The potassium iodide liquid filter (UV23) consists of 10 g/l potassium iodide in pure water. This filter is suited to check the stray light of a spectrometer in the spectral range between 210 nm and 270 nm, its cut off is at about 259 nm.
Two regulations - two measuring methods
Currently there are different methods used in the European Pharmacopoeia (EP) and in the United States Pharmacopeia (USP) to determine the stray light.
When checking for stray light according to Ph. Eur., the potassium iodide filter (UV23) is measured against the reference filter (UV12) filled with pure water. The measured absorbance value at the wavelength 250 nm must be ≥ 3.0 Abs.
In USP <857> (update Dec. 2022), two possible procedures are described for checking for stray light:
Here the potassium iodide filter with a path length of 10 mm is measured against the potassium iodide reference filter with a path length of 5 mm. The stray light value can now be calculated from the absorbance maximum obtained, using the following formula:
Sλ = 0.25 x 10-2∆A
The following acceptance criteria apply:
∆A ≥ 0.7 Abs and Sλ ≤ 0.01
Here the potassium iodide filter (UV23) with a path length of 10 mm is measured against the reference filter (UV12) filled with pure water. The measured absorbance value at the recommended wavelength 220 nm should be ≥ 2.0 Abs.
Procedure B of USP 857 corresponds to the procedure of the European Pharmacopoeia
Experiences from daily practice show that the values determined in this stray light measuring method are instrument-dependent. This means the wavelength of the peak varies depending on the spectrophotometer performance. It is important for the user to know that the measured maximum absorption in the checked range is decisive for this checking method and that it should be higher than 0.7 Abs.
|EP-Set: 667107 (UV23/UV12) Request this article
|Checking stray light in the spectral range of 210 - 270 nm according to EP and USP
|Potassium Iodide 10 g/l in pure water
|Cut-off wavelength: 259 nm
Slit width: 2 nm; with DAkkS calibration certificate
|Cut-off wavelength: fix
Slit width: 1 and 2 nm