Comparative evaluation of sample digestion techniques for Atomic Absorption Spectroscopy

Trace metals analysis by Atomic Absorption Spectroscopy requires the introduction of the sample into the system as a homogeneous liquid. It is necessary to digest solids for complete extraction of analyte(s) of interest into the solution. Commonly used practices are:

  • Acid digestion
  • Fusion
  • Ashing

The technique adopted should be safe, reproducible and afford to save time. The article discusses the advantages and disadvantages of these popular techniques.

Samples ready for digestion in sealed tubes

Samples ready for digestion in sealed tubes

Acid Digestion

Acid digestion is the most popular of the three techniques. Such digestions are classified into two categories, namely, open digestions and closed digestions. Open digestions are conducted in fume hoods using hot plates whereas closed digestions are carried out using microwave digestion systems.

Characteristic Open Digestion Closed Digestion
Corrosive fumes Evolved- Need to be ventilated Pose no harm
Digestion temperature Limited by the boiling point of acid Pressurised digesters permit higher temperatures
Sample size For low concentrations larger sample sizes can be handled Large sample sizes not required
The number of acids Requires large quantities Smaller quantities are sufficient
Loss of volatile analytes Yes –Possibility exists No loss of volatile elements
Risk of contamination Yes from surrounding environment No contamination risk
Time is taken for digestions Long- hours to days Less than an hour for even difficult to digest samples

Open acid digestions are in common use in most laboratories but closed system microwave assisted digestions are cost-effective and are gaining popularity.

Fusion

Fusion with alkali metal salts is popular. Such methods find the use for digestion of inorganic materials such as minerals, clays, silicates, and refractories.

Fusions can lead to clogging of nebulizer tubing if suspended solids are not removed completely from solutions. Further, the concentrations could be high and further dilutions may become necessary thereby increasing chances of errors due to dilutions.

Ashing

 Heating the material to non- combustible ash for analyzing its elemental composition is known as ashing. The advantages and disadvantages of ashing are briefly mentioned.

Advantages:

Large sample sizes can be taken if concentrations are low.

 Saving on the cost of reagents in sample preparation

Disadvantages:

Loss of volatile analytes on heating

Contamination from crucible material

Contamination from inside muffle furnace during heating

Material loss due to air currents when the muffle furnace door is opened or closed

Evolution of toxic vapors during ashing

It can be seen that microwave digestion offers distinct advantages over other contemporary techniques both in terms of complete quantitative digestion and time-saving. However, both ashing and fusion still find use even today when other techniques are not feasible.

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.

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