Off-line preconcentration and inductively coupled plasma sector field mass spectrometry simultaneous determination of Cd, Co, Cu, Mn, Ni, Pb and Zn mass fractions in seawater: Procedure validation☆
Graphical abstract
Introduction
Monitoring the quality of the marine environment relies principally on the analysis of abiotic matrices, such as seawater, suspended matter and sediments, as well as marine biota samples, for a variety of toxic and carcinogenic substances at various levels of concentrations. The presence of contaminants in the environment, even at low levels, is of great concern due to their specific chemical properties and effects. Monitoring the content of trace elements in seawater is the prerequisite for developing accurate environmental assessments and for evaluating the effectiveness of pollution control. Besides, distributions in the water column reflect the interaction of biogeochemical processes and the physical dynamics of the ocean, and some trace elements may be seen as tracers of major chemical input pathways to the world's oceans. The incorporation of metals into sinking particulate matter results in their deposition to deep ocean sediments, where they can be used as paleoceanographic proxies to reveal aspects of the chemical, biological, and physical structure of the past ocean [1,2]. Several of them are essential micronutrients for phytoplankton growth and play important roles in ocean biogeochemical cycles [3,4]. Trace elements such as Co, Cd, Cu, Mn, Ni and Zn), display similar behaviour to the macronutrients, nitrate, phosphate and silicate, indicative of their involvement in biological cycles [[5], [6], [7]] and may influence phytoplankton community composition [1,8,9]. In addition, some trace metals can be used as tracers for source inputs (e.g. manganese (Mn) for lateral transport from continental margins; [10], and lead (Pb) for anthropogenic inputs [11]).
Monitoring this information in different places and at different moments in time can only be done if, obviously, the data produced are comparable and reliable. It is achievable only when measurement results are traceable to a common system of reference and obtained with validated measurement procedures. Metrological traceability is the property of a measurement result whereby the result can be related to a reference (possibly from an internally established system such as the SI) through a documented unbroken chain of calibrations, each contributing to the measurement uncertainty [12]. This is closely linked to the degree of granularity to which the analytical procedure applied is described, and this description conditions the trust that can be put in data reported in the scientific literature. Thus, quantifying simultaneously and on a routine basis traceable to SI mass fractions for trace elements in seawater, at ng kg−1 level under about 35 g kg−1 salinity conditions, remains a challenging analytical task and a challenging endeavour all together.
Under this framework ICP-MS is a technique of choice owing to its low detection limits, wide linear dynamic range and capability to perform multi-element analysis using isotopic signals [13,14]. In addition, the application of aerosol desolvating devices as sample introduction systems has been shown to enhance sensitivity and limit oxide interferences during ICP-MS measurements [15,16]. Nevertheless, the determination of trace metal mass fractions in seawater using ICP-MS still presents unique challenges that are rarely encountered with other types of measurand.
Analysis of undiluted seawater, containing approximately 3% dissolved solids, generates physical, matrix or spectral interferences. None of these interferences can be easily solved only by diluting the sample [17]. In addition, the routine aspiration of seawater, even when diluted, will rapidly lead to instrumental drift, signal suppression and clogging of the sample introduction system of the instrument (from salts being deposited in and around the sampler and skimmer cone orifices as well as on the ion optics). Even when sample dilutions of 10–20 folds are applied, the interface region will still need regular maintenance. Therefore, preliminary concentration of the analytes and/or matrix separation steps are necessary [18,19]. Methods that have been developed include co-precipitation [20,21], solvent extraction [22,23] and solid phase extraction [[22], [23], [24], [25], [26], [27], [28]]. Chelating resins with functional groups such as 8-hydroxyquinoline (8-HQ) [24], nitrilotriacetic acid (NTA) [29] and iminodiacetate (IDA) [30] are advantageous due to their ease and speed of use as well as their affinity for reversibly binding multiple metals with high efficiency and recovery. Several recent studies have successfully used a resin containing both ethylenediaminetriacetate (EDTriA) and IDA functional groups for multi-element analyses [18,19]. A fully automated commercially available system integrating columns with this resin is already commercially available and implemented for matrix separation and preconcentration of the biogeochemically important trace elements from seawater samples [18,19,[31], [32], [33]].
The first aim of this study was to describe the implementation of a thorough validation scheme, following international guidelines, for an analytical procedure fit for the purpose of large scale monitoring studies requiring the simultaneous SI traceable determinations of trace metal mass fractions in seawater samples. To the best of our knowledge, this has not yet been shown in the literature. The second aim of this study was to apply this validated procedure to the characterisation of the ERM®-CA403 candidate certified reference material as the IAEA contribution to this certification.
Section snippets
Reagents and materials
High quality deionised water from Milli-Q system (Millipore Corp., USA) was used throughout this work. Chemicals used in the present study were of ultrapure grade, and included Ultrex II nitric acid, 67–69% HNO3 and hydrochloric acid 36% HCl from J. T. Baker (Germany), TraceSELECT®Ultra ammonia ≥25% and acetic acid ≥99.0% from Fluka (Switzerland). Working standard solutions (for Cd, Cu, Co, Pb, Mn, Ni, Zn) were prepared on a daily basis by stepwise gravimetric dilution of single-element stock
Procedure validation
The validation of the analytical procedure developed in the present study, was done according to the recommendations given in the guidelines for the validation of the measurement procedure, namely the ISO/IEC 17025 standard on “General requirements for the competence of testing and calibration laboratories” [40] and the Eurachem Guide “Fitness for purpose of analytical methods” [41].
The selectivity, linearity, and working range of the calibration curve, limit of detection, repeatability and
Conclusions
The validation of an analytical procedure for quantifying on a routine basis multiple trace metal mass fractions in seawater samples was proposed. It is based on ICP-SFMS combined with off-line matrix separation and analytes pre-concentration (seaFAST-pico™ system), requiring on average 12 minutes/sample for the simultaneous determination of the Cd, Co, Cu, Mn, Ni, Pb and Zn mass fractions. When applied to 10 mL seawater samples pre-concentration factors for all trace elements are up to 25
Acknowledgments
The agency is grateful for the support provided to its Environment Laboratories by the Government of the Principality of Monaco. The IAEA-NAEL in Monaco operates under an agreement between the IAEA and the Government of the Principality of Monaco.
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