Microtrap modulated flame ionization detector for on-line monitoring of methane.

A microtrap for on-line analysis of trace methane was developed. Silicosteel tubing, 15 cm long, 1.02 mm i.d. and 1.59 mm o.d. was packed with Carbosphere 80/100 mesh and placed between the standard gas source and the flame ionization detector (FID) to be used as a preconcentrator and an injector. The sample stream was passed continuously through the microtrap that was heated at a fixed interval by an electric pulse. To achieve the highest adsorption and desorption efficiency, parameters affecting the response of the system were optimized, and adsorption temperature in the range of 25 to -50 degrees C was studied. The response from the microtrap as compared to that from direct-flame ionization detector was enhanced by as much as 260 times. The on-line microtrap system showed good linearity (r2 > 0.99), low detection limit (28.3 ppbv) and good long term stability (relative standard deviation, RSD of less than 5.0%). Therefore, this simple device is suitable for on-line analysis of trace methane and similar small molecules in the environment as in high purity gases.