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Certified Reference Materials for Checking for Stray Light

Stray light (false light) is light being detected but does not belong to the bandwidth of the chosen measuring wavelength. The effect of stray light is caused by light scattering, diffraction or malfunction of the instrument. Stray light causes a decrease in the measurable absorbance range and reduces the linear relationship between concentration and absorbance of the instrument.

Stray light can be a problem at any wavelength, but if energy throughput of an instrument decreases, as when moving into the UV range, the influence of increasing stray light will more and more affect the measured values.

To measure stray light, filters are needed which would ideally absorb all light of the wavelength at which the measurement is to be performed and transmit higher wavelengths (so called cut-off filters). Hellma Analytics stray light filters do not allow light with wavelengths lower than a certain wavelength to pass (cut-off wavelength). Therefore any indication of light transmission below the cut-off wavelength must be stray light.

To measure the amount of stray light realistically, a filter with a cut-off wavelength as near as possible to the measuring wavelength should be chosen. The following table shows the stray light filters offered by Hellma. They are suitable for stray light measurements at cut-off wavelengths of 200 nm, 259 nm,325 nm and 385 nm or lower.

Checking for Calibration Standard Pharmacopeia ASTM
EP DAB USP
Stray Light Potassium Chloride Liquid Filter X X X
Sodium Iodide Liquid Filter X
Sodium Nitrite Liquid Filter X

According to European Pharmacopeia (8th edition) checking for stray light is described as follows:

„Stray light may be detected at a given wavelength with suitable filters or solutions: for example the absorbance of a 12 g/l solution of potassium chloride R in a 1 cm cell increases steeply between 220 nm and 200 nm and is greater than 2.0 at 198 nm when compared with water as compensation liquid. Suitable certified reference materials may also be used.“

According to USP 857 (39th edition) checking for stray light is described as follows:

“When using a 5 mm path length cell (filled with the same filter) as the reference cell and then measuring the 10 mm cell over the required spectral range, analysts can calculate the stray light value from the observed maximum absorbance using the formula”.

This means, for a single beam photometer, first the reference filter with 5 mm path lenth (filled with the same solution) is measured, afterwards the 10 mm stray light filter is measured for the same required spectral range. For a two beam photometer, the stray light filter with a path length of 10 mm is measured against the reference filter (filled with the same solution) with a 5 mm path length .

The stray light value can now be calculated with the received absorption maximum with the following formula:
Sλ = 0.25 x 10^(-2A) λ

Herefore the following conditions are valid:
Aλ ≥ 0.7 Abs and Sλ ≤ 0.01

Aλ = measured maximum absorption at wavelegnth λ
Sλ= calculated stray light value at wavelength λ

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