Inductively coupled plasma mass spectrometry is widely used to solve various problems of elemental analysis, including in archeology, due to its high sensitivity, wide dynamic range, and the possibility of simultaneous determination of almost all elements of the Periodic Table. In 1987, just 4 years after the appearance of the first mass spectrometers, the first work was issued on the ICP–MS determination of 12 elements (Co, Ni, Cu, Zn, As, Sn, Sb, Te, W, Pb, Bi, and Hg) in archaeological silver specimens weighing from 3.5 to 15.6 mg [1]. Over the next 30 years, several dozens of papers have appeared, in which ICP–MS analysis of various historical artifacts made of silver and gold are discussed.

In analyzing archaeological samples, a combination of ICP–MS with laser ablation (LA–ICP–MS) has become widespread, because it can minimize damage to unique samples of both silver [2–6] and gold [2, 5, 7–11] during the analysis. However, LA–ICP–MS, along with undoubted advantages, has several serious disadvantages, for example, the effects of the fractionation of elements, matrix effects associated with different degrees of absorption of laser radiation by different materials, and the lack of adequate standard samples. For these reasons, the number of elements to be determined in LA–ICP–MS is often smaller compared to the conventional ICP–MS method involving the preliminary dissolution of the test samples, and the error in determining impurity elements is usually higher. In the case of sample dissolution, the problem associated with the inhomogeneity of the sample is solved, and the high precision of the results of measurements is ensured. The ease of preparation of calibration samples and rapid measurements are also significant advantages of the conventional ICP–MS method, which involves the analysis of solutions.

Since silver and gold are widely used in the present-day industry, methods for their analysis are well developed, including ICP–AES [12–15]. The conditions for transferring samples to a solution in ICP–AES could also be used for the ICP–MS analysis of archaeological silver and gold samples, but there are two severe limitations. First, the sample weight in these methods, as a rule, ranges from 200 mg for silver [15] and 500 mg for gold [14], up to 20 g in the case of silver [16], which is often utterly unacceptable from the point of view of the preservation of historical artifacts made of precious metals. Second, the ICP–AES methods are focused on the analysis of relatively pure samples of silver and gold with the concentration of impurity elements less than 0.01–0.05 wt %, while archaeological samples of silver and gold may contain a large number of alloying elements or be alloys. In this case, the use of conventional dissolution procedures can lead to errors in the results of analysis. For example, in ICP–MS analysis of gold samples with high silver content, the dissolution of samples only in a mixture of HCl and HNO3 leads to the formation of insoluble silver chloride [17]. Silver chloride can also capture other elements [15], which leads to significant errors in determining the composition of the gold sample. Similar problems of incomplete dissolution can occur in the analysis of silver products containing gold or platinum elements, if one uses only nitric acid to dissolve them.

For these reasons, we developed two new procedures for analyzing silver and gold samples weighing a few milligrams, based on the autoclave dissolution of the samples and the subsequent analysis of the resulting solutions by ICP–AES and ICP–MS. These procedures are used to analyze samples obtained by drilling from the inner surface of silver and gold artifacts of the Borodino treasure stored in the State Historical Museum (Fig. 1), as well as other silver and gold archaeological samples.

Fig. 1.
figure 1

Spears of the Borodino treasure (silver, gold).

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EXPERIMENTAL

Reagents and glassware. We used the following concentrated acids: HNO3 (nitric acid 65%; max 0.0000005% of Hg; GR, ISO) and H2SO4 (sulfuric acid 95–97% for analysis EMSURE) manufactured by Merck (Germany), HCl (hydrochloric acid 37%; max 0.0000005% of Hg; PA-ACS-ISO) manufactured by Panreac (Spain). Deionized water with a specific resistance of 18.2 MΩ cm was used to prepare solutions.

For the storage of solutions, glass measuring flasks with ground-glass stoppers according to GOST 1770-74 and 15- and 50-mL disposable polyethylene test tubes from Labcon (United States) and Deltalab (Spain) were used. The glassware was presoaked for 4–5 days in 5% HNO3 and washed with deionized water prior to use. The samples were dissolved in an open system using quartz 20-mL glasses with “watch glass” covers.

