Stray light (stray radiant power ratio or SRPR) is an easily undetected source of error in UV-visible spectrophotometry, and when present causes depressed absorbance readings at high absorbance values
Instrument manufacturers usually quote stray light at one or sometimes two wavelengths, but it can and will vary over the range of the instrument, so there can be uncertainty about measurement accuracy at other wavelengths.
It also often increases with time due to the ageing of optical components and light sources.
Pharmaceutical GxP (where x = L, M, etc) protocols would therefore suggest that it is important to know the level of stray light as near as possible to the measurement wavelength, and the newly revised test method ASTM E387-04 specifies a range of liquid filters that can be used to measure stray light at six different wavelengths across the UV and visible spectrum.
Most of the standards are salt solutions.
For long-term stability and reliability it is preferable to seal such test solutions into the cuvettes (cells) that will be used for measurement.
Starna has been supplying heat-sealed solution standards for calibrating spectrophotometers for many years.
However one of the stray light standards - used to check stray light at 280nm, an important wavelength for DNA and protein measurements - is liquid acetone, which is also heat- sealed.
According to John Hammond, Starna's technical and marketing manager for analytical products, the company is the only supplier that can offer complete sets of sealed stray light standards for E387-04.
All Starna CRMs are covered by a lifetime guarantee providing they have been used and stored correctly, and have been re-certified every two years.
