A calibration gas generation method and apparatus for generating a selectively humidified calibration gas to a measurement probe includes a delivery conduit having a conduit inlet adapted to receive a carrier gas stream and a conduit outlet for delivering a calibration gas stream. The apparatus is provided with a first injection unit having a first intake in fluid communication with a first reservoir and a first outlet in fluid communication with the delivery conduit, the first reservoir being adapted to hold a first analyte in liquid form, and a second injection unit having a second intake in fluid communication with a second reservoir and a second outlet in fluid communication with the delivery conduit, the second reservoir being adapted to hold a humidificant in liquid form. The apparatus further includes at least one vaporizer downstream of the first and second outlets and upstream of the conduit outlet for converting the analyte and humidificant liquids to vapor form and delivering a calibration gas including the carrier gas, analyte vapor, and humidificant vapor to the conduit outlet.