A new method for high-precision palladium isotope analyses of iron meteorites and other metal samples

This paper presents a new method for high precision Pd isotope analyses in iron meteorites. First, Pd is separated from the sample matrix by a novel two-stage anion exchange procedure after which isotopic measurements are carried out using MC-ICP-MS. Analyses of doped standard solutions show that isobaric interference from Ru and Cd can be adequately corrected for Ru/Pd < 0.0005 and Cd/Pd < 0.025. This is frequently achieved using the presented separation method. The puri ﬁ ed Pd fraction after ion exchange chromatography is also su ﬃ ciently devoid of Ni (Ni/Pd < 0.04), Zr (Zr/Pd < 0.0002) and Zn (Zn/Pd < 0.06) for precise and accurate measurements because these elements produce molecular interference on the masses of the Pd isotopes. An external reproducibility of 1.29 for 3 102 Pd, 0.22 for 3 104 Pd, 0.11 for 3 106 Pd, and 0.27 for 3 110 Pd is calculated based on the repeated analyses of ﬁ ve independently processed aliquots of the IAB iron meteorite Toluca. The method was veri ﬁ ed by the analysis of three metals from IVB iron meteorites and the results show excellent agreement with previous data. The new method enables accurate analysis of all Pd isotopes, and in particular 102 Pd, which is of major interest for cosmochemical applications.


