When a Universal Reflectance Accessory (URA) is installed it is important that the accessory be aligned properly so that there is not a step due to accessory or instrumental misalignment. It is important that the validation samples selected be free of artifacts and cover the frequently used ordinate ranges of the accessory. The NIST mirror satisfies this criteria for the high %R ranges. The first surface reflection of quartz (protected from the back surface reflection) works well for low reflectance.
A good sample to use for back side protected quartz is a self-attenuating microcuvette. The shiny black surface of the microcuvette is a thin quartz window place over optically black glass. This makes a useable easily available first surface quartz sample.
Below are spectra of the NIST mirror measured on a properly aligned URA over a series of angles that comprise the angular range (8 degrees to 68 degrees) of the accessory. An angular profile such as this will validate the alignment if there is not a step that exceeds 0.2 %R relative to the reflectance intensity.
Below are spectra of a black microcuvette measured on a properly aligned URA over a series of angles that comprise the angular range of the accessory. Note that there is a noise step due to the lower ordinate values but no step due to misalignment. An angular profile such as this will validate the alignment if there is not a step that exceeds 0.2 %R relative to the reflectance intensity.
If a black self-attenuating microcuvette is not available, then a simple 1 cm clear quartz cuvette can be used instead. The ordinate values are not as low as a black cuvette because of the added reflection from the back side of the quartz wall, but it can still give useful results.
Below are spectra of the 1 cm quartz cuvette measured on a properly aligned URA over a series of angles that comprise the angular range of the accessory. Note that there is a noise step due to the lower ordinate values but no step due to misalignment. An angular profile such as this will validate the alignment if there is not a step that exceeds 0.2% R relative to the reflectance intensity.
