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.

IMG_0679
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.

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  1. 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.

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

    1. 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.

  3. dear Dr Deepak
    with respect

    I work in dairy company in chemical laboratory section, I have a question
    Please direct me, for prepare sample of dairy products for HPLC & GC
    i know we don’t need microwave acid digestion for GC & HPLC,
    what is the best method for our dairy product to prepare the sample for GC ? and HPLC?
    for example we have problem for in water soluble vitamin measuring separation from our products.
    thanks for your attention
    BR
    jalali

    1. Microwave digestions are not at all recommended for digestion of samples for chromatographic analysis. Microwave digestion rapidly raises the sample temperature and samples for chromatographic analysis will most likely degrade at such temperatures . The most common sample dissolution technique is to immerse the sample tube or flask in a water filled ultrasonic bath and sonicate for the recommended time to allow complete dissolution.

  4. HELLO Sir,

    Greetings from Nirav Patel India.

    I wish to know proper method for High purity quartz analysis as its digestive methodology and analytical instrument selection and adjustment .

    Thank you

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