Fourier Transform Infrared (FTIR) Spectrometer – Overview & Application in Lubricant Analysis
A Fourier Transform Infrared (FTIR) Spectrometer is used for chemical composition analysis of lubricating oils, greases, fuels, and petroleum products. It identifies contaminants, degradation products, and additive depletion by analyzing how molecules absorb infrared (IR) light.
1. FTIR Applications in Lubricant & Oil Analysis
✅ Oxidation & Degradation Monitoring – Detects oil oxidation, nitration, and thermal breakdown.
✅ Additive Depletion Analysis – Identifies loss of anti-wear, antioxidant, or detergent additives.
✅ Contaminant Detection – Identifies water, fuel, glycol, or soot contamination.
✅ Base Oil Differentiation – Differentiates PAO, Group I-IV base oils, and mineral oils.
✅ Wear & Corrosion Products – Detects sulfates, phosphates, and organic acids.
2. FTIR Spectroscopy Test Method
Applicable Standards:
- ASTM E1252 – Standard Practice for FTIR Spectroscopy
- ASTM D7414 – FTIR Method for Lubricants
- ASTM D7844 – Monitoring Lubricant Degradation
3. FTIR Spectrometer Setup & Test Procedure
Step 1: Sample Preparation
1️⃣ Choose the Sample Type:
- Liquid Samples (Oils, Fuels): Use a liquid transmission cell.
- Greases & Solids: Use an ATR (Attenuated Total Reflectance) accessory.
2️⃣ Prepare the Sample Cell:
- Liquid: Use a KBr or ZnSe cell (0.1mm–1mm path length).
- Grease/Solid: Directly apply on ATR crystal.
Step 2: FTIR Measurement
3️⃣ Set FTIR Spectrometer Parameters
- Wavenumber Range: 4000 cm⁻¹ to 400 cm⁻¹
- Resolution: 4 cm⁻¹
- Number of Scans: 16–64
4️⃣ Run Background Scan (for reference correction).
5️⃣ Place Sample in FTIR Cell & Start Scanning.
Step 3: Spectral Analysis & Interpretation
| Peak Position (cm⁻¹) | Functional Group | Lubricant Significance |
|---|---|---|
| 3650–3200 | O-H (Water, Glycol) | Water contamination detection |
| 2950–2850 | C-H (Hydrocarbons) | Base oil composition |
| 1740–1700 | C=O (Oxidation Products) | Oil oxidation level |
| 1650–1620 | C=C (Unsaturated Compounds) | Nitration or degradation |
| 1170–900 | P=O (Phosphates, ZDDP Additives) | Anti-wear additive analysis |
| 1600–1450 | Sulfates, Aromatics | Fuel contamination, degradation |
4. Example Test Results & Interpretation
| Lubricant Condition | FTIR Peak Observations | Conclusion |
|---|---|---|
| Good Oil | Normal hydrocarbon peaks, no oxidation peaks | ✅ Oil in good condition |
| Oxidized Oil | Strong peak at 1740 cm⁻¹ (C=O) | ❌ Oxidation detected |
| Fuel Contamination | Aromatic peaks at 1600–1450 cm⁻¹ | ⚠️ Possible fuel dilution |
| Water Contamination | O-H stretching at 3400 cm⁻¹ | ❌ Water detected |
| Additive Depletion | Loss of 1170 cm⁻¹ (ZDDP peak) | ⚠️ Anti-wear additive loss |
5. FTIR Advantages in Lubricant Analysis
✔ Non-Destructive – No need for sample preparation (with ATR).
✔ Fast Analysis – Results in minutes.
✔ Small Sample Volume – Requires only 1–2 drops.
✔ High Sensitivity – Detects chemical changes in ppm levels.
Example FTIR Spectra for Used Oil Analysis
Below are FTIR spectra examples for different lubricant conditions, including new oil, oxidized oil, contaminated oil, and additive depletion.
1. FTIR Spectrum of Fresh (New) Oil
- Key Peaks:
- C-H Stretching (2950–2850 cm⁻¹): Base oil hydrocarbons
- P=O (1170–900 cm⁻¹): Anti-wear additives (ZDDP)
- No oxidation (C=O at 1740 cm⁻¹)
- No water contamination (O-H at 3400 cm⁻¹)
📈 Spectra Characteristics: Smooth baseline, distinct additive peaks, no major degradation signs.
2. FTIR Spectrum of Oxidized Oil
- Key Changes Compared to Fresh Oil:
- Strong peak at 1740 cm⁻¹ (C=O stretch): Oil oxidation products (carbonyls, aldehydes, ketones)
- Broad peak at 3400 cm⁻¹ (O-H stretch): Oxidation byproducts (acids, alcohols)
- Increased absorbance at 1650 cm⁻¹ (C=C stretching): Nitration products
📈 Spectra Characteristics: Oxidation peak (1740 cm⁻¹) increases as the oil ages.
3. FTIR Spectrum of Water-Contaminated Oil
- Key Changes Compared to Fresh Oil:
- Strong, broad peak at 3400 cm⁻¹ (O-H stretch): Water presence
- Reduction in hydrocarbon peaks (2950–2850 cm⁻¹)
- Possible increase in sulfate peaks (1150–1000 cm⁻¹) if corrosion occurs
📈 Spectra Characteristics: A sharp increase in the 3400 cm⁻¹ region signals water intrusion.
4. FTIR Spectrum of Fuel-Diluted Oil
- Key Changes Compared to Fresh Oil:
- Increased absorbance at 1600–1450 cm⁻¹: Aromatic hydrocarbons from fuel
- Shift in hydrocarbon peaks (2950–2850 cm⁻¹): Fuel mixing with base oil
- Lower viscosity leads to altered baseline
📈 Spectra Characteristics: Increased aromatic peaks indicate fuel contamination.
5. FTIR Spectrum of Additive Depletion
- Key Changes Compared to Fresh Oil:
- Loss of P=O peak (1170 cm⁻¹): ZDDP (anti-wear) depletion
- Reduction of detergent/dispersant peaks (1000–900 cm⁻¹)
- Possible increase in oxidation peaks (1740 cm⁻¹) due to lost antioxidants
📈 Spectra Characteristics: Lower absorption in the additive regions (900–1200 cm⁻¹).
Conclusion & Interpretation
| FTIR Peak Region (cm⁻¹) | Lubricant Issue | Typical Causes | Action Required |
|---|---|---|---|
| 1740 (C=O) | Oxidation | High temp, aging | Replace oil |
| 3400 (O-H) | Water Contamination | Coolant leak, humidity | Remove water, filter oil |
| 1600–1450 (Aromatics) | Fuel Dilution | Injector leakage | Identify fuel contamination source |
| 1170 (P=O) | Additive Loss | Oil aging | Use fresh oil or replenish additive |