Wavenumber accuracy
Checks the Raman shift (x-axis) scale by measuring a standard with well-known band positions, such as cyclohexane, polystyrene or paracetamol (4-acetamidophenol), and comparing the observed bands with the reference values.
Why this is checked
Library identification depends on band positions. If the Raman shift axis drifts, for example because the laser wavelength changes slightly or the spectrograph moves, authentic materials can fail and spectra cannot be compared with libraries built on other units.
Typically a quick check daily or before use, with a full calibration every 6–12 months and after repair, as defined in your SOP.
Learn how the Raman worksStandards & tools
- Cyclohexane (spectroscopic grade) in a glass vial, with reference band positions from ASTM E1840 or the pharmacopoeia
- Alternatively polystyrene, paracetamol (4-acetamidophenol) or naphthalene, as named in your SOP and the current USP <858> / Ph. Eur. 2.2.48
- Silicon wafer (first-order band near 520.7 cm⁻¹) for a quick laser-wavelength check if used by your SOP
Prerequisites
- Laser warmed up and stable (often 15–30 minutes for bench systems)
- Built-in x-axis calibration (e.g. neon lamp) completed if the instrument requires it
- Standard of known purity, within expiry; vial clean
Step by step
- 1Place the cyclohexane vial in the holder (or the standard in front of the probe).
- 2Acquire a spectrum using the integration time and number of accumulations in the SOP; avoid detector saturation.
- 3Pick the peak maxima of the reference bands (e.g. near 801, 1028, 1266, 1444 and 2853 cm⁻¹).
- 4Record observed positions and calculate the difference from the reference values.
Example observation table
| Reference band (cm⁻¹) | Observed (cm⁻¹) | Difference (cm⁻¹) | Result |
|---|---|---|---|
| 801.3 | 801.8 | +0.5 | Pass |
| 1028.3 | 1027.6 | −0.7 | Pass |
| 1266.4 | 1266.9 | +0.5 | Pass |
| 1444.4 | 1445.1 | +0.7 | Pass |
| 2852.9 | 2853.8 | +0.9 | Pass |
How to calculate the result
- Band near 2852.9 cm⁻¹ observed at 2853.8 cm⁻¹
- Difference = 2853.8 − 2852.9 = +0.9 cm⁻¹
- |0.9| is within a typical tolerance of about ±1.5 cm⁻¹, so the band passes
Pass if
- Every reference band within the tolerance in your SOP, typically about ±1.5 cm⁻¹ for cyclohexane or polystyrene bands; some SOPs allow wider limits at high shift or for low-resolution handheld units. See the current USP <858> / Ph. Eur. 2.2.48.
What to do
- Label the instrument 'Out of calibration' and stop identity testing.
- Repeat the built-in x-axis calibration, check the standard and repeat once.
- If it still fails, call service to check the laser wavelength and spectrograph alignment.
- Assess identity results since the last passing check; confirm library performance after repair.
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.