Synthetic standard solutions are not ideal for interlaboratory calibration because of the difficulty of ensuring their integrity over time. The risk always remains that solutions on analysis have not retained their prepared composition because of either evaporation or contamination during transport and storage. Evaporation, which affects element concentrations equally, is only a minor problem for element ratio standards, whereas contamination affects individual elements unequally and has serious consequences for element ratios. Plastics such as polypropylene and high-density polyethylene, manufactured catalytically using a polymerization procedure involving MgCl2 supported Ziegler-Natta catalysts [Masuda et al., 1997], represent an obvious contamination risk for Mg/Ca standard solutions. The potential for contamination is minimized by circulating concentrated standard solutions in acid cleaned plastic bottles, but there will always be uncertainty in the integrity of small volumes of standard solutions over time. An ideal reference material for interlaboratory calibration would be a solid standard with well characterized Mg/Ca (and Sr/Ca) ratios, similar to typical foraminifera calcite. This would circumvent the potential problems inherent in the circulation of liquid standards. A series of three reference standards, covering the range of Mg/Ca ratios in foraminifera, would enable analysts to check the sensitivity and linearity of their techniques across the range.
 Requirements for any solid standard are that (1) it has Mg/Ca (and Sr/Ca) within the range of foraminifera samples; (2) it is homogeneous, both within and between batch samples; (3) it has a matrix of pure calcite with no contribution to Mg/Ca and Sr/Ca ratios from other mineral phases; and (4) it is readily available in a form suitable for use.
 We have investigated commercial reference materials to see whether existing carbonate standards could be used as reference materials for Mg/Ca in foraminiferal calcite. Table 2 lists element concentrations and calculated Mg/Ca and other element ratios for a selection of certified reference materials (CRMs). In general, the Mg concentrations of available calcite and limestone certified reference materials are too high to match foraminiferal calcite, and these materials contain significant Al, Fe, Si and Ti from other mineral phases (Table 2).
 Among the certified reference materials listed in Table 2 one standard closely meeting the requirements for foraminiferal calcite is ECRM 752-1 (alternative name BCS-CRM 393), a limestone CRM issued by the Bureau of Analyzed Samples Ltd. UK. The calculated Mg/Ca ratio of ECRM 752-1 is 3.9 mmol/mol, within the range of typical planktonic foraminifera samples. Calculated Al/Ca, Fe/Ca, Si/Ca and Ti/Ca ratios, indicating the presence of contaminant silicate minerals and associated non-carbonate Mg, are low, although higher than observed in cleaned foraminifera [Barker et al., 2003]. The certified element concentrations of ECRM 752-1 are insufficiently precise to permit direct use as a foraminiferal Mg/Ca CRM; propagation of the concentration errors produces a 6.8% (r.s.d.) error on the calculated Mg/Ca ratio. However, if this material is sufficiently homogeneous it has potential as an Mg/Ca consistency standard for use within and between laboratories.
 ECRM 752-1 was prepared from a Derbyshire, UK, limestone and is supplied as a powder, ground to pass a 75 μm sieve. Enquiries of the manufacturer confirmed that this material was prepared in a single batch, but is packed according to demand with individual bottles labeled with a packing lot number (Bureau of Analyzed Samples, personal communication). Determination of its Mg/Ca homogeneity, both within the calcium carbonate, and the contribution from accessory mineral phases, is necessary in order to assess its suitability as a reference material for foraminiferal Mg/Ca. We performed homogeneity tests on two separate bottles of ECRM 752-1, taken from the same packing lot number, 0973, using sample weights in the range 0.1 mg to 1000 mg.
3.1. Analytical Methods
 Replicate aliquots of 10, 50,100, 250, 500 and 1000 mg were weighed from each of two 100g bottles of ECRM 752-1 into acid cleaned (10% HNO3, overnight) and dried low-density polyethylene (LDPE) bottles. Samples were dissolved in 0.075M HNO3 in line with foraminiferal sample preparation. No sample treatment was employed before dissolution. Dissolution volumes were maintained in proportion to sample weights to give constant [Ca2+] of ∼400 μg/g (i.e., 10 mg in 10 mL, 50 mg in 50 mL, etc). In the case of smaller sample sizes, 1.0 and 0.1 mg, replicate aliquots were weighed into acid cleaned polypropylene microcentrifuge tubes and dissolved in 1 mL or 0.5 mL 0.075M HNO3, respectively. The powder dissolved easily, with no particles visible to the naked eye remaining and solutions were analyzed both with and without centrifugation. 0.5 mL aliquots were centrifuged using an Eppendorf model 5415 C microcentrifuge (10 mins. >= 6000 rpm). Between six and twelve replicate weighings were analyzed after centrifugation at each weight, but not all samples were analyzed without centrifuging. Solutions were diluted to [Ca2+] = 60 μg/g and element ratios determined by ICP-OES using a Varian Vista Axial instrument following the procedure of de Villiers et al. . The analytical and instrumental conditions are summarized in Table 3. Solutions were always analyzed (both with or without centrifugation) on the same day as samples were dissolved, although the entire experiment was performed over a number of days. The instrument was calibrated using the standard solutions described in Section 2. Dilution of samples and standard solutions to constant [Ca] permits instrument calibration using the intensity ratio method, with a Ca concentration of 60 μg/g chosen [de Villiers et al., 2002]. Within run precisions of 0.3% or better were obtained for replicate Mg/Ca determinations of a solution (Q5) containing [Ca2+] = 60 μg/g, Mg/Ca = 5.130 mmol/mol, Sr/Ca = 2.088 mmol/mol and no correction was applied for instrument drift during a run. Daily instrument calibration differences produced mean values for solution Q5 ranging from −0.73% to + 0.30% of its expected Mg/Ca ratio and results were normalized to the Mg/Ca ratio for solution Q5 to account for this variation.
