A simple analytical device was developed for performing non-instrumental immunoassay. The device consists of membrane with antibody-coated zones and filter pad acting as an absorbent body. Very low concentrations of DDT in food samples were estimated by using an improved catalysed reporter
deposition I-CARD method. The signal amplification involved biotinylated tyramine and avidin–alkaline phosphatase (ALP) conjugate. 5-Bromo-4-chloro-3-indolyl-phosphate in conjunction with nitro blue tetrazolium was used for colour visualisation. Semi-quantitative results were obtained
by visual comparison of the colour intensity of the sample spot with those of reference standards. Quantitative estimation is possible by densitometry using CCD camera. A batch of five extracted samples could be analysed in a single kit within 30 minutes. Spiked samples were analysed without
sample cleanup. The possible matrix interference was analysed. The results were comparable with gas chromatography and CCD analyses. The method is well suited for visual screening of food samples for DDT under field conditions.
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