PRACTICAL TASK P11. UV/VIS WAVELENGTH ACCURACY Outline Monochromators can lose alignment, meaning the reported wavelength is not correct. For example, setting the wavelength to 500.0 nm may in fact result in measuring the absorbance at 501.2. With the normal broad peaks in the UV/VIS region, this may not cause that much error in the absorbance measurement, but it is nevertheless still an error. To check this requires a material of accurately known wavelengths, preferably with narrow peaks. Various compounds are used, and usually in the form of a glass plate, meaning no preparation is required. Holmium oxide is the most common. Procedure (to be done on either of the Cary spectrophotometers) 1. Obtain a holmium filter. 2.
Set up the instrument in Scan mode to cover the wavelength range of the peaks in the table below. 3.
Run a baseline with nothing in either cell. 4.
Record the spectrum of the filter. 5.
Accurately record the wavelengths of the peaks closest to each of the specified wavelengths. Holmium glass wavelengths 279.3 453.4 360.8 536.4 418.5 637.5 The measured wavelength should be within ± 0.2 nm of the specified values. Reporting Requirements Use the standard Performance Check reporting sheet.