EVALUATION OF A DIFFUSIVE SAMPLING METHOD FOR THE DETERMINATION OF ATMOSPHERIC AMMONIA

A diffusive sampling method is presented for the determination of atmo spheric levels of ammonia based on the collection of ammonia at the ph osphoric acid coated bottom of small glass vials and subsequent flow i njection analysis of the trapped ammonium. In order to avoid effects o f varying wind velocities on the sampling rate, the tube-type samplers are covered with a hydrophobic microporous membrane. The experimental ly obtained uptake rate of the diffusive samplers (i.e. 7.8 mi min(-1) ) is in excellent agreement with the theory. The analytical characteri stics of the method with respect to sensitivity, selectivity, dynamic range, precision and practicability are evaluated in detail. As the de tectability of the proposed method is related to both the mass collect ed during exposure and the mass sensitivity of the detection step, the role of microanalytical procedures in general, and significance of me ticulous control of contamination in particular are discussed. Under o ptimum conditions the gas-phase detection limit achievable (in terms o f time independent exposure dose value) is about 2.3 x 10(3) mu g m(-3 ) s(-1), which permits measurements at low mu g m(-3)-levels with a ti me resolution of a few hours. Application of the method to indoor and outdoor measurements proved its suitability for routine purposes.