Sorting or separation of cells can be carried out using a number of physical and biological agencies. Perhaps the most extensively and frequently used, but least appreciated, is gravity. Red cells settle out to form the haematocrite layer in heparinized blood leaving the white cells in the buffy coat. Variations on this theme include density gradient sedimentation (Ficol and Ficol–Paque) and cell electrophoresis. The latter uses charge in combination with gravity in the separation process. Magnetism can also be used. Macrophages which have ingested iron particles fall prey to such manoeuvres. Biological procedures include complement lysis of specific antibody tagged unwanted cells leaving the cells of interest intact. Also, magnetic beads coated with antibody can be used to select positively for the cells of interest. Some of these procedures do not yield particularly pure sub-fractions and they will not be discussed further because this book is about flow cytometry. However, they should not be forgotten as they can, and frequently should, be used in conjunction with flow technology (see sections 6.3 and 6.4).

Ink-jet writing

Ink-jet writing (Sweet, 1965) was developed in the early and mid 1960s to attempt to cut down on the noise generated by ‘classical’ mechanical printing and to be able to write on fragile objects. One of the latter applications was date stamping individual eggs. This is notoriously difficult with traditional mechanical stamping devices which inevitably result in a significant proportion of breakages. In ink-jet writing the ink is forced at high speed through a fine nozzle and the jet breaks up into droplets.