Assessment of Dowex 1-X8-based Anion-exchange Procedures for the Separation and Determination of Ruthenium, Rhodium, Palladium, Iridium, Platinum and Gold in Geological Samples by Inductively Coupled Plasma Mass Spectrometry Ian Jarvis*a, Marina M. Totland†a and Kym E. Jarvisb a School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey, UK KT1 2EE b NERC ICP-MS Facility, Centre for Analytical Research in the Environment, Imperial College at Silwood Park, Buckhurst Road, Ascot, Berkshire, UK SL5 7TE Synthetic multielement solutions of the platinum group metals (PGE: Ru; Rh; Pd; Ir; Pt) and gold, with analysis by ICP-AES and ICP-MS, have been used to study the behaviour of the precious metals on Dowex 1-X8 resin.Simple solutions of precious-metal chlorocomplexes showed near-complete adsorption ( > 99%) of most elements, and only minor breakthrough of Ru and Ru ( Å 5%). Solutions pre-treated with acid mixtures typically used to decompose geological samples, demonstrated that perchloric acid adversely affects the adsorption of the PGEs on the resin.Solutions treated with HF–HNO3–HCl maintained good retention of Ir, Pt, Au ( > 99%), Pd ( > 94%) and Ru ( > 90%), but displayed significant loss (up to 40%) of Rh. A two-step procedure was necessary to elute the precious metals from the resin: 0.3 mol l21 thiourea prepared in 0.1 mol21 HCl removed Ru, Pd, Pt, Au, and some Rh: 12 mol l21 HCl eluted remaining Rh and all Ir.Recoveries ranged from 50 to 100%. At low levels, the determination of PGE and Au in the thiourea fraction by ICP-MS was compromised by high levels of total dissolved solids (TDS), which necessitated dilution of the eluate prior to analysis. The TDS was reduced by decomposing thiourea with HNO3 and removing SO4 22 by precipitation of BaSO4, but this led to lower and more erratic results, and increased contamination. An assessment of the optimised procedure employing geological reference materials PTM-1, PTC-1 and SARM7, indicated that acceptable results should be attainable for ICP-MS determination of most elements in geological samples containing high concentrations ( > 1 mg g21) of the PGE, for which decomposition of thiourea is unneccessary.The addition of a decomposition step led to low recovery of all elements except Ir, which was present entirely in the HCl eluate. The method is viable for the determination of Ir in a range of geological materials, but modifications will be required if it is to be extended to the other precious metals.Keywords: Platinum group element; gold; cation exchange chromatography; geological sample; inductively coupled plasma mass spectrometry The commonest methods1–3 used for the determination of the platinum group elements (PGE: Ru; Rh; Pd; Os; Ir; Pt) and gold are based on fire assay, which involves fusing 10–100 g samples with large amounts of alkali flux and collecting the PGE into a nickel sulfide or, for Au, a lead button.The nickel sulfide button is usually dissolved in acid to remove the nickel and sulfur, leaving a residue of PGE sulfides which may be analysed directly by neutron activation analysis (NAA). More commonly, the precipitate is taken into solution for analysis by AAS, direct coupled plasma-atomic emission spectroscopy (DCP-AES), or ICP-AES. Although fire assay is used extensively, it is highly labour intensive, and one of its most serious limitations is a dependence of the quality of results on the experience of the analyst.A more robust method would clearly be advantageous. Fire assay is also not well suited to the analysis of small ( < 5 g) samples, which may be required for some geological studies, such as investigations of precious metal mobility in sediment cores.4 We have previously described5 a new combined microwave digestion–minifusion method with analysis by ICP-MS, that yields quantitative data for Ru, Rh, Pd, Ir, Pt and Au in mineralised samples, but is limited by modest lower limits of determination in samples of 0.2–1 mg g21 (ppm).The objective of the present study was to investigate anion-exchange separation procedures for the PGE and Au, that would allow the determination of these elements at the lower levels (1–10 ng g21; ppb) found in most geological materials. Similar techniques have been employed by others6–8 with varying success, to determine individual or small groups of precious metals in a variety of sample types.Dowex 1-X8 is a strong base anion-exchanger with a styrene divinyl benzene polymer skeleton to which tertiary ammonium groups have been bound.7 Dowex 1-X8 exhibits a high selectivity for the PGE and Au, and it has been shown7,9 that it will remove these elements quantitatively from HCl solutions. However, the resin has not been used previously to simultaneously separate and preconcentrate the entire group of PGE and Au from solutions of geological materials, prior to their analysis by ICP-MS.The aims of this study were to: (a) develop experimental conditions under which 100% of the PGE and Au present in solution would be absorbed onto the Dowex 1-X8 resin, with minimal adsorption of competing ions; (b) find a set of conditions to elute 100% of the adsorbed PGE and Au; (c) develop a methodology to quantify the PGE and Au at ng ml21 levels in the eluate using ICP-MS.The first objective imposes limitations on the solution chemistry of the digested geological materials that are to undergo separation, while the second is known7 to be difficult to achieve from strong base cationexchange resins. Osmium was not included in the study because its volatility precludes its determination in most solutions of geological materials prepared for other precious metal analysis. 2,3,10 Experimental Instrumentation A JY 70 Plus (Jobin-Yvon, Longjumeau, France) inductively coupled plasma atomic emission spectrometer, located at † Present address: Atomic Energy of Canada Ltd., Chalk River Laboratories, Chalk River, Ontario, Canada KOJ 1JO.Analyst, January 1997, Vol. 122 (19–26) 19Kingston University, was used to determine the PGE and Au at high levels ( > 50 ng ml21) during method development. A VG Elemental PlasmaQuad PQ2 Plus (Fisons Instruments, Winsford, Cheshire, UK) ICP-MS, formerly at Royal Holloway University of London, was employed for lower level ( < 50 ng ml21) work.Operating conditions for the two instruments are given in Tables 1 and 2. In both cases, an external drift monitor (generally, a calibration standard solution) was used to correct for signal changes with time. Spectral lines used for PGE and Au determination by ICPAES were selected experimentally. Tables of spectral data11–13 were used to identify the most sensitive wavelengths for each element.A 1 mol l21 ARISTAR HCl (Merck, Lutterworth, UK) blank solution, a 1 mg ml21 solution of the analyte in the same matrix, and a multi-element solution containing all of the precious metals at 1 mg ml21 (both prepared using aliquots of single-element 1000 mg ml21 precious metal standard solutions, supplied in 20% v/v HCl; Johnson Matthey, Royston, UK), were each scanned14 across a 0.12 nm window, centred on the spectral line selected. The emission lines chosen (Table 3) were the most sensitive lines having no interference from other elements in the group.As only simple solutions of the precious metals were to be analysed by ICP-AES, interferences from other elements were not considered. Subsequently, calibration was achieved using five synthetic multi-element PGE and Au standards, prepared using aliquots of the 1000 mg ml21 singleelement standard solutions. Solutions analysed were generally in 0.1–1 mol l21 ARISTAR HCl; matrix effects were minimised by matching the acidity and salt content of samples and standard solutions.Detection limits (Table 3) calculated as 3s standard deviations of eleven determinations of a 1 mol l21 ARISTAR HCl blank, demonstrate that ICP-AES is well suited to the quantification of PGE and Au at concentrations in excess of 50 ng ml21. The ICP-MS was optimised using 59Co and 238U to give maximum sensitivity whilst minimising interferences, particularly refractory oxide species. The PGE and Au lie in two distinct parts of the mass range, 96–110 and 184–198 u.Some isotopes have an isobaric overlap and these were not used for analysis. The mass spectrometer was scanned from 98–200 u, with masses between 112–179 u being skipped to maximise the integration time spent on PGE and Au isotopes. During data processing, the existence of polyatomic or oxide interferences was determined by comparing isotopic ratios for elements with their theoretical values; only isotopes free from all spectroscopic interferences (Table 3) were used for quantification. External calibration of the ICP-MS for PGE and Au determinations was accomplished using multielement standards prepared in dilute HCl from 1000 mg ml21 single-element standard solutions; in all cases, the acid concentration was matched to the samples.A single standard (ranging between 20 and 200 ng ml21, depending on the expected concentration in the samples) and a blank were used as calibration points.It was noted that some elements displayed a significant memory effect (Au and, to a lesser extent, Pd), and extended washout times were necessary to avoid sample carryover. For example, following a solution containing 200 ng ml21 of the PGE and Au, a 3–4 min washout (using maximum pumping speed) with 1 mol l21 HCl was required to minimise the background level of Au. To avoid cross-contamination between high- and lowconcentration solutions, the integrals for all elements were monitored throughout the analytical run, and wash times were extended after solutions containing high levels of the precious metals had been analysed.Blanks were measured to establish the background level of these elements throughout the run, and blank corrections were applied where necessary. The measured detection limits for all elements by ICP-MS (Table 3) were better than 0.22 ng ml21. It is worth noting that since this study was completed, a newer instrument fitted with an Enhanced Performance Interface (Fisons Instruments) has been installed, which yields detection limits for the PGE and Au between 0.006 and 0.05 ng ml21.However, these do not translate directly to improved lower limits of determination for samples, because the new instrument displays poorer tolerance of total dissolved solids (TDS), necessitating higher dilution factors for solutions. Nevertheless, with these exceptionally low detection limits, ICP-MS is ideally suited to the determination of low concentrations of PGEs and Au in simple solutions.ICPMS is also far less prone to interferences than most other instrumental techniques.15,16 However, limitations on the types of solution analysed are imposed by the level of TDS; upper limits of approximately 0.2% TDS may be aspirated into an ICP-MS instrument without causing significant instrumental drift and/or matrix effects, so solutions had to be diluted to an appropriate level prior to analysis.Anion-exchange Experiments The Dowex 1-X8 resin (Bio-Rad Laboratories, Hemel Hempstead, UK) used here was nominally in chloride form (i.e., with chlorine counter-ions) and graded from 200–400 mesh (74–37 mm). However, the resin supplied contained a much Table 1 ICP-AES operating parameters for PGE and Au determination Instrument Jobin-Yvon JY 70 Plus Plasma power 1000 W Reflected power < 5 W Coolant gas flow 12 l min21 Auxiliary gas flow 0 l min21 Sheath gas flow 0.2 l min21 Nebuliser gas flow 0.35 l min21 Nebuliser Meinhard TR-50-C1 concentric glass Sample uptake 1 ml min21 Spray chamber JY Scott-type, double pass Spectrometer Czerny–Turner monochromator Grating Holographic 3600 grooves mm21 Integration period 10 s Table 2 ICP-MS operating parameters for PGE and Au determination Instrument VG PlasmaQuad PQ2 Plus Plasma power 1300 W Reflected power < 1 W Coolant gas flow 14 l min21 Auxiliary gas flow 0.5 l min21 Nebuliser gas flow 0.75 l min21 Nebuliser de Galan high dissolved solids Sample uptake rate 0.5 ml min21 Spray chamber Surrey design, single-pass, water-cooled at 4 °C Scan regions 98–111 and 180–200 u Table 3 Detection limits for the PGE and Au by ICP-AES and ICP-MS.Values calculated as 3s standard deviations for 11 determinations of a 1 mol l21 ARISTAR HCl blank ICP-AES ICP-MS Wavelength/ Detection limit/ Mass Detection