The rate constants for hydroxyl radical reaction with multiple protonated forms of free diethylenetriaminepentaacetic acid {DTP A) as well as with DTPA complexed with a number of lanthanide metal ions were determined using electron pulse radio lysis and transient absorption spectroscopy. The rate constants for the most protonated DTP A species were ≈ 2.5 x 10 ⁹ M^-1s^-1, but for H₂-DTPA ^3- a higher value of 4.1 x 10⁹ M^-1s ^-1 at 20-23 °C was obtained. Thermodynamic parameters were also determined for these protonated DTPA species. The metal-complexed DTPA rate constants at pH 3.0-5.0 were typically faster than for the free DTP A values, and varied depending on the metal IOn.

The complexation kinetics ofDTPA with multiple lanthanide metal ions was measured using a stopped-flow system. The measured rate constants ranged from 10³-10⁴ M^-1s^-1. Thermodynamic parameters were also determined for both the complexation ofEu ³⁺ and Lu³⁺. It was observed that as the ionic radius of the metal ion being complexed increased, so did the complexation rate constant.

This study will increase understanding of the chemistry occurring within TALSPEAK, which is necessary in order to optimize this process.