Microwave-assisted digestion methods: towards greener approaches for plasma-based analytical techniques

In recent years, the evolution of instrumental techniques such as inductively coupled plasma (ICP)-based techniques has allowed multielemental analytical capability at trace levels and for a variety of matrices. Such information can be strongly affected by matrix-related effects causing spectral and non-spectral interferences and several alternatives have been proposed to overcome these drawbacks. Sample preparation methods with high digestion efficiency have been the most convenient for subsequent ICP analysis. Conventionally, high digestion efficiency can be obtained following protocols based on the addition of a relatively high amount of concentrated reagents combined with operational conditions using high temperature and pressure. Modern instrumentation and a better understanding of chemical reactions related to sample digestion have enabled the achievement of high digestion efficiency even using dilute reagents. In this sense, this review presents the main advances for the digestion of organic samples, based on the reduction of reagents without impairing digestion efficiency for further multielement determination by ICP-based techniques. Recent approaches are highlighted, such as the use of dilute acid solutions with the aid of O2, H2O2 and UV-assisted digestion, high pressure flow digestion, and combustion methods, all assisted by microwave radiation. The main features and applications reported in the past ten years are also covered. Finally, a critical view of the suitability of metrics for the evaluation of greener digestion methods, mainly for matrices considered hard to be digested using conventional protocols, is presented.