A microfluidic lab-on-chip derivatisation technique has been optimized to achieve a rapid, automated and sensitive determination of ambient gaseous formaldehyde when used in combination with GC-MS. The method used a Pyrex micro-reactor comprising three inlets and one outlet, gas and fluid splitting and combining channels, mixing junctions, and a 2.0 m long, 620 µm internal diameter reaction micro-channel. The micro-reactor integrated three functions, that of: (1) mixer and reactor, (2) heater, and (3) preconcentrator. The flow rates of the gas sample and derivatisation solution and the temperature of the micro-reactor were optimized to achieve a near real-time measurement with a rapid and high efficiency derivatisation step following gas sampling. The enhanced phase contact area-to-volume ratio and the high heat transfer rate in the micro-reactor resulted in a fast and high efficiency derivatisation reaction. Calibration showed good linearity in the range of 26 to 331 ppb with correlation coefficients R ² = 0.988 and 0.997 for PFPH and PFBHA derivatives. For low gas phase formaldehyde mixing ratios (<26 ppb) the derivatisation solution could be repeatedly recycled through the chip enabling pre-concentration of the derivative - hydrazone. The calibration curves for this recycling approach also showed good linearity from 4.0 to 26 ppb with method detection limits (MDLs) of 2.1 ppb and 1.1 ppb for PFPH and PFBHA derivatives. The feasibility of the technique was assessed using measurements of laboratory ambient air, with formaldehyde the predominant carbonyl compound at a 12.5 ppb level. The proof of principle experiments demonstrated the potential of the approach for on-line measurements of other carbonyls including acetaldehyde, acetone and propionaldehyde.