limit/ Element nm ng ml21 (u) ng ml21 Ru 245.66 14 101 0.22 Rh 233.48 11 103 0.03 Pd 340.46 13 105 0.17 Ir 224.27 9.0 193 0.07 Pt 214.42 29 195 0.11 Au 242.80 5.9 197 0.06 20 Analyst, January 1997, Vol. 122wider range of grain-sizes than the nominal fraction, including a significant amount of very fine particles, possibly produced during packaging and handling.These were removed by slurrying the resin with deionised water, allowing the resin to settle for a short time, and then decanting off the suspended fines with the supernatant. This was repeated at least three times to remove most of the fine material.The resin was batch-cleaned prior to use by slurrying it with 6 mol l21 AnalaR HCl (Merck), allowing it to stand for 10–20 min, and then decanting off the acid. This procedure was repeated at least three times, or until no further discolouration of the acid was observed. After pouring off the last portion of the cleaning acid, the resin was slurried with 1 mol l21 ARISTAR HCl, and stored ready for use. For each ion-exchange experiment, following loading on the column, the resin was conditioned by passing 100 ml (later increased to 500 ml) 1 mol l21 ARISTAR HCl through the column.Only new resin was used for this work. To evaluate possible anion-exchange procedures, two parameters were measured: (1) percentage of the PGE and Au not adsorbed onto the resin, termed ‘breakthrough’; (2) percentage of the adsorbed metals eluted from the column, termed ‘recovery’. Unless stated otherwise, Merck ARISTAR reagents and ultra-pure (better than 18 MW cm) deionised water were used throughout this study.Adsorption of the PGEs and Au on Dower 1-X8 resin The PGEs must be present in appropriate oxidation states to ensure strong adsorption onto Dowex 1-X8 resin,7 and the charge of the PGE-complex may change depending on the ligands attached to the metal. Preliminary evaluation of the resin was conducted using stable solutions of PGE anionic chlorocomplexes, supplied at 1000 mg ml21 and stored in 20% v/v HCl solutions (Johnson Matthey, Royston, UK). Breakthrough of each PGE and Au was determined using synthetic multielement solutions produced by diluting aliquots of 1000 mg ml21 standards with 1 mol l21 HCl to yield 500 mg and 1 mg spikes of each element, diluted to 10 ml.Column dimensions were chosen based on previous studies6–9,17–23 to hold a settled resin bed of 1 cm diameter, 10 cm long. The percentage breakthrough of each element was determined by analysing the 1 mol l21 HCl eluate (25 ml) collected as the spike solutions were loaded onto the column.Solutions generated during experiments with the 500 mg spike were sufficiently concentrated to enable analysis by ICP-AES. This high level was chosen to determine whether the capacity of the resin was likely to be exceeded in normal use. The proportion of each element not retained on the column was small: < 0.1% Pd, Pt, Au; 0.2% Ir; 4.6% Rh; 7.3% Ru. A level of precious metals much closer to that expected for geological materials (1 mg) was used to evaluate breakthrough at lower concentrations. In this instance, absolute breakthrough was expected to be very small, so ICP-MS was used as the analytical finish.Results showed no significant breakthrough of Pd, Pt, Au ( < 0.1%), Ir (0.1%) and Rh (0.8%), and < 10% breakthrough of Ru from the 1 mg spike. While the above results verified the adsorption characteristics of Dowex 1-X8 described in the literature, they represent a simplified situation.The chemical state of the PGE and Au following digestion of geological materials5,24 may not be identical to those found in standard solutions. The effects of digestion procedures on the chemical state of individual PGE and Au needed to be established experimentally. Knowledge of the exact chemical state of each element is not necessary, provided that the PGE and Au are completely adsorbed onto the resin.It is not possible to measure breakthrough of the PGE and Au directly with geological materials, because the high concentrations of matrix elements passing through the columns preclude the determination of low levels of precious metals in the eluate. To estimate possible breakthrough when real samples are separated by this method, synthetic solutions were treated in an identical manner to geological samples. In this way, the effects of various acids used in digestion procedures could be evaluated. An HF–HClO4-based acid attack is one of the most commonly used methods to digest geological samples.24–29 To assess the affects of these acids on column retention, a 1 mol l21 HCl solution containing 500 mg of PGE and Au was treated as follows: (1) 8 ml of 16 mol l21 HNO3, 4 ml of 29 mol l21 HF, 4 ml of 12 mol l21 HClO4 were added in a PTFE beaker, evaporated at 200 °C to incipient dryness, a further 2 ml of HClO4 (12 mol l21) were added, evaporated to near-dryness, and the final solution made to 10 ml with 1 mol l21 HCl; (2) 4 ml of 16 mol l21 HNO3, 4 ml of 29 mol l21 HF, 4 ml of 12 mol l21 HClO4 were added and evaporated to incipient dryness, 8 ml of 12 mol21 HCl were added and evaporated to neardryness (twice), and the final solution also made to 10 ml with 1 mol l21 HCl.Breakthrough from these solutions were measured by ICP-AES from 1 cm diameter, 10 cm long columns of Dowex 1-X8. Breakthrough increases significantly following acid pre-treatment (Table 4).Losses at this stage were reduced when the solution was evaporated twice with 12 mol l21 HCl, instead of HClO4. Nevertheless, breakthrough of Rh and Ir remained very high, 58 and 18%, respectively. A comparison made using different elution volumes of the HClO4- evaporated solution (Table 4), demonstrated that the amount of breakthrough increased dramatically when 65 ml of 1 mol l21 HCl was eluted compared to 25 ml; in the former case, no Rh was retained on the resin.The level of breakthrough and the dependence on elution volume are clearly unacceptable for this separation method, precluding the use of HClO4-based digests prior to cation-exchange chromatography. A series of 1 mol l21 HCl solutions spiked with PGE and Au were evaporated in PTFE beakers at 100 °C on a hotplate, in the presence of 10 ml of 29 mol l21 HF, 15 ml of 12 mol l21 HCl and 5 ml of 16 mol l21 HNO3. This acid mixture has been demonstrated5 to be effective for the digestion of preciousmetal- bearing geological samples.Three masses of PGE and Au spike (10, 1, 0.1 mg) showed greatly reduced losses following the application of this acid mixture (Table 5), and subsequent evaporation (twice) with 4 ml of 12 mol l21 HCl to ensure conversion of elements to chloride form. Final solutions were approximately 10 ml of 1 mol l21 HCl; 25 ml of 1 mol l21 HCl were eluted and analysed by ICP-MS. Breakthrough was considered to be acceptably low ( < 10%) for Ru, Pd, Ir, Pt and Au, but the loss of Rh was high (27–40%).Elution conditions Dowex 1-X8 resin has a high affinity for the PGE and Au in dilute HCl solutions, but increased acid concentrations favour their removal.7 Distribution coefficients fall significant with Table 4 Breakthrough (%) of 500 mg of PGE and Au on Dowex 1-X8 anionexchange resin following pretreatments with perchloric acid. HClO4 = evaporated with HNO3, HF, HClO4; HClO4 added and evaporated.HCl = evaporated with HNO3, HF, HClO4; HCl added and evaporated. Both final solutions in 1 mol l21 HCl HClO4 HCl No pretreatment Element 25 ml 65 ml 25 ml 25 ml Ru 12 18 6.1 7.3 Rh 80 100 58 4.6 Pd 0.4 1.0 < 0.1 < 0.1 Ir 9.4 31 18 0.2 Pt 47 53 2.0 < 0.1 Au 0.4 1.0 < 0.1 < 0.1 Analyst, January 1997, Vol. 122 21increasing acid molarity, although there remains significant adsorption of some PGE even in concentrated HCl. Exceptions are Ir3+ and Rh3+ which have distribution coefficients of < 1 in 12 mol l21 HCl, and Ru4+, which is slightly higher at 10.7 The inference from these observations is that concentrated HCl will elute only part of the Ru, Rh and Ir adsorbed on the column; increasing the concentration of the counter ion in the system will not be sufficient to elute the entire group of elements.Thiourea has been successfully used to elute Pd, Pt and Au from strong base anion-exchange resins;7 selected PGE and Au are either reduced and/or complexed by thiourea, and the resulting complexes have low affinity for the resin.Elution experiments were conducted using 500 mg of each PGE and Au loaded in 10 ml of 1 mol l21 HCl. These solutions are coloured, and it was possible to get a quick assessment of elution conditions by observing the migration of coloured bands on the columns. Using this approach, it became apparent that although a portion of the PGE and Au could be eluted with thiourea, some remained on the column.It was concluded that a two-step procedure was necessary to elute the PGE and Au from a Dowex 1-X8 column. Three 25 ml aliquots of 0.3 mol l21 thiourea (4.7 g AnalaR thiourea dissolved in 200 ml of deionised water, acidified to 0.1 mol l21 HCl using 1.7 ml of 12 mol l21 HCl; selected based on work by Korkisch7) were eluted through the Dowex 1-X8 columns used for the breakthrough study. Solutions from the 500 mg experiments were diluted four-fold prior to ICP-AES analysis; 1 mg solutions were diluted 40-fold and analysed by ICP-MS.Dilution was necessary to reduce the high levels of TDS from the thiourea, to levels acceptable for each technique. 16 Aliquots of 25 ml of 12 mol l21 HCl were then eluted through each column, collected, diluted 10-fold to reduce the acid concentration, and analysed by ICP-AES or ICP-MS. A total of 100 ml of 12 mol l21 HCl was collected for the 500 mg experiment, and 125 ml from the 1 mg spike.Elution profiles (Fig. 1) show that most Ru, Pd, Pt and Au are eluted by 75 ml of 0.3 mol l21 thiourea, while Rh is only partly eluted and Ir remains bound to the resin. Remaining Rh and Au are completely eluted from the column with 12 mol l21 HCl, and Ir is removed by this eluent. Elution profiles are instructive for determining the rate of elution of elements and the relative efficacy of eluents, but overall recovery is the critical measure of the usefulness of a procedure.Recoveries for the 500 mg spike were 92% for Ru, and better than 97% for all other elements. Slightly lower recoveries ( Å 85%) were measured for Ir and Rh at the 1 mg level. High recoveries of gold were caused by carry-over effects during analysis; these were minimised in later experiments by increasing washout times and uptake rates during the wash period. To optimise our procedure, various parameters were studied including the: effect of temperature on elution efficiency; volume of eluent required; possibility of combining thiourea and 12 mol l21 HCl into a single step.The volume of thiourea required to elute the PGEs and Au is dependent on the rate of formation of thiourea complexes. These are easily identified in concentrated ( > 100 mg ml21) solutions because they form strong colours. Experiments showed that the rate of formation of the thiourea complex varied between elements; the colour of solutions began to change almost immediately upon the addition of individual PGE and Au (each treated separately) to a 0.3 mol l21 thiourea in 0.1 mol l21 HCl.Colours continued to change (orange Ru solutions changed to green and then blue) when left standing for 1 h. Some elements precipitated when left to stand for several hours, not surprisingly, since thiourea has been used 30 to quantitatively precipitate some of the PGE and gold, and facilitate their separation. However, the formation of insoluble thiourea complexes was considered to be unlikely during ion-exchange, because the level of PGE and Au in solution would be very low, and precipitation only occurs from concentrated solution.Furthermore, fresh thiourea is continually added to the column, so the equilibrium for formation of these complexes is always shifted towards dissociation. Our experiments indicated that the recovery of the PGE and Au from a Dowex 1-X8 column is governed by both kinetic and thermodynamic factors.These were evaluated further by increasing the elution temperature using a column with a jacket through which heated water was passed. A solution of 1 mol l21 HCl containing 500 mg of each of the PGEs and Au was loaded onto the column at room temperature, and the eluate analysed to determine the concentration of each element remaining on the column. The column temperature was raised to 50 °C and a solution of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, also heated to 50 °C, was eluted through the column (the eluate was collected as three 25 ml fractions), followed by the usual concentrated HCl elution step.A control experiment was run at room temperature ( Å 20 °C). The only improvement observed for elution at 50 °C was the complete recovery of Rh in the thiourea fraction (Fig. 2). The recovery of Ir in the thiourea fraction did not increase, so a 12 mol l21 HCl elution step was still required; indeed, elution of Ir with concentrated HCl seemed to be hindered by previous elution with heated thiourea (Fig. 2). Table 5 Breakthrough (%) of PGE and Au on Dowex 1-X8 anion-exchange resin after evaporated with HNO3, HF and HCl Mass of each PGE and Au Element 10 mg 1 mg 0.1 mg Ru 4.8 9.0 9.5 Rh 27 40 36 Pd 1.8 6.0 6.0 Ir < 0.1 < 0.1 < 0.1 Pt < 0.1 2.3 < 0.1 Au < 0.1 0.4 0.5 Fig. 1 Elution profiles (cumulative % recovery) for 500 mg (squares) and 1 mg (circles) spikes of the PGEs and Au from a 10 cm long, 1 cm diameter Dowex 1-X8 column using a two-step elution procedure. 