Evaluation of tangential flow ultrafiltration procedures to assess trace metals bound to marine dissolved organic matter

Abstract

A procedure based on two-dimensional size exclusion chromatography (SEC) and anion exchange chromatography (AEC) with UV (205 nm) and ICP-MS detection was used to assess dissolved organic matter (DOM) and trace metals associated to DOM in surface seawater. Marine DOM was isolated by tangential ultrafiltration (UF) using two different polyethersulfone membranes exhibiting different molecular weight cut-off (MWCO), 3 and 10 kDa. The procedures require a volume of seawater sample of 100 L, and marine DOM of molecular weight higher than 3 and 10 kDa (UF membranes of 3 and 10 kDa MWCO) was pre-concentrated in 0.5 L (retentate), which implies a pre-concentration factor of 200. Retentate fractions obtained after the different UF procedures were further desalted by using HI Trap desalting mini-columns before two-dimensional SEC/AEC. Apparent molecular weights of isolated compounds after SEC with UV detection ranged from 1.5 kDa (close to the permeable volume of the SEC column fixed by injecting vitamin B12) to 16 and 22 kDa (UF membranes of 3 and 10 kDa MWCO, respectively). Further AEC/UV characterization of SEC fractions showed a large group of macromolecules eluted at 4.5 min, and small signals at shorter retention times (2.5 and 3.5 min). In addition, AEC experiments of the isolated SEC fractions when using 10 kDa MWCO UF membranes showed a group of substances eluted at high retention times (13 min). SEC hyphenation with ICP-MS proved the existence of several trace elements (Ni, Co, Cu, Zn, Mn, Mo and Sr) bound to the isolated marine DOM. Mass balance studies after analyzing the retentate and permeate fractions for trace elements indicate good recoveries (close to 100 %) for elements such as Mo, Sr, Ba and Zn when performing the UF with both 3 and 10 kDa MWCO membranes. However, recoveries from 36 to 81 % were obtained for the remaining studied elements after either UF procedure. SEC-ICP-MS experiments showed percentages of metals bound to the isolated marine DOM ranging from 0.055 and 0.077 % (Zn) to 4.1 10−4 and 1.4 10−4 % (Sr).