What is Microwave Digestion?

To measure the quantity of a metal in a sample, the metal must first be dissolved completely into a solution (called an analyte) before it can be analysed using AA (atomic absorption) or ICP Inductively Coupled Plasma. For semi quantitative analysis, hot plate digestion is an option but for high accuracy and for digestion of volatile elements such as Mercury, Arsenic, Tin  etc., then microwave digestion is the preferred method of digestion.

Microwave digestion has become an increasingly popular method to dissolve metals as the digestion process can be completed in minutes, rather than the hours it takes with hot plate digestion. The success of this method is due to the high temperatures and pressures created by using microwaves. Also the complete sample is captive in the vessel so micro levels can be measured later in the analysis.

On the other hand, because the extreme conditions created by microwaves can be very dangerous it is vital that the process is not only safe, but that there is no contamination or loss of the analyte during the digestion process.

To ensure this high level of safety and due to the corrosive nature of acids and the high pressures and temperatures attained in microwave digestion, the material used to manufacture the containment vessels must exhibit certain characteristics:

  • Resistance to aggressive acids
  • Able to withstand temperatures ranging up to 260oC
  • Excellent insulation capacity (dielectric properties)
  • A high degree of hydrophobicity (water repellence)
  • No memory effects of the digested material in the vessel walls.

To achieve the high integrity required in containment vessels for microwave digestion, it has been usual to use a synthetic material called PTFE, better known as TeflonÒ. A second generation material known as TFMTMPTFE has now been developed and is used in the Berghof Speedwavefour.

The benefits of microwave digestion using vessels manufactured from TFMTMPTFE are as follows:

  • The vessels can tolerate higher temperatures and pressures. This ability minimises the deterioration of the vessel, due to the extreme conditions required in the process.
  • The precise dimensions of the vessels created by the high quality machine finish means that excessive pressures are not required to seal the vessel and there is an even distribution of microwaves throughout the vessel.
  • Since digestion pressure can be vented to a fume cupboard, no capping station is required.
  • Due to the isostatic compression moulding process developed by Berghof, these vessels are not considered as consumables (as are vessels produced by other manufacturers) and thus are covered by warranty.

The modified features of TFMTMPTFE mean that microwave digestion can take place at higher temperatures and pressures than vessels made of PTFE and in a much safer environment.

Another important consideration of the digestion process is to avoid spoiling the sample digestion by over-pressure of the sample which will result in the safety rupture disc being broken and the sample lost through the vent system. This can occur if the sample undergoes a fast exothermic reaction. The Speedwave Four system minimises this occurrence by using  a ‘fast’ sample temperature   measurement and control system on every vessel. This provides early reduction of Microwave power in the very early stage of the exothermic reaction which means a safe digestion through the exothermic reaction phase.