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A detailed examination of the performance of a double focusing (DF) sector field ICP-MS (Element, Finnigan MAT) for the ID-ICP-MS determination of cadmium in biological and environmental samples is described and compared with a quadrupole-based instrument (HP4500, Hewlett Packard). Experimental parameters governing the instrumental precision and accuracy for isotope ratio measurements of cadmium in the DF-ICP-MS are first characterized. Systematic errors, including dead time, mass bias effects and spectroscopic interferences, can be easily corrected in the two instruments studied. Detector dead time has only to be taken into account using the DF-ICP-MS. Typical isotope ratio precisions of 0.2-0.3% can be obtained routinely with both instruments. However, the precision of the measurements in the DF-ICP-MS can be improved (<0.1%) by increasing the number of scans. Moreover, for very low cadmium concentrations the DF-ICP-MS at low resolution setting (R=300) can provide better isotope ratio precision than the quadrupole ICP-MS because of its higher sensitivity. Both ICP-MS instruments were successfully applied to the determination of very low levels of cadmium by ID-ICP-MS (by both conventional and on-line ID modes) in biological and environmental certified reference materials, including Freeze-Dried Urine NIST SRM-2670, Horse Kidney IAEA H-8, Lichens BCR TP-25, Marine Sediment PACS-1 and Riverine Water SLRS-3. Combined uncertainty associated with the determinations is addressed.

Experimental parameters governing the instrumental precision and accuracy for isotope ratio measurements of cadmium in ICP-MS, including sampling time, mass bias, detector dead-time and spectroscopic interferences, were first characterised. The precision achieved for the isotope ratio determination of Cd was around 0.2% (RSD forn=5) both for 111:112 and 111:114 ratios. Two alternative flow approaches for the determination of ultratrace concentrations of cadmium by isotope dilution (ID) were explored and compared with the more conventional ID methodology: first, on-line mixing of the sample solution with the spike solution just before the ICP-MS nebuliser using a peristaltic pump and, second, the generation of volatile cadmium species using sodium tetraethylborate by merging zones flow injection ICP-MS. The three approaches were successfully applied to the determination of ultratrace levels of cadmium in biological and environmental certified reference materials (NIST SRM 2670 Freeze-Dried Urine, IAEA H-8 Horse Kidney, BCR TP-25 Lichens, PACS-1 Marine Sediment and SLRS-3 Riverine Water). The on-line ID method proved to be the most convenient for the determination of cadmium in such samples because it is fast, provides similar results to those of conventional ID and requires less sample preparation.

The generation of a volatile Bi compound by its reaction in aqueous solution with sodium tetraethylborate(iii) has been achieved using a flow injection (FI) system with injection of Bi and NaBEt₄ solutions in a ‘merging zones’ mode. This vapour generation (VG) system has been successfully interfaced with an atomic absorption spectrometric (AAS) detector for the determination of low levels of the metal. A detection limit of 0.8 μg l⁻¹ (0.56 ng) of Bi was obtained after optimization of the chemical and instrumental parameters. This volatile species generation with AAS detection showed a precision of ±2.9% at the 40 μg l⁻¹ level of Bi and a sample throughput of about 100 samples h⁻¹ was observed. Interference studies were carried out for many potentially interfering elements. The proposed FI–VG-AAS method proved to be very selective for the determination of Bi and so it has been successfully applied to the determination of ultratraces of Bi in human urine after simple 1+1 dilution without any other preteatment; the detection limit in real urine samples was 1.6 μg l⁻¹. Bismuth recoveries of 100% were found in spiked urine samples and also excellent agreement was achieved between results obtained by FI–VG-AAS and those obtained by ICP-MS for real urine samples. Finally, the proposed procedure was applied to the study of urinary clearance of Bi in three different people after intake, at therapeutic doses, of colloidal Bi subcitrate (CBS).