Reference samples. For the preparation of calibration solutions for ICP–AES and ICP–MS measurements, we used solutions of the following multielement and single-element standards from High-Purity Standards (United States):

—An ICP-AM-6A ICP Analytical Mixture multielement standard solution with a weight concentration of 100 mg/L of Li, Be, B, Na, Mg, Al, Si, K, Ca, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Sr, Cd, Sb, Ba, Tl, and Pb.

—An ICP-MS-68-A multielement standard solution with a weight concentration of 10 mg/L of Al, As, Ba, Be, Bi, B, Cd, Ca, Ce, Cs, Cr, Co, Cu, Dy, Er, Eu, Gd, Ga, Ho, In, Fe, La, Pb, Li, Lu, Mg, Mn, Nd, Ni, P, K, Pr, Re, Rb, Sm, Sc, Se, Na, Sr, Tb, Tl, Th, Tm, U, V, Yb, Y, and Zn.

—An ICE-MS-68-B multielement standard solution with a weigh concentration of 10 mg/L of Sb, Ge, Hf, Mo, Nb, Si, Ag, Ta, Te, Sn, Ti, W, and Zr.

—An ICP-MS-68-С multielement standard solution with a weight concentration of 10 mg/L of Au, Ir, Os, Pd, Pt, Rh, and Ru.

—Standard single-element solutions of B, Al, P, S, V, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Sr, Rh, Ag, Ba, Pt, Au, and Hg with a weight concentration of 1000 mg/L.

The accuracy of the analysis of silver and gold samples was controlled using a set of standard samples of silver SN2/GSO 10738-2015 and standard samples of gold Zl4aK1/GSO 10589-2015, Zl4aK2/GSO 10590-2015, and Zl4a-6/SOP 0279-2015, produced by Yekaterinburg Non-Ferrous Metals Processing Plant, Ru-ssia.

Apparatus. The autoclave decomposition of the samples was carried out in the autoclave sample opening system developed in the Institute of Microelectronics Technology Problems and High-Purity Materials, Russian Academy of Sciences, which is a modification of the well-proven MKP-05 system (ANKON-AT-2, Russia). The system enables heating of 30-mL Teflon reaction chambers to a maximum temperature of 240°C at a pressure of 20 MPa (200 bar). The thermostat unit for six autoclaves is equipped with two independent thermocouples and an automatic control unit based on a TRM-251 programmable PID temperature controller (OVEN-K, Russia). The control unit implements a five-stage program for heating the thermostat; the temperature, the duration of heating and maintaining the selected temperature are set at each stage. RP-1 electric plates with a variable output (Tom’analit, Russia) were used for heating and evaporation of solutions. A CE224-C analytical balance (Sartogosm, Russia) was used for weighing.

For the AES analysis of the obtained solutions, an iCAP-6500 Duo spectrometer (Thermo Scientific, United States) was used at the following operating parameters: generator output power, 1250 W; VeeSpray nebulizer; cyclone glass spray chamber; argon plasma forming gas flow rate, 12 L/min; argon auxiliary gas flow rate, 0.5 L/min; argon flow rate in the nebulizer, 0.57 L/min; and sample flow rate, 1.8 mL/min. The list of elements under determination, the lines used, and the method of observation of the plasma are given in Table 1.

Table 1.   List of elements determined by ICP–AES, used spectral lines and their types, and measurement conditions
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For the mass spectrometric determination of the elements, an X-7 quadrupole mass spectrometer (Thermo Scientific, United States) was used at the following parameters of operation: generator output power, 1300 W; set of standard nickel cones; concentric atomizer PolyCon; quartz conical spray chamber cooled to 3°C; plasma-forming argon flow rate, 13 L/min; auxiliary argon flow rate, 0.9 L/min; flow rate of argon in the atomizer, 0.95 L/min; and sample flow rate, 0.8 mL/min. The detected elements and isotopes used are listed in Table 2.

Table 2.   List of elements determined by ICP–MS and the isotopes used
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For laser sampling, a UP266 MACRO unit (New Wave Research, United States) was used at the following operating parameters: wavelength, 266 nm; pulse duration, 4 ns; pulse repetition frequency, 10 Hz; pulse energy, 5 mJ; and crater diameter, 100 µm. The sampling was carried out by purging the chamber with a helium flow at 0.6 L/min, which was mixed with an argon flow at 0.6 L/min before being fed into the mass spectrometer. The resulting aerosol was analyzed using an X-2 quadrupole mass spectrometer (Thermo Scientific, United States). The 197Au16O+ polyatomic ion was used as an internal standard.

