METTLER TOLEDO Presents New Automated Stray Light and Performance Verification in UV/VIS Spectroscopy

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The US Pharmacopeia has revised a chapter on UV/Vis Spectroscopy and proposed a new method for the test of stray light. Learn about the advantages of this new method and how to automate performance verification on a UV/Vis Spectrophotometer.

The CertiRef module from METTLER TOLEDO offers a full US Pharmacopoeia compliant solution for the automated performance verification of the UV/VIS Excellence Spectrophotometers.

In UV/VIS spectroscopy, regular performance verification is essential to ensure accurate and reliable instrument performance. Widely accepted guidelines for performance verification of spectrophotometers are described in the US Pharmacopeia. The recommended tests include the check of photometric accuracy and repeatability, wavelength accuracy and repeatability, instrument resolution as well as stray light measurement. Recently, the US Pharmacopeia revised a chapter on ultra-violet visible spectroscopy and adapted the test for stray light. Here, we compare the methods for measuring stray light according to the current and previous versions of the US Pharmacopeia and highlight METTLER TOLEOD's new solution - CertiRef™.

Effects on Measurement

Stray light is defined as the light reaching the detector, which is not part of the analysis. Therefore, the absorbance of the sample results will be lower than what they actually are. The larger the observed stray light energy, the larger the error recorded by the spectrograph.

USP Requirements for Stray Light
USP requires the stray light to be smaller than 1% transmittance. The traditional measurement technique prescribed by the Pharmacopeia is based on the Specific Wavelength Method, which looks at the absorbance of a material with a sharp cut-off wavelength against water as the blank. The idea behind this concept is that all light is absorbed below the cut-off and all light is transmitted above.

The new proposed method, the Solution Filter Ratio Method, uses the same cut-off filter materials but requires that the material be measured with a path length of 1cm against the same material with a path length of .5 cm as a blank. With smaller wavelengths towards the cut-off wavelength, the absorbance of the measurement increases and shows a peak whose height serves as the criterion for the amount of stray light present.

Assessment of Both Methods
The Solution Filter Ratio Method gives two valuable advantages over the Specific Wavelength Method:

1.)    More accurate absorbance measurements: the peak absorbance is measured in contrast to a selected wavelength close to the cut-off wavelength
2.)    Optimal Range of Spectrophotometer: measurements are taken at lower absorbance values

CertiRef™ Automatic Verification Test
Performance verification on a regular basis can be extremely tedious and time-consuming. A simple solution is to use modules for automated performance verification, including certified reference materials. The CertiRef module from METTLER TOLEDO offers a full US Pharmacopoeia compliant solution for the automated performance verification of the UV/VIS Excellence Spectrophotometers. Once the tests are complete, comprehensive report documents are automatically generated, minimizing user interference.

For more information on these topics, please download our Whitepaper on Stray Light and Performance Verification and visit our CertiRef Automatic Verification Test.


METTLER TOLEDO is a leading global manufacturer of precision instruments. The Company is the world’s largest manufacturer and marketer of weighing instruments for use in laboratory, industrial and food retailing applications. The Company also holds top-three market positions in several related analytical instruments markets and is a leading provider of automated chemistry systems used in drug and chemical compound discovery and development.

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