⁻¹, symmetric at 2870 cm⁻¹) and methylene (asymmetric at 2920 cm⁻¹, symmetric at 2850 cm⁻¹).
- Weak, sharp peaks above 3000 cm⁻¹ indicate unsaturated C-H (sp²/sp hybridization): alkenyl/aromatic =C-H at 3000~3100 cm⁻¹ (e.g., ethylene at 3080 cm⁻¹, benzene at 3030 cm⁻¹) or alkynyl ≡C-H (single sharp peak at 3300 cm⁻¹, e.g., acetylene).
- No C-H peaks in 2500~3600 cm⁻¹ suggest inorganic oxides/salts (e.g., MoO₂, ZnO).
Step 3: Analyze the Carbonyl Region (1900~1600 cm⁻¹)
Carbonyl (C=O) stretching peaks are among the strongest and most stable in FTIR spectra, enabling unambiguous identification of carbonyl compounds:
| Carbonyl Type | Peak Position (cm⁻¹) | Peak Shape Characteristics | Supporting Verification Peaks |
| Acyl Chloride C=O | 1780~1810 | Strong doublet | No O-H/N-H peaks; no C-O peaks in fingerprint region |
| Anhydride C=O | 1750~1860 | Strong doublet (60 cm⁻¹ spacing) | Strong C-O-C peaks at 1050~1300 cm⁻¹ (fingerprint region) |
| Ester C=O | 1735~1750 | Strong, sharp singlet | C-O-C doublet at 1050~1300 cm⁻¹ (fingerprint region) |
| Ketone C=O | 1700~1725 | Strong, sharp singlet | No O-H/N-H peaks; no C-O peaks in fingerprint region |
| Aldehyde C=O | 1720~1740 | Strong, sharp singlet | Aldehyde C-H doublet at 2720 cm⁻¹ and 2820 cm⁻¹ |
| Carboxylic Acid C=O | 1680~1720 | Strong, slightly broad singlet | Extremely broad O-H peak at 2500~3300 cm⁻¹ |
| Amide C=O | 1630~1690 | Strong, broad singlet | N-H peaks at 3300~3500 cm⁻¹ (doublet for primary amides, singlet for secondary) |
If no strong peak appears in 1900~1600 cm⁻¹, the sample contains no carbonyl groups—skip carbonyl compound speculation.
Step 4: Analyze the Double Bonds/Skeleton Vibration Region (1600~1300 cm⁻¹)
Focus on identifying double bonds, benzene rings, and amine/amide structures:
- Benzene rings: Four characteristic skeleton vibration peaks at 1450 cm⁻¹, 1500 cm⁻¹, 1580 cm⁻¹, and 1600 cm⁻¹. The strongest peaks (1500 cm⁻¹ and 1600 cm⁻¹) confirm benzene rings, with further validation from aromatic =C-H peaks at 3000~3100 cm⁻¹.
- Alkene C=C: Weak, sharp peaks at 1620~1680 cm⁻¹ (sp² hybridization). Conjugated alkenes (e.g., styrene) show lower peak positions (1600~1650 cm⁻¹) and stronger intensity; isolated alkenes (e.g., ethylene) have higher positions (1650~1680 cm⁻¹) and weaker intensity.
- Amine/Amide C-N: Medium-strong broad peaks at 1030~1230 cm⁻¹ (C-N stretching). Combine with N-H peaks at 3300~3500 cm⁻¹ to confirm: amide C-N peaks (1200~1300 cm⁻¹) vs. amine C-N peaks (1030~1150 cm⁻¹).
Step 5: Analyze the Fingerprint Region (1300~600 cm⁻¹)
The fingerprint region has dense, unique peaks—ideal for verifying structural details and identifying substitution patterns:
- Single bond stretching:Strong peaks at 1000~1300 cm⁻¹ indicate C-O bonds (alcohols, ethers, esters, carboxylic acids): alcohols (1050~1200 cm⁻¹, e.g., ethanol at 1050 cm⁻¹), ethers (1000~1100 cm⁻¹, e.g., diethyl ether at 1050 cm⁻¹), and esters (1050~1300 cm⁻¹ doublet, confirmed with C=O peaks).
- Peaks at 700~800 cm⁻¹ correspond to C-X bonds (halogens): C-Cl (750~800 cm⁻¹), C-Br (650~700 cm⁻¹).
- C-H bending:Alkene =C-H bending peaks at 650~1000 cm⁻¹: cis-alkenes (strong peak at 650~700 cm⁻¹), trans-alkenes (strong peak at ~950 cm⁻¹).
- Benzene ring substitution patterns (650~900 cm⁻¹): monosubstituted (doublet at 700/750 cm⁻¹), ortho-disubstituted (single peak at 750 cm⁻¹), meta-disubstituted (doublet at 700/780 cm⁻¹), para-disubstituted (single peak at 800~850 cm⁻¹).
- A single peak at 1370 cm⁻¹ (no isopropyl); a doublet (1370/1385 cm⁻¹) indicates isopropyl (1:1 intensity ratio) or tert-butyl (1:2 intensity ratio).