22 Analyst, January 1997, Vol. 122A manually operated two-step elution is more cumbersome and requires more operator attention than a single-step procedure. Attempts were made to find experimental conditions that would enable the PGE and Au to be eluted using a single solvent. When 0.3 mol l21 thiourea in 12 mol l21 HCl was used as an eluent (Fig. 2), total recovery from 125 ml of eluent was generally lower than that achieved by the two-step method.Consequently, a two-step elution operated at room temperature was judged to yield the best results. Ion-exchange column dimensions The 1 cm diameter and 10 cm long Dowex 1-X8 resin bed employed in our initial studies, has been used by previous workers.7 However, the high capacity of the resin demonstrated by minimum breakthrough of PGE and Au at the 500 mg level, suggested that it might be possible to reduce column length while retaining good absorption on the resin.The recovery of the PGE and Au from a shorter, 1 cm diameter, 5 cm long column was examined. The volume of reagents used was varied to establish the effect of each reagent on the overall recovery of each element. Two experiments were undertaken: (a) 50 ml of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, then 150 ml of 12 mol l21 HCl; (b) 75 ml of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, followed by 75 ml of 12 mol l21 HCl. The eluate was collected in 25 or 50 ml fractions and analysed by ICP-AES. Ruthenium, Rh, Pd, Pt and Au were completely recovered by both procedures (Fig. 3). The recovery of Ir, which is primarily eluted in the 12 mol l21 HCl fraction, is highly dependent on eluate volume, 73% was recovered in 75 ml, increasing to 89% in 150 ml, so the larger volume of concentrated HCl was used in all further experiments. Results for the shorter column of Dowex 1-X8 showed that recoveries were not improved by reducing the length of the resin bed, while breakthrough experiments demonstrated that adsorption efficiency decreased (around 8% Ru and 18% Rh were not retained), so a 10 cm long column was confirmed as being optimum.Cleaning Dowex 1-X8 resin When using ion-exchange resins, procedures are needed to clean new, and if possible regenerate previously used, resin. A common approach is to clean the resin with reagents used for the elution step (in this case, thiourea and 12 mol l21 HCl). This approach is commonly used in chromatographic techniques to ensure that no additional contamination is obtained from the resin when the solvent is changed.The percentage breakthrough of the PGE and Au was compared after the resin had been cleaned with: (a) 0.3 mol l21 thiourea in 0.1 mol l21 HCl; and (b) 6 mol l21 HCl. Breakthrough was assessed as previously (called here a single pass), as well as for a double pass of the sample solution. For the double pass, the eluate containing the PGE and Au from a single pass through the column was collected and passed through the column again.The second eluate was then analysed (the same final volume was eluted for both experiments). Comparing the levels of breakthrough for single passes (Table 6), shows that there was significantly increased loss of Rh (from 5 to 17%) from resin pre-treated with thiourea, compared to that washed only with HCl. The loss of Ru was Fig. 2 Elution profiles (cumulative % recovery) for 500 mg spikes of the PGE and Au from a 10 cm long, 1 cm diameter Dowex 1-X8 column using different elution conditions.Fig. 3 Elution profiles (cumulative % recovery) for 500 mg spikes of the PGE and Au from a short (5 cm long, 1 cm diameter) Dowex 1-X8 column using : (a) 50 ml of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, then 150 ml of 12 mol l21 HCl (circles); (b) 75 ml of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, followed by 75 ml of 12 mol l21 HCl (squares). Table 6 Breakthrough (%) of 500 mg PGE and Au for Dowex 1-X8 resin cleaned with thiourea solution or HCl Thiourea HCl Element Single pass Double pass Single pass Ru 6.6 13 7.3 Rh 17 26 4.6 Pd 0.4 0.2 < 0.1 Ir < 0.1 0.4 0.2 Pt < 0.1 0.2 < 0.1 Au < 0.1 < 0.1 < 0.1 Analyst, January 1997, Vol. 122 23approximately the same in both cases ( Å 7%), while there was no significant breakthrough of Pd, Ir, Pt and Au. The level of breakthrough increased significantly with a double pass on the thiourea-cleaned column. This may have been caused by the initial eluate removing residual thiourea trapped on the resin, which then eluted additional Ru, Rh, and some Pt on the second pass.Breakthrough of small amounts of Ir cannot be attributed to this mechanism, but might have been caused by a change in oxidation state from Ir4+ to Ir3+ during the procedure. It was observed that as the first eluate was passed through the column a second time, the coloured band of PGE and Au moved rapidly down the column.This means that breakthrough is very sensitive to small changes in elution volume. Clearly, only resin which had not been in contact with thiourea provides a reliable anion-exchange medium, so reuse of resin cannot be recommended. This is not a serious limitation, since the cost of Dowex 1-X8, rather than analytical-grade (AG) resin, is not high. Blank levels of each PGE and Au were determined for Dowex 1-X8 resin after batch cleaning with 6 mol l21 HCl.The column was prepared as previously and eluted with three fractions of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, followed by five 25 ml fractions of 12 mol l21 HCl. The solutions were analysed by ICP-MS and the total amount of each element was obtained by summing the fractions. Small amounts (ng) of Ru (18), Rh (8), Pd (40), Ir (14) and Pt (22) were found, which would only be significant if the method was applied to the determination of very low levels of precious metals.Concentrations of Au (490 ng) were higher, illustrating the need to carefully clean the resin prior to use. This level of Au was not significant for the 500 mg experiments, but the cleaning step was lengthened to include preconditioning of the column with 500 ml of 1 mol l21 HCl for later, low-level work. Determination of the PGE and Au at low levels Relatively high levels of the PGE and Au were generally used during the initial development of the ion-exchange method.This enabled ICP-AES to be used as the analytical finish, which is more precise and less prone to analytical problems caused by high and variable levels of TDS in solutions than ICP-MS. Experiments on solutions containing 1 mg spikes, however, necessitated diluting the thiourea fraction to reduce the level of TDS presented to the instrument. Diluted samples were below the lower limit of quantitation for ICP-AES, and close to those for ICP-MS, so the ion-exchange method developed so far would only be applicable to geological samples containing high ( > 1 mg g21) PGE and Au contents (calculated based on a 1 g sample size).There was a requirement, therefore, to concentrate the precious metals in the eluate (the volume eluted could not be reduced, since it was the minimum necessary to completely elute the PGE and Au), and/or eliminate the need for dilution to reduce the TDS and acid concentrations. The TDS content of the thiourea eluate could be reduced by decomposing the thiourea with nitric acid: H2NCSNH2 + HNO3 » NH3 + COx + SOx (1) where x = 1, 2, 3, or 4, as appropriate.The ammonia, carbon monoxide and carbon dioxide are lost by volatilisation. The main species remaining in solution after decomposition with HNO3 is sulfate (SO4 22). Unfortunately, sulfate solutions are not well suited to analysis by ICP-MS because the Ni sampling cone rapidly degrades, causing signal instability during analysis. Two possibilities were considered for the analysis of these solutions by ICP-MS: (1) use of a Pt-tipped sampling cone, which is not damaged by low levels of SO4 22; (2) removal of the sulfate before analysis.The first option was not feasible because Pt was one of the elements being sought, so precipitation of BaSO4 was investigated as a means of removing excess sulfate from solution. The solubility of BaSO4 is 2.2 mg ml21 in cold water.31 It was calculated that 75 ml of 0.3 mol l21 thiourea would produce 0.225 mol 121 SO4 22, assuming that there is complete conversion of S in thiourea to SO4 22.This estimate is almost certainly too high, since it takes no account of the loss of volatile lower-order sulfur–oxygen species during the decomposition step, so a maximum of 5.5 g of BaCl2·2H2O is needed to precipitate the sulfate. The following procedure was used to decrease the TDS of the thiourea fraction: (1) each 0.3 mol l21 thiourea solution was reduced to Å 10 ml in a 100 ml Pyrex beaker by evaporation at 95 °C on a sand bath, and allowed to cool; (2) fuming AnalaR HNO3 was added dropwise (2–3 ml) until effervescing ceased; (3) the resulting solution was evaporated at 95 °C to < 1 ml, to remove excess HNO3, and then diluted to 5–10 ml with deionised water; (4) AnalaR BaCl2 (5.5 g BaCl2·2H2O dissolved in Å 30 ml of H2O) was added, and the solution stirred to ensure complete precipitation; (5) the BaSO4 precipitate was removed by vacuum filtration through a Whatman (Whatman, Maidstone, Kent, UK) 0.45 mm cellulose nitrate filter membrane, using a large diameter (47 mm) filter funnel; (6) the filtered solution was evaporated on a sandbath at 95 °C to Å 5 ml, transferred into a 10 ml calibrated flask, and made up to volume with 0.5 mol l21 HCl.To evaluate the effectiveness of the HNO3–BaSO4 method, solutions of thiourea were spiked with known amounts of the PGE and Au, and treated as above. ICP-AES analyses of the treated solutions showed high levels of Ba (up to 1000 mg ml21) and S (approximately 400 mg ml21), indicating incomplete conversion of the sulfur in the thiourea to sulfate ions.Although the level of Ba and S was below the 2000 mg ml21 TDS limit imposed for solutions being analysed by ICP-MS, the presence of such high levels of individual elements caused signal suppression, necessitating the need for all thiourea fractions to be analysed by the standard additions method.This enabled more accurate analyses to be obtained, but quadrupled the number of solutions that had to be processed. Results for duplicate 1 mg experiments were in poor agreement (Table 7), making assessment of the results difficult. In general, PGE and Au in the spiked thiourea solutions showed moderate to good (70–100%) recoveries of Rh, Pd, Ir, Pt and Au at the 10 and 1 mg levels, but low values (50–70%) were obtained from the 0.1 mg spike, except for Au which was completely recovered in all three cases.Ruthenium recovery ranged from 70% for 10 mg, to 53% for the 0.1 mg solution. In a second experiment, 10, 1 and 0.1 mg PGE and Au solutions prepared in 1 mol l21 HCl were loaded onto Dowex 1-X8 columns, the initial eluate was collected to determine the breakthrough, and the precious metals were eluted with 75 ml of thiourea solution and 125 ml of 12 mol l21 HCl. The thiourea fraction was treated using the HNO3–BaSO4 method, while the concentrated HCl fraction was evaporated to incipient dryness and made up to 10 ml in 0.5 mol l21 HCl.The initial eluate, treated thiourea