Z height dimension of spectrophotometer cells / cuvettes

To find the z height of many common makes/models of instruments, please use our Cell Height Finder.

What is the "z height" of a spectrophotometer cell/cuvette?

The z height dimension of a spectrophotometer cell / cuvette is defined as the distance from the base of the cell to the centre of the sample compartment window or aperture (i.e. the area through which the light passes).

The z height is normally specified for small volume cuvettes and flow cells, where there is a restricted window aperture.

For example the z height should be specified for the following  Starna cuvettes / cells: Types 16 & 26 sub-micro, Type 19 ultra-micro, Types 73, 74, 75, 583, 584, 576, 577, & 585 flow and tablet dissolution.

Z dimension


Why is the Z Dimension important?

It is essential when using a a small volume cell / cuvette that the sample is positioned within the centre of the instrument light beam ("centre beam height" of the instrument) - otherwise you will likely obtain erroneous or noisy measurements. Any light energy which does not pass through your sample should preferably be blocked (or masked) by black material. If the cell is not properly masked then the measurement sensitivity will likely be diminished (See our article on Why black walled cells? for more information).

Why does z height vary between instruments?

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 optical and engineering decisions taken to ensure that the instrument performs to it's published specifications.

Where can I find the z height of my instrument?

The correct ‘z’ height or centre beam height for an instrument should be detailed in the literature supplied by the manufacturer with the instrument.

We have also compiled a tool for finding the z-height of many common models/makes of instruments on our Cell Height Finder page.

How can I determine the Z Dimension of my instrument myself?

Some instruments have cell holders with different z heights, or where the z height can be varied in respect to the alignment with the instrument light beam. It is therefore useful to know how to determine the z height oneself.

Method 1

You can test for the Z dimension of your instrument yourself very easily. Simply follow these steps: 
1) Cut a piece of dense white card or paper 12 mm wide and 100 mm long (approximately). 
2) Make a hole in the centre of the paper about 3 mm in diameter (just push a ballpoint pen through the paper) 8.5 mm from the bottom of one end and 15 mm 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.

Method 2

Alternatively if you can easily access the sample compartment, a simple method to determine this height would be to set the instrument to measure at a visible wavelength (546 nm for example), insert a piece of white card into the sample compartment and then mark the card with a pen/pencil where the beam intersects. The distance from the bottom of the card to the mark should give the approximate height of the beam.