Comparative evaluation of sample digestion techniques for Atomic Absorption Spectroscopy

Samples ready for digestion in sealed tubes

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.

CharacteristicOpen DigestionClosed Digestion
Corrosive fumesEvolved- Need to be ventilatedPose no harm
Digestion temperatureLimited by the boiling point of acidPressurised digesters permit higher temperatures
Sample sizeFor low concentrations larger sample sizes can be handledLarge sample sizes not required
The number of acidsRequires large quantitiesSmaller quantities are sufficient
Loss of volatile analytesYes –Possibility existsNo loss of volatile elements
Risk of contaminationYes from surrounding environmentNo contamination risk
Time is taken for digestionsLong- hours to daysLess 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.

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  1. I want to convert plastic materials into homogenous liquids sample for AAS analysis.could you guide me in which digestion method is good for this purpose ? and which techniques flame or graphite AAS should I use ??? Please guide me .

    1. You can use recommended methods if available for material specific analysis. Otherwise you can try acid digestion at room or elevated temperatures.Adoption of flame or graphite furnace atomization will depend on required concentration estimations. If results fall in ppm levels flame is recommended. However, for studies in ppb levels you should consider graphite furnace atomization.

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