A gas-liquid chromatographic method to determine size spectra of droplets of an aerially applied non-volatile spray mix diposited on Kromekote cards

A non-volatile oil-based spray mix of a low-vapour-pressure insecticide, aminocarb, containing an oil-soluble red dye was applied at a dosage rate of 70 g AI in 1.5 litre ha-1, using a fixed wing aircraft equipped with four 'Micronair' AU3000 atomizers, over a 1000 X 500 m spray block selected in Bathurst, New Brunswick, Canada. Spray was applied twice, at an interval of five days, to provide a total dosage rate of 140 g AI in 3.0 litre ha-1. Spray mass recovery was assessed on glass plates and droplets were collected on 'Kromekote' cards, both at ground level. The stain sizes were grouped into different categories. The area containing the stains was excised, and the aminocarb present was quantified by gas-liquid chromatography (GLC). The mass of aminocarb per droplet in each stain size category was evaluated. From the mass, the spherical droplet diameter (d), number and volume median diameters (DN.5 and DV.5 respectively), a new parameter [mass (of aminocarb) median diameter] (DM.5), and the droplet size spectra were calculated. The DM.5 for the first application was 56 micrometer, which was identical to the DV.5, whereas the DN.5 was smaller at 45 micrometer. The corresponding values for the second application were: DM.5 = DV.5 = 63 micrometer, but the DN.5 was 53 micrometer. Because the spray mix was non-volatile, all the droplet size spectra parameters were identical both at spray release height and at ground level. The present study has provided, for the first time in the literature, a novel method to determine directly the spherical diameters of the droplets deposited on artificial samplers, without having to go through the tedious procedures of spread factor measurements under laboratory conditions. In fact, the present study has made it possible to calculate spread factors under field conditions, by using the stain diameters measured and the spherical diameters calculated from the aminocarb concentration levels.