Introduction
Palladium belongs to the main component elements that are predicted to condense from a gas of solar composition, together with elements such as Fe and Ni. It is therefore considered moderately refractory. 1,2 Palladium is also a highly siderophile element (HSE) and, together with other HSEs, strongly partitions into metal fractions during metal-silicate differentiation, which leads to enrichments in iron meteorites and severe depletion in silicates and the Earth's mantle (e.g., ref. [2][3][4]. Hence, Pd isotope variations could prove a useful and powerful tool for addressing a number of key questions outlined below. First, in recent years it has become obvious that many elements display small, but well resolvable, nucleosynthetic variations (0.1 per mil range) in meteorites compared to terrestrial samples. Meteorite parent bodies, Mars, and the Earth display unique isotope compositions for a range of elements (e.g., Cr, Ti, Ni, Zr and Mo; e.g., ref. [5][6][7][8]. These variations stem from the heterogeneous distribution of presolar dust in the solar system carrying highly anomalous isotopic compositions that were synthesised in various stellar environments. They are extremely useful for meteorite provenance studies and provide important constraints on mixing processes in the early solar system. Iron meteorites are thought to represent the core of asteroids that formed, and were subsequently destroyed, throughout the solar system during the early stages of planet formation. 9 Nucleosynthetic isotope variations in iron meteorites are reported for Ni, Ru, Mo, W and Pd (e.g., ref. [10][11][12][13][14][15][16][17]. Palladium is an ideal element to further investigate these variations because it features six stable isotopes that are produced in different stellar environments: one p-process isotope ( 102 Pd), one s-process isotope ( 104 Pd), one predominately r-process isotope ( 110 Pd) and three isotopes ( 105 Pd, 106 Pd, and 108 Pd) that contain a mixture of s-and r-process components. 18 Accurate measurements of the p-process isotope 102 Pd are necessary to differentiate between the s-process and rprocess variations that would otherwise be indistinguishable in meteorites. A recent Pd isotope study investigating IVB iron meteorites 16 has reported that Pd nucleosynthetic isotope variations are smaller than those of Ru and Mo, and suggested that this reects selective destruction of their carrier phases in the solar nebula. However, further high precision Pd isotope analyses of other iron meteorite groups are needed to better constrain this observation.
Moreover, meteorites are exposed to galactic cosmic rays (GCR) during their travel in space. Modelling of GCR exposure for iron meteorites shows that signicant isotopic shis can be induced in many elements, including Pd. 19 Thus, it is important to quantify the GCR effects on Pd isotopes, because Pd isotope variations can be the result of both nucleosynthetic processes and exposure to GCR in space. Isotopic shis are caused by the capture of secondary neutrons produced by nuclear reactions in meteorites due to irradiation by GCR ðe:g:; 104 46 Pd þ 1 0 n/ 105 46 Pd þ gÞ. The magnitude of these reactions depends on several factors: (i) isotope-specic properties such as the neutron capture cross section; (ii) meteorite properties such as the matrix composition, original depth of the sample within the meteoroid and time of exposure to GCR; and (iii) epithermal burnout of other elements resulting in the production of unstable isotopes that decay to the isotopes of interest ðe:g:; 103 45 Rh þ 1 0 n/ 104 45 Rh/ 104 46 Pd þ 0 À1 bÞ. All Pd isotopes possess relatively small neutron capture cross sections resulting in small ($<0.23) isotopic shis, even for samples with an optimal sample depth and large exposure times. Nevertheless, epithermal burnout of 103 Rh can lead to larger (>13) isotopic shis in 104 Pd, particularly due to the similar abundance of Rh and Pd in iron meteorites. Epithermal burnout of 107,109 Ag also occurs but due to the low Ag/Pd ($<1 Â 10 À4 ) ratio in iron meteorites this reaction is negligible with current precision.
Another motivation to understand and quantify GCR effects in Pd isotopes is the Pd-Ag dating system (e.g. ref. 20 and 21). This chronometer is based on the short-lived isotope 107 Pd, which decays to 107 Ag with a half-life of 6.5 Ma. Cosmogenic production of the short-lived isotope 107 Pd could affect the accuracy of the Pd-Ag dating system. 19,22 High precision Pd isotope analyses, together with the well-established Pt GCR neutron dosimeter, should enable a thorough evaluation of the GCR effects on Pd-Ag chronometry.
Finally, Pd isotopes are a potentially powerful tool to determine the origin of Pd and other HSEs in the Earth. There are two end member scenarios to explain the origin of the HSEs in the Earth's mantle. One proposes that the concentration of HSEs in the mantle reects the addition of a so-called 'late veneer' to the Earth, post core formation (e.g., ref. 23 and 24). The second scenario states that the Pd (and other HSEs) concentration of the mantle may be achieved during metal-silicate fractionation and core formation in a deep magma ocean, negating the need for a late veneer. 3,25 The terrestrial nucleosynthetic signature relative to meteorites may provide constraints on the source of Pd in the Earth, particularly when correlated with other elements (e.g., Mo 10 and Ru 5,12 ).
In order to achieve accurate high precision Pd isotope measurements, it is important that Pd is thoroughly separated from matrix elements, which can cause interference and matrix effects during analysis. For example, Pd isotopes suffer from isobaric interference from Ru and Cd isotopes (Table 1). While Cd abundances are low in iron meteorites, Ru can occur in concentrations similar to Pd (e.g., ref. 26). In particular, high precision measurements of the low abundance 102 Pd (1.02%) are hampered due to the high abundance of 102 Ru (30%), if Ru is not sufficiently removed prior to mass spectrometry analysis. Molecular interference from matrix elements (i.e., Ni, Zn, and Zr) can also affect the precision and accuracy of the isotopic analyses. Existing procedures for Pd separation from geological matrices are either aimed at abundance determination and do not sufficiently remove matrix elements to enable accurate high precision isotope analyses, 27,28 or fail to efficiently separate Pd from Ru. 16 Here we present a new method for high precision Pd isotope measurements involving a two-step ion exchange procedure to separate Pd from an iron meteorite matrix. Initial separation of Pd is achieved using a modied version of the ion exchange procedure of Rehkämper and Halliday, 28 described in further detail by Hunt et al. 29 This method allows the collection of both Pd and Pt from the same sample aliquot, enabling direct comparison with the well-established Pt neutron dosimeter. The Pd elution from the rst ion exchange column requires further purication before isotopic measurements and a novel procedure to achieve this goal is presented here. First, Ru is removed from the Pd fraction by utilising its volatile nature. Then Pd is separated from the remaining matrix elements using an anion exchange column. This yields a nal Pd fraction that is sufficiently devoid of matrix elements, including Ru, to allow for high precision isotopic measurements of all isotopes via multi-collector ICP-MS (MC-ICP-MS). The accuracy of our method was veried by processing one IAB, three IVB iron meteorites and terrestrial standard solutions. Our new analytical method achieves the necessary precision and accuracy to enable the thorough evaluation of GCR effects and nucleosynthetic isotope variations in iron meteorites.