and HCl fractions were all analysed separately by ICP-MS. Recoveries were calculated by summing analyses Table 7 Recovery (%) of the PGE and Au from thiourea solutions. ICP-MS determinations after decomposition in HNO3 and removal of sulfate by precipitation of BaSO4. Averages and standard deviations for the 1 mg spike are based on determination of two solutions Mass in spike/mg Element 10 1 0.1 Ru 69 67 ± 7 53 Rh 85 67 ± 12 53 Pd 87 76 ± 22 57 Ir 99 96 ± 8 69 Pt 103 95 ± 13 72 Au 110 110 ± 29 100 24 Analyst, January 1997, Vol. 122of the thiourea and HCl fractions, and expressing results as a percentage of the amount in the original spike. The 1 mg experiment was performed in triplicate to establish the reproducibility of the method. The best results (Table 8) were obtained from the 1 mg spikes, with combined breakthrough and recovery of Ru, Rh, Pd and Pt generally totalling > 90%.However, the standard deviation of the measured recovery was relatively high, around 2–20%. The recovery of Au was 70% (with no breakthrough), while 67% Ir was eluted and 6% lost through breakthrough. The range of results obtained, however, included one run with 90% recovery of all PGE and Au. Lower recoveries were observed for all elements except Ru and Pt for the 10 mg solutions (Table 8). The 0.1 mg experiment yielded recoveries of > 100% for all elements, indicating contamination during handling of these solutions. The variable, and generally low, recovery of the PGE and Au from the thiourea solution following treatment with HNO3 and BaCl2 is attributed to coprecipitation and/or occlusion of the PGE and Au with the BaSO4 precipitate.Contamination was also encountered while determining the level of PGE and Au eluted from the Dowex 1-X8 resin in a blank run. Blank levels of the PGE and Au obtained for two columns are given in Table 9.Method blanks were also obtained by processing a 1 mol l21 HCl solution in an identical method to a geological sample (incorporating a digestion step using the method of Totland et al.,5 and anion-exchange), and analysing the eluent after decomposing the thiourea; again, two sets of results are presented because of the large difference obtained. Runs with high blank levels of the PGE and Au, were generally caused by high concentrations in the thiourea fraction.However, analysis of 75 ml of thiourea solution processed using the HNO3–BaSO4 method (Table 9), indicated low PGE and Au concentrations in the reagents. The highly variable blank levels, therefore, were not caused by contamination or interferences arising from the reagents used, but were probably a result of the extensive handling of solutions required in the procedure. This makes subtraction of a true blank difficult. Geological Reference Materials Although developed using synthetic solutions of the PGE and Au, the low levels of breakthrough and good recovery of several elements, indicated that the method should be applicable to the separation and determination of these elements in geological materials.To assess this, a study was undertaken using geological reference materials. Nickel copper matte PTM-1 (CCRMP, Canadian Certified Reference Materials Project, Energy Mines and Resources, Ottawa, Canada) contains relatively high levels of the PGE and Au, ranging from 0.34 mg g21 Ir to 5.8 mg g21 Pt, so this material was chosen to evaluate the basic anion-exchange procedure described above.In this case, the thiourea and 12 mol l21 HCl fractions could simply be diluted prior to analysis by ICP-MS. Three reference materials were used to evaluate the procedure employing the decomposition of thiourea: CCRMP materials PTM-1 and PTC-1 (sulfide flotation concentrate); Council for Mineral Technology (MINTEK, South African Bureau of Standards, Pretoria, South Africa) platinum ore, SARM7.Samples were prepared using a microwave aciddigestion procedure, described in detail elsewhere.5 Briefly, the method employs 1 g samples and acid digestion with 20 ml of aqua regia and 10 ml of 29 of mol l21 HF in Ultem-jacketed Teflon PFA sealed-vessels, heated at elevated pressure (200 psi; Å 1.4 MPa) in an MDS-2000 microwave oven (CEM Corporation, Matthews, NC, USA).Samples are subsequently evaporated to near-dryness, digested in 1 mol l21 HCl, filtered, and the insoluble residues fused with small amounts of 1 + 1 Na2O2 + Na2CO3 (silicate samples) or Na2O2 (sulfides), before being dissolved in 1 mol l21 HCl. Filtrate and dissolved residue solutions are combined to give 10–20 ml of 1 mol l21 HCl, which is suitable for loading directly onto the anion-exchange column. Data for PTM-1 obtained by direct analysis of the thiourea fraction following anion-exchange separation were (mg g21): Ru 0.3; Rh 1.3; Pd 9.8; Ir 0.3; Pt 6.4; Au 2.6. When compared to results (Table 10) obtained following acid digestion and Table 9 Blank values (ng) obtained from Dowex 1-X8 columns following digestion of thiourea and preconcentration of HCl eluents prior to analysis by ICP-MS.Method blank includes a microwave digestion procedure.5 Values for a decomposed thiourea blank are included for comparison Column blank Method blank Element A B A B Thiourea blank Ru < 2 190 36 70 < 2 Rh < 0.3 150 < 0.3 39 < 0.3 Pd < 2 120 11 42 1.5 Ir 10 180 35 120 < 0.7 Pt 370 180 77 230 1.2 Au < 0.6 450 17 280 0.7 Table 8 Breakthrough and recovery (%) of the PGEs and Au after anionexchange separation.ICP-MS determinations following decomposition of thiourea and preconcentration of the HCl eluents. Averages and standard deviations for the 1 mg spike are based on three replicates Mass in spike/mg 10 1 0.1 Break- Re- Break- Re- Break- Re- Element through covery through covery through covery Ru 4.8 84 9.0 ± 2.3 79 ± 2 9.5 140 Rh 27 41 40 ± 15 49 ± 14 36 120 Pd < 0.1 65 2.3 ± 4.0 87 ± 11 < 0.1 180 Ir 1.8 50 6.0 ± 3.6 67 ± 22 6.0 200 Pt < 0.1 96 0.4 ± 0.2 97 ± 14 < 0.1 230 Au < 0.1 38 < 0.1 70 ± 18 < 0.1 107 Table 10 Results for geological reference materials (mg g21) obtained following acid digestion, alkali fusion and anion-exchange separation with decomposition of thiourea, compared to acid digestion and fusion only,5 and reference values Element Ion exchange Digestion Reference PTC-1— Ru 0.29 ± 0.11 0.50 ± 0.07 0.65 Rh 0.30 ± 0.15 0.480 ± 0.089 0.62 ± 0.70 Pd 2.3 ± 1.2 11.1 ± 1.2 12.7 ± 0.7 Ir 0.21 ± 0.02 0.11 ± 0.01 0.1 Pt 2.40 ± 0.09 1.70 ± 0.14 3.0 ± 0.2 Au 0.52 ± 0.12 0.38 ± 0.21 0.65 ± 0.10 PTM-1— Ru 0.36 ± 0.06 0.670 ± 0.029 0.5 Rh 0.33 ± 0.06 0.940 ± 0.025 0.9 ± 0.2 Pd 6.1 ± 0.6 7.60 ± 0.12 8.1 ± 0.7 Ir 0.38 ± 0.22 0.35 ± 0.04 0.3 Pt 4.5 ± 0.5 4.90 ± 0.08 5.8 ± 0.4 Au 0.90 ± 0.02 1.500 ± 0.045 1.8 ± 0.2 SARM7— Ru 0.19 ± 0.11 0.360 ± 0.027 0.430 ± 0.057 Rh 0.049 ± 0.003 0.230 ± 0.007 0.240 ± 0.013 Pd 1.30 ± 0.14 1.230 ± 0.095 1.530 ± 0.032 Ir 0.11 ± 0.03 0.110 ± 0.016 0.074 ± 0.012 Pt 2.90 ± 0.37 3.40 ± 0.30 3.740 ± 0.045 Au 0.170 ± 0.013 0.290 ± 0.094 0.310 ± 0.015 Analyst, January 1997, Vol. 122 25fusion of the insoluble residue without an ion-exchange step,5 and with reference values, these data demonstrate acceptable, if marginally high, recovery of Rh, Pd, Ir and Pt.The value for Ru is low, but the level of Ru in the thiourea solution was close to the limit of detection for the ICP-MS, making the assessment inconclusive. Gold yielded a high value, suggesting a continuing contamination problem. Results for three preparations of PTM-1, and duplicate preparations of PTC-1 and SARM7 (Table 10), obtained following anion-exchange with decomposition of thiourea show, with a few exceptions, low recoveries of Ru, Rh, Pd, Pt and Au when compared to digestion only and reference data.These elements are eluted in the thiourea fraction, and it is believed that the treatment used to reduce the TDS was the cause of the poor recovery due, at least in part, to occlusion of a portion of the PGE and Au in the BaSO4 precipitate. This conclusion is supported by the complete recovery of most elements in PTM-1 when the thiourea fraction was analysed directly.Furthermore, Ir data (Table 10) are in good agreement with reference values. Iridium is eluted entirely with the 12 mol l21 HCl fraction, producing a simple matrix that poses no analytical difficulties by ICP-MS. Conclusions Our experiments demonstrate that the PGE and Au may be quantitatively adsorbed onto Dowex 1-X8 anion-exchange resin, and eluted using a two-stage procedure: thiourea to elute most Ru, Rh, Pd, Pt, Au; concentrated HCl to complete elution of these elements, and to elute all Ir.Evaluation of the procedure using geological reference materials showed encouraging results. In particular, the method has been successfully applied to the separation and determination of Ir in three rock reference materials by ICP-MS. The application of our method to the entire group of PGE and Au is limited principally by difficulties associated with analysis of the thiourea fraction. The extra dilution required for direct analysis of this eluate by ICP-MS, leads to limits of quantitation in samples26 of around 1 mg g21 for Ru, Rh, Pd, Pt and Au, which are similar to those achievable5 without separation from matrix elements.Reduction of the TDS in the first eluate was undertaken by decomposing thiourea with fuming HNO3, causing the loss by volatilisation of NH3 and CO2. However, high concentrations of sulfate ions remaining in solutions prevented their analysis by ICP-MS, because of the risk of corroding the Ni sampling cone.Removal of sulfate by precipitation with Ba was of limited success, producing erratic and generally low values for elements eluted in this fraction. It is concluded that precipitation is unsuitable for the analytical method, because the potential for coprecipitation and/or occlusion of the PGE and Au is too high and unpredictable. Although the use of isotope dilution methods could be used to compensate for low recoveries of Ru, Pd, Ir and Pt,32 extensive handing required at this stage led to sporadic contamination and difficulties in producing reliable procedural blanks, which is less easily addressed.To apply our method to the separation and determination of low levels of the PGE and Au, an alternative method for analysing the thiourea fraction is required. Potential ways to achieve this include electrothermal vaporisation or flow injection ICP-MS. These techniques may be used to directly analyse solutions with high levels of TDS, but their development is non-trivial and is beyond the scope of this study.Funding by RTZ Mining and Exploration Ltd. and enthusiastic support from Drs. C. Carlon and N. Badham (RTZ) are gratefully acknowledged. The operation of the ICP-MS laboratory as an analytical facility, located at Imperial College Centre for Analytical Research in the Environment, is supported by the UK Natural Environment Research Council (NERC). References 1 Hall, G. E. M., and Bonham-Carter, G. F., J. Geochem.Explor., 1988, 30, 255. 2 Van Loon, J. C., and Barefoot, R. R., Analytical Methods for Geochemical Exploration, Academic Press, San Diego, CA, 1989. 3 Van Loon, J. C., and Barefoot, R. R., Determination of the Precious Metals—Selected Instrumental Methods, Wiley, Chichester, 1991. 4 Colodner, D. C., Boyle, E. A. Edmond, J. M., and Thomson, J., Nature, 1992, 358, 402. 5 Totland, M. M., Jarvis, I., and Jarvis, K. E., Chem. Geol., 1995, 124, 21. 6 Ali-Bazi, S. J., and Chow, A., Talanta, 1984, 31, 815. 7 Korkisch, J., Handbook of Ion Exchange Resins: Their Application in Inorganic Analytical Chemistry, CRC Press, Boca Raton, FL, 1989, vol. 3. 8 Marhol, M., in Comprehensive Analytical Chemistry, ed. Svehla, G., Wilson and Wilson’s, Prague, 1982, vol. XIV, p. 580. 9 Korkisch, J., and Klakl, H., Talanta, 1968, 15, 339. 10 Totland, M. M., Jarvis, I., and Jarvis, K. E., Chem. Geol., 1993, 104, 175. 11 Boumans, P. W. J. M., Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry, 2nd edn., Pergamon, Oxford, 1984, vol. 2. 12 Boumans, P. W. J. M., Line Coincidence Tables for Inductively Coupled Plasma Atomic Emission Spectrometry, 2nd edn., Pergamon, Oxford, 1984, vol. 1. 13 Winge, R. K., Fassel, V. A., Paterson, V. J., and Floyd, M. A., Inductively Coupled Plasma-Atomic Emission Spectroscopy—An Atlas of Spectral Information, Elsevier, Amsterdam, 1985. 14 Totland, M. M., PhD Thesis, Kingston University, Kingston upon Thames, 1993. 