Aplicaciones de la Casa de Moneda como recurso turístico

Several methods for the analysis of strontium isotopes exist and different types of mass spectrometry can be utilized for the analysis of strontium isotopes from human and animal remains to address various research questions (Balter et al. 2008; Bentley et al. 2007a; Booden et al. 2008; Nowell and Horstwood 2009; Richards et al. 2008; Simonetti et al. 2008). The most common means of measuring strontium isotope compositions of these material types are ICP-MS and MC-ICP-MS (in solution mode or through the direct ablation of solid-state samples via the coupling of a laser to the mass spectrometer) and TIMS. The use of chemical separation procedures and isotopic analysis via TIMS represents the longest and most widely used method for strontium isotope analysis in

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archaeological contexts (Ezzo et al. 1997; Grupe et al. 1997; Price et al. 1994a; Price et al. 1994b).

This approach was taken within this research project for a variety of reasons. Several decades of analysis via TIMS, has indicated that this robust method routinely produces precise, reliable, and consistent results (Nowell and Horstwood 2009). TIMS is also relatively inexpensive, especially if one prepares, processes, and analyzes their own samples. Furthermore, the purported advantages of LA-ICP-MS, namely minimization of sample destruction and speed of sample processing relative to other factors, particularly data quality, have not been borne out by recent research. More explicit discussions of the pros and cons of various methods of strontium isotope analyses, can be found in (Copeland et al. 2008; Copeland et al. 2010; Nowell and Horstwood 2009; Simonetti et al. 2008).

4.4.1 TIMS - Filaments

Strontium isotopes ratios within samples were measured on a TIMS by using annealed rhenium ribbons mounted onto metal sample loading brackets. Sample strontium is imbedded within a tiny ceramic bead of tantalum salts, which is loaded onto the filament and eventually into the TIMS machine. The ceramic bead thus generated reduces the rate of strontium evaporation, causing the Sr to evaporate at higher temperatures. This higher temperature causes greater thermal ionization and thus provides higher ionization efficiencies.

The first step in this process is to make rhenium filaments onto which the sample can be loaded and then placed into the machine where it can be heated through the application of an electric current (all previously used Re is removed and the filament holder is cleaned with a file or an abrasive drill). This is accomplished by welding a ~1.2 mm strip of rhenium ribbon onto the filament loading bracket using a high-precision welder with a current of 4 amps. ‘Filaments’ in this terminology consist of a small metal loading bracket with a thin rhenium ribbon welded onto it.

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The metal brackets themselves and their associated plates and shields are all reusable but the ribbons themselves are not. The hardware (brackets, plates, and shields) are pre-cleaned with the following protocol: 1) The hardware is washed by scrubbing the surface with a combination of demi-water and powdered aluminum, and then rinsed thoroughly with demi-water; 2) Then the hardware is boiled in demi-water for 25 minutes and then boiled in Milli-Q water for 25 minutes; 3) The hardware is then dried in an oven at 100 o_{C for 1 hour.}

The filaments are then further cleaned by removal of contaminants (from the air and water) by subjecting them to high temperatures at vacuum; this is known as ‘out- gassing’. The out-gassing procedure consists of loading the filaments into a designated chamber within a ThermoScientific Bakeout Device. This machine de-pressurizes the internal chamber containing the filaments to a pressure of < 2 x 10-6 _{bar and applies a} high current to all of the filaments. This increases the surface temperature to 1.5-2.0k o_C and thus evaporates and/or ionizes any surface contaminants on the Re filament. This step is essential because it is necessary to remove (as much as possible) any potential source of strontium other than the sample which is to be analyzed. These steps and others are thus designed to provide confidence that the measurements produced are derived from the sample itself and not from other sources.

4.4.2 Loading Samples

Strontium samples were loaded onto the prepared filaments according to the following procedures. 1) dried samples were dissolved in 2 μl of 3 N HNO3; 2) filaments were attached to a specially designed loading device which allows a current to be passed through the filament; 3) a 1.1 A current is applied to the filament and a small amount of parafilm is melted onto the filament in two places to produce ‘dams’ that prevent the loaded sample from spreading across the entire filament; 4) the current is reduced to 0.9A and 2 μl of TaCl5 and 1 μl of H3PO4 is placed onto the filament; 5) 2 μl of nitrated sample (or standard) are then placed onto the filament and the mixture is allowed to dry down; 6) the current is sequentially increased to 1.2, 1.5, and 2 amps to dry down the sample, burn

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off the parafilm, and evaporate the phosphoric acid, respectively; 7) the sample is heated to a dull red color to fully dry the sample; 8) the loading bracket with samples is then placed onto a loading turret which is subsequently placed into the source chamber of the TIMS and at least one external standard (NBS-987) is analyzed with every 12 samples.

