Measuring Deltamethrin Levels on LLINs

The cyanopyrethroid field test (CFT) was used to measure surface levels of deltamethrin on 5 pieces of LLIN out of the same 10 used for the cone bioassays^{13 (link)}. From the rectangular nets, the pieces were obtained from sides A, B2, C, D2 and E2, and on conical nets from sides A, C, D, G, and H (Figs. 5, 6). In brief, for each net piece, two 13-mm diameter Whatman 597 filter paper disks were wiped using magnets (magnetizing force = 35,000 Oersted) 30 times across 90 mm of net material on both the outer and inner surfaces. The amount of deltamethrin on both filter papers was measured by comparison with filter paper disks containing known quantities of deltamethrin (calibration standard). The sample disks and the calibration disks containing known amounts of deltamethrin were placed in 24-well polystyrene flat-bottomed tissue culture plates. A 0.2 ml aliquot of a solution containing 30 mg/ml of 1,2-dinitrobenzene and 4-nitrobenzaldehyde dissolved in methyl cellosolve (2-methoxyethanol) was added to each well. After allowing the disks to soak in the reagent for five minutes, the colorimetric reaction was activated with the addition of 0.05 ml 0.4 N sodium hydroxide. The reaction was allowed to proceed for five minutes whereupon the intensity of the purple color was recorded by digital photography using a standard digital camera. Deltamethrin concentrations were determined by comparing the color intensity of the sample disks to the calibration standards using image analysis techniques as described previously^{13 (link)}.