Benefits of Microwave Digestion over Open Acid Digestions

Trace metal analysis by Atomic Absorption Spectroscopy or ICP requires that the sample be in liquid state before introducing into the system. At times samples come in liquid form such as drinking water, beverages, liquor which require little or no sample treatment prior to analysis. However, samples can also be complex matrices such as body tissue, hair, blood, fats & oils, minerals, rocks, soils, ceramics, oxides, coal, etc. such samples are extracted using acid digestions for removal of metals prior to analysis.

The objective is to achieve complete decomposition of the solid matrix while preventing loss or contamination of the analytes.

Wet chemical digestions utilize various mineral acids such as HNO3, HCL, HF, H2SO4, HClO4 or combinations of such acids. Digestions can be carried out in either open system under atmospheric pressure or under elevated temperature using microwave digestions.

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Microwave Digestion System

Conventional open wet sample digestions of solid samples are carried out in vessels containing the sample and large volume of acids. The mixture is heated inside a fume hood for long periods using hot plate or a heating mantle. The heating is stopped when the analyst feels that digestion is complete. In contrast microwave digestion offers digestions of both inorganic and organic matrices under controlled conditions.

Microwave digestion systems offer several distinct advantages over open digestions:

  • Biggest benefit is time saving. Simultaneous heating of 8 – 12 samples is possible and reaction times are typically less than an hour in comparison to 5 – 12 hours or even more for open digestions.
  • Time taken for a digestion is dependent on temperature.Open digestions are limited to boiling points of acids used whereas closed digestion systems can be operated at higher temperatures of 250 – 300OC
    due to ability to withstand higher pressure.
  • Acid consumption is lower in microwave digestion
  • Microwave digestion systems offer greater extraction efficiency whereas in open system digestion extraction can be incomplete depending on discretion of the analyst.
  • No exposure of analyst to corrosive acid fumes as compared open digestions
  • Loss of volatile elements such as Hg or Pb can take place in open digestion whereas in closed system digestions loss of volatile elements is prevented.
  • A greater risk of contamination from external sources exists in open digestion whereas in closed systems the risk is non-existent

Closed microwave digestion systems have only one disadvantage, namely, explosion and cracking of digestion tubes due to simultaneous build – up of pressure alongwith increase of temperature. However, microwave systems have inbuilt sensors for both temperature and pressure. The microwave power is automatically controlled or shut-off when pressure reaches the maximum limit.

Microwave closed system advantages far outweigh the disadvantage and is therefore a very useful addition to high throughput laboratories.

Please share your experiences of handling difficult digestions and leave your comments.

About Dr. Deepak Bhanot

Dr Deepak Bhanot is a seasoned professional having nearly 30 years expertise beginning from sales and product support of analytical instruments. After completing his graduation and post graduation from Delhi University and IIT Delhi he went on to Loughborough University of Technology, UK for doctorate research in analytical chemistry. His mission is to develop training programs on analytical techniques and share his experiences with broad spectrum of users ranging from professionals engaged in analytical development and research as well as young enthusiasts fresh from academics who wish to embark upon a career in analytical industry.

Comments

  1. jungudo babangida says:

    I am a fresh graduate of geology and am appointed as laboratory technologist , however, I am having a problem in rock digestion and extraction for AAS analysis , please help and support me on how are will become expert in doing that. thank you.

    • Hi Jungudo,
      I would recommend joining of our certificate programme on AAS.Besides your sampling issues with rock samples you are sure to find solutions to your other doubts on the technique and its applications.The course will be launched very shortly. Please keep a lookout on its announcement on our site.

  2. jungudo babangida says:

    Dear Dr Deepak now i am planning to travel to UK for AAS PG990 Graphite furnace training please I need your advice sir.

    • Dear Jungudo,
      Wishing you all the best for the visit to UK for your training.In case you have technical querries you are welcome to address me an I shall try to assist you to the best of my abilities.

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