NIR · Calibration procedure
Photometric linearity
Checks that the NIR spectrometer's absorbance (log 1/R) response is linear by measuring a set of certified reflectance standards of different reflectance values.
PURPOSE
Why this is checked
Quantitative NIR models (moisture, assay, blend uniformity) assume the instrument's response is linear over the absorbance range of the samples. A non-linear detector makes predictions biased, especially for strongly absorbing samples.
Frequency
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 worksYOU WILL NEED
Standards & tools
- Set of certified diffuse reflectance standards covering about 2–99% reflectance (e.g. carbon-doped polymer standards at about 99, 75, 50, 20, 10 and 2% R)
- For transmission instruments, certified neutral-density filters as permitted by the SOP
BEFORE YOU START
Prerequisites
- Wavelength accuracy check passed
- Instrument warmed up; background recorded on the high-reflectance (white) standard
- Standards clean and within certificate validity; handle by the edges only
PROCEDURE
Step by step
- 1Record a background on the white reference.
- 2Measure each reflectance standard in turn, from highest to lowest reflectance.
- 3For each, read the mean observed absorbance (log 1/R) at the wavelengths or over the range stated in the SOP.
- 4Plot observed absorbance against certified absorbance and calculate slope, intercept and correlation coefficient by least squares.
RECORD
Example observation table
| Standard (% R) | Certified A = log(1/R) | Observed A |
|---|---|---|
| 99 | 0.004 | 0.006 |
| 75 | 0.125 | 0.128 |
| 50 | 0.301 | 0.305 |
| 20 | 0.699 | 0.704 |
| 10 | 1.000 | 1.008 |
| 2 | 1.699 | 1.712 |
CALCULATION
How to calculate the result
Certified A = log₁₀(100 ÷ %R); least-squares fit: Observed A = slope × Certified A + intercept
- 50% R standard: certified A = log₁₀(100 ÷ 50) = 0.301
- Mean certified A = 0.638; mean observed A = 0.644
- Slope = Σ(x − x̄)(y − ȳ) ÷ Σ(x − x̄)² = 2.0520 ÷ 2.0392 = 1.006
- Intercept = ȳ − slope × x̄ = 0.6438 − 1.006 × 0.638 = +0.002
- Correlation coefficient r = 0.99999
- Slope 1.006 is within 1.00 ± 0.05 and intercept 0.002 is within 0.00 ± 0.05: pass
ACCEPTANCE CRITERIA
Pass if
- Typically slope 1.00 ± 0.05 and intercept 0.00 ± 0.05 for the plot of observed against certified absorbance, as described in USP <1119>; confirm the limits in your SOP and the current chapter.
- For instruments used only for qualitative identity, some SOPs apply a simpler photometric check; follow your approved procedure.
IF IT FAILS
What to do
- Label the instrument 'Out of calibration' and stop quantitative NIR testing.
- Check the standards for contamination or damage and that the correct certified values were used; repeat once.
- Check lamp energy and detector; call service if the failure is confirmed.
- Assess quantitative results since the last passing calibration.
MORE
Other NIR calibrations
In LimsPulse · Calibration Management
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.