A simplified hot-wire cutting aid for PVC trays used in radioimmunoassay.

The ease and convenience of a hot-wire cutter for the separation of wells from PVC radioimmunoassay trays has been demonstrated.' In constructing a similar apparatus, it was felt that a worthwhile simplification could be achieved by arranging to push the PVC tray past a fixed hot wire. The wire can simply be held under spring tension, and the springloaded telescopic arm previously employed' be eliminated. We feel that this modification may be of interest to laboratories without access to sophisticated workshop facilities. The general layout of the apparatus is shown in Fig. 1 (a) and (b). The baseboard is laminated from two layers of 12 mm plywood. Basically the apparatus consists of a heated wire stretched between two rails which guide the PVC tray as it is pushed past the wire. Each rail has a shoulder which permits the tray to sit proud of a rubber centrifuge block moulded to hold the wells of the tray. The rails are each machined in two pieces from solid PVC, but could readily be laminated from sheet plastic or plywood. Two PVC strips are screwed between the tops of the rails to prevent the tray from rising during the cutting operation. A laminated PVC block fits between the rails and is used to push the tray and support block past the heated wire by means of a handle of 20 mm diameter PVC rod. The outer rail is drilled to clear a brass terminal rod mounted through the upper layer of the baseboard. One end of the 26 g Nichrome wire is held in a horizontal hole in this terminal by a set screw. The wire is then threaded through a second brass rod set into the inner rail and is looped and secured to a spring. The spring exerts a force of 1150 g on the cold wire. By angling the spring away from the line of the wire between the terminals, a good electrical contact with the inner terminal is ensured. Screw threads on the terminals allow the height of the cutting wire to be varied. An isolating transformer with a nominal output of 12 V and 8 A is fitted, as previously suggested.' To achieve optimal heating with a 120 mm cutting wire, a further 275 mm is wired in series to provide the appropriate resistance. This is conveniently done