title: Understanding microwave vessel contamination by chloride species.
abstract: Microwaves are widely used to assist digestion, general sample treatment and synthesis. The use of aqua regia is extensively adopted for the closed vessel mineralization of samples prior to trace element detection, leading to the contamination of microwave vessels by chlorine containing species. The latter are entrapped in the polymeric matrix of the vessels, leading to memory effects that are difficult to remove, among which the risk of silver incomplete recoveries by removal of the sparingly soluble chloride is the predominant one. In the present paper, we determined by mass spectrometry that hydrogen chloride is the species entrapped in the polymeric matrix and responsible for vessel contamination. Moreover, several decontamination treatments were considered to assess their efficiency, demonstrating that several cleaning cycles with water, nitric acid or silver nitrate in nitric acid were inefficient in removing chloride contamination (contamination reduction around 90%). Better results (≈95% decrease) were achieved by a single decontamination step in alkaline environment (sodium hydroxide or ammonia). Finally, a thermal treatment in a common laboratory oven (i.e. without vacuum and ventilation) was tested: a one hour heating at 150°C leads to a 98.5% decontamination, a figure higher than the ones obtained by wet treatments which requires comparable time. The latter treatment is a major advancement with respect to existing treatments as it avoids the need of a vacuum oven for at least 17h as presently proposed in the literature.
authors.affiliation: Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italy.
authors.forename: Sandro
authors.initials: S
authors.lastname: Recchia
authors.affiliation: Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italy.
authors.forename: Davide
authors.initials: D
authors.lastname: Spanu
authors.affiliation: Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italy.
authors.forename: Davide
authors.initials: D
authors.lastname: Bianchi
authors.affiliation: Dipartimento di Scienze Teoriche e Applicate, Università degli Studi dell'Insubria, via Dunant 3, 21100 Varese, Italy.
authors.forename: Carlo
authors.initials: C
authors.lastname: Dossi
authors.affiliation: Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italy.
authors.forename: Andrea
authors.initials: A
authors.lastname: Pozzi
authors.affiliation: Dipartimento di Scienza e Alta Tecnologia, Università degli Studi dell'Insubria, via Valleggio 11, 22100 Como, Italy.
authors.forename: Damiano
authors.initials: D
authors.lastname: Monticelli
keywords: Chloride contamination; Cleaning; Microwave digestion; Silver