Z Dimension of Spectrophotometer Cells
What does Z Dimension mean? Z dimension
Z Dimension is defined as the distance from the bottom of the cell compartment in your instrument to the center of the light beam. 
Why Z Dimension is Important?
Each instrument manufacturer makes engineering decisions for the design of their instrument to assure that it performs optimally. The height, width, focus and placement of the light beam in any instrument is based on the engineering decisions to assure that the instrument performs to their published specifications. It is important to you, as the instrument user, only when you are attempting to use a small sample volume.
You must assure that your smaller volume is placed in the centre of the light beam and that any light which does not pass through your sample is blocked (or masked) by black material. If the cell is not properly masked then the sensitivity will be eroded. Z Dimension of various manufacturers
We have compiled this list of Z Dimensions from some of the instrument manufacturers. If you find a discrepancy in this list, please let us know and we will investigate.

Manufacturer Z Dimension Manufacturer Z Dimension
Beckman® 8.5 mm Bio-Rad 8.5 mm
Hewlett Packard® 15 mm Hitachi® 8.5 mm
Perkin-Elmer® 15 mm Pharmacia 15 mm
Shimadzu® 15 mm Spectronics® 8.5 mm
Varian® 15 or 20 mm Shimadzu 15 mm
Thermo Spectronic 8.5 & 15
How can I determine the Z Dimension of my instrument?
You can test for the Z dimension of your instrument very easily. Simply follow these steps: 
1) Cut a piece of dense paper 12mm wide and 100mm long. 
2) Make a hole in the centre of the paper about 3mm in diameter (just push a ballpoint pen through the paper) 8.5mm from the bottom of one end and 15mm from the bottom of the other. 
3) Turn on your spectrophotometer in %T mode and set to any reasonable wavelength. 
4) Place the piece of paper with the hole in the end into the cell holder as if it were a cell. Whichever end (either 8.5mm or 15mm) transmits light will tell you the Z dimension of your instrument. 
5) If neither works then make up another piece of paper with holes placed at different dimensions and determine which works best.