Reagents & materials
Hydrochloric (HCl) and nitric (HNO 3 ) acids used in this study were twice distilled, while hydrouoric (HF) acid used was once distilled in dedicated Savillex Teon stills. The nal concentration of these acids aer distillation is $9.6, $13.9 and $30 M for HCl, HNO 3 and HF respectively. Merck Suprapur® perchloric (HClO 4 ) acid (70%) and Merck Millipore Bromine Suprapur® (99.9999%) were used without further purication. Reagents were mixed using 18.4 U cm À1 water supplied by a Millipore™ (Milli-Q©) system. All acids and acid mixtures were titrated before use to ensure accurate molarities. BioRad AG1-X8 resin (200-400 mesh, chloride form) was utilised and batch-cleaned before use. 29

Sample digestion
The sample digestion procedure is described by Hunt et al. 29 and a short description of the procedure is given here. Prior to digestion and where necessary, the meteorite samples were sawn using a boron carbide blade operated with ethanol as a cooling uid. Weathering and fusion crusts were removed using silicon carbide paper. The sample dissolution and ion exchange procedures were performed in laminar ow hoods in a clean laboratory environment. Prior to dissolution, the samples were submerged in ethanol in an ultrasonic bath, followed by leaching in cold 2 M HCl for 5 minutes. The samples were dissolved in a 2 : 1 mixture of concentrated HNO 3 and HCl at 100 C for 48 hours. They were then dried and re-dissolved in concentrated HCl at 100 C for 48 hours, which yielded clear solutions of brown colour, indicating that they were fully dissolved.

Ion-exchange procedure
Palladium was puried in a two-stage ion exchange procedure. The rst stage is adapted from Rehkämper and Halliday 28 and described in detail by Hunt et al. 29 Aer acid digestion, up to 0.3 g of each sample was reuxed at 110 C in a 2 : 1 mixture of concentrated HCl and HNO 3 (aqua regia) for $48 hours before being evaporated to dryness. Next the samples were reuxed overnight ($18 hours) in 0.5 M HCl + 10% Br 2 -water at 110 C before being cooled to room temperature. Each aliquot was loaded onto a glass column with 1.25 ml of pre-cleaned BioRad AG1-X8 resin, which was preconditioned with 0.5 M HCl + 10% Br 2 -water ( Table 2). The loading was followed by the addition of 1 M HCl + 10% Br 2 -water, 0.8 M HNO 3 + 10% Br 2 -water, and concentrated HCl to elute matrix elements including Fe, Ni and Ru ( Table 2). Palladium was eluted from the column in 10 ml of hot (90 C) 8 M HNO 3 and nally Pt was eluted in 14 ml of 13.5 M HNO 3 . The Pd fraction was taken to dryness in preparation for the Ru evaporation stage. The Pt fraction was further processed following the procedure described by Hunt et al. 29 Ruthenium was removed from the Pd fraction via volatilisation. The Pd fraction from the rst ion exchange column was reuxed in 2 ml aqua regia for 48 hours at 110 C, aer which the solution was allowed to cool before 0.3 ml HClO 4 was added. The solution was then dried at 210 C. This step was repeated twice to ensure maximum Ru loss before the second ion exchange column. Tests revealed that evaporation of Ru aer the second ion exchange column was much less efficient.
The second ion exchange column was designed to remove matrix elements (notably Fe, Mo, Ru, Ni, and Zr) that remained in the Pd fraction aer the rst column. In preparation for the second column the Pd fractions were reuxed overnight ($18 hours) in 1 ml 4 M HF before being taken to dryness and again reuxed overnight in 1 ml of 4 M HF. Teon columns (with an internal diameter of 5 mm) were loaded with 0.5 ml BioRad AG8-X1 resin and rinsed with 10 ml 0.8 M HNO 3 , 10 ml concentrated HCl, 10 ml HNO 3 , and nally 20 ml 6 M HCl, before being preconditioned with 8 ml 4 M HF ( Table 2). The sample was loaded onto the column, and rinsed with 1 ml 4 M HF. Most non-transition metals have very low absorption in HF onto the anion resin and eluted directly. 30,31 This was followed by 5 ml of 6 M HNO 3 to elute remaining Ru and Mo. Subsequently, 4 ml concentrated HCl was added to reduce tailing of Ru into the Pd fraction. Palladium was then eluted in 4 ml concentrated HCl followed by 6 ml 13.5 M HNO 3 . The nal Pd fraction was generally devoid of elements that potentially form molecular interference (see Section 4.1-2) to enable accurate isotopic determination. This sequence of HCl and HNO 3 minimised the tailing of Pd that occurred otherwise. The Pd cuts were taken to dryness, and then reuxed in 1 ml 5 M HNO 3 at 110 C overnight before evaporation and preparation for isotopic analyses.

