Use a clean rectangular cuvet that is free of cracks. Plastic cuvets are acceptable when absorbance measurements will be recorded in the visible range of the spectrum (about 400 to 700 nm wavelengths), but quartz cuvets are necessary if measurements are made at lower wavelengths or if the sample liquid will destroy a plastic cuvet.
To ensure a constant path length, the cuvet should be placed the same way in the sample compartment for each measurement. Therefore, it is important to mark which face of the cuvet is placed in the path of the light. One common method of marking cuvets is to cover one face of the cuvet with tape, but any indentifying mark that does not permanently damage the cuvet is acceptable. Some brands of glass or quartz cuvets have only one clear path, so you will not need to place tape on the cuvet.