Autoclave decomposition of samples.Silver. Before decomposition, silver samples were kept for 5–10 min in 10–15% HCl to remove surface contaminants, washed 2–3 times with deionized water, and dried. The autoclave decomposition of the samples was carried out in batches of 4–5 samples, the weight of which was 10 mg. A control (blank) experiment was performed in each batch, and a standard reference sample of composition was decomposed together with the test samples in each second batch. The samples were placed in the reaction chambers of the autoclaves; 1 mL of HNO3 was added to the samples. The reaction chambers were heated on the hot plate until the solution boiled and the samples were completely or partially (depending on their composition) dissolved. After cooling, portions of HCl (0.5 mL) and HNO3 (0.5 mL) were added to the reaction chambers, and the chambers were closed with lids and sealed in titanium enclosures of the autoclaves. The autoclaves were placed in an electric heater and heated at 160°C for 1 h. After cooling, the autoclaves were opened, and the resulting solutions were transferred to plastic test tubes. The silver chloride precipitates were washed with 2–3 mL of deionized water, and this water was combined with the previous solutions from autoclaves. The volume of the resulting solutions was adjusted to 10 mL (solutions with index a). Portions of H2SO4 (2 mL) were added to the reaction chambers, and the chambers were closed with lids and sealed in titanium enclosures of the autoclaves. The autoclaves were placed in an electric heater and heated at 160°C (1 h), 180°C (1 h), 200°C (1 h), and 220°C (1 h). After cooling, the autoclaves were opened, the solutions from the reaction chambers were transferred to plastic test tubes, and the volume of the solutions was adjusted to 10 mL (solutions with index b). The remains of silver chloride at the bottom of the reaction chambers were discarded. The solutions obtained from the reaction chambers, underwent the above procedures and those containing no sample were used as blank samples. Before starting the measurements, solutions with indices a and b were diluted tenfold, and an internal standard (cesium) was introduced. The concentrations of elements in both diluted and initial solutions were determined by ICP–AES, and ICP–MS was used for only diluted solutions.

Gold. Before decomposition, gold samples were kept for 5–10 min in 10–15% HCl to remove surface contaminants, washed 2–3 times with deionized water, and dried. The autoclave decomposition of the samples was carried out in batches of 4–5 samples, the weight of which was 10 mg. A control (blank) experiment was performed in each batch, and a standard reference sample of composition was decomposed together with the test samples in each second batch. The samples were placed in reaction chambers of the autoclaves; portions of HCl (0.5 mL) and HNO3 (1 mL) were added, and the chambers were closed with lids and sealed in titanium enclosures of the autoclaves. The autoclaves were placed in an electric heater and heated at 160°C for 1 h. After cooling, the autoclaves were opened, and the resulting solutions were transferred to plastic test tubes. If precipitates (silver chloride) formed during the dissolution, they were washed with 2–3 mL of deionized water, and this water portions were combined with the previous solutions. The volume of the resulting solutions was adjusted to 10 mL (solutions with index a). Portions of H2SO4 (2 mL) were added to the reaction chambers, and the chambers were closed with lids and sealed in titanium enclosures of the autoclaves. The autoclaves were placed in an electric heater and heated at 160°C (1 h), 180°C (1 h), 200°C (1 h), and 220°C (1 h). After cooling, the autoclaves were opened; the reaction chambers were placed on the hot plate. The solutions were boiled for 15–20 min, cooled, and transferred to plastic test tubes; and the volume of the solutions was adjusted to 10 mL (solutions with index b). The solutions obtained from the reaction chambers, underwent the above procedures and those containing no sample were used as blank samples. Before starting the measurements, solutions with indices a and b were diluted tenfold, and an internal standard (cesium) was introduced. The concentrations of elements in both diluted and initial solutions were determined by ICP–AES, and ICP–MS was used for only diluted solutions.