Yields and procedural blanks
The yield from the rst column was typically 70-80%, based on Pd concentrations of iron meteorites from the literature compared to the recovered Pd aer the separation procedure. Yields for the second column were oen >70%, although yields as low as 50% were also observed. The total yield from both columns was generally >50%. In most cases the nal Ru/Pd ratio of samples processed with the method outlined here was <0.0005; however, on rare occasions samples with higher Ru/Pd ratios were observed. These samples were passed through the second column again to reduce their Ru/Pd ratio. Up to 20% of the Ru loaded onto the second ion exchange column can be eluted in the Pd fraction. This highlights the importance of the Ru volatilisation step, where 80-100% of the Ru remaining aer the rst ion exchange column is lost. Cadmium concentrations are low in iron meteorites (<20 ppb (ref. 32)) and negligible amounts of Cd (Cd/Pd ratios < 0.00001) were determined in the nal Pd fraction. Procedural blanks for the ion exchange procedures were routinely collected and were always <1.2 ng g À1 sample. No blank correction was therefore necessary since typically >300 ng of Pd was collected for each sample.

Instrumentation and data collection protocols
All measurements were carried out at ETH Zürich using a Thermo Fisher Scientic Neptune Plus MC-ICP-MS operated in low resolution mode. Standard H-cones were used and samples were introduced into the plasma using a Cetac Aridus II desolvating system and a nebuliser with an uptake rate of 100 ml min À1 . All Pd isotopes were measured simultaneously with 10 11 U resistors, while also monitoring 101 Ru and 111 Cd using 10 12 U resistors (Table 1). Before each sample/standard measurement, an on-peak baseline (OPB) was measured using a solution containing an acid matrix identical to the subsequent sample/standard. Each OPB and standard/sample measurement consisted of a 30 s electronic baseline measurement followed by collection of 60 integrations (4.7 s each). A peak centre routine was performed immediately prior to each sample/ standard analysis. Between measurements, the sample introduction system was washed with 0.5 M HNO 3 for $20 min to reduce the background signal to below 1/50 th of the original standard/sample signal. The samples were bracketed by measurements of the NIST SRM 3138 Pd standard solution at concentrations that were generally within 15% of the sample. All analyses were carried out in a 0.5 M HNO 3 acid matrix, and solutions were diluted to achieve a signal of 5 to 7 V on 105 Pd, typically 100 ng ml À1 Pd, when possible. A single measurement typically consumed 1 ml ($100 ng of Pd) of solution and took 10 minutes to complete. The sensitivity of the instrument was between 240 and 340 V ppm À1 Pd. The concentrations of matrix elements in the sample solution were checked during each analytical session prior to sample analysis.

