ESP particle collector (1) for collecting particles in a particle containing gas stream, comprising an inlet section (4), a collector section (6), and an electrode arrangement (8), the inlet section comprising a flow tube (10) defining a gas flow channel (12) therein bounded by a guide wall (24) extending between an entry end (14) and a collector end (16) that serves as an inlet to the collector section (6), the entry end comprising an inlet (28) for the particle gas stream and a sheath flow inlet portion (26) for generating a sheath flow around the particle gas stream, the collector section comprising a housing (18) coupled to the flow tube, and a collector plate (20) mounted therein having a particle collection surface (23). The ESP particle collector is configured to allow optical analysis of the collector plate particle collection surface to measure particles collected thereon. The electrode arrangement comprises at least a base electrode (8a) positioned below the collection surface and a counter-base electrode (8b) positioned at a separation distance L2 above the collection surface such that an electrical field is generated between the electrodes configured to precipitate said particles on the collection surface, wherein the electric field is in a range of 0.1 kV per mm to 1.5 kV per mm, with an absolute voltage on any said electrode that is less than 10 kV, and wherein a ratio ratio I of a radius LI of said inlet at the collector end divided by said separation distance L2 is in a range of 0.8 to 1.2.