At its 162nd session in November 2018, the Ph. Eur. Commission adopted a new version of one of its major general methods. Ultraviolet and visible absorption spectrophotometry (2.2.25) has undergone extensive revision and is published in the 10th Edition (implementation date 1 January 2020). This general method is one of the original cornerstones of pharmacopoeial testing, as it has multiple applications ranging from identification to content determination.
However, UV-Vis spectroscopy techniques have evolved over the years leading to a decline in the popularity of direct detection in cuvettes. Thus, while the method is still referred to in many general texts and in more than 500 individual monographs, the number of these references has nonetheless dropped in the last few decades. This can be explained by the rising popularity of infrared spectroscopy as a replacement for UV-Vis absorption spectrophotometry-based identification tests and also by the phasing out of assays based on UV-Vis absorbance in favour of titrations or HPLC methods. Nonetheless, UV-Vis spectroscopy is such a versatile analytical technique that it has found more contemporary applications, and is still used indirectly in most individual monographs, particularly as a detection technique after chromatographic separation.
The text has been completely overhauled by a sub-group of ad hoc specialists in UV-Vis spectroscopy within the Ph. Eur. General Methods Working Party. As mentioned, it now covers not just cuvette-based measurements but also more integrated applications when the UV-Vis detector is coupled with a chromatographic system or used as part of process analytical technology (PAT). In addition to this extension of its scope, other major changes to the chapter include:
- improved structure of the text;
- more detailed description of equipment and its operation;
- recognition of the specificities of the different uses of UV-Vis absorption spectrophotometry methods with recommendations and requirements for each;
- a new section on system suitability testing;
- introduction of nicotinic acid as an alternative to potassium dichromate (REACH Annex XIV) for control of absorbance accuracy;
- clear and comprehensive requirements for the control of equipment performance (wavelength accuracy, absorbance accuracy, photometric linearity, limit of stray light, resolution/spectral bandwidth), whilst retaining some flexibility in the choice of reference materials.
The revised chapter on Ultraviolet and visible absorption spectrophotometry (2.2.25) is published in the 10th Edition of the Ph. Eur. (available in July 2019) along with many other new, revised or corrected texts.