Interference correction and data reduction
Prior to the interference correction, all analyses were background-corrected using the OPB collected before each sample/standard analysis. The analyses were corrected for instrumental mass fractionation (b) using the exponential law, 33 and were internally normalised to 108 Pd/ 105 Pd ¼ 1.18899. 34 Isobaric interference from Ru ( 102 Pd and 104 Pd) and Cd ( 106 Pd, 108 Pd, 110 Pd) was corrected using the following procedure. First, the b value for 108 Pd/ 105 Pd ¼ 1.18899 was calculated using the measured intensities on mass 105 ( 105 Pd) and mass 108 ( 108 Pd + 108 Cd). The contribution to mass 108 from 108 Cd was calculated using this b value and the measured intensity on mass 111 ( 111 Cd) together with isotopic abundances of Cd from the study of Rosman et al. 35 The calculated 108 Cd signal was then subtracted from the measured signal on mass 108 and a new b for 108 Pd/ 105 Pd was calculated. This correction procedure was repeated until b converged. The nal b value was then used to correct for 106 Cd and 110 Cd using 111 Cd and for interference from 102 Ru and 104 Ru using 101 Ru as an interference monitor together with isotopic abundances from the study of Huang and Masuda. 36 The results are reported relative to 105 Pd in epsilon notation (3), i.e., the deviation of the sample from the average of two bracketing NIST SRM 3138 Pd standards, given in parts per 10 000.

Isobaric interference
The isobaric interference on Pd isotopes is caused by Ru and Cd isotopes ( Table 1). The analysis of NIST SRM 3138 standard solutions doped with Ru and Cd (Table 4 and Fig. 1) demonstrates that Ru/Pd and Cd/Pd ratios of up to 0.001 and 0.025 (respectively) can be accurately corrected within our external reproducibility (discussed in Section 4.3). Above these thresholds the correction breaks down and Ru is under-corrected, yielding positive values for 3 102 Pd and 3 104 Pd (Fig. 1A), while Cd is over-corrected, generating negative values, most noticeably on 3 110 Pd (Fig. 1B).

