Pollution due to human activities make an attempt on the health of many elements: air, water and soil. The effect of the soil pollution on the food chain are various and very dangerous. Heavy metals and dioxins for example are carcinogenic and alter the nervous and nephritic system. A lot of solvents cause fatigue, sickness and headache. This is way we introduce you the following article:

"Sample preparation for the determination of inorganic micro pollutants in soils and waters: microwaves digestion of solid matrices"

The increasing environmental pollution entails the contamination of ground water that cannot be used for civil usage anymore because of the presence of heavy metals, nitrates, hydrocarbons and insecticide. This contamination results from the indiscriminate use of chemical fertilizers, herbicides and pesticides in agriculture and civil dump that through the soil penetrate into the underground waters.

It is fundamental to allow for the fragile balance of the ecosystems and the strict interdependence of them with the quality of waters.

That’s why it is becoming more a more crucial to study the principles of microwave sample preparation and the current legislation concerning the environmental pollution and to analyze the latest technological innovation of the sector. In fact, the analytical and screening step is assuming more relevance and a series of techniques has been developed to give accurate results in a fraction of time.
In particular, the technological research & development has been focused on the manufacturing of instruments for sample preparation since it is the most restricting barrier of the whole analytical process.
Therefore, microwave instruments has been designed to improve the analytical quality and the efficiency of the sample preparation processes.
The reliability of analytical methods depends on the quantitative conversion of a solid in a homogeneous solution.
The traditional methods for wet sample preparation need matric dissolutions using a large reagent's volume the between 15 and 100ml. This mixture is heated for several hours with hot plates or ovens. This step finished when the analyst decides, “at sight”, that the digestion is completed. The disadvantages of this type of digestion are: big amount of employed reagents, probable sample contamination due to the working environmental and even serious safety problems since analysts are exposed to vapor reagents.

On the other hand, the method of microwave decomposition uses a substantially different technology and physical approach and it grants a considerable procedure efficiency with respect of the maximum safety of the operator.

Considering the advantages offered by microwave in terms of quality, efficiency and reproducibility, US-EPA has developed methods of preparation that allow for this innovative technology.
These procedures contribute to the rapid execution of the necessary analysis regarding a contaminated water or soil sample in order to activate procedures to safeguard public health and environmental.

This methodologies can be divided in two groups: the first, for the metal analysis in water samples (US EPA 3015). The second one, for the analysis of metals in soils (US EPA 3051, US EPA 3052).

The method US-EPA3052 is very interesting: it describes the procedure for the total sample digestion. In particular, the method refers to the treatment of matrices containing silicates (soils, sediments) and organic matrices (polluted soils by oil and hydrocarbons). To digest these kind of matrices it is allowed to use acid mixture where a second reagent such as HCl, HF, H2O2 or H2O can be added in a second step to the nitric acid.

The table #1 offers a reports of the results obtained digesting a certified river sediment sample.

The very interesting aspect of this preparation procedure is the description of the separation methods of the exceeding hydrofluoric acid after digestion: the method counts to use an accessory that can evaporate the solution avoiding the sample to be manipulated for the change of vessel. This process is able to eliminate the exceeding free fluoride without the loss of any volatile elements.

The graphic here below shows a different temperature profile that can be obtained comparing the heating mechanism of hot plate and microwave assisted evaporation.

The microwave technique can be applied also for the research on organic pollutants. For example, the most commonly used methodology is the US-EPA3546: its application enables the sample preparation needed for chromatographic analysis. The main pollutants that can be extracted are pesticides, herbicides, phenols, PCB and PCDD/PCDF.
The method foresees a maximum time of treatment of 30 minutes with a volume of about 30 ml.
The unquestioned advantages are, apart from time-reduction, the efficiency of the process in which solvent and solute are at the same extraction temperature.

Instrumentation

Microwave ovens are born in the first half of ‘900.
In 70s the technology has evolved and prices, initially very high, started to decrease. From that basis, the Milestone ETHOS 1 has born. The latest progress in the microwave field has been built in this unit.

The success of the modern microwave systems is due to two fundamental aspects: the high technological level and the maximum ease of use. Every part of it, software or hardware has been built reaching the maximum quality standards allowing the operator to optimize the process productivity of sample preparation. These units are equipped with last generation sensors for temperature and pressure control in all vessels.

A particular attention has been paid to the engineering of the various type of vessels both in the materials’ choice and in the safety systems: they can release only the excess pressure from the vessel. http://www.milestonesrl.com/analytical/products-microwave-digestion-ethos-1-vessels-technology.html

Future Possibilities and versatility of the technique

Analytical laboratories needs different productivities and matrices are very heterogeneous; therefore, the manufactures decided to develop and produce carousels with different technical specifications both for temperature and pressure that can be reached. These solutions increase the versatility of microwave platforms and offer the best solution for all the application fields.
The Milestone ETHOS 1 further develop this concept, since it is not dedicated totally to acid digestion, but also to solvent extraction, evaporation and microwave fusion. Four major applications in one microwave system: the Milestone ETHOS 1.

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Bibliography

Kok L. P., Boon M. E. (2003), Microwaves for the Art of Microscopy, Coulomb Press Leyden, Leiden, Netherland.

Richter R.,(2003), Clean Chemistry – Techniques for the Modern Laboratory, Milestone Press, Monroe, USA.

Agazzi A., Pirola C., (2000), Fundamentals, methods and future trends of environmental microwave sample preparation, Microchemical Journal 67, 337-341

Grillo G ENI Monterotondo RM, Pedroni G.ENI Divisione E&P San Donato Milanese MI, (2009), Enciclopedia degli idrocarburi, Fenomeni di inquinamento in suoli e acque, monitoraggio e analisi di rischio, Volume III, Nuovi sviluppi: energia, trasporti, sostenibilità, 10.3, Fenomeni di inquinamento in suoli e acque, monitoraggio e analisi di rischio, 955-971,

Treccani Link D. D., Kingstom H. M. “Skip” (2000). Use of Microwave- Assisted Evaporation for the complete recovery of volatile species of inorganic trace analytes, Analytical Chemistry, 5.6