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

Product Description:

Stray light, also called Stray Radiant Energy or Power, is any light reaching the instrument detector other than that selected by the monochromator. It can be due to optical imperfections or stray reflections within the monochromator itself or to light leaks in the optical system. The detector cannot discriminate between the analytical wavelength and the stray light, so the stray light introduces an error in the measured absorption. The stray light is not absorbed even at high concentrations of the absorbing species, so its effect is a negative deviation from the linear relationship between concentration and absorbance (the Beer-Lambert law) on which most quantitative determinations are based.

Stray light is wavelength and instrument dependent. It can be present at any wavelength but is most noticeable when the energy throughput of the system is relatively low, for example in the far UV region. At these wavelengths, any deterioration in the instrument optics or UV light source will exaggerate the apparent stray light. Checking the instrument in the far UV region, even if this is not the area for which it will be primarily used, is an excellent way to monitor the condition of the instrument optics.

A range of materials allows stray light to be estimated at different wavelengths:

These reference materials allow you to detect the presence of stray light in your instrument. Each material cuts off all light below a specified wavelength. Any light detected by the instrument below that wavelength must, by definition, be stray light. In this USP approved method of estimating stray light, also known as the Meilenz method, the 10 mm reference material cell is scanned using the 5 mm cell as a blank. The resulting spectrum is in the form of a peak. The peak wavelength will vary according to the optical configuration and stray light characteristics of the instrument under test, but the absorbance value at the peak must exceed 0.7 A to meet the requirements of the USP. This graph shows scans of the 10mm potassium chloride reference cell on three different instruments, using the 5 mm potassium chloride cell as blank. The peak wavelength is different on each instrument due to their different stray light characteristics, but in all three cases the absorbance maximum is greater than 0.7 A, so all three instruments satisfy the USP stray light requirement.

Suggestions for Use:

The procedure for using the stray light references is similar for all materials. Insert the 10 mm stray light reference cell in the sample beam cell holder and the supplied 5 mm blank cell in the reference beam cell holder of the spectrophotometer. Scan over the stated usable range and note the absorbance at the peak. This absorbance value at the peak must exceed 0.7 A to meet the requirements of USP Chapter <857>.

Periodically scan with the same instrument configuration and compare the results. Over time you will have a data trail for your instrument that will make the detection and correction of any problems relating to stray light much more effective.