Table 3. Analytical and Instrumental Conditionsa
|Sample Weight, mg||0.075M HNO3, mL||Dissolution Vessel||[Ca], μg/g|
|1000||1000||1000 mL LDPE bottle||400|
|500||500||500 mL LDPE bottle||400|
|250||250||250 mL LDPE bottle||400|
|100||100||125 mL LDPE bottle||400|
|50||50||60 mL LDPE bottle||400|
|10||10||15 mL LDPE bottle||400|
|1||1||1.5 mL PP microcentrifuge tube||400|
|0.1||0.5||0.5 mL PP microcentrifuge tube||80|
|All solutions diluted to constant Ca concentration for analysis|| || ||60|
|Instrument: Varian Vista Simultaneous ICP-OES|
| || ||RF power||1.2 kW|
| || ||Plasma gas flow||15 L/min|
| || ||Auxiliary gas flow||1.5 L/min|
| || ||Nebulizer gas flow||1.0 L/min|
|Nebulizer|| ||Glass expansion, Micromist||0.2 mL/min|
|Spraychamber|| ||Glass expansion, Cinnabar Cyclonic|| |
|Measurement|| ||Integration time||5 s|
| || ||Replicates||6|
 The Mg/Ca and other element ratios measured in material from the two bottles of ECRM 752-1, using sample weights in the range 10–1000 mg, are summarized in Table 4. The mean Mg/Ca ratios and standard deviations obtained for each set of analyses, including those using smaller sample weights down to 0.1 mg, are plotted against sample weight in Figure 1a.
Figure 1. Homogeneity of ECRM 752-1, measured element ratios versus sample weight: (a) Mg/Ca, (b) Fe/Ca, (c) Mn/Ca. Open symbols, not centrifuged; solid symbols, centrifuged after dissolution. Red, first bottle; blue, second bottle.
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Table 4. ECRM 752-1: Average Element Ratios Obtained Using Sample Weights in the Range 10–1000 mga
| r.s.d., %||0.87||1.29||19.5||13.7||4.2||27.8||40.3|
| r.s.d., %||0.35||5.25||22.6||9.7||3.7||20.2||33.9|
| s.d.||0.018||0.004|| ||0.007||0.004|| || |
| r.s.d., %||0.47||1.92|| ||9.21||2.83|| || |
| n||59||59|| ||59||59|| || |
| s.d.||0.012||0.008|| ||0.003||0.004|| || |
| r.s.d., %||0.33||4.47|| ||4.5||3.1|| || |
| n||59||59|| ||59||59|| || |
|Detection limit|| || ||0.13||0.024||0.006||0.049||0.007|
 Results after centrifuging agree well, both within and between the two bottles analyzed, over the range of sample weights from 10 mg to 1000 mg and confirm the homogeneity of Mg/Ca within the readily soluble carbonate material. Inhomogeneity would be indicated by increasing scatter in the data with decreasing sample size but this was not observed, the standard deviations for the sets of analyses remaining similar over the five orders of magnitude weight range (Figure 1a). However, higher Mg/Ca ratios were consistently measured for the smallest sample weights. Small samples, 1.0 and 0.1 mg, were dissolved in polypropylene microcentrifuge tubes and it is likely that sufficient Mg was extracted from the polypropylene to produce these higher Mg/Ca ratios. An addition of <0.5 ng Mg to 0.1 mg sample would be required to increase its Mg/Ca ratio from 3.75 mmol/mol, the average for all samples weighing from 10 to 1000 mg, to 3.77 mmol/mol, the average for 0.1 mg. This contribution is within the range of Mg blank values measured in this laboratory for polypropylene microcentrifuge tubes (Eppendorf SafeLok) after acid cleaning.