15 Jarvis, K. E., Gray, A. L., and Houk, R. S., Handbook of Inductively Coupled Plasma Mass Spectrometry, Blackie, Glasgow, 1992. 16 Jarvis, I., and Jarvis, K. E., Chem. Geol., 1992, 95, 1. 17 Branch, C. H., and Hutchison, D., J. Anal. At. Spectrom., 1986, 1, 433. 18 Busch, D. D., Prospero, J. M., and Naumann, R. A., Anal. Chem., 31, 884. 19 De Laeter, J. R., and Mermelengas, N., Geostand. Newsl., 1978, 2, 9. 20 Hodge, V., Stallard, M., Koide, M., and Goldberg, E.D., Anal. Chem., 1986, 58, 616. 21 Kraus, K. A., Nelson, F., and Smith, G. W., J. Phys. Chem., 1954, 58, 11. 22 Morgan, J. W., Anal. Chim. Acta, 1965, 32, 8. 23 Petrie, R. K., and Morgan, J. W., J. Radioanal. Chem., 1982, 74, 15. 24 Totland, M. M., Jarvis, I., and Jarvis, K. E., Chem. Geol., 1992, 95, 35. 25 Chao, T. T., and Sanzolone, R. F., J. Geochem. Explor., 1992, 44, 65. 26 Jarvis, I., in Handbook of Inductively Coupled Plasma Mass Spectrometry, ed. Jarvis, K. E., Gray, A.L., and Houk, R. S., Blackie, Glasgow, 1992, pp. 172–224. 27 Potts, P. J., A Handbook of Silicate Rock Analysis, Blackie, London, 1987. 28 Potts, P. J., in Analysis of Geological Materials, ed. Riddle, C., Marcel Dekker, New York, 1993, pp. 123–220. 29 Sulcek, Z., and Povondra, P., Methods of Decomposition in Inorganic Analysis, CRC Press, Boca Raton, FL, 1989. 30 Singh, S., Mathur, S. P., Thakur, R. S., and Lal, K., Orient. J. Chem., 1987, 3, 203. 31 CRC Handbook of Chemistry and Physics, ed.Weast, R. C., Astle, M. J., and Beyer, W. H., CRC Press, Boca Raton, FL, 68th edn., 1987. 32 Enzweiler, J., Potts, P. J., and Jarvis, K. E., Analyst., 1995, 120, 1391. Paper 6/06169I Received September 9, 1996 Accepted November 1, 1996 26 Analyst, January 1997, Vol. 122 Assessment of Dowex 1-X8-based Anion-exchange Procedures for the Separation and Determination of Ruthenium, Rhodium, Palladium, Iridium, Platinum and Gold in Geological Samples by Inductively Coupled Plasma Mass Spectrometry Ian Jarvis*a, Marina M.Totland†a and Kym E. Jarvisb a School of Geological Sciences, Kingston University, Penrhyn Road, Kingston upon Thames, Surrey, UK KT1 2EE b NERC ICP-MS Facility, Centre for Analytical Research in the Environment, Imperial College at Silwood Park, Buckhurst Road, Ascot, Berkshire, UK SL5 7TE Synthetic multielement solutions of the platinum group metals (PGE: Ru; Rh; Pd; Ir; Pt) and gold, with analysis by ICP-AES and ICP-MS, have been used to study the behaviour of the precious metals on Dowex 1-X8 resin.Simple solutions of precious-metal chlorocomplexes showed near-complete adsorption ( > 99%) of most elements, and only minor breakthrough of Ru and Ru ( Å 5%). Solutions pre-treated with acid mixtures typically used to decompose geological samples, demonstrated that perchloric acid adversely affects the adsorption of the PGEs on the resin. Solutions treated with HF–HNO3–HCl maintained good retention of Ir, Pt, Au ( > 99%), Pd ( > 94%) and Ru ( > 90%), but displayed significant loss (up to 40%) of Rh.A two-step procedure was necessary to elute the precious metals from the resin: 0.3 mol l21 thiourea prepared in 0.1 mol21 HCl removed Ru, Pd, Pt, Au, and some Rh: 12 mol l21 HCl eluted remaining Rh and all Ir. Recoveries ranged from 50 to 100%. At low levels, the determination of PGE and Au in the thiourea fraction by ICP-MS was compromised by high levels of total dissolved solids (TDS), which necessitated dilution of the eluate prior to analysis.The TDS was reduced by decomposing thiourea with HNO3 and removing SO4 22 by precipitation of BaSO4, but this led to lower and more erratic results, and increased contamination. An assessment of the optimised procedure employing geological reference materials PTM-1, PTC-1 and SARM7, indicated that acceptable results should be attainable for ICP-MS determination of most elements in geological samples containing high concentrations ( > 1 mg g21) of the PGE, for which decomposition of thiourea is unneccessary.The addition of a decomposition step led to low recovery of all elements except Ir, which was present entirely in the HCl eluate. The method is viable for the determination of Ir in a range of geological materials, but modifications will be required if it is to be extended to the other precious metals. Keywords: Platinum group element; gold; cation exchange chromatography; geological sample; inductively coupled plasma mass spectrometry The commonest methods1–3 used for the determination of the platinum group elements (PGE: Ru; Rh; Pd; Os; Ir; Pt) and gold are based on fire assay, which involves fusing 10–100 g samples with large amounts of alkali flux and collecting the PGE into a nickel sulfide or, for Au, a lead button.The nickel sulfide button is usually dissolved in acid to remove the nickel and sulfur, leaving a residue of PGE sulfides which may be analysed directly by neutron activation analysis (NAA).More commonly, the precipitate is taken into solution for analysis by AAS, direct coupled plasma-atomic emission spectroscopy (DCP-AES), or ICP-AES. Although fire assay is used extensively, it is highly labour intensive, and one of its most serious limitations is a dependence of the quality of results on the experience of the analyst. A more robust method would clearly be advantageous.Fire assay is also not well suited to the analysis of small ( < 5 g) samples, which may be required for some geological studies, such as investigations of precious metal mobility in sediment cores.4 We have previously described5 a new combined microwave digestion–minifusion method with analysis by ICP-MS, that yields quantitative data for Ru, Rh, Pd, Ir, Pt and Au in mineralised samples, but is limited by modest lower limits of determination in samples of 0.2–1 mg g21 (ppm).The objective of the present study was to investigate anion-exchange separation procedures for the PGE and Au, that would allow the determination of these elements at the lower levels (1–10 ng g21; ppb) found in most geological materials. Similar techniques have been employed by others6–8 with varying success, to determine individual or small groups of precious metals in a variety of sample types. Dowex 1-X8 is a strong base anion-exchanger with a styrene divinyl benzene polymer skeleton to which tertiary ammonium groups have been bound.7 Dowex 1-X8 exhibits a high selectivity for the PGE and Au, and it has been shown7,9 that it will remove these elements quantitatively from HCl solutions.However, the resin has not been used previously to simultaneously separate and preconcentrate the entire group of PGE and Au from solutions of geological materials, prior to their analysis by ICP-MS. The aims of this study were to: (a) develop experimental conditions under which 100% of the PGE and Au present in solution would be absorbed onto the Dowex 1-X8 resin, with minimal adsorption of competing ions; (b) find a set of conditions to elute 100% of the adsorbed PGE and Au; (c) develop a methodology to quantify the PGE and Au at ng ml21 levels in the eluate using ICP-MS.The first objective imposes limitations on the solution chemistry of the digested geological materials that are to undergo separation, while the second is known7 to be difficult to achieve from strong base cationexchange resins.Osmium was not included in the study because its volatility precludes its determination in most solutions of geological materials prepared for other precious metal analysis. 2,3,10 Experimental Instrumentation A JY 70 Plus (Jobin-Yvon, Longjumeau, France) inductively coupled plasma atomic emission spectrometer, located at † Present address: Atomic Energy of Canada Ltd., Chalk River Laboratories, Chalk River, Ontario, Canada KOJ 1JO.Analyst, January 1997, Vol. 122 (19–26) 19Kingston University, was used to determine the PGE and Au at high levels ( > 50 ng ml21) during method development. A VG Elemental PlasmaQuad PQ2 Plus (Fisons Instruments, Winsford, Cheshire, UK) ICP-MS, formerly at Royal Holloway University of London, was employed for lower level ( < 50 ng ml21) work. Operating conditions for the two instruments are given in Tables 1 and 2. In both cases, an external drift monitor (generally, a calibration standard solution) was used to correct for signal changes with time.Spectral lines used for PGE and Au determination by ICPAES were selected experimentally. Tables of spectral data11–13 were used to identify the most sensitive wavelengths for each element. A 1 mol l21 ARISTAR HCl (Merck, Lutterworth, UK) blank solution, a 1 mg ml21 solution of the analyte in the same matrix, and a multi-element solution containing all of the precious metals at 1 mg ml21 (both prepared using aliquots of single-element 1000 mg ml21 precious metal standard solutions, supplied in 20% v/v HCl; Johnson Matthey, Royston, UK), were each scanned14 across a 0.12 nm window, centred on the spectral line selected.The emission lines chosen (Table 3) were the most sensitive lines having no interference from other elements in the group. As only simple solutions of the precious metals were to be analysed by ICP-AES, interferences from other elements were not considered.Subsequently, calibration was achieved using five synthetic multi-element PGE and Au standards, prepared using aliquots of the 1000 mg ml21 singleelement standard solutions. Solutions analysed were generally in 0.1–1 mol l21 ARISTAR HCl; matrix effects were minimised by matching the acidity and salt content of samples and standard solutions. Detection limits (Table 3) calculated as 3s standard deviations of eleven determinations of a 1 mol l21 ARISTAR HCl blank, demonstrate that ICP-AES is well suited to the quantification of PGE and Au at concentrations in excess of 50 ng ml21. The ICP-MS was optimised using 59Co and 238U to give maximum sensitivity whilst minimising interferences, particularly refractory oxide species.The PGE and Au lie in two distinct parts of the mass range, 96–110 and 184–198 u. Some isotopes have an isobaric overlap and these were not used for analysis. The mass spectrometer was scanned from 98–200 u, with masses between 112–179 u being skipped to maximise the integration time spent on PGE and Au isotopes.During data processing, the existence of polyatomic or oxide interferences was determined by comparing isotopic ratios for elements with their theoretical values; only isotopes free from all spectroscopic interferences (Table 3) were used for quantification. External calibration of the ICP-MS for PGE and Au determinations was accomplished using multielement standards prepared in dilute HCl from 1000 mg ml21 single-element standard solutions; in all cases, the acid concentration was matched to the samples.A single standard (ranging between 20 and 200 ng ml21, depending on the expected concentration in the samples) and a blank were used as calibration points. It was noted that some elements displayed a significant memory effect (Au and, to a lesser extent, Pd), and extended washout times were necessary to avoid sample carryover.For example, following a solution containing 200 ng ml21 of the PGE and Au, a 3–4 min washout (using maximum pumping speed) with 1 mol l21 HCl was required to minimise the background level of Au. To avoid cross-contamination between high- and lowconcentration solutions, the integrals for all elements were monitored throughout the analytical run, and wash times were extended after solutions containing high levels of the precious metals had been analysed. Blanks were measured to establish the background level of these elements throughout the run, and blank corrections were applied where necessary.The measured detection limits for all elements by ICP-MS (Table 3) were better than 0.22 ng ml21. It is worth noting that since this study was completed, a newer instrument fitted with an Enhanced Performance Interface (Fisons Instruments) has been installed, which yields detection limits for the PGE and Au between 0.006 and 0.05 ng ml21. However, these do not translate directly to improved lower limits of determination for samples, because the new instrument displays poorer tolerance of total dissolved solids (TDS), necessitating higher dilution factors for solutions.Nevertheless, with these exceptionally low detection limits, ICP-MS is ideally suited to the determination of low concentrations of PGEs and Au in simple solutions. ICPMS is also far less prone to interferences