The application discloses several methods of detecting particles moving in a gas flow, a detection means (70), a method of improving the linearity of the signal produced by a triboelectric particle detector, a method to test a signal path of an emission monitor, an emission monitor for detecting particles, and a method of detecting faults in a triboelectric probe of system for detecting particles entrained in a gas flow. The particle detection means (70) includes a gas flow deflection means (40) having an upstream face (43) and a downstream face (45) and a detector (42, 44) associated exclusively with the downstream face (45). Deflecting the gas flow creates vortices that have little effect on the large particles, but cause the small particles to move close to the detectors on the downstream face. Therefore the downstream detectors (42, 44) are particularly sensitive to light particles. The method improves of small particles and provides accurate particle counts.