Fluorometric assays for DNA deposited on filters

Procedures for collecting DNA from dilute solutions as small spots of the insoluble cetyltrimethylammonium salt deposited on either glass fiber or Millipore filters are described. This method of concentration facilitates the fluorescent assay of DNA, either by a microreaction in which the spots on glass fiber are punched out, reacted with a small volume of 3,5-diaminobenzoic acid, and then assayed in a microcuvette or, alternatively, by spotting the DNA onto black Millipore filters and soaking these in ethidium bromide so that the fluorescence of each spot can be measured by direct scanning. These methods are capable of detecting DNA amounts as low as, respectively, 50 and 10 ng. The most important advantage in this procedure is not, however, its intrinsic sensitivity but, rather, the ability to recover and assay DNA from very dilute solutions. The use of silver both to stain DNA on filters and to enhance the sensitivity of the 4',6-diamidino-2-phenylindole fluorescence method of estimating DNA is also briefly described.