than most other instrumental techniques.15,16 However, limitations on the types of solution analysed are imposed by the level of TDS; upper limits of approximately 0.2% TDS may be aspirated into an ICP-MS instrument without causing significant instrumental drift and/or matrix effects, so solutions had to be diluted to an appropriate level prior to analysis.Anion-exchange Experiments The Dowex 1-X8 resin (Bio-Rad Laboratories, Hemel Hempstead, UK) used here was nominally in chloride form (i.e., with chlorine counter-ions) and graded from 200–400 mesh (74–37 mm).However, the resin supplied contained a much Table 1 ICP-AES operating parameters for PGE and Au determination Instrument Jobin-Yvon JY 70 Plus Plasma power 1000 W Reflected power < 5 W Coolant gas flow 12 l min21 Auxiliary gas flow 0 l min21 Sheath gas flow 0.2 l min21 Nebuliser gas flow 0.35 l min21 Nebuliser Meinhard TR-50-C1 concentric glass Sample uptake 1 ml min21 Spray chamber JY Scott-type, double pass Spectrometer Czerny–Turner monochromator Grating Holographic 3600 grooves mm21 Integration period 10 s Table 2 ICP-MS operating parameters for PGE and Au determination Instrument VG PlasmaQuad PQ2 Plus Plasma power 1300 W Reflected power < 1 W Coolant gas flow 14 l min21 Auxiliary gas flow 0.5 l min21 Nebuliser gas flow 0.75 l min21 Nebuliser de Galan high dissolved solids Sample uptake rate 0.5 ml min21 Spray chamber Surrey design, single-pass, water-cooled at 4 °C Scan regions 98–111 and 180–200 u Table 3 Detection limits for the PGE and Au by ICP-AES and ICP-MS.Values calculated as 3s standard deviations for 11 determinations of a 1 mol l21 ARISTAR HCl blank ICP-AES ICP-MS Wavelength/ Detection limit/ Mass Detection limit/ Element nm ng ml21 (u) ng ml21 Ru 245.66 14 101 0.22 Rh 233.48 11 103 0.03 Pd 340.46 13 105 0.17 Ir 224.27 9.0 193 0.07 Pt 214.42 29 195 0.11 Au 242.80 5.9 197 0.06 20 Analyst, January 1997, Vol. 122wider range of grain-sizes than the nominal fraction, including a significant amount of very fine particles, possibly produced during packaging and handling.These were removed by slurrying the resin with deionised water, allowing the resin to settle for a short time, and then decanting off the suspended fines with the supernatant. This was repeated at least three times to remove most of the fine material. The resin was batch-cleaned prior to use by slurrying it with 6 mol l21 AnalaR HCl (Merck), allowing it to stand for 10–20 min, and then decanting off the acid.This procedure was repeated at least three times, or until no further discolouration of the acid was observed. After pouring off the last portion of the cleaning acid, the resin was slurried with 1 mol l21 ARISTAR HCl, and stored ready for use. For each ion-exchange experiment, following loading on the column, the resin was conditioned by passing 100 ml (later increased to 500 ml) 1 mol l21 ARISTAR HCl through the column. Only new resin was used for this work.To evaluate possible anion-exchange procedures, two parameters were measured: (1) percentage of the PGE and Au not adsorbed onto the resin, termed ‘breakthrough’; (2) percentage of the adsorbed metals eluted from the column, termed ‘recovery’. Unless stated otherwise, Merck ARISTAR reagents and ultra-pure (better than 18 MW cm) deionised water were used throughout this study. Adsorption of the PGEs and Au on Dower 1-X8 resin The PGEs must be present in appropriate oxidation states to ensure strong adsorption onto Dowex 1-X8 resin,7 and the charge of the PGE-complex may change depending on the ligands attached to the metal.Preliminary evaluation of the resin was conducted using stable solutions of PGE anionic chlorocomplexes, supplied at 1000 mg ml21 and stored in 20% v/v HCl solutions (Johnson Matthey, Royston, UK). Breakthrough of each PGE and Au was determined using synthetic multielement solutions produced by diluting aliquots of 1000 mg ml21 standards with 1 mol l21 HCl to yield 500 mg and 1 mg spikes of each element, diluted to 10 ml.Column dimensions were chosen based on previous studies6–9,17–23 to hold a settled resin bed of 1 cm diameter, 10 cm long. The percentage breakthrough of each element was determined by analysing the 1 mol l21 HCl eluate (25 ml) collected as the spike solutions were loaded onto the column. Solutions generated during experiments with the 500 mg spike were sufficiently concentrated to enable analysis by ICP-AES.This high level was chosen to determine whether the capacity of the resin was likely to be exceeded in normal use. The proportion of each element not retained on the column was small: < 0.1% Pd, Pt, Au; 0.2% Ir; 4.6% Rh; 7.3% Ru. A level of precious metals much closer to that expected for geological materials (1 mg) was used to evaluate breakthrough at lower concentrations. In this instance, absolute breakthrough was expected to be very small, so ICP-MS was used as the analytical finish.Results showed no significant breakthrough of Pd, Pt, Au ( < 0.1%), Ir (0.1%) and Rh (0.8%), and < 10% breakthrough of Ru from the 1 mg spike. While the above results verified the adsorption characteristics of Dowex 1-X8 described in the literature, they represent a simplified situation. The chemical state of the PGE and Au following digestion of geological materials5,24 may not be identical to those found in standard solutions.The effects of digestion procedures on the chemical state of individual PGE and Au needed to be established experimentally. Knowledge of the exact chemical state of each element is not necessary, provided that the PGE and Au are completely adsorbed onto the resin. It is not possible to measure breakthrough of the PGE and Au directly with geological materials, because the high concentrations of matrix elements passing through the columns preclude the determination of low levels of precious metals in the eluate. To estimate possible breakthrough when real samples are separated by this method, synthetic solutions were treated in an identical manner to geological samples. In this way, the effects of various acids used in digestion procedures could be evaluated.An HF–HClO4-based acid attack is one of the most commonly used methods to digest geological samples.24–29 To assess the affects of these acids on column retention, a 1 mol l21 HCl solution containing 500 mg of PGE and Au was treated as follows: (1) 8 ml of 16 mol l21 HNO3, 4 ml of 29 mol l21 HF, 4 ml of 12 mol l21 HClO4 were added in a PTFE beaker, evaporated at 200 °C to incipient dryness, a further 2 ml of HClO4 (12 mol l21) were added, evaporated to near-dryness, and the final solution made to 10 ml with 1 mol l21 HCl; (2) 4 ml of 16 mol l21 HNO3, 4 ml of 29 mol l21 HF, 4 ml of 12 mol l21 HClO4 were added and evaporated to incipient dryness, 8 ml of 12 mol21 HCl were added and evaporated to neardryness (twice), and the final solution also made to 10 ml with 1 mol l21 HCl.Breakthrough from these solutions were measured by ICP-AES from 1 cm diameter, 10 cm long columns of Dowex 1-X8. Breakthrough increases significantly following acid pre-treatment (Table 4). Losses at this stage were reduced when the solution was evaporated twice with 12 mol l21 HCl, instead of HClO4. Nevertheless, breakthrough of Rh and Ir remained very high, 58 and 18%, respectively.A comparison made using different elution volumes of the HClO4- evaporated solution (Table 4), demonstrated that the amount of breakthrough increased dramatically when 65 ml of 1 mol l21 HCl was eluted compared to 25 ml; in the former case, no Rh was retained on the resin. The level of breakthrough and the dependence on elution volume are clearly unacceptable for this separation method, precluding the use of HClO4-based digests prior to cation-exchange chromatography.A series of 1 mol l21 HCl solutions spiked with PGE and Au were evaporated in PTFE beakers at 100 °C on a hotplate, in the presence of 10 ml of 29 mol l21 HF, 15 ml of 12 mol l21 HCl and 5 ml of 16 mol l21 HNO3. This acid mixture has been demonstrated5 to be effective for the digestion of preciousmetal- bearing geological samples. Three masses of PGE and Au spike (10, 1, 0.1 mg) showed greatly reduced losses following the application of this acid mixture (Table 5), and subsequent evaporation (twice) with 4 ml of 12 mol l21 HCl to ensure conversion of elements to chloride form.Final solutions were approximately 10 ml of 1 mol l21 HCl; 25 ml of 1 mol l21 HCl were eluted and analysed by ICP-MS. Breakthrough was considered to be acceptably low ( < 10%) for Ru, Pd, Ir, Pt and Au, but the loss of Rh was high (27–40%). Elution conditions Dowex 1-X8 resin has a high affinity for the PGE and Au in dilute HCl solutions, but increased acid concentrations favour their removal.7 Distribution coefficients fall significant with Table 4 Breakthrough (%) of 500 mg of PGE and Au on Dowex 1-X8 anionexchange resin following pretreatments with perchloric acid.HClO4 = evaporated with HNO3, HF, HClO4; HClO4 added and evaporated. HCl = evaporated with HNO3, HF, HClO4; HCl added and evaporated. Both final solutions in 1 mol l21 HCl HClO4 HCl No pretreatment Element 25 ml 65 ml 25 ml 25 ml Ru 12 18 6.1 7.3 Rh 80 100 58 4.6 Pd 0.4 1.0 < 0.1 < 0.1 Ir 9.4 31 18 0.2 Pt 47 53 2.0 < 0.1 Au 0.4 1.0 < 0.1 < 0.1 Analyst, January 1997, Vol. 122 21increasing acid molarity, although there remains significant adsorption of some PGE even in concentrated HCl. Exceptions are Ir3+ and Rh3+ which have distribution coefficients of < 1 in 12 mol l21 HCl, and Ru4+, which is slightly higher at 10.7 The inference from these observations is that concentrated HCl will elute only part of the Ru, Rh and Ir adsorbed on the column; increasing the concentration of the counter ion in the system will not be sufficient to elute the entire group of elements.Thiourea has been successfully used to elute Pd, Pt and Au from strong base anion-exchange resins;7 selected PGE and Au are either reduced and/or complexed by thiourea, and the resulting complexes have low affinity for the resin. Elution experiments were conducted using 500 mg of each PGE and Au loaded in 10 ml of 1 mol l21 HCl.These solutions are coloured, and it was possible to get a quick assessment of elution conditions by observing the migration of coloured bands on the columns. Using this approach, it became apparent that although a portion of the PGE and Au could be eluted with thiourea, some remained on the column. It was concluded that a two-step procedure was necessary to elute the PGE and Au from a Dowex 1-X8 column. Three 25 ml aliquots of 0.3 mol l21 thiourea (4.7 g AnalaR thiourea dissolved in 200 ml of deionised water, acidified to 0.1 mol l21 HCl using 1.7 ml of 12 mol l21 HCl; selected based on work by Korkisch7) were eluted through the Dowex 1-X8 columns used for the breakthrough study.Solutions from the 500 mg experiments were diluted four-fold prior to ICP-AES analysis; 1 mg solutions were diluted 40-fold and analysed by ICP-MS. Dilution was necessary to reduce the high levels of TDS from the thiourea, to levels acceptable for each technique. 