Background Interference in FTIR Analysis: Sources and Effective Elimination Methods

In FTIRアクセサリー, background interference has a direct impact on baseline flatness, peak shape accuracy, and the reliability of both qualitative and quantitative analysis. Uncontrolled interference can obscure weak absorption bands, distort spectral features, and compromise reproducibility, particularly in trace or high-precision measurements.

To support routine laboratory operation and troubleshooting, the most common sources of background interference and their corresponding mitigation strategies are summarized below.

Table 1. Common FTIR Background Interference and Practical Countermeasures

Interference Category	Typical Spectral Features	Primary Cause	Recommended Action
Atmospheric H₂O	Broad band near 3400 cm⁻¹; peak at 1640 cm⁻¹	High ambient humidity	Background scan synchronization; dry gas purging; humidity control
Atmospheric CO₂	Peaks at 2360, 2340, 667 cm⁻¹	CO₂ concentration fluctuations	Background subtraction under identical conditions
Impure or moist KBr	Water and carbonate absorption	Undried or low-purity KBr	Use ≥99.9% KBr; drying and desiccated storage
Window contamination	Residual or random peaks	Sample residue on windows	Thorough cleaning and proper storage
Sample impurities	Additional absorption bands	Moisture, solvents, additives	Sample purification and drying
Particle scattering	Baseline elevation, peak broadening	Oversized particles	Fine grinding or mull method
Instrument instability	Baseline drift, noise	Aging source or detector issues	Maintenance, calibration, parameter optimization

This table is intended as a quick diagnostic reference for FTIR users encountering abnormal baselines or unexpected absorption features.

1. Atmospheric Component Interference (Most Common)

Source of Interference

Atmospheric water vapor (H₂O) and carbon dioxide (CO₂) exhibit strong and characteristic infrared absorption bands:

• H₂O: Broad absorption near 3400 cm⁻¹ and a bending vibration around 1640 cm⁻¹
• CO₂: Strong absorption bands at 2360 cm⁻¹, 2340 cm⁻¹, and 667 cm⁻¹

These absorption features frequently overlap with sample peaks, significantly affecting the analysis of low-concentration or moisture-sensitive samples.

ソリューション

Synchronized Background and Sample Scanning
Before each sample measurement, a background scan should be collected under identical environmental conditions using an empty optical path or a blank substrate. FTIR software then subtracts atmospheric H₂O and CO₂ contributions from the sample spectrum.

Purification of the Infrared Optical Path

• Continuously purge the optical path with dry, high-purity nitrogen or dry air
• Install desiccants such as anhydrous calcium chloride or molecular sieves and replace them regularly

Laboratory Environmental Control

• Maintain laboratory relative humidity below 50%
• Prevent moisture exposure of samples and optical components
• Minimize personnel movement near the instrument to reduce short-term CO₂ concentration fluctuations

2. Sample Carrier or Diluent Interference

Source of Interference

For solid samples analyzed using the KBr pellet method, insufficiently pure or moisture-contaminated KBr introduces unwanted absorption bands, particularly from water and carbonate impurities.
For liquid samples, poorly cleaned KBr or NaCl window plates may retain residues from previous samples, resulting in spectral contamination.

ソリューション

Selection of High-Purity Carrier Materials

• Use spectroscopy-grade KBr with a purity of ≥ 99.9%
• Dry KBr powder at 110 °C for at least 24 hours prior to pellet preparation
• For liquid cells, select window materials such as KBr, NaCl, or ZnSe based on sample polarity and solvent compatibility

Carrier Pre-treatment

• Grind dried KBr to a particle size below 2 μm to reduce scattering effects
• Clean window plates with anhydrous ethanol or acetone, dry thoroughly, and store in a desiccator

Blank Subtraction Method
Collect a background spectrum using a pure KBr pellet or a clean blank window plate to subtract the intrinsic absorption of the carrier material.

3. Interference Originating from the Sample Itself

Source of Interference

Sample Impurities
Moisture, residual solvents, plasticizers, or additives present in the sample can generate additional absorption peaks that interfere with functional group identification.

Light Scattering Effects
If solid sample particles are larger than the infrared wavelength, scattering occurs, leading to baseline elevation and peak broadening, particularly in the high-wavenumber region.

ソリューション

Sample Pretreatment and Purification

• Solid samples: Remove impurities via recrystallization, extraction, or distillation
• Liquid samples: Eliminate solvents using vacuum evaporation or dry with anhydrous sodium sulfate
• For water-containing samples, differential spectroscopy may be applied when analyzing regions outside water absorption bands

Reduction of Sample Particle Size

• Grind solid samples to particle sizes below 2 μm
• Alternatively, use the mull method with paraffin oil to minimize scattering

4. Instrument System Interference

Source of Interference

• Aging infrared light sources or power supply fluctuations causing baseline drift
• Detector noise due to temperature instability or contamination, particularly for MCT detectors
• Dust or residues on mirrors and beam splitters affecting optical transmission

ソリューション

Routine Instrument Maintenance and Calibration

• Regularly inspect and replace aging light sources
• Ensure stable power supply conditions
• Maintain detectors at specified operating temperatures; MCT detectors require liquid nitrogen cooling
• Clean optical components using lint-free wipes and absolute ethanol

Optimization of Measurement Parameters

• Increase the number of scans (e.g., 64 or 128) to improve signal-to-noise ratio
• Use a resolution of 4 cm⁻¹ for routine measurements to balance noise and peak clarity

Practical Takeaway

From an application perspective, effective control of FTIR background interference relies on a structured workflow:

Environmental control → Sample and carrier preparation → Background synchronization → Instrument optimization

Applying these measures systematically allows FTIR users to obtain stable baselines, well-defined absorption peaks, and reproducible spectra suitable for routine analysis and advanced research.