Determination of Trace Noble Metals in Natural Samples Using Hyphenated Methods

Abstract

New ecological and geochemical problems call for the sophistication of techniques for analyzing platinum samples [1]. Special attention is given to the search for and study of new potential sources of nontraditional platinum raw materials among organic substances at different stages of metamorphism (coals, carbonaceous shales, iron-manganese concretions, etc.) [2,3]. The study of the anomalous iridium distribution in some sedimentary rocks and the control of environmental contamination with platinum, iridium, and rhodium because of the use of combustion catalysts are also urgent tasks [4]. These problems are complicated by the test samples including high concentrations of organic substances and low concentrations of noble metals, which occur as various species insufficiently studied yet [5,6].The most common natural species of noble metals are native metals (including invisible colloidal metals), as well as intermetallides, sulfides, tellurides, selenides, arsenides, and antimonides [4,7]. Being siderophils, noble metals form compounds with iron and iron-group elements, particularly chromites. In carbon-containing natural samples, organic compounds of noble metals can also be found, along with the most common native species and solid solutions (e.g., platinum-copperiron) [2,6]. The ecological contamination with platinum and palladium is usually due to the presence of metal nanoparticles in dust and soil [8]. In at least the two latter cases, acid decomposition can be the most efficient method for solubilizing noble metals (provided the complete oxidation of the organic matter).Inductively coupled plasma-mass spectrometry (ICP-MS) (including its isotopic dilution version), electrothermal atomization-atomic absorption spectrometry (ETA-AAS), and inductively coupled plasma-atomic emission spectrometry (ICP-AES) offer most promise for the instrumental determination of trace noble metals [4,5,[8][9][10]. The potentials of each method used in the combined analytical schemes depend on the analyte nature and concentration, as well as on the presence of interfering matrix components, which affect the choice of the determination parameters.The possibilities of different instrumental methods in determining noble metals, as found from the generalized data of some analytical laboratories, are presented in Table 1 [11]. It can be seen that the actual detection limits for noble metals are relatively similar. At the same time, each method has its own unique features; these will be considered below.In spite of the high tolerance for the matrix compositions of test solutions and high efficiency of multielement determination, the ICP-AES is insufficiently sensitive for the direct determination of noble metals in solutions after the decomposition of most of the natural samples. Hyphenated techniques involving the separation of matrix components allow ICP-AES to be used for determining trace noble metals, although some problems arose. For example, the determination of gold is characterized by a significant and rapidly appearing memor...

Trends in Sorption Preconcentration Combined with Noble Metal Determination

“…[32][33][34][35] Microwave techniques are especially effective for the dissolution of samples with a high content of organic matter, for example, carbonaceous shales, biological materials, pharmaceuticals and others. 9,36,37 The quantitative conversion of noble metals into suitable complexes, along with complete dissolution, is another critical step in almost all analytical procedures. The sorption preconcentration of noble metals is usually carried out from hydrochloric acid solutions (frequently 0.1 -3 M), in which these metals exist as sorption-active chloride or aqua-chloride anionic complexes.…”

“…For ETAAS, FAAS, ICP-AES and ICP-MS analyses, the dissolved substances are preferable. 37 In many cases the elution of noble metals by acids or thiourea is carried out. 22,29,35,45,[127][128][129][130][131][132] However, the determination of noble metals in thiourea solutions can be hampered, and additional options, such as high dilution, are necessary.…”

“…142 An additional restriction of the method is exacting requirements to the saline composition of analyzed solutions, usually less 0.2%. 2,37 A search for an optimum way of solution introduction into the plasma is continuing. To improve the nebulization efficiency, alternative introduction techniques, such as electrothermal atomization, ultrasonic nebulization, and thermonebulization are proposed.…”

“…30,35,37,44,45,132,135,138,[160][161][162][163][164] Analyses of the features of instrumental techniques show that the use of selective sorbents is preferable for the development of efficient determinations of noble metals. Complexing sorbents ensure the quantitative recovery of noble metals, and the concentrates obtained are free from most impurities, which allows one to enhance the reliability and sensitivity of noble metal detection.…”

“…The greatest influence on noble metal determination, especially iridium and ruthenium, is exerted from the iron, nickel and copper, which are often present in real objects as basic components. 37 The sorption preconcentration is often able to eliminate matrix effects. Many combined techniques based on sorption, including on-line preconcentration, are proposed for noble metal determination by FAAS or ETAAS.…”