Calibration with sulphur standards
Builds a calibration line from standards of known sulphur content in a matching matrix, checks its linearity, and confirms it with an independent check standard.
Why this is checked
Both XRF and UVF analysers are comparative: they convert a signal to sulphur content using a calibration made from standards. A correct, matrix-matched calibration across the working range is needed so results near the 10 mg/kg specification limit (or higher limits for heavier fuels) are reliable.
Full calibration typically when the instrument is installed, after major maintenance (new UVF combustion tube, XRF tube/detector service), when the check standard fails, or at the interval in your SOP; check standards daily.
Learn how the Sulphur analyser worksStandards & tools
- Certified sulphur standards covering the range, e.g. di-n-butyl sulphide or dibenzothiophene in isooctane, toluene or mineral oil, matched to the sample type
- Sulphur-free solvent or white oil as blank
- Independent check standard (different lot or supplier) near the specification limit
Prerequisites
- Instrument warmed up and stable; UVF gas flows and furnace temperature correct
- Balance calibrated if standards are prepared gravimetrically
- Syringe or autosampler checked for volume accuracy (UVF)
Step by step
- 1Choose a calibration range that brackets the expected samples (for example 0–50 mg/kg for ULSD by UVF).
- 2Measure the blank and each standard in replicate (for example three injections each for UVF).
- 3Fit the calibration line (signal against concentration) by least squares.
- 4Check the correlation coefficient and residuals of each point.
- 5Measure the independent check standard and calculate its recovery.
- 6Save the calibration and record standard lot numbers.
Example observation table
| Standard (mg/kg S) | Mean response (units) |
|---|---|
| 0 (blank) | 0.012 |
| 5 | 0.538 |
| 10 | 1.071 |
| 25 | 2.648 |
| 50 | 5.301 |
| Check std 10.0 mg/kg | 1.112 |
How to calculate the result
- 1
Least-squares fit of the five points: slope = 0.10576 units per mg/kg, intercept = 0.0103, r = 0.99999.
- 2Check standard: found = (1.112 − 0.0103) ÷ 0.10576 = 10.42 mg/kg.
- 3Recovery = 10.42 ÷ 10.0 × 100 = 104.2%.
- 4
With an illustrative SOP limit of 90–110% at this level, the calibration is accepted.
Pass if
- Correlation coefficient typically ≥ 0.999 (or as in your SOP), with no point showing a large residual.
- Independent check standard within the SOP limits (for example ±10% near 10 mg/kg) or within the method's expected precision.
- Blank response low and stable.
What to do
- Check standards for evaporation or contamination and prepare fresh ones.
- UVF: check combustion tube for soot, gas flows, membrane dryer and syringe; XRF: check film, helium and tube stability.
- Repeat the calibration; do not analyse samples until the check standard passes.
LimsPulse schedules this calibration, records the result against limits, and blocks the instrument everywhere if it fails or goes overdue.