Fourier Transform Infrared Spectroscopy (FTIR)

• 1. **Sample Preparation:** Minimal sample preparation is a key advantage. For ATR-FTIR, sample is simply placed on the ATR crystal. For transmission, solids may require KBr pellet pressing or casting as thin films . 2. **Instrument Setup:** FTIR spectrometer parameters are set, including: - Spectral range (typically 4000–400 cm⁻¹) - Number of scans (e.g., 16–64 scans for signal averaging) - Resolution (typically 4–8 cm⁻¹) - Background collection (reference spectrum) 3. **Background Scan:** A background spectrum is collected without the sample to account for atmospheric (CO₂, H₂O) and instrument contributions. 4. **Sample Measurement:** The sample is placed in the instrument, and IR light interacts with it. For ATR, the sample is pressed onto the crystal surface
• for transmission, light passes through the sample. 5. **Interferogram Collection:** The detector records the interferogram—a plot of light intensity versus mirror position . 6. **Fourier Transform:** A computer mathematically transforms the interferogram into a spectrum (intensity vs. wavenumber) . 7. **Spectral Analysis:** The resulting spectrum is analyzed by: - **Qualitative identification:** Comparing to reference spectral libraries (computerized search with certainty ratings) or manual interpretation of key absorption bands . - **Quantitative analysis:** Using Beer-Lambert law (A = εcl) for concentration determination, requiring calibration with standards . 8. **Interpretation & Reporting:** An experienced analyst reviews computer matches, identifies functional groups, and prepares the final report with conclusions .

- **ASTM E168** – Standard Practices for General Techniques of Infrared Quantitative Analysis - **ASTM E1252** – Standard Practice for General Techniques for Obtaining Infrared Spectra for Qualitative Analysis - **ASTM E573** – Standard Practices for Internal Reflection Spectroscopy - **ASTM E334** – Standard Practice for General Techniques of Infrared Microanalysis - **ISO 14558** – Plastics – Determination of residual monomers and other components by FTIR - **ISO/TS 19022** – Medical devices – FTIR analysis for polymer characterization - **Ph. Eur. 2.2.24** – European Pharmacopoeia IR spectroscopy - **USP <197>** – United States Pharmacopeia IR spectroscopy

- **FTIR Spectrometer:** Interferometer-based system with IR source (e.g., globar), beam splitter, moving mirror, and detector (DTGS or MCT) - **Sampling Accessories:** - **ATR Attachment:** Diamond, ZnSe, or Ge crystal for minimal/no sample preparation - **Transmission cell:** For liquids and gases - **KBr pellet die and press** - **Reflectance accessories:** Specular, diffuse, or grazing angle - **Micro-FTIR:** IR microscope for analyzing spots as small as 25 µm diameter - **Spectral Libraries:** Computerized databases of reference spectra - **Software:** Data acquisition, Fourier transform processing, library search, and chemometric analysis software

- **Qualitative Parameters:** - Functional group identification (e.g., O-H, C-H, C=O, N-H, C-O, C=C) - Material identity / polymer type determination - Contaminant or impurity identification - Structural confirmation - **Quantitative Parameters:** - Concentration of specific components (using Beer-Lambert law) - Percent composition in mixtures - Degree of cross-linking or curing - Oxidation or degradation levels - **Spectral Parameters:** - Absorption band positions (wavenumber, cm⁻¹) - Peak intensity (absorbance or transmittance) - Spectral library match certainty (%)

• - **Spectral range:** Typically 4000–400 cm⁻¹ (mid-IR region)
• some instruments cover 7000–350 cm⁻¹ - **Resolution:** 0.5–16 cm⁻¹ (typical routine analysis uses 4 cm⁻¹) - **Sample size:** Microgram to milligram quantities - **Concentration range:** From trace (ppm) levels to pure materials, depending on sample preparation

• - **Wavenumber accuracy:** ± 0.01 cm⁻¹ (with calibration) - **Wavenumber reproducibility:** ± 0.1 cm⁻¹ - **Detection limit:** Varies by material and sampling technique: - ATR-FTIR: ~ 1–5% for contaminants with strong absorptions - Transmission: ~ 0.1–1% (with appropriate sample preparation) - Contamination detection limit: 1–2% for materials with very different IR spectra
• may not detect 10% if spectra are similar - **Concentration accuracy:** Typically ± 2–5% with proper calibration

• - **Non-destructive:** Does not alter or consume the sample - **Minimal sample preparation:** Especially with ATR-FTIR—measure directly - **Rapid analysis:** Complete spectrum in 1–5 minutes - **Versatile:** Suitable for solids, liquids, gases, powders, pastes, and films - **High sensitivity:** Capable of detecting low concentrations and small sample sizes - **Qualitative and quantitative:** Provides both identification and concentration information - **Unique molecular fingerprint:** Every compound has a characteristic spectrum - **Broad applications:** Used across virtually all industries and scientific disciplines - **Computer-aided identification:** Automated library searching with match certainty ratings - **Cost-effective:** Low cost per analysis
• no expensive reagents or consumables

• - **Cannot detect all molecules:** Symmetrical molecules like N₂, O₂, and homonuclear diatomics are IR-inactive - **Moisture interference:** Water absorbs strongly in the IR region, masking key signals in wet samples - **Overlapping spectra:** Complex mixtures or large molecules may have overlapping peaks, requiring expert interpretation and advanced software - **Quantitative challenges:** Accurate concentration measurement requires careful calibration and may be affected by sample conditions - **Surface-sensitive (ATR):** ATR-FTIR only probes a thin surface layer (few micrometers), may not represent bulk composition of heterogeneous samples - **Spectral matching limitations:** Computer-selected library matches can be misleading
• an experienced analyst must verify results - **Carbon interference:** Samples with high carbon black content may not yield usable spectra - **Not for metals:** Metals do not absorb IR light and cannot be directly analyzed - **Hard samples:** Some samples may be too hard to make sufficient contact with the ATR crystal - **Sample heterogeneity:** ATR only probes the immediate surface
• bulk properties may differ

• - **Minimum sample size:** A single resin pellet or microgram quantities for ATR-FTIR . For ATR, any sample visible to the naked eye is typically sufficient . - **Solids:** Should be clean, dry, and free from surface contaminants. For transmission, samples must be thin enough to allow IR transmission (typically < 50 µm). - **Powders:** Finely ground
• can be mixed with KBr and pressed into a pellet for transmission analysis. - **Liquids:** A few drops (50–100 µL) are sufficient for ATR analysis
• no preparation needed. - **Surface contaminants:** Can be collected via solvent wash and analyzed after solvent evaporation . - **Caution:** Samples with high carbon black or carbon fiber content strongly absorb IR and may not yield usable spectra . - **Metals:** Cannot be analyzed by standard FTIR as they do not absorb IR light .

- **Routine analysis:** 1–3 working days (depending on sample load) - **Urgent/Express analysis:** 24 hours (available upon request) - **Instrument scan time:** 1–5 minutes per sample (multiple scans for signal averaging may take longer)

- Official test certificate/report with detailed results - FTIR spectrum (absorbance or transmittance plot) - Table of peak positions and corresponding functional groups - Spectral library search results (with match certainty) - Quantitative analysis results (if requested) - Raw interferogram and spectral data files (if requested) - Graphical comparison with reference standards (if specified)