The prevention of cross-contamination in PBI determinations.

‘THEUSE of heavy aluminum foil in our ashing procedure for PB1 (1), introduced 3 years ago, has proved extremely successful in preventing cross-contamination. A Temco double-door muffle furnace is used for which three steel buckets were made; small buckets, 4 in. wide, 6 in. long, and 6 in. deep, and one large bucket, 6 in. wide, 8 in. long, and 6 in. deep. A steel shovel was made to aid the technician in introducing and removing the buckets from the furnace. The tubes are prepared as follows. A piece of aluminum foil, about 10 in. long and 4#{189} in. wide is cut. A pair of tubes (duplicate specimens) is placed approximately 21/2 in. from the top edge of the foil. The sides are folded together and the foil wrapped up from the bottom. The covering stands at least 21/2 in. above the top of the tubes. The aluminum foil should extend to at least this height to prevent the vapors from flowing into neighboring tubes and causing contamination of other specimens. The rising vapors in the wrapped tubes are held within the aluminum foil cylinder and directed back down into the tube. The foil-wrapped tubes are stood upright in the bucket and heated for 3 hr. at 600#{176} ± 10#{176}, after which they are removed from the furnace and cooled to room temperature. The used foil is discarded.