Microwave digestion

Microwave digestion is a chemical technique used to decompose sample material into a solution suitable for quantitative elemental analysis via inductively coupled plasma mass spectrometry (ICP-MS), atomic absorption spectroscopy, or atomic emission spectroscopy (including ICP-AES). It is commonly used in analyses for heavy metals.

A vast array of materials (e.g. plants, food, rocks, etc.) can be transformed by exposure to microwave energy in a closed vessel. The increased temperatures and pressures, in combination with reactive compounds (usually strong acids) produce an aqueous solution in which the elements are fully dissolved. This solution is then diluted and analyzed.

Generally, sample material is combined with a concentrated strong acid or a mixture thereof, most commonly using nitric acid, hydrochloric acid and/or hydrofluoric acid, in a closed vessel. The vessel and its contents are then exposed to microwave irradiation, raising the pressure and temperature of the solution mixture. The elevated pressures and temperatures within a low pH sample medium increase both the speed of thermal decomposition of the sample and the solubility of elements in solution. Organic compounds are removed during decomposition. Once these elements are in solution, it is possible to quantify elemental concentrations within samples.

Microwaves can be programmed to reach specific temperatures or ramp up to a given temperature at a specified rate. The temperature in the interior of the vessel is monitored by an infrared external sensor or by a optic fiber probe, and the microwave power is regulated to maintain the temperature defined by the active program. The vessel solution must contain at least one solvent that absorbs microwave radiation, usually water. The specific blend of acids (or other reagents) and the temperatures vary depending upon the type of sample being digested. Often a standardized protocol is followed, such as an Environmental Protection Agency Method.

Before microwave digestion technology was developed, samples were digested using less convenient methods, such as heating vessels in an oven, typically for at least 24 hours. The use of microwave energy allows for fast sample heating, reducing digestion time to as little as one hour.

References

Bibliography

• Kingston, H. M.; Jassie, Lois B. (1988), Introduction to Microwave Sample Preparation: Theory and Practice, ACS Professional Reference Book Series, American Chemical Society, ISBN 9780841214507