 The differences between results for centrifuged and non-centrifuged samples clearly demonstrate the effect of suspended insoluble material, carried through the nebulizer into the plasma torch, on Mg/Ca ratios. Non-centrifuged samples contain higher and more variable Mg/Ca (Table 4, Figure 1a) consistent with higher Fe/Ca (Figure 1b), demonstrating the presence of other mineral phases in this material. Fe/Ca falls to consistent but non-zero values on centrifugation (Figure 2a), suggesting that Fe is associated both with the readily soluble carbonate and with the insoluble suspended material. In contrast, Mn/Ca ratios (Figure 1c) show very little difference between centrifuged and non-centrifuged determinations. Results for Al/Ca, Si/Ca and Ti/Ca (Table 4) show the presence of these elements in the suspended material. When analyzed after centrifuging, concentrations of these elements fall close to detection limits by ICP-OES. Detection limits, estimated as three times the standard deviation of the blank were, Al, 0.005 μg/g, Fe, 0.002 μg/g, Mn, 0.001 μg/g, Si, 0.002 μg/g, Ti, 0.001 μg/g, giving element ratio detection limits at [Ca2+] = 60 μg/g of Al/Ca, 0.126 mmol/mol, Fe/Ca, 0.024 mmol/mol, Mn/Ca, 0.006 mmol/mol, Si/Ca, 0.049 mmol/mol, Ti/Ca, 0.007 mmol/mol. As element concentrations approach the detection limits, calculated element ratios become unreliable because of the domination of the intensity signal by baseline noise and any interferences on the measured wavelengths. We therefore used intensity data for these elements to demonstrate the effect on Mg/Ca from non-carbonate phases. Figure 2b shows Mg/Ca plotted against Al intensity, after correction of the Al intensity signal at 396.15 nm for the contribution from 60 ppm Ca in solution. The correlation between Mg/Ca and Al indicates aluminosilicate minerals and also shows differences between the two bottles. Similarly, a plot of Mg/Ca against Si (Figure 2c), after subtraction of the Si intensity of the acid blank, clearly demonstrates the effect of un-dissolved silicate phases on Mg/Ca determinations. The close correlation of Al with Si (Figure 2d) reinforces this conclusion. The contaminant silicate material must have remained mainly in suspended form to permit centrifugation to lower the Mg/Ca and minor element ratios, but its contribution to the dissolved phase depends on its mineralogy, the acid used and the time interval between the addition of acid and removal of the dissolved phase after centrifuging. The reproducibility of Mg/Ca and Fe/Ca after centifuging (Figures 1a and 1b) implies that any dissolution of contaminant silicate minerals was also reproducible under the conditions used. More detailed experiments would be necessary to quantify the contribution from suspended silicate minerals to the dissolved phase under different conditions.
Figure 2. The effect on Mg/Ca of the contribution from insoluble aluminosilicate minerals: (a) Mg/Ca versus Fe/Ca, (b) Mg/Ca versus Al, (c) Mg/Ca versus Si, (d) Al versus Si. Open symbols, not centrifuged; partially filled symbols, centrifuged after dissolution. Red, first bottle; blue second bottle.
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 Measurements of the Sr/Ca ratio (Table 4) of this material confirmed the calculated ratio of 0.19 mmol/mol (Table 2), much lower than Sr/Ca of 1.0–1.5 mmol/mol typically found in foraminiferal calcite.
 Mean measured Mg/Ca ratios of 3.849 mmol/mol (0.034 s.d., 0.87% r.s.d.) and 3.810 mmol/mol (0.013 s.d., 0.35% r.s.d.,) were obtained for 34 determinations from each of the two bottles of ECRM 752-1 when analyzed without centrifugation. The differences in Mg/Ca within and between the two bottles being a result of the contribution of Mg from undissolved aluminosilicate minerals. In comparison, where samples were centrifuged after dissolution, average values of Mg/Ca = 3.749 mmol/mol (0.018 s.d., 0.48% r.s.d.) and 3.750 mmol/mol (0.012 s.d., 0.32% r.s.d.) were obtained on 59 determinations from each of the two bottles of ECRM 752-1 tested.
 Measurement precisions more than an order of magnitude better than those calculated from the reference analysis certificate reflect partly the improved precision of modern methods, but also the use in this study of a single technique applied to two bottles of material taken from the same packing lot. Investigation of many bottles from more than one packing lot would be expected to give worse reproducibility. However, it is clear from this study (see Figure 2) that the major contribution to inhomogeneity within this material is from contaminant silicate minerals; not visible to the naked eye but visible microscopically as small particles within the powder. Homogeneity of the readily soluble calcite material across many bottles may therefore not be significantly different from that established using the two bottles tested in this study. The dissolution protocol employed is important, the primary objective being to check instrumental calibrations. It is obviously necessary for laboratories to record a batch number and, if discrepancies are found, it would be a simple exercise for collaborating laboratories to exchange material. It is preferable to dissolve a sample weight in the range 10–100 mg to give a high Ca concentration and analyze an aliquot, rather than dissolving a sample weight more typical of foraminifera analyses, in order to minimize the effect of the Mg blank from small dissolution vials (Figure 1a).