Type 23 cells are identical to Standard Rectangular fluorometer cells as listed on page 26 except that instead of having an open rectangular hole at the top, a block is fused to the top of the cell with a ground hole to receive a PTFE stopper. This provides a more suitable seal for volatile liquids. A PTFE stopper is supplied with each cell. Two stoppers are supplied with cells which have a path length of 40mm or greater. A PTFE stopper is supplied with each cell.