Molecular interference
The molecular interference from hydrides (Rh and Ag), argides (Ni and Zn) and oxides (Zr and Mo) can cause spectral interference on several Pd isotopes ( Table 1). The doped Pd standard solutions were analysed in order to constrain the level of these elements that can be tolerated in the nal Pd fraction without jeopardizing the accuracy of the data. Interference from Ni (on 101 Ru, 102 Pd and 104 Pd) and Zn (on 104 Pd and 106 Pd) argides produces isotopic shis outside of our external reproducibility at Ni/Pd > 0.04 and Zn/Pd > 0.06 (Table 3 and Fig. 2). The samples processed through our ion exchange procedure rarely yielded Ni/Pd ratios above 0.005 and no sample was above the threshold ratio of 0.04. For Zn/Pd, ratios below 0.06 were consistently achieved aer the ion exchange chemistry (Table 3 and Fig. 2B). Our analyses showed that the main source of Zn in our solutions was contamination during the nal stages of sample preparation and it is therefore important to monitor the Zn/Pd ratio of samples before every analysis. Our doping tests revealed that production of x Zr 16 O depends on the instrumental settings and that ZrO/Zr ratios up to 0.2 can be produced. It is therefore vital to determine ZrO/Zr prior to each analytical session. Typically, the instrument was calibrated to achieve ZrO/Zr ratios < 0.02. Zirconium/Pd ratios below 0.0002 do not induce isotopic shis outside of our external reproducibility, when ZrO/Zr ¼ 0.02 (Table 3 and Fig. 2C). Generally, most samples yielded Zr/Pd ratios below 0.00015 aer ion exchange chromatography, which is below the threshold if the production of ZrO is minimised. No isotopic shis were observed for Mo/Pd ratios below 0.34 (Table 3), while Mo/Pd in sample solutions aer ion exchange chemistry is typically below 0.005. Additionally, standards with Pt/Pd ratios of up to 0.22 did not introduce resolvable isotopic shis. This ratio is higher than the typical values obtained for iron meteorites aer our ion exchange procedure. Rhenium and Ag can cause both isobaric interference in the form of hydrides and tailing effects on adjacent isotopes (Table 1). However, doping tests at up to Rh/Pd ¼ 0.5 and Ag/Pd ¼ 0.28 show no resolvable isotopic shis. These levels are well above what is observed in samples (Rh/Pd < 0.01, Ag/Pd < 0.0001) aer ion exchange chemistry. The samples with elemental ratios exceeding the stated thresholds (Table 3) were re-processed through the second ion exchange procedure to further purify the samples and achieve accurate results.   set yielded no resolvable differences from the unprocessed bracketing standard (Table 4). This veries the accuracy of our data and demonstrates that (i) isotope fractionation did not occur during the ion exchange procedure beyond that corrected for using the internal normalisation procedure, and (ii) that all isobaric and molecular interference was either accurately corrected for or absent in the analysed fractions. IAB iron meteorites. Repeated measurements of ve independently processed sample aliquots of the IAB iron meteorite Toluca (ETH meteorite collection) were performed to assess the sample reproducibility of our method for a sample with a natural matrix. The means of all aliquots overlap within uncertainty for all isotopes (Table 5 and Fig. 4), which indicates that there is no isotopic difference between the aliquots. The reproducibility calculated based on our entire Toluca data set (n ¼ 20; 2 sd) is 1.29 for 3 102 Pd, 0.22 for 3 104 Pd, 0.11 for 3 106 Pd, and 0.27 for 3 110 Pd (Fig. 4). These uncertainties are similar or slightly higher compared to those calculated from the doped synthetic solutions (Table 4), which we attribute to the more complex matrix of the natural samples. Since they were  obtained on natural samples, we consider their uncertainty as a better approximation of the overall sample reproducibility and applied it for the inference tests described in Section 4.1 and 4.2.

Reproducibility and accuracy
IVB iron meteorites. Three IVB iron meteorites (Hoba, BM 19030, 976, loaned by the Natural History Museum London; Santa Clara and Tawallah Valley, ETH collection), which were analysed in a previous Pd study, 16 were also processed to test the reproducibility and accuracy of our method. The 3 106 Pd values of these samples are all identical to the terrestrial value, within uncertainty ( Fig. 5C and Table 5). All IVB meteorites show a resolved 3 104 Pd decit relative to the Earth (Fig. 5B) and wellresolved excesses of a similar magnitude in 3 110 Pd ($+0.6; Fig. 5D). Santa Clara is within uncertainty of the terrestrial value for 3 102 Pd while Hoba shows a very small negative offset compared to the terrestrial value (Fig. 5A). Tawallah Valley shows a resolvable positive offset for 3 102 Pd that is not within uncertainty of the other two samples; however, this sample has a high Ni/Pd ratio ($0.03) that could affect accuracy at the given precision. The 3 104 Pd, 3 106 Pd and 3 110 Pd values are in excellent agreement with those reported by Mayer et al. 16 (Fig. 5) for the same meteorites, verifying the accuracy of our method. Mayer et al. 16 observe large isotopic offsets for 3 102 Pd and state that they reect inaccurate correction of the isobaric 102 Ru interference due to high Ru/Pd ratios in their sample solutions aer the ion exchange procedure. Our data conrm their conclusion, because, in contrast to the previous work, our new analytical procedure yields low Ru/Pd ratios that are below the threshold at which the interference correction breaks down (Tables 3-5). The IVB iron meteorites contain relatively high amounts of Ru (Ru/Pd > 2 (ref. 26)) and this also veries the ability of our method to separate and/or correct for Ru interference, even for samples with high Ru/Pd ratios.

