Calibration Standards for Checking the Wavelength Accuracy
The ideal standard to measure wavelength accuracy has very narrow, well defined peaks at a series of wavelengths throughout the UV and visible range. There are several standards suitable for wavelength calibration, which are described in different regulations. The following table shows an overview of the calibration standards offered by Hellma and their reference in the most important regulations:
| Checking for | Calibration Standard | Pharmacopeia | ASTM | ||
| EP | DAB | USP | |||
| Wavelength Accuracy | Holmium Oxide Liquid Filter | X | X | X | X |
| Holmium Oxide Solid Filter | X | X | |||
| Didymium Glass Solid Filter | X | X | |||
According to European Pharmacopeia checking of the wavelength accuracy is described as follows: “Verify the wavelength scale using the absorption maxima of holmium perchlorate solution R [...]. The permitted tolerance is ±1 nm for the ultraviolet range and ±3 nm for the visible range”.
European Pharmacopeia specifies four wavelengths for holmium perchlorate solution filters: 241.15 nm, 287.15 nm, 361.5 nm and 536.3 nm. The spectrum contains some additional peaks which can also be used for checking the wavelength in principle.
Compared with the holmium oxide liquid filter, the holmium oxide glass filter possesses a slightly weaker pronounced spectrum. Particularly in the deep UV range the holmium peaks are affected by the absorbance of the carrier glass. A similar effect can be observed with the didymium glass filter.
For common wavelength accuracy standards, because the peaks are not ideally symmetrical, changes in the slit width of the instrument will slightly affect the measured position of the peaks. Because of the narrow bands of the peaks the heights of the measured maxima will vary with a change of the slit width. Therefore standards for checking the wavelength accuracy are in general not suited for checking the photometric accuracy.