Digesters break down samples into their basic constituents for analysis. While they function in a similar fashion to biological digestion, the constituents are not used for food, rather for study. Sample digestion is often involved in hunts for trace metals, to prepare for such analyses as atomic absorption or inductively coupled plasma spectroscopy. This classification of devices includes those used for laboratory research, sample testing, and process study. Large-scale troughs, tanks, or silos used for sewage treatment, treatment of agricultural waste, or methane abatement are not covered.
The two most common types of digesters are those that function via acid digestion or microwave digestion (although custom, specialized, and proprietary technologies are available).
In acid digestion, the sample is digested under elevated pressure (100-500 psi) and temperature (50-180 degrees C) conditions using conventional acids, such as nitric and hydrochloric acids.
Microwave digesters heat microwave-absorbing reagents containing a sample inside a pressurized, microwave-transparent container. Pressurization allows higher temperatures to be achieved, thus increasing the speed of digestion. Pressure and temperature inside the control vessels may be monitored and used to determine the amount of microwave energy applied.
Laboratory digesters are all of a batch-type design. Sample material, generally organic matter, is placed within the device and is allowed to digest. The retention time depends on temperature and other factors. Once the digestion is complete, the effluent is removed and the process is repeated. Installation of a fume hood is helpful to remove gases produced by the digestion process.
One of two methods is used to process samples:
- open flask digestion
- closed flask digestion
As the name implies, in open flask digestion, the reaction takes place in an open container. This is the more commonly used method; calling for lower pressure to perform the procedure, although gas build up is more likely. Closed flask reaction takes place in a closed container. The elevated pressures that can be achieved in closed reaction vessels decrease digestion time. Generally, high-pressure vessels are used for biological and organic chemical samples, while lower pressure vessels are used for soil/sediment, environmental samples, and catalysts.
Digesters can be preset to process a batch of samples over a specific period of time. Automated operation frees key laboratory personnel from managing the long and tedious digestion processes and provides a head start on analysis.
Kristian Bjornard / CC BY-SA 2.0
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