As part of a commercial collaboration with Laporte Pigments, Technical Center UK, we have investigated the magnetite content of printer and photocopier toners of both Laporte's own materials and that of competing companies.

Magnetite forms a pivotal role in laser printing technologies. The main component of toner is a resin which when heated (or forced under pressure into the paper) binds the toner to the paper. Approximately $ 10\%$ of toner is the actual colourant, usually carbon. These colourants must be able to hold an electronic charge which allows them to be attracted to photoreceptors which then electrostatically transfer the toner to the charged paper.

If the colourant is of a low charge it may not stick to the developer roll and so a small amount of magnetic material is added and the developer roll contains a small permanent magnet. The magnetic field then helps hold the toner particles to the developer roll. Magnetite is the most common magnetic field creating material used.

The ability of the magnetite additive to increase the acceptance of toner onto the roll improves the reliability of the printer, reduces toner usage and the build up of machine dirt (which can have damaging effects on the printer's internal mechanisms). Toners containing magnetic materials are also important components of Magnetic Ink Character Recognition (MICR) systems where characters are read directly from documents with magnetic sensors.

Laporte asked us to evaluate the ratio of Fe$ _3$O$ _4$ to Fe$ _2$O$ _3$ in the toner samples, using spectra recorded at room temperature and $ 77\ensuremath{\unskip\,\mathrm{K}}$.

Dr John Bland, 15/03/2003