Cisplatin, mono- and diaquacisplatin were measured in aquatic samples and in diluted urine of a cancer patient by HPLC-ICP-MS. On-line IDMS was applied for accurate, species unspecific quantification. Limits of detection of 0.74, 0.69 and 0.65 µg L−1
(3 s criterion) were calculated for cisplatin, monoaqua- and diaquacisplatin, respectively. Degradation kinetics of 6 × 10−6 M cisplatin were determined over a period of 48 h in solutions containing 100, 50 and 0 mg L−1 chloride, showing the suitability of the HPLC-ICP-MS method for kinetic model studies. The first order rate constants k1 of cisplatin aquation for the three chloride concentrations were 1.79 × 10−5, 1.68 × 10−5 and 2.06 × 10−5 s−1. For cisplatin anatation (second order reverse reaction), rate constants of k−1
= 6.5 × 10−3, 5.8 × 10−3 and 4.1 × 10−3 M−1 s−1 could be assessed. At low chloride levels, no equilibrium was established between cisplatin and its degradation products. It was found that the intermediately formed mono- and diaquacisplatin-products started to decay after several hours. Diluted urine of a cancer patient contained the parent drug cisplatin and a considerable fraction of highly active monoaquacisplatin, as well as several unknown platinum species.