Wavelength accuracy
Checks that the wavelength (or wavenumber) scale of the NIR spectrometer is correct by measuring the band positions of a certified reference material such as polystyrene or a rare-earth oxide standard.
Why this is checked
Libraries and quantitative models are built on spectra recorded at defined wavelengths. A shift in the wavelength scale makes good material look different from its library entry, causing false failures, or biases a quantitative prediction. Wavelength accuracy is also essential when a library or model is transferred to another instrument.
Typically every 6–12 months with external certified standards, and after lamp replacement, major repair or relocation; built-in checks are run daily, as defined in your SOP.
Learn how the NIR worksStandards & tools
- Certified polystyrene reference standard (reflectance or transmission), or
- Rare-earth oxide reflectance standard (e.g. NIST SRM 1920 series or an equivalent traceable standard), or
- Rare-earth doped glass standard (e.g. NIST SRM 2036) for transflectance; water vapour lines may be used for FT-NIR as allowed by the chapter
Prerequisites
- Instrument warmed up as per the manufacturer and the daily performance check passed
- Standard clean, undamaged and within its certificate validity
- Measurement mode (reflectance or transmission), resolution and number of scans set as in the SOP
Step by step
- 1Record a background (reference) scan on the white reflectance standard or empty beam as the SOP requires.
- 2Place the wavelength standard on the sample window (or in the holder) and record its spectrum.
- 3Use the peak-pick function, with the same algorithm stated on the certificate, to find the band maxima.
- 4Record the observed position of each certified band (typically three or more bands across the range).
- 5Calculate the difference between each observed and certified value and compare with the tolerance.
Example observation table
| Band | Certified position (nm, example) | Observed (nm) | Difference (nm) | Tolerance (nm) | Result |
|---|---|---|---|---|---|
| 1 | 1143.6 | 1143.9 | +0.3 | ±1.0 | Pass |
| 2 | 1680.3 | 1680.0 | −0.3 | ±1.0 | Pass |
| 3 | 2166.1 | 2166.8 | +0.7 | ±1.5 | Pass |
How to calculate the result
- Band 3: certified 2166.1 nm, observed 2166.8 nm
- Difference = 2166.8 − 2166.1 = +0.7 nm
- |0.7| ≤ 1.5 nm, so the band passes
Pass if
- Each observed band within the tolerance in your SOP, typically about ±1 nm below roughly 1900 nm and ±1.5 nm at longer wavelengths (or the equivalent in cm⁻¹ for FT-NIR); see the current USP <856> / Ph. Eur. 2.2.40 and the standard's certificate.
- Use the certified values from your own standard's certificate, not the example values above.
What to do
- Label the instrument 'Out of calibration' and stop identity and quantitative testing.
- Clean the standard and window, let the instrument stabilise and repeat once.
- If it still fails, call service (laser, interferometer or grating alignment).
- Assess the impact on identity and model results since the last passing calibration; after repair, recalibrate and confirm libraries and models still perform.
Other NIR calibrations
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.