Specificity of noble metals dynamic sorption preconcentration on reversed-phase sorbents
Graphical abstract
Introduction
The determination of noble metals (NMs) in complex matrices, e.g. rocks or depleted ores, still remains a difficult and significant task, despite the application of highly sensitive analytical methods such as inductively coupled plasma-mass spectrometry (ICP-MS). The NM content in the Earth's crust is extremely low and is estimated to be 1–10 ng g−1 for Pt, 0.1–3 ng g−1 for Pd, <0.5 ng g−1 for Rh, <1 ng g−1 for Ru, 0.3–5 ng g−1 for Ir, and <0.1 ng g−1 for Os [1]. The concentration of NMs in the solution to be analyzed after digestion of rock samples is further reduced by one to two orders of magnitude. The use of ICP-MS does not solve the problem of measuring such low contents because of substantial polyatomic interferences caused by matrix elements. Such interferences seriously affect the results of analysis if relatively inexpensive quadrupole mass-spectrometers are used. Therefore, the stage of NM preconcentration is required.
Dynamic sorption preconcentration systems appear to be most promising because extraction of elements occurs in the closed volume. This reduces the possibility of contamination and provides the chance to develop an on-line scheme of NM determination in complex solutions after rock sample digestion.
The use of polymeric sorbents with basic functional groups allows selective extraction of NMs from solution with concentrations of matrix elements [2]. The drawback of these sorbents is the low velocity of interaction with NM complexes due to the kinetic inertness of the latter. It is especially true for Ir, Rh and Ru. Usually, for extraction of inert complexes, one needs the heating of a solution with a sorbent or the use of microwave radiation to intensify the extraction [3]. Additionally, NM desorption from such complexes is in most cases problematic. It requires the digestion of the sorbent-concentrate or the use of aggressive regents, which are not recommended to introduce directly in the ICP interface [2], [3], [4], [5].
An alternative system might be a reversible scheme based on reversed-phase sorbents. Extraction of NM is based on the fact that NM-chlorcomplexes interact with protonated N-containing reagents with hydrophobic substituents and form stable associates, which can be extracted on non- or low-polar sorbent surfaces. The NM complexes are desorbed from the surface of such sorbents by polar organic solvents [6], [7]. The efficiency of these schemes was demonstrated for the extraction of Pd and Pt from “simple” samples – road dust [6] and aerosol filters [7]. The final solutions after rock digestion are more complex in composition, which may decrease the efficiency of NM sorption preconcentration.
So far the quantitative reversible sorption preconcentration of all NMs has been reported in only one publication [8]. In this paper, the desorption of MNs was performed in two stages and large volumes (more than 150 mL) of thiourea and concentrated HCl were used. Such eluate cannot be directly introduced in plasma, which requires additional pretreatment. The goal of this work is the search for a preconcentration scheme for quantitative sorption of Au, Pd, Pt, Ir, Rh and Ru and their quantitative desorption in a form suitable for direct introduction in plasma of ICP-MS. To achieve this goal, the efficiency of different “sorbent–reagent–eluent” schemes was investigated.
Section snippets
Solutions, reagents, eluents and sorbents
Nitric acid (65%, Suprapur grade, Merck, Germany) was used for the preparation of standard solutions and the mobile phase. Hydrochloric acid (37%, reagent grade, ChimMed, Russia) was used for the preparation of standards, PGM model solutions, reagents and diluents. Sulfuric acid (96%, reagent grade, ReaChim, Russia) was used for the production of hydrogen chloride gas and cleaning the glassware.
Tributylamine (TBA, synthesis grade, Merck, Germany) and 4-(n-octyl)diethylenetriamine (ODETA) were
Sorption on sorbents without own N-atoms
At the first stage of this work, it was proposed that sorption systems based on extraction of hydrophobic NM compounds on the surface of non-polar or low-polar sorbents are most promising for reversible preconcentration. NM chlorocomplexes present in the hydrochloric acid solution form stable ion associates with protonated N-containing reagents of different compositions and structures. Hydrophobic ion associates are extracted on the surface of non- or low-polar sorbents and may be desorbed with
Conclusion
The properties of sorbents with different degrees of hydrophobicity were studied. The use of StrataX and StrataX-AW for sorption of NM chlorocomplexes was proposed and investigated for the first time. The extraction of Au, Pd, Pt, Ir, Rh and Ru chlorocomplexes from hydrochloric acid solutions on hyper-crosslinked polysterene MN-200, StrataX and StrataX-AW in the form of ion associates with TBA and ODETA was investigated.
It was found that Pd, Pt and Au were quantitatively and reversibly
Acknowledgments
This work is financially supported by the fundamental research program of the Presidium of Russian Academy of Sciences, “Development and upgrading of the chemical analytical techniques and investigations of the material structure”, and by the Russian Foundation for Fundamental Research (RFFR) by Grants #12-03-00418 and #12-03-00804-a.
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