A liquid scintillation counter specifically designed for samples deposited on a flat matrix

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Abstract
A prototype liquid scintillation counter has been designed to count samples deposited as a 6*16 array on a flat matrix. Applications include the counting of labelled cells processed by a cell harvester from 96-well microtitration plates onto glass fibre filters and of DNA samples directly deposited onto nitrocellulose or nylon transfer membranes (e.g. 'Genescreen' NEN) for genetic studies by dot-blot hybridisation. The whole filter is placed in a bag with 4-12 ml of scintillant which is sufficient to count all 96 samples. Nearest-neighbor intersample cross talk ranged from 0.004% for 3H to 0.015% for 32P. The background was low: 1.4 counts/min for glass fibre and 0.7 counts/min for 'Genescreen' in the 3H channel, whilst for 14C the respective figures were 5.2 and 4.3 counts/min. The counting efficiency for 3H-labelled cells on glass fibre was 54% (E2/B-2053) and 26% for tritiated thymidine spotted on 'Genescreen' (E2/B=980). Similar 14C samples gave figures of 97% (E2/B=1775) and 81% (E2/B=1526) respectively. It was also found possible to count the electron emission from samples containing 125I and 51Cr. The saving in scintillant and vials is considerable and the small sample size reduces the volume of radioactive waste by more than 95%.