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What is Microwave Digestion

 

WHAT IS ‘MICROWAVE DIGESTION

SARAH DEVERELL, a chemist for AnchorCert Analytical explains the technologies known as ICP and microwave digestion.

Until 1975 only gold and silver was required to be assayed and hallmarked in the UK and assay offices delivered the necessary testing using traditional cupellation (fire assay) and titration methods respectively.

The advent of platinum as a metal requiring hallmarking under the Hallmarking Act 1973 presented a challenge to the assay offices as it could not be accurately assayed using either of the existing methods. Investment in equipment and expertise was required and the Birmingham Assay Office acquired new technology, ‘Inductively Coupled Plasma – Optical Emission Spectrometry’, commonly known as ICP, and skilled analytical chemists.

ICP analysis takes a very different approach from cupellation and titration. The two traditional methods follow a chemical process which gradually removes all other elements from the sample until only fine gold or silver are present and the weight of the final pure sample can be measured against the original alloy.

The technology can analyse several elements simultaneously giving an accurate reading not only for platinum content but other elements present also. While most samples are assayed for hallmarking using X-ray Fluorescence now (see the March 2015 edition of Jewellery Focus), analysis by ICP–OES remains the reference method for platinum and palladium.

THE TECHNIQUE
In order to test by ICP the sample has to be completely dissolved. The solution is then passed through a nebuliser to create a fine spray which is sprayed onto a heat source for molecules to dissociate into free atoms.

The heat ionises the atom and the resulting ion in an excited electronic state produces characteristic line spectra which identify the elemental composition/concentration as every element will display different, characteristic spectra.

This data is captured electronically and analysed by comparing the results against known standards before being exported for reporting.

Having invested a five-figure sum in the purchase of the ICP, the Assay Office had equipment which could analyse any solution. The ICP was one of the catalysts which spawned the development of AnchorCert Analytical, the laboratory division of the Assay Office which has widened its scope to test precious and costume jewellery and textiles and leather to ensure they comply with consumer product safety legislation.

MICROWAVE DIGESTION
Metals including platinum dissolve relatively easily on a hot plate after the addition of aqua regia. Materials such as paints, plastic, glass and ceramics, can be dissolved by this method, using nitric acid, but the length of time required for complete dissolution can stretch to hours as opposed to minutes and is not commercially viable. Most of my time is spent processing component samples for lead and cadmiumm and ‘Microwave Digestion’ equipment is now a crucial part of kit in AnchorCert Analytical.

The micro-digestor gives us total control over the dissolution.

  1. The sample is weighed and placed in a microwave-compatible tube with appropriate acid and a small amount of water to dilute it.
  2. The tube is loaded into a firm support frame and the cap applied tightly using a wrench to ensure pressure is maintained.
  3. A temperature probe is inserted into one of the test tubes to continually monitor the temperature reached.
  4. The operator can select the appropriate pressure, temperature and duration to ensure the samples are fully dissolved. This will vary according to the number and nature of samples to be tested.
  5. The temperature is normally 210 degrees centigrade and is increased rapidly so the sample undergoes an intensive heating process.
  6. This entire process takes around 45 minutes as opposed to potentially several hours heating on the hotplate. It also delivers a more accurate result as there is no loss or oxidation of the metal of interest.

Once the dissolution is complete the solution will be batched up ready for processing by ICP. The new versions of the ICP are a major step ahead of those originally purchased for platinum analysis. In addition to delivering a higher level of accuracy they are also more user friendly and have a bigger capacity.

The equipment can work throughout the night by use of an auto-sampler, with personnel loading up batches of up to 480 samples to be processed automatically.

The sophisticated technology used is impressive and represents a major investment, necessary to those who need to deliver highly accurate results. However, results depend upon the knowledge and experience of the anaytical chemists who run the sophisticated equipment and interpret the huge amount of data which it produces.

In the case of lead and cadmium, an accurate result is also dependent upon the skill and dexterity of the ever-patient samplers who spend a considerable amount of time skilfully separating the material components, which may include glitter, sequins and different coloured layers of plastic, from beads, hair clips and other jewellery components. As ever the high quality outcome cannot be delivered entirely by the technology. The experience and expert intervention of those who are using it is still crucial.

 

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