FTIR · Calibration procedure
Signal-to-noise
Measures the noise on a '100% line' (two backgrounds ratioed against each other) and converts it into a signal-to-noise ratio to compare with the manufacturer's specification.
PURPOSE
Why this is checked
Low signal-to-noise hides weak bands and makes library matching less reliable. A falling ratio is often the first sign of a weak source, a fogged beam splitter or a failing detector.
Frequency
Full calibration typically every 6 months or yearly; a quick polystyrene check is often done daily or before use, as defined in your SOP.
Learn how the FTIR worksYOU WILL NEED
Standards & tools
- No physical standard: the test uses the empty beam (or a clean ATR crystal) under defined conditions
- Practice described in ASTM E1421 and the manufacturer's OQ procedure
BEFORE YOU START
Prerequisites
- Instrument fully warmed up and stable
- Conditions fixed as in the specification (e.g. resolution 4 cm⁻¹, a stated scan time or number of scans, a stated wavenumber region)
PROCEDURE
Step by step
- 1Collect a background spectrum with the empty beam.
- 2Immediately collect a second spectrum under the same conditions and ratio it against the first to give a 100% transmittance line.
- 3Measure the peak-to-peak (or RMS) noise in %T over the specified region (e.g. 2200–2000 cm⁻¹, where there are no atmospheric bands).
- 4Calculate signal-to-noise and repeat as many times as your SOP requires.
RECORD
Example observation table
| Run | Region (cm⁻¹) | Peak-to-peak noise (%T) | S/N (peak-to-peak) |
|---|---|---|---|
| 1 | 2200–2000 | 0.0030 | 33,333 : 1 |
| 2 | 2200–2000 | 0.0028 | 35,714 : 1 |
CALCULATION
How to calculate the result
S/N = 100 ÷ peak-to-peak noise (in %T) on the 100% line
- Peak-to-peak noise over 2200–2000 cm⁻¹ = 0.0030 %T
- S/N = 100 ÷ 0.0030 = 33,333 : 1
- Compare with the manufacturer's specification for the same resolution, scan time and region
ACCEPTANCE CRITERIA
Pass if
- Signal-to-noise not less than the manufacturer's specification (or your SOP limit) for the stated conditions. Values differ widely between models and conditions, so there is no single compendial number.
IF IT FAILS
What to do
- Check the desiccant and humidity; replace if exhausted.
- Check source energy and interferogram amplitude; run alignment.
- Arrange service for source, beam splitter or detector if it still fails.
MORE
Other FTIR 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.