User Guide

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



These Reference Materials are described in the US Pharmacopeia (Chapter <857> to qualify the Stray Light (or Stray Radiant Energy) of ultraviolet spectrometers.

Note: The European and most other Pharmacopoeias recommend the Specified Wavelength method which requires different references.

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.

USP Filter Ratio


In the Filter Ratio (or Meilenz) Method described in USP Chapter <857>, a 10mm cell of the appropriate cut-off solution is scanned against a 5 mm cell of the same solution in the instrument's reference beam. Under these conditions the resulting difference spectrum is in the form of a peak, as shown in these spectra obtained on three different instruments:


The absolute wavelength of the peak will depend on the optical characteristics of the instrument under test, but to satisfy the USP stray light requirement, the absorbance at the peak must exceed 0.7A. In this example, all three instruments comply with the USP requirement.
A range of materials covers wavelengths from 190 nm to 385 nm:

Material Cut-off
Potassium Chloride 190-205 nm
Sodium Iodide 210-259 nm
Potassium Iodide 210-259 nm
Acetone 250-320 nm
Sodium Nitrite 300-385 nm



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.