4.4.3 TIMS – Operating Parameters

All samples were analyzed for strontium isotope composition with a thermal ionization mass spectrometer (TIMS, ThermoFinnigan MAT 262 RPQ plus) at the Faculty of Earth and Life Sciences of the VU, Free University Amsterdam, The Netherlands. The source chamber with loaded samples (and standards and blanks) is pumped down to a vacuum of < 8 x 10-8 _{torr and a current is slowly applied to the appropriate filament. Samples are} generally run at a current of 2.5-3.2 amps, producing temperatures of roughly 1400-1600 o_{C. The TaCl5/H3PO4 mixture, within which the sample is loaded, creates a tiny ceramic-} like bead which causes the strontium atoms to ionize at higher temperatures and thus tends to increase the ionization efficiency compared to previous methods that use only H3PO4. The ionization is accomplished thermally, hence the name TIMS, and after a brief interval to permit the burning off of any contaminants (including any residual rubidium), Sr isotope analysis can begin.

The ionization process generates charged particles that can then be manipulated via electromagnetic forces. The ion beam is thus focused and the particles spatially separated by mass such that ions of different masses can be collected and measured via Faraday collectors within the instrument. To avoid biases generated from potential differences between collector cups, for the first year of the project samples were measured with a dynamic triple-jumping method. All measurements were automatically corrected, using an exponential correction factor, to an 86_Sr/88_{Sr value of 0.1194.} Measurements were only accepted when the 85_Rb/86_{Sr ratio was below 0.0002, indicating} minimal presence of mass interference from rubidium. A minimum of 60 ratios per sample were collected, more if necessary, with a goal of obtaining a standard error of 0.000010 (2σ) or less for each sample. A small percentage (<3%) of samples produced

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standard errors (2σ) > than 0.000020 and were re-analyzed, thus samples included herein yielded signatures with a standard error of < 0.00002 (2σ). After the first year, samples were measured in static mode once it became clear that measurement precision of ±0.00002 was insubstantial in terms of the regional Sr isotope variations.

4.4.4 TIMS – Standards

Each set of samples on the TIMS included one or more standards, in addition to about 10- 12 samples and blanks. For external reproducibility and quality control, we used the certified strontium carbonate (SrCO3) reference material (NBS/NIST) SRM-987. Repeat measurements of the international standard over the period of analyses yielded a mean 87_Sr/86_{Sr value of 0.710250 ± 0.00003 (1σ, n=81). Over the course of the project, samples} and standards were analyzed using two different analytical modes. Analyses of the international standard (SRM-987) in dynamic mode produced more consistent results with a mean 87Sr/86Sr value of 0.710231 ± 0.000008 (1σ, n=27) that is within error of the accepted value and a lower standard deviation. Thus all samples analyzed in dynamic mode were not corrected (see Appendix A and B). Over the three year duration of the project, analyses of the international standard (SRM-987) in static mode produced slightly more variable results with a mean 87_Sr/86_{Sr value of 0.710259 ± 0.000033 (1σ,} n=54). This period included changes in collector performance that led to drift in the Faraday cups used for Sr isotope analysis. These changes caused transient variations in the values of the standard increasing the variance in the standard data. Consequently, all samples analyzed in static mode were normalized (in reference to the in-run value of the external standard) to the generally accepted 87_Sr/86_{Sr value for the standard reference} material of 0.710240.

93 4.4.5 TIMS – Blanks

For each series of samples that underwent chemical processing and strontium separation at least one total procedural blank was analyzed to test for the presence of ambient or intrusive strontium which may have potentially contaminated any of the reagents, hardware, or samples. Total procedural blanks were spiked after strontium separation with calibrated 84Sr spike [serial code # 20080117]. Analyses of 84Sr/86Sr ratios were conducted with a minimum of one block (10 single scans) in static mode with the TIMS and the overall strontium concentration was calculated using the principles of isotopic fractionation (Faure and Mensing 2005). Over the course of our analyses, total procedural blanks yielded Sr concentrations typically less than 200 pg. Such low concentrations are considered negligible and insignificant relative to the Sr concentrations of the samples themselves, with loaded samples ranging from roughly 0.2 to 1.0 µg. Thus the typical sample contains at least a thousand times more strontium than the typical blank.