Samples and reagents are heated on a hotplate, in a sand bath or in a microwave-heated system. The sample and reagents are in open vessels made of glass, quartz, glassy carbon or PTFE. The gas vapors which are produced are either extracted or caught by a reflux condenser and returned to the reaction mixture.
The maximum reaction temperature is dictated by the boiling point of the acid mixture at ambient pressure.
High sample throughput, very good for inhomogeneous samples
Any reaction gases can evaporate unhindered (no build-up of pressure)
Dis-advantages
Maximum temperature is limited to the boiling point of the reagent mixture; poor decomposition quality
Long decomposition time
Use of H2SO4 to increase the temperature of the mixture
Difficult to produce pure sulfuric acid
Interferes with many subsequent analysis methods due to its high viscosity and forms sulfates with low solubility when mixed with many metals (e.g. Cd, Pb, etc.), which leads to values which are lower than the actual values
High consumption of reagents, high blank values
Evaporated reagents must be refilled during the decomposition
Possible contamination from reagents and the environment
Produces corrosive air in the laboratory
Loss of volatile elements Hg (elemental) As, B, Cr, Ge, Pb, Sn, Te, Ti, Zn, Zr (as halogen compounds) Os, Rh, Ru (under oxidative conditions) Se, Te (under reductive conditions)
Large vessels (i.e. 250 ml) with large surfaces, adsorption effects and loss of analytes
Time-consuming cleaning required after decomposition
Closed Digestion
Samples and reagents are heated in closed pressure vessels made of fluoroplastics (PFA, PTFE, PTFE-TFM) or quartz. The pressure vessels can be heated conventionally (autoclave, heating block) or using microwave technology. The reaction is controlled via temperature and pressure sensors.
The maximum reaction temperature depends on the thermal stability and pressure stability of the vessel material used.
Sample amount
0.1 to 1 g (organic sample matrix) 0.5 to 5 g (inorganic sample matrix)
Reagent consumption
2 to 10 mL
Digestion duration
0.2 to 1 h
Use
All materials
Matrices which are difficult to dissolve
For all measuring methods (spectrometry, voltammetry)
Benefits
High temperatures (up to 300°C depending on the instrument and vessel type)
High pressures (up to 80 bar depending on the instrument and vessel type)
Complete decomposition results due to the high temperatures and pressures
Simple acid mixtures; HNO3 is sufficient for organic samples
H2SO4 or HClO4 are not required
Low reagent volumes, reduction of blank values and reagent costs
Short reaction times
No loss of volatile elements
Does not lead to corrosive air in the laboratory
Small vessels and inert materials, extremely low adsorption effects
Dis- advantages
Higher purchase cost
Limited sample weight; higher weights produce larger amounts of reaction gases which remain in the vessels and lead to pressure which must be controlled
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