A shining example

When it comes to analysing impurities in silver to make sure your bling is the real thing - ICP spectrometry and a separate sampling and excitation accessory make the perfect couple

Silver is one of the oldest metals known to humanity and is a precious metal which is still very important in minting, jewelry and industry. Silver refinery plants receive low assay silver from suppliers and have to analyse the silver, gold and platinum group metals (PGM) content as well as toxic and harmful elements which may interfere with the refinery process. Impurities present in silver may require the reprocessing of contaminated products, resulting in metal and cost losses. As a consequence, the maximum concentration of impurities allowed in saleable silver bullion is ppm and sub-ppm levels in solid. Extensive purification and sensitive analyses are required in order to ensure greatest product quality.

This article discusses the limitations associated with traditional technologies used for silver analysis and introduces inductively coupled plasma (ICP) spectrometry coupled with a separate sampling and excitation accessory (SSEA) as a powerful alternative to provide rapid, timely analysis of impurities in silver. An application example is presented to describe the use of this unique solution and present data from its implementation for the impurity analysis of high and low grade silver.

DC Arc spectrometry has been traditionally used for this type of analysis. It is a sensitive method capable of determining low-level impurities in solid silver. However, matrices containing even small quantities of selenium and tellurium in silver may show errors, which also happens when analysing lower grade silver.

Flame atomic absorption (AA) spectrometry and an inductively coupled plasma optical emission spectrometer (ICP-OES) provide suitable solutions for precious metal bullion, lower grade metal and base metal analysis. ICP spectrometry is a standard method for analysis of silver, particularly in Russia and the other successor states of the former USSR. The ICP-OES configuration is most suited to pure silver analysis than DC Arc spectrometry due to its multi-element capabilities, relatively low spectral emissions from the silver matrix and improved sensitivity.

Graph 1: Pd 340.458nm calibration

Nevertheless, the technique is associated with a few important shortcomings in relation to silver being analysed after the metal dissolution in diluted nitric acid (1:1). Given that not all impurities dissolve in nitric acid - for example gold and rhodium - possible filtration and separate treatment for insolubles may be required, which can be a very time consuming procedure. In addition, the limited solubility of silver chloride when trying to dissolve with aqua regia may necessitate filtering. If the sample is not filtered and the whole sample is subjected to the aqua regia, a lengthy precipitation separation of the silver is often required. As a result, the silver needs to be separated prior to the aqua regia treatment since it has limited solubility in hydrochloric acid. Furthermore, the precipitation procedure can produce errors in the impurity analysis by co-precipitating portions of the analyte traces. In such cases, either a complexing agent or excess chlorine addition is required but impurities may still be lost.
 
ICP spectrometry coupled with an SSEA has emerged as a viable alternative to the methods described above.

Graph 2: Bismuth 190.241nm calibration

 
The SSEA utilises a high voltage spark to ablate a metal or conductive sample, producing a dry aerosol which is transferred to the plasma using the ICP’s main argon carrier gas supply at a reduced pressure. The metal or sample vapor then undergoes the same processes of excitation and emission as a liquid sample. Only a small part of the sample is ablated and the sample can be used repeatedly until the surface is covered with spark sites. The sample then simply requires a repeat of the initial surface preparation and skimming before reuse.

Spark ablation is an approved standard method of analysis - according to the

"Combining the duo spectrometer, with an SSEA enables close monitoring of all stages of the silver refinery process. This configuration achieves fast and sensitive multi-elemtn determination of impurities.

Russian State Standard (GOST) - for the analysis of gold. The technique involves minimal sample preparation while the metal sample surface only needs to be lathed following a quick and simple process. Sample preparation and analysis are completed in 20 minutes. Additionally, the amount of sample consumed is small. With precious metals and metal CRMs so expensive, minimised consumption of a sample is preferred. Metal CRMs are also expensive. When utilising spark ablation, metal CRMs may be reused many times over. Spark ablation produces a much larger sampling relative to laser ablation, thus making it ideal for bulk and solid metal sampling whereas laser ablation is better suited for non-conductive samples and samples which require analysis at precise locations.

Any metal sample including steels, PGM, gold and other conductive materials can be sampled with the ablative process. This rapid, efficient and easy to use process is ideal for the direct analysis of noble metals and other conductive substances by ICP, provided that the detector used can analyse trace intensities among the high background signals. The complex spectral matrices produced by solid metal sampling make full wavelength coverage and a high resolution optical ICP essential. Full wavelength coverage allows maximum choice of alternate wavelengths to reduce interferences and high resolution makes those interferences less likely.

Table 1: Correlation between found and certified values (all units in % w/w). SD is the Standard Deviation of the replicates of each element.

The open joint-stock company Kolyma State Refinery has been refining precious metals since 1998. The enterprise is on the roster of the organisations authorised to refine precious metals. Production proceeds with minimal irrecoverable losses of precious stones. 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.

For this application, the Thermo Scientific iCAP 6500 Duo spectrometer was chosen due to its enhanced sensitivity in axial view and the ability to couple with, and fully support, a conductive metal spark accessory like the SSEA. Silver and lead are not spectrally rich and signal to background is improved in axial view. The SSEA requires a powerful ICP spectrometer to enable the advanced features of electronic control and triggering. The iCAP 6500 Duo uses a CID camera which, unlike CCD cameras, is bloom-resistant and can analyse for trace elements in the presence of high intensity emissions whilst allowing for full wavelength coverage. The company invested in this configuration to replace their DC Arc spectrometer and overcome the problems associated with this technology when used for silver analysis.

Table 2: Results of a typical analysis with duplicates (1472-2, 1472-2’) and detection limits (all units in % w/w)

The calibration standards used were acquired from a library of Russian State Reference Samples of silver. The concentration values of the standards were certified as percentage concentration and applied as direct calibration values. The silver wavelengths at 232.468nm and 233.137nm were used as internal standard lines to maximize the precision of the method and minimize the effect of transport efficiencies.

Both liquid and solid samples of reference materials were utilised to build a calibration. Liquid standards can be used in conjunction with solid samples by means of a purpose-designed twin inlet spray chamber. The use of a liquid calibration requires that a conversion factor be established between the liquid and solid sample responses, and is useful in cases where suitable solid standards are not readily available. Calibrations were regularly checked against standard Quality Control samples (QCs).

Calibration graphs (Graphs 1 and 2) with excellent correlation coefficients were generated by this method. All data shows an exceptionally accurate and stable analysis. Liquid calibration standards CH-4 and CH-7 are shown in Graph 2; the rest of the standards are solids.

The results in Table 1 show the excellent correlation between the measured values and the reference values of two Russian State reference materials.

Table 2 presents the results of some samples in % weight units. The value of silver (Ag) was determined by difference, i.e. the impurities measured were subtracted from 100% to give a final silver concentration. The detection limit was determined by the repeated analysis (10 replicates) of a high-purity silver sample, then multiplying the standard deviation of those results by 3 to produce a 3σ detection limit. Samples # 1472-2 and #1472-2’ are duplicate samples to demonstrate the repeatability of the method.

Combining the Duo spectrometer, with an SSEA enables close monitoring of all stages of the silver refinery process. This configuration achieves fast and sensitive multi-element determination of impurities at ppm and sub-ppm levels in both solid and liquid silver matrices with only minimal sample preparation, while also being capable of determining the silver content itself. Future developments of the solution will focus on categorising further materials such as pure copper and hard alloys, researching the effect of other carrier gases such as helium and nitrogen and adjusting the spray chamber conditions using a continuous spray of a dilute acid.