沢水,雪などの淡水に含まれる微量の鉛を同位体希釈法を用いる質量分析法で定量した。試料中の鉛の分離は208Pbスパイクを試料水へ加えて同位体平衡としたのち,pH7.5～8.5でジチゾン抽出を行なった.抽出した鉛はレニウムフィラメントの上へシリカゲルとともに固定し,固体試料用質量分析計で同位体イオン流強度を求め,その強度比から同位体希釈法により,その鉛含量を測定した.鉛含量は沢水で2×10-2μg/kg,雪で3μg/kgであった.本法は1～1000×10-9g鉛の定量にじゅうぶんな感度と精度をもち,試薬からの鉛汚染は0.85×10-9g鉛にすぎなかった.
Trace lead concentrations in natural stream waters and snow were determined by isotope dilution mass spectrometry.<BR>Owing to the extremely low concentrations of lead, cleaning of sampling containers and analytical laboratory wares were carried out with special care. Sampling bottles of conventional polyethylene were cleaned by 2 solutions of cold concentrated nitric acid and two solutions of 1% nitric acid. Chemical wares of FEP teflon and quartz were cleaned by 3-day exposures to two solutions of hot concentrated nitric acid and a 3-day exposure to 1% nitric acid.<BR>After the sampling in the field, samples were kept frozen until time of analysis.<BR>A known amount of <SUP>208</SUP>Pb spike was added to about 200 m<I>l</I> of acidified sample solution and cooked one day at 55°C in order to assure the isotopic equilibration of spike and sample lead. Lead was then extracted with 44 m<I>l</I> of dithizone-chloroform solution(3.8 mg dithizone/250 m<I>l</I> chloroform) at pH7.58.5 Lead was backextracted into an aqueous layer of 10 m<I>l</I> of 0.1 <I>N</I> nitric acid. The lead extraction by 5 m<I>l</I>of dithizone solution was repeated in the presence of 1 m<I>l</I>of 25% ammonium citrate and 2 m<I>l</I> of 1% potassium cyanide at pH7.58.5 After the back-extraction with 1 <I>N</I> nitric acid solution, the sample was mounted on a rhenium filament with silica gel and phosphoric acid. The isotopic ratios were measured in a solid source mass spectrometer (12 inch radius, single focussing, thermal emission source, equipped with electron multiplier) at the California Institute of Technology. The lead concentrations were 0.02 μg/kg in stream water and 3 μg/kg in snow. The error is considered to be within 3%. The lead contamination from reagents was 0.85×10<SUP>-9</SUP> g Pb per one extraction. No lead contamination was detected from the sampling bottle.