16 Aliquots of 25 ml of 12 mol l21 HCl were then eluted through each column, collected, diluted 10-fold to reduce the acid concentration, and analysed by ICP-AES or ICP-MS. A total of 100 ml of 12 mol l21 HCl was collected for the 500 mg experiment, and 125 ml from the 1 mg spike. Elution profiles (Fig. 1) show that most Ru, Pd, Pt and Au are eluted by 75 ml of 0.3 mol l21 thiourea, while Rh is only partly eluted and Ir remains bound to the resin.Remaining Rh and Au are completely eluted from the column with 12 mol l21 HCl, and Ir is removed by this eluent. Elution profiles are instructive for determining the rate of elution of elements and the relative efficacy of eluents, but overall recovery is the critical measure of the usefulness of a procedure. Recoveries for the 500 mg spike were 92% for Ru, and better than 97% for all other elements. Slightly lower recoveries ( Å 85%) were measured for Ir and Rh at the 1 mg level.High recoveries of gold were caused by carry-over effects during analysis; these were minimised in later experiments by increasing washout times and uptake rates during the wash period. To optimise our procedure, various parameters were studied including the: effect of temperature on elution efficiency; volume of eluent required; possibility of combining thiourea and 12 mol l21 HCl into a single step. The volume of thiourea required to elute the PGEs and Au is dependent on the rate of formation of thiourea complexes. These are easily identified in concentrated ( > 100 mg ml21) solutions because they form strong colours.Experiments showed that the rate of formation of the thiourea complex varied between elements; the colour of solutions began to change almost immediately upon the addition of individual PGE and Au (each treated separately) to a 0.3 mol l21 thiourea in 0.1 mol l21 HCl. Colours continued to change (orange Ru solutions changed to green and then blue) when left standing for 1 h.Some elements precipitated when left to stand for several hours, not surprisingly, since thiourea has been used 30 to quantitatively precipitate some of the PGE and gold, and facilitate their separation. However, the formation of insoluble thiourea complexes was considered to be unlikely during ion-exchange, because the level of PGE and Au in solution would be very low, and precipitation only occurs from concentrated solution.Furthermore, fresh thiourea is continually added to the column, so the equilibrium for formation of these complexes is always shifted towards dissociation. Our experiments indicated that the recovery of the PGE and Au from a Dowex 1-X8 column is governed by both kinetic and thermodynamic factors. These were evaluated further by increasing the elution temperature using a column with a jacket through which heated water was passed. A solution of 1 mol l21 HCl containing 500 mg of each of the PGEs and Au was loaded onto the column at room temperature, and the eluate analysed to determine the concentration of each element remaining on the column.The column temperature was raised to 50 °C and a solution of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, also heated to 50 °C, was eluted through the column (the eluate was collected as three 25 ml fractions), followed by the usual concentrated HCl elution step. A control experiment was run at room temperature ( Å 20 °C).The only improvement observed for elution at 50 °C was the complete recovery of Rh in the thiourea fraction (Fig. 2). The recovery of Ir in the thiourea fraction did not increase, so a 12 mol l21 HCl elution step was still required; indeed, elution of Ir with concentrated HCl seemed to be hindered by previous elution with heated thiourea (Fig. 2). Table 5 Breakthrough (%) of PGE and Au on Dowex 1-X8 anion-exchange resin after evaporated with HNO3, HF and HCl Mass of each PGE and Au Element 10 mg 1 mg 0.1 mg Ru 4.8 9.0 9.5 Rh 27 40 36 Pd 1.8 6.0 6.0 Ir < 0.1 < 0.1 < 0.1 Pt < 0.1 2.3 < 0.1 Au < 0.1 0.4 0.5 Fig. 1 Elution profiles (cumulative % recovery) for 500 mg (squares) and 1 mg (circles) spikes of the PGEs and Au from a 10 cm long, 1 cm diameter Dowex 1-X8 column using a two-step elution procedure. 22 Analyst, January 1997, Vol. 122A manually operated two-step elution is more cumbersome and requires more operator attention than a single-step procedure.Attempts were made to find experimental conditions that would enable the PGE and Au to be eluted using a single solvent. When 0.3 mol l21 thiourea in 12 mol l21 HCl was used as an eluent (Fig. 2), total recovery from 125 ml of eluent was generally lower than that achieved by the two-step method. Consequently, a two-step elution operated at room temperature was judged to yield the best results. Ion-exchange column dimensions The 1 cm diameter and 10 cm long Dowex 1-X8 resin bed employed in our initial studies, has been used by previous workers.7 However, the high capacity of the resin demonstrated by minimum breakthrough of PGE and Au at the 500 mg level, suggested that it might be possible to reduce column length while retaining good absorption on the resin.The recovery of the PGE and Au from a shorter, 1 cm diameter, 5 cm long column was examined. The volume of reagents used was varied to establish the effect of each reagent on the overall recovery of each element.Two experiments were undertaken: (a) 50 ml of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, then 150 ml of 12 mol l21 HCl; (b) 75 ml of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, followed by 75 ml of 12 mol l21 HCl. The eluate was collected in 25 or 50 ml fractions and analysed by ICP-AES. Ruthenium, Rh, Pd, Pt and Au were completely recovered by both procedures (Fig. 3). The recovery of Ir, which is primarily eluted in the 12 mol l21 HCl fraction, is highly dependent on eluate volume, 73% was recovered in 75 ml, increasing to 89% in 150 ml, so the larger volume of concentrated HCl was used in all further experiments.Results for the shorter column of Dowex 1-X8 showed that recoveries were not improved by reducing the length of the resin bed, while breakthrough experiments demonstrated that adsorption efficiency decreased (around 8% Ru and 18% Rh were not retained), so a 10 cm long column was confirmed as being optimum.Cleaning Dowex 1-X8 resin When using ion-exchange resins, procedures are needed to clean new, and if possible regenerate previously used, resin. A common approach is to clean the resin with reagents used for the elution step (in this case, thiourea and 12 mol l21 HCl). This approach is commonly used in chromatographic techniques to ensure that no additional contamination is obtained from the resin when the solvent is changed. The percentage breakthrough of the PGE and Au was compared after the resin had been cleaned with: (a) 0.3 mol l21 thiourea in 0.1 mol l21 HCl; and (b) 6 mol l21 HCl.Breakthrough was assessed as previously (called here a single pass), as well as for a double pass of the sample solution. For the double pass, the eluate containing the PGE and Au from a single pass through the column was collected and passed through the column again. The second eluate was then analysed (the same final volume was eluted for both experiments).Comparing the levels of breakthrough for single passes (Table 6), shows that there was significantly increased loss of Rh (from 5 to 17%) from resin pre-treated with thiourea, compared to that washed only with HCl. The loss of Ru was Fig. 2 Elution profiles (cumulative % recovery) for 500 mg spikes of the PGE and Au from a 10 cm long, 1 cm diameter Dowex 1-X8 column using different elution conditions. Fig. 3 Elution profiles (cumulative % recovery) for 500 mg spikes of the PGE and Au from a short (5 cm long, 1 cm diameter) Dowex 1-X8 column using : (a) 50 ml of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, then 150 ml of 12 mol l21 HCl (circles); (b) 75 ml of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, followed by 75 ml of 12 mol l21 HCl (squares).Table 6 Breakthrough (%) of 500 mg PGE and Au for Dowex 1-X8 resin cleaned with thiourea solution or HCl Thiourea HCl Element Single pass Double pass Single pass Ru 6.6 13 7.3 Rh 17 26 4.6 Pd 0.4 0.2 < 0.1 Ir < 0.1 0.4 0.2 Pt < 0.1 0.2 < 0.1 Au < 0.1 < 0.1 < 0.1 Analyst, January 1997, Vol. 122 23approximately the same in both cases ( Å 7%), while there was no significant breakthrough of Pd, Ir, Pt and Au. The level of breakthrough increased significantly with a double pass on the thiourea-cleaned column. This may have been caused by the initial eluate removing residual thiourea trapped on the resin, which then eluted additional Ru, Rh, and some Pt on the second pass.Breakthrough of small amounts of Ir cannot be attributed to this mechanism, but might have been caused by a change in oxidation state from Ir4+ to Ir3+ during the procedure. It was observed that as the first eluate was passed through the column a second time, the coloured band of PGE and Au moved rapidly down the column. This means that breakthrough is very sensitive to small changes in elution volume. Clearly, only resin which had not been in contact with thiourea provides a reliable anion-exchange medium, so reuse of resin cannot be recommended.This is not a serious limitation, since the cost of Dowex 1-X8, rather than analytical-grade (AG) resin, is not high. Blank levels of each PGE and Au were determined for Dowex 1-X8 resin after batch cleaning with 6 mol l21 HCl. The column was prepared as previously and eluted with three fractions of 0.3 mol l21 thiourea in 0.1 mol l21 HCl, followed by five 25 ml fractions of 12 mol l21 HCl.The solutions were analysed by ICP-MS and the total amount of each element was obtained by summing the fractions. Small amounts (ng) of Ru (18), Rh (8), Pd (40), Ir (14) and Pt (22) were found, which would only be significant if the method was applied to the determination of very low levels of precious metals. Concentrations of Au (490 ng) were higher, illustrating the need to carefully clean the resin prior to use. This level of Au was not significant for the 500 mg experiments, but the cleaning step was lengthened to include preconditioning of the column with 500 ml of 1 mol l21 HCl for later, low-level work.Determination of the PGE and Au at low levels Relatively high levels of the PGE and Au were generally used during the initial development of the ion-exchange method. This enabled ICP-AES to be used as the analytical finish, which is more precise and less prone to analytical problems caused by high and variable levels of TDS in solutions than ICP-MS.Experiments on solutions containing 1 mg spikes, however, necessitated diluting the thiourea fraction to reduce the level of TDS presented to the instrument. Diluted samples were below the lower limit of quantitation for ICP-AES, and close to those for ICP-MS, so the ion-exchange method developed so far would only be applicable to geological samples containing high ( > 1 mg g21) PGE and Au contents (calculated based on a 1 g sample size). There was a requirement, therefore, to concentrate the precious metals in the eluate (the volume eluted could not be reduced, since it was the minimum necessary to completely elute the PGE and Au), and/or eliminate the need for dilution to reduce the TDS and acid concentrations.The TDS content of the thiourea eluate could be reduced by decomposing the thiourea with nitric acid: H2NCSNH2 + HNO3 » NH3 + COx + SOx (1) where x = 1, 2, 3, or 4, as appropriate. The ammonia, carbon monoxide and carbon dioxide are lost by volatilisation.The main species remaining in solution after decomposition with HNO3 is sulfate (SO4 22). Unfortunately, sulfate solutions are not well suited to analysis by ICP-MS because the Ni sampling cone rapidly degrades, causing signal instability during analysis. Two possibilities were considered for the analysis of these solutions by ICP-MS: (1) use of a Pt-tipped sampling cone, which is not damaged by low levels of SO4 22; (2) removal of the sulfate before analysis.The first option was not feasible because Pt was one of the elements being sought, so precipitation of BaSO4 was investigated as a means of removing excess sulfate from solution. The solubility of BaSO4 is 2.2 mg ml21 in cold water.31 It was calculated that 75 ml of 0.3 mol l21 thiourea would produce 0.225 mol 121 SO4 22, assuming that there is complete conversion of S in thiourea to SO4 22. This estimate is almost certainly too high, since it takes no account of the loss of volatile lower-order sulfur–oxygen species during the decomposition step, so a maximum of 5.5 g of BaCl2·2H2O is needed to precipitate the sulfate.The following procedure was used to decrease the TDS of the thiourea fraction: (1) each 0.3 mol l21 thiourea solution was reduced to Å 10 ml in a 100 ml Pyrex beaker by evaporation at 95 °C on a sand bath, and allowed to cool; (2) fuming AnalaR HNO3 was added dropwise (2–3 ml) until effervescing ceased; (3) the resulting solution was evaporated at 95 °C to < 1 ml, to remove excess HNO3, and then diluted to 5–10 ml with deionised water; (4) AnalaR BaCl2 (5.5 g BaCl2·2H2O dissolved in Å 30 ml of H2O) was added, and the solution stirred to ensure complete precipitation; (5) the BaSO4 precipitate was removed by vacuum filtration through a Whatman (Whatman, Maidstone, Kent, UK) 0.45 mm cellulose nitrate filter membrane, using a large diameter (47 mm) filter funnel; (6) the filtered solution was evaporated on a sandbath at 95 °C to Å 5 ml, transferred into a 10 ml calibrated flask, and made up to volume with 0.5 mol l21 HCl.To evaluate the effectiveness of the