How to Clean and Safely Remove HF from Acid Digestion Solutions for Ultra-Trace Analysis: A Microwave-Assisted Vessel-Inside-Vessel Protocol

Abstract

The complete dissolution of silicate-containing materials, often necessary for elemental determination, is generally performed by microwave-assisted digestion involving the forced use of hydrofluoric acid (HF). Although highly efficient in dissolving silicates, this acid exhibits many detrimental effects (e.g., formation of precipitates, corrosiveness to glassware) that make its removal after digestion essential. The displacement of HF is normally achieved by evaporation in open-vessel systems: atmospheric contamination or loss of analytes can occur when fuming-off HF owing to the non-ultraclean conditions necessarily adopted for safety reasons. This aspect strongly hinders determination at the ultra-trace level. To overcome this issue, we propose a clean and safe microwave-assisted procedure to induce the evaporative migration of HF inside a sealed “vessel-inside-vessel” system: up to 99.9% of HF can be removed by performing two additional microwave cycles after sample dissolution. HF migrates from the digestion solution to a scavenger (ultrapure H2O) via a simple physical mechanism, and then, it can be safely dismissed/recycled. The procedure was validated by a soil reference material (NIST 2710), and no external or cross-contamination was observed for the 27 trace elements studied. The results demonstrate the suitability of this protocol for ultra-trace analysis when the utilization of HF is mandatory.

Atomic spectrometry update – a review of advances in environmental analysis

“…A novel signal-enhancement strategy for LIBS analysis of soil combined 201 APGD and cylindrical (plasma) connement. The LODs of 2,31,21,35,49,67,43,20 and 18 mg kg −1 for Ba, Cu, Eu, La, Lu, Ni, Ti, Y and Yb, respectively, were signicantly better than the values (10,133,102,175,262,356,246,158 and 105 mg kg −1 , respectively) achievable using conventional LIBS. An alternative approach involved 202 addition of 15% KI to soil samples to increase plasma temperature and electron density.…”

“…The authors suggested that microscopic cracks in the Teflon, observed with SEM/EDS, allowed penetration of elements below the Teflon surface, and they proposed the incorporation of additional bar-cleaning steps to avoid transfer of adsorbed PTEs to subsequent samples. A protocol designed to remove HF from samples following microwave digestion had 133 the aim of preventing the introduction of impurities frequently observed with open-vessel HF evaporation. The ‘vessel-inside-vessel’ technique involved sample digestion (with HF and HNO 3 ) in a small (5 mL) loosely-capped inner PFA vessel, after which HF was transferred from the inner vessel into the larger (70 mL) outer PTFE sealed vessel by means of two further microwave cycles at 500 W for 10 min.…”

Tackling the Challenging Determination of Trace Elements in Ultrapure Silicon Carbide by LA-ICP-MS

“…For each digestion batch, 3 vessels were used for SiC samples and 2 for reference blanks. The adopted strategy initially involved the exploitation of the “Vessel-Inside-Vessel” technique [ 41 , 42 ] to avoid memory effects [ 43 ]; digestion took place in small capped containers of perfluoroalkoxy (PFA, internal volume = 5 mL) placed inside commercial PTFE vessels. The sample (mass approximately equal to 50 mg) was introduced into the PFA vessel together with the mixture of acids, while 5 mL of ultrapure H 2 O was placed outside of it (i.e., inside the PTFE vessel).…”