Charles D. Russell

Polarographic reduction of pertechnetate

Abstract Oxidation states were identified for the products of electrolytic reduction of pertechnetate at the mercury cathode in a variety of media, and for the products of reoxidation upon reversing the cell current. Tast polarography, pulse polarography and triangular-wave voltammetry were employed. The first cathodic wave in acidic phosphate was identified as Tc(VII) → Tc(III) by means of the ratio between it and the Tc(III) → Tc(IV) reoxidation wave obtained on anodic-sweep pulse polarography. This wave was then used as a standard to determine values of n in other media of the same ionic strength. The first wave in acidic media occurred at pH-dependent potential and was found generally to correspond to the formation of Tc(III). The Tc(III) could in some media be reoxidized at the electrode to Tc(IV) or Tc(V). For the second wave in acidic media, which occurred at −0.9 V, diffusion current measurements indicated that n ≤7 with a catalytic component preventing determination of the exact value. In alkaline or unbuffered media the first wave occurred at −0.8 V and a second wave was sometimes seen at −1.0 V. The wave heights in alkaline media, while generally proportional to concentration of pertechnetate and varying with drop time in the manner expected for a diffusion current, often corresponded to non-integral values of n . In certain basic media the reduction of pertechnetate led to the formation of an insoluble oxidizable surface film. Under all conditions investigated the reduction of pertechnetate proved to be irreversible.

Carrier electrochemistry of pertechnetate: application to radiopharmaceutical labelling by controlled potential electrolysis at chemically inert electrodes.

Abstract A method is reported for labelling radiopharmaceuticals with 99mTc by direct reduction of 99mTcO4− at chemically inert electrodes without contamination of the agent by any reducing agent. This is distinct from previous electrolytic labelling methods, in which reduction of TcO4− was mediated by chemically reactive tin or zirconium species produced from the electrode metal. From polarographic and voltammetric measurements using carrier concentrations of the long lived isotope 99Tc, appropriate ranges of electrode potential for electrolytic reduction were determined for platinum, gold, mercury and glassy carbon electrodes in a variety of complexing and non-complexing media. The reduction potentials proved to depend only on the pH and the electrode composition and not on the presence of complexing agents. Thus from polarograms or voltammograms of the substrate to be labelled in buffers of varying pH one can determine whether or not there exists a combination of electrode potential and pH for which pertechnetate is reduced but not the substrate. In alkaline media pertechnetate was reduced at approximately −0.8 V (referred to the saturated calomel electrode) for all four electrodes. In acid media reduction occurred more readily (at less negative potentials) at mercury than at gold, platinum, or glassy carbon. In acid media the reduction potential depended on pH for all four electrodes and reduction occurred concurrently with hydrogen evolution on platinum and gold. Mercury was found to have a much wider potential range for electrolytic reduction of technetium than did platinum, gold or glassy carbon at any pH. From these data reaction conditions were predicted and experimentally confirmed for the electrolytic labelling of tetracycline and EDTA with carrier free 99mTc.

Tracer electrochemistry of pertechnetate: Chelation of 99mTc by EDTA after controlled-potential reduction at mercury and platinum cathodes

Abstract Technetium pharmaceuticals are usually prepared by reduction of TcO 4 - in the presence of a chelating agent. As a prototype for an electrolytic method, the reduction of tracer concentrations of 99m TcO 4 - was studied at Hg and Pt cathodes with and without EDTA. A tin-free chelate could be produced with less than 2% free TcO 4 - or hydrolyzed Tc in 5 min of electrolysis. The effects of pH and electrode potential on the yield and purity of chelate were determined by thin-layer chromatography. At Hg, Tc complexes were formed in complexing media at a potential approximating the polarographic half-wave potential so that the effect of pH could be predicted from previous measurements. In non-complexing media, deposition of Tc onto the electrode occurred at −0.7 to −1.1 V vs S.C.E. independent of pH. Electrodeposition was inhibited in complexing media. At Hg cathodes, the yield and purity of Tc-EDTA were comparable with those attained in parallel experiments using SnCl 2 as the reductant. By either method the yield and purity exceeded 98% using 0.1 M EDTA. When the EDTA concentration was lowered to 0.001 M both methods yielded a product contaminated by significant quantities of free TcO 4 − and hydrolyzed Tc. Pt electrodes gave less satisfactory results. At Hg, but not at Pt, loss of activity from solution to electrode generally occured only at potentials more negative than those used for labelling. Electrolytic labelling can be an effective means for preparing Tc chelates free from foreign cations such as Sn +4 , and electrochemical measurements performed at carrier concentrations of TcO 4 − can be used to select reacion conditions for tracer labelling.

Mean ionic charge on two components of technetium pyrophosphate prepared using stannous chloride

Ionic charge, complexation equilibria, and acid-base equilibria can be measured at tracer concentrations by ion-exchange chromatography. To characterize better the principal components found in [99mTc]pyrophosphate preparations used for diagnostic bone and heart scanning, these were studied in two ion-exchange chromatographic systems: (A) DEAE-cellulose in pyrophosphate form at pH 6.0-7.5, and (B) DEAE-cellulose in perchlorate form at pH 3.7-4.8. The chromatographic retention times were measured as a function of pH and electrolyte concentration, and compared with those of trisoxalatochromium(III) complex, which was chosen as reference ion because of its similarity in charge and retention to the Tc-pyrophosphate complexes. The pyrophosphate complexes were even more sensitive to electrolyte concentration than was the triply negative reference ion: the calculated mean net charge in the mobile phase for the two principal Tc-pyrophosphate species were -4.5 +/- 0.5 and -4.9 +/- 0.5 at pH 4.3, and -11.2 +/- 1.3 and -10.1 +/- 1.0 at pH 7.0. It can be concluded that the two principal radioactive components in clinical Tc-pyrophosphate preparations both bear a high, pH-dependent negative charge.

A single-injection, two-sample method for measuring renal 99mTc-MAG3 clearance in both children and adults

We present a method for estimating 99mTc-MAG3 clearance from both a single injection and two blood samples that is valid for both adults and children. It was obtained by fitting a scaled two-compartment model (having only two adjustable parameters) to adult and pediatric data from multiple centers.

Estimation of ERPF in adults from plasma clearance of 131I—hippuran using a single injection and one or two blood samples

The plasma clearance of [131I]o-iodohippurate, which is an estimate of effective renal plasma flow, can be measured from one or two blood samples drawn after an i.v. bolus of tracer. The most widely used method is based on a single sample. Here, it is shown that the error (relative to a complete curve of six or more blood samples) can be halved by using two samples rather than one. Since the one-sample method is satisfactory for most clinical purposes, the two-sample method is recommended only when special accuracy is needed.

Chronopotentiometric oxidation of substituted acetates in acetonitrile

Abstract The chronopotentiometric behavior of tetrabutylammonium salts of twenty substituted acetates was studied at platinum anodes in acetonitrile. In every case a diffusion-controlled wave was obtained, suitable for analytical use. Quarter-wave potentials and diffusion coefficients are reported. Waves were obtained on current reversal in most cases, thought to indicate concomitant oxidation of traces of water but not influencing analytical results. Only a single step was observed with monocarboxylates, but dicarboxylates (such as the dinegative malonate ion) gave two-step waves.