In a spectrophotometer, stray light is light that passes by the sample and falls directly on the detector. This can lead to incorrect measurement results. Stray light may be caused by scattering or diffraction, by poor optical alignment, the use of incorrect or damaged cuvettes, incorrectly fitted sampling accessories or damaged seals around a light-tight sample chamber.
Stray light is problematic, as it reduces the range of measurable absorbance and impairs the linearity between concentration and absorbance. Cut-off filters (filters with a strictly defined spectrum) are required to check the device for stray light.
The following liquid filters are due to their very sharp spectrum excellent for qualifying the stray light of the spectrophotometer according to the regulations of the pharmacopoeias. The procedure is the same for all stray light filters.
In the European Pharmacopoeia (8th edition) the checking of 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 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."
Hellma Analytics stray light filters do not allow light to pass through them below a certain wavelength (cut-off wavelength). Any transmittance values displayed in the cut-off wavelength range therefore represent stray light. According to the Ph. Eur method, the measurement is made against the reference filter which is filled with water. The exception is the filter with acetone as it is measured against air.
In USP <857> (39th edition), the checking of stray light is described as follows:
Original text: “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 that in a single-beam spectrophotometer, the reference filter with 5 mm path length (filled with the same solution) is measured. The 10 mm stray light filter is then measured for the same required spectral range. In a double-beam spectrophotometer, the stray light filter with 10 mm path length is measured against the reference filter (filled with the same solution) with 5 mm path length.
The stray light value can now be calculated from the absorbance maximum obtained, using the following formula:
Sλ = 0.25 x 10-2Aλ
The following acceptance criteria apply: Aλ ≥ 0.7 Abs and Sλ ≤ 0.01
Aλ = absorbance measured at peak maximum at wavelength λ
Sλ= stray light value calculated at wavelength λ
The measurement indicated on the calibration certificate for the wavelength at the peak maximum refers only to the measurement taken with the UV-Vis-NIR spectrophotometer shown on the calibration certificate. This wavelength is instrument-dependent due to the different optical components installed and the resulting differences in performance, it is not applicable to other UV-Vis-NIR spectrophotometers. The indicated measurement of the wavelength at the peak maximum is not suitable for checking the wavelength scale.