An impurity detector (10) atomises a liquid sample by injecting it into a chamber (11) with a gas. In the chamber (11) the liquid is heated, preferably by heating the gas prior to injection, to cause all the liquid to evaporate. The solid impurities contained in the liquid are thus entrained in the gas and carried to a detector region (17). The chamber is arranged so that all the gas and all the particles arrive at the detector region (17). A particle detector (19) measures the number and size of the particles and from that measurement, the concentration of impurities in the sample can be determined. A first measurement is made with the liquid being fed directly to the impurity detector (10), and then a second measurement is made with the liquid being filtered by a filter unit (64) prior to being fed to the impurity detector (10). This permits two separate measurements to be made, and thus calculation of the amounts of soluble and granular impurities present. The system permits on-line operation, and produces accurate results.