User Guide

For an introduction to the application and use of Certified Reference Materials in absorption spectrophotometry




These Reference Materials are described in the European Pharmacopoeia (Chapter 2.2.25) to qualify the Stray Light (or Stray Radiant Energy) of ultraviolet spectrometers. These filters are also approved for instrument qualification by other standardisation bodies, including:

  • American Society for Testing and Materials, ASTM
  • British Pharmacopoeia, BP
  • Deutsches Arzneibuch (German Pharmacopoeia), DAB
  • Therapeutic Goods Administration (Australia), TGA
  • Japanese Pharmacopoeia, JP
  • Pharmacopoeia of the Peoples Republic of China, PPRC

Note: The US Pharmacopeia recommends the Filter Ratio or Meilenz method which requires different references.


 Note: The US Pharmacopeia now recommends the alternative Filter Ratio or Meilenz method which requires different references. This method is simpler to use with high performance, low stray light instruments, especially those with double monochromators. The Specified Wavelength method is, however , still accepted for instrument qualification. 

Stray light, also called Stray Radiant Energy or Power, is any light reaching the detector that is outside the Spectral Band Width selected for analysis by the monochromator. It is not absorbed by the analyte, so its effect is a negative deviation from the linear relationship between concentration and absorbance (the Beer-Lambert law), i.e lowered concentration results.

Stray light is usually measured using cut-off filters or solutions that cut off all light near the analytical wavelength, and transmit at all higher wavelengths. Below the certified cut-off wavelength, within the indicated usable range, any light transmitted must be stray light.

Starna liquid cut-off filters allow stray light to be checked at a range of wavelengths from 200 nm to 390 nm.

The European Pharmacopoeia states that at a path length of 10mm, a 12g/l aqueous solution of potassium chloride must have an absorbance ≥ 2A at 198 nm.


Material Cut-off
Potassium Chloride 201 nm
Sodium Chloride 201 nm
Lithium Carbonate 227 nm
Sodium Iodide 260 nm
Potassium Iodide 260 nm
Acetone 326 nm
Sodium Nitrite 391 nm


Note that Potassium Iodide and Sodium Iodide have identical cut-off characteristics and are effectively interchangeable.

Starna liquid stray light references are supplied permanently sealed by heat fusion into 10 mm high quality far UV quartz cells.  A water blank cell is supplied with each reference or set of references 


A Certificate of Calibration and Traceability and full instructions for use are provided with each Reference Material. Starna Scientific is accredited to both ISO 17034 (4001) as a Reference Material producer, and ISO/IEC 17025 (0659) as a Calibration Laboratory for optical reference measurements. Starna offers a Lifetime Guarantee on all its Reference Materials.