Instrument for generation of volatile organic compounds for the development of chemical sensors and evaluation of tube-type diffusive sampling

Abstract

An automatic calibration apparatus for the dynamic generation of organic vapours was developed. The accurately controlled stream of nitrogen was drawn at a low flow-rate through a thermostated container filled with the standard substance, thus generating a continuous stream of saturated vapour of the compound. The compound holder vessel was thermostated at −16°C. A large stream of pure carrier gas was mixed with a low stream of substance in a mixing chamber for dilution. The fittings were manufactured from PTFE, and tubes were made of special PTFE with an inert inner surface to eliminate the wall adsorption and to decrease the cross-diffusion. Moisture interferences were reduced using a Nafion membrane filter. The vapour generator was validated by diffusive sampling and gas chromatographic methods. Standard mixtures have been prepared containing toluene at concentrations ranging from 3 to 3000 ppm. The combined uncertainty of preparative and analytical error components associated with the concentration of the analytes at the 95% confidence level typically ranges from 2 to 5% relative, depending upon the concentration. The measured and the calculated values were compared and good correlation ( r 2>0.99) was found.