On-line separation and preconcentration of chromium species in seawater

A study was undertaken to ascertain the analytical capabilities of combined HPLC-ICP-AES with ultrasonic nebulization or HPLC-ICP-MS systems to perform on-line separation of Cr III-Cr VI and on-line preconcentration of Cr VI. Tetrabutylammonium bromide was used as the ion-pair forming agent and a reverse phase C 18 column was used for separation. By optimizing the conditions of separation and preconcentration as well as plasma parameters a method could be developed for the determination of Cr III and Cr VI in seawater by HPLC-ICP-AES with ultrasonic nebulization. In this matrix detection limits (3σ criterion) turned out to be 4.6 ng/ml for Cr III and 3.7 ng/ml for Cr VI, while the RSD is 2%–3% at the level of 100 ng/ml Cr. As the preconcentration of Cr VI is regarded, the detection limit of ICP-AES depends, among others, on the sample volume loaded onto the column. It resulted to be 0.20 ng/ml for an initial volume of 2 ml, while the RSD at 10 ng/ml Cr VI was ± 3.8%. On the other hand, the high salinity of seawater and C content of the separation eluent pose serious problems with ICP-MS detection. In fact, in this matrix only Cr VI preconcentration was achieved with a detection limit of 0.12 ng/ml for Cr VI (initial volume 2 ml). Due to isobaric interferences, the isotope at mass 50 (relative abundance 4.31%) was preferred.