Silver is one of the oldest metals known to humanity and is still very important in minting, jewelry, and industry. Silver can be obtained from pure deposits and ores and, like other precious metals, needs to be refined to remove any impurities to leave a quality end product. Refinery plants receive low-assay silver, which needs to be analyzed for silver, gold, and platinum group metals (PGMs) content and for any harmful or toxic elements that may hinder refinery processes. The maximum concentration of impurities allowed in saleable silver bullion is at ppm and sub-ppm levels in the solid. Extensive purification and sensitive analyses are therefore required to produce high-quality material. It is essential for plants to constantly monitor impurity levels to retain high yields and avoid the need to reprocess contaminated products.
The United States is one of the main producers of silver, along with Mexico and South America. U.S. regulations state that only an alloy of at least 92.5% can be marketed as "silver." Refined silver in bar form has three different grade levels indicated by the degree of fineness of the silver. Grade 99.90 silver has a minimum fineness of 999.0 and is often referred to as bullion or commercial bar silver. Grade 99.95 subsequently has a fineness level of at least 999.5. Grade 99.99 silver, also known as premium-grade silver, has a fineness level of 999.9 or above.
The American Society for Testing and Materials (ASTM) standards currently feature methods such as atomic spectroscopy and direct current (DC) arc spectrometry for metal analysis. However, recently gold, silver, and PGMs have been analyzed using a spark ablation/inductively coupled plasma (ICP) method, which is increasing in popularity as a standard method of metal analysis, particularly in Russia, and is expanding to other metals such as nickel and stainless steel.
This article discusses the traditional methods of silver analysis and the limitations that arise from using these technologies. It also introduces ICP spectrometry used in conjunction with a separate sampling and excitation accessory (SSEA) as an alternative that offers rapid and timely analysis of impurities in silver. This powerful solution is also demonstrated through an application example drawing on data collected from its implementation in the analysis of impurities in silver.
Traditional methods and their shortcomings
Traditional methods of silver analysis include atomic spectroscopy, which is the current preferred method of pure silver analysis due to the low spectral emission from the silver matrix compared to other techniques. DC arc spectrometry has also been used for this type of analysis and is capable of detecting low-level impurities in solid silver. However, matrices containing even small quantities of impurities may show errors, especially when analyzing low-grade silver.
ICP is a standard method for analyzing silver and is widely used in regions across Russia due to its multielement capabilities, reduced matrix effects, and better sensitivity. Silver can be analyzed after the dissolution of the metal in diluted nitric acid (1:1). Not all elements are soluble in nitric acid; for example, gold and rhodium may require subsequent aqua regia treatment. If the sample has not been filtered and the whole sample is subjected to aqua regia, a lengthy precipitation separation of the silver is often required. This can produce errors in the purity analysis by coprecipitating portions of the trace analytes. In such cases either a complexing agent or excess chlorine addition is required; however, the impurities may still be lost. Silver has limited solubility in hydrochloric acid and therefore should ideally be separated before the aqua regia treatment. These processes can be lengthy and ultimately increase the cost of purification.
ICP spectrometry with an SSEA has emerged as a unique solution to traditional methods and addresses the procedural problems that may arise when using these existing technologies.
Overcoming the limitations
SSEA utilizes a high-voltage spark to ablate the surface of the metallic or conductive sample, producing a dry aerosol, which is then transferred to the plasma using the ICP instrument's main argon carrier gas supply at a reduced rate. The resulting metal or sample vapor then undergoes processes of excitation and emission within the plasma. Only a small portion of the sample is ablated and thus the sample can be used several times. Once the surface is covered with spark sites, the sample simply requires a repeat of the surface preparation and skimming with a lathe before reuse.
Spark ablation samples a much larger surface area relative to laser ablation, thus making it ideal for bulk and solid metal sampling, whereas laser ablation is better suited for nonconductive samples and those that require analysis at precise microscopic locations. The SSEA and ICP combination allows for ablative sampling of any metal including the direct analysis of noble metals and other conductive substances, provided that the detector can analyze trace intensities among the high background signals.
Refineries and mints in Russia use the Thermo Scientific iCAP 6000 Series spectrometers (Thermo Fisher Scientific, Cambridge, U.K.) coupled with SSEA for the analysis of gold, a method that is included in the forthcoming version of Russian State Standard (GOST) for the analysis of gold. This alternative technique offers plants fast and efficient analysis with a completion time frame of 20 min with minimal sample preparation and sample consumption.