Cosmochemical implication for IAB and IVB iron meteorites
The well-resolved decits in 3 104 Pd ($À0.2) and excess in 3 110 Pd (+0.6) for the three IVB meteorites most likely reect the presence of a nucleosynthetic s-process decit/r-process excess ( 104 Pd is an s-process isotope and 110 Pd is an r-process isotope) coupled with the effects of GCR. Both GCR and an s-process decit/r-process excess will increase the 3 110 Pd ratio of a sample. The three IVB meteorite samples share a very similar GCR exposure history, 16 which resulted in this relatively constant positive offset. Similarly, the negative offsets in 3 104 Pd (Fig. 5) are likely generated by a combination of GCR and nucleosynthetic effects, as discussed by Mayer et al. 16 Studies of p-process isotopes of other elements such as Mo 10 and Ru 12 allow an s-process decit to be resolved from an r-process excess; however, the precision of our 3 102 Pd data is not sufficient to distinguish between an s-process decit and an rprocess excess without further investigation. Thus a more thorough evaluation of the GCR effects in our samples is needed to elucidate the details. Combining Pd with Pt isotope analyses obtained on the same sample aliquots will act as a neutron dosimeter, and allow for a decisive statement about the respective magnitude of the nucleosynthetic and GCR effects.
In contrast, the Pd isotope analyses of the IAB meteorite Toluca yield identical isotope compositions to the terrestrial standard. This indicates that our Toluca sample was not strongly exposed to GCR. This is in agreement with a study of the GCR exposure of the Toluca 1 aliquot by Hunt et al. 39 using Pt isotopes. Moreover, the data also testify to the absence of nucleosynthetic variations in IAB meteorites, which conrms results from other elements, such as Mo 10 and Ru. 12 Number aer the sample name denotes sample aliquot and the letter indicates duplicate analyses of the sample aliquot. Each aliquot was processed separately through the entire separation procedure. b Uncertainties in the individual analyses are reported as the 2s standard errors (2 se) on the mean of the individual ratios obtained in a single analysis. The 2 se uncertainty in the mean of an aliquot is calculated as the 2 sd of the aliquot or the 2 sd of the Toluca mean (whichever is larger) divided by the square root of n. For means calculated based on analyses from multiple aliquots the uncertainty is reported as the 2 sd. c Mean calculated based on the entire data set of that sample. d Tawallah Valley has a high Ni/Pd ratio ($0.03) which may affect the accuracy of the 3 102 Pd data.

Conclusions
We present a new analytical method for separating Pd from iron meteorites for high precision isotope analysis. Our separation procedure yields sample solutions with minimal amounts of Ru and other elements (e.g., Zn and Zr) that produce interference during mass spectrometry. Following initial Pd separation from the iron meteorite matrix using an anion exchange resin, Ru is volatilised and removed from the sample. A second anion exchange column is employed to remove remaining matrix elements including Ru. The new analytical procedure yields an external reproducibility (2 sd) of 1.29 for 3 102 Pd, 0.22 for 3 104 Pd, 0.11 for 3 106 Pd, and 0.27 for 3 110 Pd based on repeated analyses of ve independently processed aliquots of the IAB iron meteorite Toluca. The method was also successfully applied to three IVB iron meteorites (Tawallah Valley, Santa Clara, and Hoba) and our Pd isotope data are in excellent agreement with those reported by Mayer et al. 16 for the same meteorites. Our new procedure achieves separation of Ru from Pd to a high degree, which allows for the accurate determination of 3 102 Pd. The Pd isotope compositions of the three IVB iron meteorites are consistent with the presence of a nucleosynthetic s-process decit/r-process excess, while the data for Toluca indicate that these effects are absent in IAB meteorites. Further work, however, is required for a thorough evaluation of GCR effects in IAB and IVB iron meteorites.