RESULTS AND DISCUSSION

In developing methods for analyzing silver and gold, the decomposition procedures described earlier [12–15] were taken as the basis. In contrast to the known procedures, the weight of the test samples was no more than 10 mg; therefore, it was necessary to ensure a low level of uncontrolled contamination from the laboratory glassware, air, reagents, etc. We also should eliminate or minimize the uncontrolled loss of the analytes through incomplete dissolution, spattering, volatility, adsorption, incomplete transfer of solutions, and other reasons. Therefore, the dissolution of the samples was carried out in autoclaves at elevated pressure and temperature up to 220°C, which ensured, on the one hand, low values of the blank experiment, and, on the other hand, the most severe dissolution conditions. Finally, to expand the range of elements to be determined and increase the reliability of their determination, the resulting solutions were analyzed by two methods: ICP–AES and ICP–MS.

The above-described ICP–AES and ICP–MS procedures were tested in analyzing a set of standard samples of silver SN2/GSO 10738-2015 and standard samples of gold Zl4aK1/GSO 10589-2015, Zl4aK2/GSO 10590-2015, and Zl4a-6/SOP 0279-2015 We used the conventional measurement procedures of ICP–AES and ICP–MS; they were described in previous works (for example, [18]) and are not given in this paper.

Upon the decomposition of standard silver samples, it was found that most of the impurity elements were entirely transferred to the solution at the first stage of autoclave decomposition (solution a). The second solution (solution b), obtained after the treatment of the silver chloride precipitate with 2 mL of sulfuric acid, contained the total amount of rhodium and up to 20% of lead and bismuth relative to their total concentration in the sample. This procedure could be modified to quantify silver. To do this, the obtained solutions b must be boiled for 15–20 min after the second heating of autoclaves with sulfuric acid, until the complete destruction of silver chloride. In this case, silver completely passed to the solution and can be determined, for example, by ICP–AES. The results of the ICP–AES and ICP–MS analysis of two standard silver samples (Table 3) confirm the excellent reproducibility and accuracy of the determination.

Table 3.   Results (µg/g) of analysis of standard reference samples of refined silver GSO 10738–2015 (СН2-4 and СН2-7)
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In decomposing standard gold samples containing up to 0.03% of silver (Zl4a-6/SOP 0279-2015), all impurity elements, including silver, were completely dissolved in the solution at the first stage of autoclave decomposition (solution a). However, in analyzing real gold samples containing appreciable amounts of silver (>1%), a precipitate of silver chloride formed, and the second stage of autoclave decomposition was required to dissolve it. In addition to silver, we observed lead and bismuth in solution b in the amount of up to 10% of their total concentration in the sample. As for rhodium, most of it was in solution a, unlike silver samples. The procedure of the autoclave decomposition of gold samples was checked by the standard addition method using reference solutions of standard samples containing all platinum elements (an ICP-MS-68-С standard solution). It was found that the loss of impurity elements during the autoclave decomposition described above did not occur. Moreover, the yield for osmium was 620 ± 10%. The reason for this, apparently, was the difference in the forms of osmium in the initial standard solution used for calibration, and in the solution obtained after autoclave decomposition. The results of the ICP–AES and ICP–MS analysis of the standard gold samples (Table 4) confirmed the excellent reproducibility and accuracy of the determination.

Table 4.   Results (µg/g) of analysis of a standard sample of gold Zl4aK2/GSO 10590–2015
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The real and standard samples of gold and silver were also analyzed by LA–ICP–MS. In most cases, there was a good agreement between the results obtained by two independent methods. For some elements, substantial differences seem to be associated with the possible nonuniform distribution of impurity elements in the samples. As an example, typical results of analysis of one of the archaeological samples of gold are presented in Table 5.

Table 5.   Comparison of the results of the ICP–AES/ICP–MS (the developed procedure) and LA–ICP–MS analysis of a gold sample
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The developed procedures for determining silver and gold samples were used to analyze archaeological silver and gold issues, including samples from the Borodino treasure. More than 60 samples of silver and gold were analyzed in total. Table 6 shows the found concentration ranges of the elements in these samples. The concentrations of elements not included in Table 6 in almost all samples of the Borodino treasure were below the limits of determination (see Tables 3 and 4). The results of analysis of silver and gold samples from the Borodino treasure are presented more fully in [19].

Table 6.   Concentration ranges (wt %) of elements in the silver and gold samples of the Borodino treasure
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