Abstract: A flue gas conditioning system generates sulfur trioxide which is injected into boiler flue gas to condition it by reducing the resistance of the particulate matter entrained in the flue gas. The conditioned flue gas is passed through an electrostatic precipitator which collects the particulate matter by electrostatic precipitation. The sulfur trioxide concentration in the flue gas is measured with a mass spectrometer downstream of where the sulfur trioxide was injected into the flue gas and upstream of the electrostatic precipitator. The concentration of the sulfur trioxide in the flue gas is measured with a mass spectrometer at a point where the injected sulfur trioxide has uniformly mixed with the flue gas. The measured sulfur trioxide concentration is used to control the flue gas conditioning system, such as by comparing the measured concentration to a desired concentration range and adjusting the amount of sulfur trioxide being injected into the flue gas accordingly.