HNO3–BaSO4 method, solutions of thiourea were spiked with known amounts of the PGE and Au, and treated as above. ICP-AES analyses of the treated solutions showed high levels of Ba (up to 1000 mg ml21) and S (approximately 400 mg ml21), indicating incomplete conversion of the sulfur in the thiourea to sulfate ions.Although the level of Ba and S was below the 2000 mg ml21 TDS limit imposed for solutions being analysed by ICP-MS, the presence of such high levels of individual elements caused signal suppression, necessitating the need for all thiourea fractions to be analysed by the standard additions method. This enabled more accurate analyses to be obtained, but quadrupled the number of solutions that had to be processed. Results for duplicate 1 mg experiments were in poor agreement (Table 7), making assessment of the results difficult.In general, PGE and Au in the spiked thiourea solutions showed moderate to good (70–100%) recoveries of Rh, Pd, Ir, Pt and Au at the 10 and 1 mg levels, but low values (50–70%) were obtained from the 0.1 mg spike, except for Au which was completely recovered in all three cases. Ruthenium recovery ranged from 70% for 10 mg, to 53% for the 0.1 mg solution. In a second experiment, 10, 1 and 0.1 mg PGE and Au solutions prepared in 1 mol l21 HCl were loaded onto Dowex 1-X8 columns, the initial eluate was collected to determine the breakthrough, and the precious metals were eluted with 75 ml of thiourea solution and 125 ml of 12 mol l21 HCl.The thiourea fraction was treated using the HNO3–BaSO4 method, while the concentrated HCl fraction was evaporated to incipient dryness and made up to 10 ml in 0.5 mol l21 HCl. The initial eluate, treated thiourea and HCl fractions were all analysed separately by ICP-MS.Recoveries were calculated by summing analyses Table 7 Recovery (%) of the PGE and Au from thiourea solutions. ICP-MS determinations after decomposition in HNO3 and removal of sulfate by precipitation of BaSO4. Averages and standard deviations for the 1 mg spike are based on determination of two solutions Mass in spike/mg Element 10 1 0.1 Ru 69 67 ± 7 53 Rh 85 67 ± 12 53 Pd 87 76 ± 22 57 Ir 99 96 ± 8 69 Pt 103 95 ± 13 72 Au 110 110 ± 29 100 24 Analyst, January 1997, Vol. 122of the thiourea and HCl fractions, and expressing results as a percentage of the amount in the original spike. The 1 mg experiment was performed in triplicate to establish the reproducibility of the method. The best results (Table 8) were obtained from the 1 mg spikes, with combined breakthrough and recovery of Ru, Rh, Pd and Pt generally totalling > 90%. However, the standard deviation of the measured recovery was relatively high, around 2–20%. The recovery of Au was 70% (with no breakthrough), while 67% Ir was eluted and 6% lost through breakthrough. The range of results obtained, however, included one run with 90% recovery of all PGE and Au.Lower recoveries were observed for all elements except Ru and Pt for the 10 mg solutions (Table 8). The 0.1 mg experiment yielded recoveries of > 100% for all elements, indicating contamination during handling of these solutions. The variable, and generally low, recovery of the PGE and Au from the thiourea solution following treatment with HNO3 and BaCl2 is attributed to coprecipitation and/or occlusion of the PGE and Au with the BaSO4 precipitate. Contamination was also encountered while determining the level of PGE and Au eluted from the Dowex 1-X8 resin in a blank run.Blank levels of the PGE and Au obtained for two columns are given in Table 9. Method blanks were also obtained by processing a 1 mol l21 HCl solution in an identical method to a geological sample (incorporating a digestion step using the method of Totland et al.,5 and anion-exchange), and analysing the eluent after decomposing the thiourea; again, two sets of results are presented because of the large difference obtained.Runs with high blank levels of the PGE and Au, were generally caused by high concentrations in the thiourea fraction. However, analysis of 75 ml of thiourea solution processed using the HNO3–BaSO4 method (Table 9), indicated low PGE and Au concentrations in the reagents.The highly variable blank levels, therefore, were not caused by contamination or interferences arising from the reagents used, but were probably a result of the extensive handling of solutions required in the procedure. This makes subtraction of a true blank difficult. Geological Reference Materials Although developed using synthetic solutions of the PGE and Au, the low levels of breakthrough and good recovery of several elements, indicated that the method should be applicable to the separation and determination of these elements in geological materials.To assess this, a study was undertaken using geological reference materials. Nickel copper matte PTM-1 (CCRMP, Canadian Certified Reference Materials Project, Energy Mines and Resources, Ottawa, Canada) contains relatively high levels of the PGE and Au, ranging from 0.34 mg g21 Ir to 5.8 mg g21 Pt, so this material was chosen to evaluate the basic anion-exchange procedure described above.In this case, the thiourea and 12 mol l21 HCl fractions could simply be diluted prior to analysis by ICP-MS. Three reference materials were used to evaluate the procedure employing the decomposition of thiourea: CCRMP materials PTM-1 and PTC-1 (sulfide flotation concentrate); Council for Mineral Technology (MINTEK, South African Bureau of Standards, Pretoria, South Africa) platinum ore, SARM7. Samples were prepared using a microwave aciddigestion procedure, described in detail elsewhere.5 Briefly, the method employs 1 g samples and acid digestion with 20 ml of aqua regia and 10 ml of 29 of mol l21 HF in Ultem-jacketed Teflon PFA sealed-vessels, heated at elevated pressure (200 psi; Å 1.4 MPa) in an MDS-2000 microwave oven (CEM Corporation, Matthews, NC, USA).Samples are subsequently evaporated to near-dryness, digested in 1 mol l21 HCl, filtered, and the insoluble residues fused with small amounts of 1 + 1 Na2O2 + Na2CO3 (silicate samples) or Na2O2 (sulfides), before being dissolved in 1 mol l21 HCl.Filtrate and dissolved residue solutions are combined to give 10–20 ml of 1 mol l21 HCl, which is suitable for loading directly onto the anion-exchange column. Data for PTM-1 obtained by direct analysis of the thiourea fraction following anion-exchange separation were (mg g21): Ru 0.3; Rh 1.3; Pd 9.8; Ir 0.3; Pt 6.4; Au 2.6. When compared to results (Table 10) obtained following acid digestion and Table 9 Blank values (ng) obtained from Dowex 1-X8 columns following digestion of thiourea and preconcentration of HCl eluents prior to analysis by ICP-MS.Method blank includes a microwave digestion procedure.5 Values for a decomposed thiourea blank are included for comparison Column blank Method blank Element A B A B Thiourea blank Ru < 2 190 36 70 < 2 Rh < 0.3 150 < 0.3 39 < 0.3 Pd < 2 120 11 42 1.5 Ir 10 180 35 120 < 0.7 Pt 370 180 77 230 1.2 Au < 0.6 450 17 280 0.7 Table 8 Breakthrough and recovery (%) of the PGEs and Au after anionexchange separation.ICP-MS determinations following decomposition of thiourea and preconcentration of the HCl eluents. Averages and standard deviations for the 1 mg spike are based on three replicates Mass in spike/mg 10 1 0.1 Break- Re- Break- Re- Break- Re- Element through covery through covery through covery Ru 4.8 84 9.0 ± 2.3 79 ± 2 9.5 140 Rh 27 41 40 ± 15 49 ± 14 36 120 Pd < 0.1 65 2.3 ± 4.0 87 ± 11 < 0.1 180 Ir 1.8 50 6.0 ± 3.6 67 ± 22 6.0 200 Pt < 0.1 96 0.4 ± 0.2 97 ± 14 < 0.1 230 Au < 0.1 38 < 0.1 70 ± 18 < 0.1 107 Table 10 Results for geological reference materials (mg g21) obtained following acid digestion, alkali fusion and anion-exchange separation with decomposition of thiourea, compared to acid digestion and fusion only,5 and reference values Element Ion exchange Digestion Reference PTC-1— Ru 0.29 ± 0.11 0.50 ± 0.07 0.65 Rh 0.30 ± 0.15 0.480 ± 0.089 0.62 ± 0.70 Pd 2.3 ± 1.2 11.1 ± 1.2 12.7 ± 0.7 Ir 0.21 ± 0.02 0.11 ± 0.01 0.1 Pt 2.40 ± 0.09 1.70 ± 0.14 3.0 ± 0.2 Au 0.52 ± 0.12 0.38 ± 0.21 0.65 ± 0.10 PTM-1— Ru 0.36 ± 0.06 0.670 ± 0.029 0.5 Rh 0.33 ± 0.06 0.940 ± 0.025 0.9 ± 0.2 Pd 6.1 ± 0.6 7.60 ± 0.12 8.1 ± 0.7 Ir 0.38 ± 0.22 0.35 ± 0.04 0.3 Pt 4.5 ± 0.5 4.90 ± 0.08 5.8 ± 0.4 Au 0.90 ± 0.02 1.500 ± 0.045 1.8 ± 0.2 SARM7— Ru 0.19 ± 0.11 0.360 ± 0.027 0.430 ± 0.057 Rh 0.049 ± 0.003 0.230 ± 0.007 0.240 ± 0.013 Pd 1.30 ± 0.14 1.230 ± 0.095 1.530 ± 0.032 Ir 0.11 ± 0.03 0.110 ± 0.016 0.074 ± 0.012 Pt 2.90 ± 0.37 3.40 ± 0.30 3.740 ± 0.045 Au 0.170 ± 0.013 0.290 ± 0.094 0.310 ± 0.015 Analyst, January 1997, Vol. 122 25fusion of the insoluble residue without an ion-exchange step,5 and with reference values, these data demonstrate acceptable, if marginally high, recovery of Rh, Pd, Ir and Pt. The value for Ru is low, but the level of Ru in the thiourea solution was close to the limit of detection for the ICP-MS, making the assessment inconclusive.Gold yielded a high value, suggesting a continuing contamination problem. Results for three preparations of PTM-1, and duplicate preparations of PTC-1 and SARM7 (Table 10), obtained following anion-exchange with decomposition of thiourea show, with a few exceptions, low recoveries of Ru, Rh, Pd, Pt and Au when compared to digestion only and reference data. These elements are eluted in the thiourea fraction, and it is believed that the treatment used to reduce the TDS was the cause of the poor recovery due, at least in part, to occlusion of a portion of the PGE and Au in the BaSO4 precipitate.This conclusion is supported by the complete recovery of most elements in PTM-1 when the thiourea fraction was analysed directly. Furthermore, Ir data (Table 10) are in good agreement with reference values. Iridium is eluted entirely with the 12 mol l21 HCl fraction, producing a simple matrix that poses no analytical difficulties by ICP-MS. Conclusions Our experiments demonstrate that the PGE and Au may be quantitatively adsorbed onto Dowex 1-X8 anion-exchange resin, and eluted using a two-stage procedure: thiourea to elute most Ru, Rh, Pd, Pt, Au; concentrated HCl to complete elution of these elements, and to elute all Ir.Evaluation of the procedure using geological reference materials showed encouraging results. In particular, the method has been successfully applied to the separation and determination of Ir in three rock reference materials by ICP-MS.The application of our method to the entire group of PGE and Au is limited principally by difficulties associated with analysis of the thiourea fraction. The extra dilution required for direct analysis of this eluate by ICP-MS, leads to limits of quantitation in samples26 of around 1 mg g21 for Ru, Rh, Pd, Pt and Au, which are similar to those achievable5 without separation from matrix elements.Reduction of the TDS in the first eluate was undertaken by decomposing thiourea with fuming HNO3, causing the loss by volatilisation of NH3 and CO2. However, high concentrations of sulfate ions remaining in solutions prevented their analysis by ICP-MS, because of the risk of corroding the Ni sampling cone. Removal of sulfate by precipitation with Ba was of limited success, producing erratic and generally low values for elements eluted in this fraction. It is concluded that precipitation is unsuitable for the analytical method, because the potential for coprecipitation and/or occlusion of the PGE and Au is too high and unpredictable. Although the use of isotope dilution methods could be used to compensate for low recoveries of Ru, Pd, Ir and Pt,32 extensive handing required at this stage led to sporadic contamination and difficulties in producing reliable procedural blanks, which is less easily addressed. To apply our method to the separation and determination of low levels of the PGE and Au, an alternative method for analysing the thiourea fraction is required. Potential ways to achieve this include electrothermal vaporisation or flow injection ICP-MS. These techniques may be used to directly analyse solutions with high levels of TDS, but their development is non-trivial and is beyond the scope of this study. Funding by RTZ Mining and Exploration Ltd. and enthusiastic support from Drs. C. Carlon and N. Badham (RTZ) are gratefully acknowledged. 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