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APIs & raw materials · Active substance or excipient (powder, crystals or liquid)

Identification by FTIR

Confirms the identity of a raw material by recording its infrared spectrum and showing that it matches the spectrum of the reference substance (or the official reference spectrum) measured the same way.

  • FTIR (ATR or KBr disc)
  • FTIR
  • Analytical balance
  • Hot air oven

Reference: General approach described in Ph. Eur. 2.2.24, USP <854> and USP <197>. The monograph states which technique (KBr disc, ATR, film) and any sample treatment to use; always follow your approved specification and STP.

PRINCIPLE

Why this method works

Each compound absorbs infrared light at a unique combination of wavenumbers, set by the vibrations of its bonds. The fingerprint region (about 1500–400 cm⁻¹) in particular is so characteristic that two different compounds almost never match. If the sample and the reference substance are measured under the same conditions and their bands appear at the same positions with similar relative intensities, the sample is identified as that substance.

Instruments used

MATERIALS

Reagents & standards

ReagentGrade / specification
Reference substancePharmacopoeial reference standard (Ph. Eur. CRS / USP RS / IP RS) or a qualified working standard; or the official reference spectrum where the monograph allows
Potassium bromideIR grade, dried (for the KBr disc technique)
Cleaning solventIsopropanol or another solvent suitable for the ATR crystal
Recrystallisation solventOnly if the monograph asks for sample and reference to be treated the same way to obtain the same crystal form
SAMPLE PREPARATION

Preparing the sample

  1. 01

    Check instrument

    Confirm calibration status and run the polystyrene check if your SOP requires it.

  2. 02

    Background

    Collect a background spectrum (clean ATR crystal or blank KBr disc).

  3. 03

    Prepare sample

    Place sample on the ATR crystal, or grind with dry KBr and press a disc.

  4. 04

    Record spectrum

    Scan the sample, typically 4000–400 cm⁻¹ at 4 cm⁻¹ resolution.

  5. 05

    Reference

    Record the reference substance in exactly the same way.

  6. 06

    Compare

    Overlay the spectra and compare band positions and relative intensities.

Procedure

  1. 1ATR technique: clean the crystal, collect a background, then place enough sample to cover the crystal. Lower the pressure clamp to the set position and record the spectrum.
  2. 2KBr disc technique: dry the potassium bromide as per the SOP. Grind about 1–2 mg of the sample with about 200–300 mg of dry KBr in an agate mortar until fine and uniform, and press into a clear disc under vacuum with the hydraulic press.
  3. 3Check the spectrum quality: the strongest band should not be flat-topped (typically below about 1.0–1.5 absorbance or above about 5–10 %T), the baseline should be reasonably flat, and water or CO₂ bands should be small.
  4. 4If the monograph specifies a sample treatment (for example drying, or dissolving and evaporating to dryness), apply the same treatment to the sample and the reference.
  5. 5Clean the crystal or die between measurements.

Standard preparation

  1. 1Prepare and record the reference substance by the same technique, on the same instrument and with the same settings as the sample.
  2. 2Where the monograph allows comparison with an official reference spectrum, use the spectrum obtained with the same technique (a KBr reference spectrum should not be compared directly with an ATR sample spectrum).

Notes for analysts

  • Use the same technique for sample and reference: ATR spectra have relatively stronger bands at low wavenumbers than transmission spectra.
  • Polymorphs, solvates and hydrates can give different spectra. This is not necessarily a failure; follow the monograph's treatment step before concluding.
  • Some salts (for example hydrochlorides) can exchange ions with KBr during grinding, which changes the spectrum. Monographs may then specify potassium chloride discs or ATR.
  • Keep KBr and discs dry: a broad band near 3400 cm⁻¹ and a band near 1640 cm⁻¹ usually mean absorbed water.
  • Print or save the overlay with the identification of instrument, technique, date and analyst for the record.
TEST CONDITIONS

Method parameters (typical)

Spectral rangeAbout 4000–400 cm⁻¹ (or 4000–650 cm⁻¹ for diamond ATR)
ResolutionTypically 4 cm⁻¹ (2 cm⁻¹ if the method requires)
Number of scansTypically 16–32 for sample and background
Display% transmittance or absorbance, same for sample and reference
TechniqueAs stated in the monograph or STP: ATR, KBr disc, liquid film or other
BEFORE YOU REPORT

System suitability & sequence

ParameterLimit
Instrument performanceWavenumber accuracy check with polystyrene film passes (daily or as per SOP)
BackgroundFresh background collected; no significant water vapour or CO₂ bands in the sample spectrum
Spectrum qualityStrongest band not saturated; adequate signal-to-noise

Typical sequence

  1. 1Polystyrene performance check (as per SOP)
  2. 2Background spectrum
  3. 3Reference substance spectrum
  4. 4Background spectrum (if time has passed or the SOP requires)
  5. 5Sample spectrum
  6. 6Overlay and comparison
ACCEPTANCE CRITERIA

What result passes

  • The spectrum of the sample shows absorption bands at the same positions as the reference substance, with similar relative intensities, across the whole range (paraphrasing Ph. Eur. 2.2.24 and USP <197>).
  • If the spectra differ only because of crystal form, the monograph's instructions for treating sample and reference are applied and the comparison repeated.
  • Where your SOP also uses a software correlation (hit-quality) value, it supports but does not replace visual comparison; the threshold is site-defined.
METHOD VALIDATION

Validation parameters (ICH Q2(R2))

ParameterTypical requirement
SpecificityThe key validation parameter: the method must distinguish the material from closely related substances (e.g. salts, analogues, other excipients)
RobustnessEffect of sample amount, ATR pressure and humidity on the spectrum is understood
Compendial methodsPharmacopoeial identification tests are verified rather than fully validated; in-house library methods need specificity data
In LimsPulse · Sample Management

Run this method in LimsPulse: specifications and STPs under version control, results calculated and checked against limits, and a signed Certificate of Analysis.

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