The open joint-stock company Kolyma State Refinery has been refining precious metals since 1998. The enterprise is on the roster of the organizations authorized to refine precious metals and is on the LBMA "Good Delivery" list of accredited melters and assayers. Production proceeds with minimal irrecoverable losses of precious metals. The Kolyma State Refinery, in the Magadan region of Russia, employs ICP spectrometry coupled with an SSEA for routine analysis of low-assay silver, high-purity silver, and other silver products. The company invested in this configuration to replace its DC arc spectrometer and to overcome the problems associated with this technology when used for analyzing silver.
Figure 1 - Thermo Scientific iCAP 6500 Duo attached to the SSEA in Kolyma State Refinery.
The Thermo Scientific iCAP 6500 Duo spectrometer (Figure 1) was chosen for this application due to its enhanced sensitivity with the axial view and ability to couple with a conductive metal spark accessory like the SSEA. The system's capabilities enable the advanced features of electronic control and triggering of the SSEA, thus being the only available ICP able to fully support this accessory.
Calibration standards from the library of Russian State Reference samples of silver were used. Their concentration values were certified as percentage concentration and applied as direct calibration values. The silver wavelengths at 232.468 nm and 233.137 nm were used as internal standard lines to maximize the precision of the method and minimize the effect of transport efficiency changes.
The iCAP 6500 uses a charge injection device (CID) detector which, unlike a traditional charge-coupled device (CCD) camera, is resistant to adjacent pixel blooming and can therefore accurately determine trace elements in the presence of high-intensity emissions. The detector's full wavelength coverage allows flexible selection of wavelengths to avoid spectroscopic interference. This is aided by the high spectral resolution capability of the spectrometer, which is essential for analyzing the complex spectra produced by solid metal sampling.
Use of the iCAP 6500 enables both liquid- and solid-phase reference samples to be used to build calibrations. The liquid-phase standards can be utilized in conjunction with solid samples by using a purpose-designed twin inlet spray chamber. The use of liquid calibration requires a conversion factor to be established between the liquid and solid sample responses, which are useful in situations in which suitable solid standards are not readily available. Calibrations were regularly checked against standard quality control samples (QCs) employing the Thermo Scientific iTEVA software's check table system.
Figure 2 - Palladium 340.458-nm calibration.
Calibration graphs (Figures 2 and 3) show that very good correlation coefficients were generated by this method and all data recorded were extremely accurate and stable. Liquid calibration standards CH-4 and CH-7 are shown in Figure 3; the remaining standards are solids.
Figure 3 - Bismuth 190.241-nm calibration.
Table 1 - Results of a typical analysis with
duplicates (1472-2, 1472-2′) and
detection limits (all units in % w/w)
Table 1 presents the concentrations in percentage weight units found in some of the samples. The silver (Ag) value was determined by concentration ratio where the silver is the main metal constituent and is used as an internal standard, and the final silver percentage is automatically calculated by difference. The detection limit was determined by the repeated analysis of a high-purity silver sample, then multiplying the standard deviation of the results by 3 to produce a 3s detection limit.
Use of the Thermo Scientific iCAP 6500 Duo spectrometer in conjunction with an SSEA provided rapid, repeatable analysis of the impurities in silver samples, enabling close monitoring of all stages of the refinery process. The configuration provided sensitive multielement determination of impurities at ppm and sub-ppm levels in both solid-phase and extracted liquid-phase silver matrices with only minimal sample preparation and usage, while also being capable of determining the silver content itself. The precision of the instrumentation allows plants to accurately determine the purity levels of silver and thus allocate the correct grade for each sample. The method described is already considered a standard method in refineries across Russia.
Spark ablation does not always need a full set of standards for calibration. Industries can overcome the limitations of the availability of solid standards suitable for calibration by producing their own set of standards based on the process and elemental constituents in their samples. Future developments of the SSEA and ICP solution will focus on categorizing further materials such as pure copper and hard alloys, researching the effect of other carrier gases such as helium and nitrogen, and adjusting the sample introduction options to maximize sensitivity and efficiency.
Mr. Buchbinder is an Applications Specialist, and Mr. Verblyudov is a Technical Engineer, Intertech Corp., Novosibirsk, Russia. Mr. Clavering is an ICP Applications Specialist, Thermo Fisher Scientific, 19 Mercers Row, Cambridge CB5 8BZ, U.K.; tel.: +44 0 1223 347498; fax: +44 0 1223 347407; e-mail: firstname.lastname@example.org.