Measurement of Atmospheric Fenitrothion and Aminocarb Concentrations Near the Spray Area

Fenitrothion and Aminocarb are the insecticides commonly used in spruce budworm control programmes. Significant amounts of the pesticides remain airborne long after completion of aerial spraying and their vapours, as well as aerosol, drift over long distances. Their concentration, dosage, range and residence time were determined using a method described below, which is sufficiently rapid to allow the processing of over 100 samples a day. Samples of airborne pesticides were collected in glass adsorber tubes 6 mm O.D. × 75m, each packed with a 1cm plug of 60/80 mesh Tenax GC (∼ 25mg). The capacity of the Tenax adsorber, defined as the maximum air volume which could be sampled before pesticide elution occurred, was determined to be over 7001 for both fenitrothion and aminocarb. The ambient air was drawn through the adsorber tubes by means of a battery-powered pump incorporated in a sampling device. Three types of these devices were used. The first sampler, designed for unattended operation, had a built-in electronic clock, which allowed us to pre-programme the sampling period and a start time. It sampled air at a rate of 21/min, usually for 4 hours, and was used to determine time-integrated concentrations (dosages). The second type of sampler also had the capability for unattended operation. An electronic timing device and a set of valves allowed it to draw air through eight consecutive adsorber tubes for predetermined short periods and was used to measure time-wise variations of concentration. The sampling rate of this device was 11/min. When very low concentrations of pesticide were anticipated and time-wise variations of concentration were to be measured, a third type of sampler was used. It sampled air at a rate of up to 81/min for short periods of time. An unlimited number of adsorber tubes could be attached consecutively to the sampler by an operator. The trapped vapours and aerosol were thermally released from the adsorber and transferred by a stream of carrier gas directly into a gas chromatograph (G.C.). The G.C. was equipped with a special adaptor for ease of adsorber coupling, and a 4-port switching valve. A thermoionic specific detector allowed concentrations in the picogram per liter range to be determined with a precision of better than ± 10%. Eight-stage Anderson 2000 Inc. cascade impactors, modified according to Crabbe et al., were used to measure drop-size distributions of pesticide formulation aerosol in a range of 1-30 μm. To eliminate experimental error caused by evaporation of pesticide from impactor plates, tris (2-ethyl hexyl) phosphate (TEHP) was added to the formulation and drop-size distribution was obtained from the TEHP deposits. The minimum amount of TEHP which could be quantitatively determined with ±5% precision was 20 pg per plate.