MIC and ICP-MS determination of elements
Samples of WPC, WPH, WPI, and casein were digested following
the operational parameters described in a previous study (Pereira
Barbosa et al., 2013). Pellets of samples were placed on a quartz
holder with a small disc of filter paper. A 6 mol L1 NH4NO3 solution
(50 lL) was added to the filter paper followed by the introduction
of a quartz holder in a quartz vessel previously charged with
6 mL of absorbing solution (10–100 mmol L1 NH4OH). After closing
and capping the rotor, vessels were pressurized with 20 bar of
oxygen. The rotor was placed inside the microwave cavity and the
heating program was started using 1400 W for 5 min and 0 W for
20 min for the cooling step. After digestion, the pressure of each
vessel was carefully released and the digests were transferred to
a 25 mL volumetric flask and diluted with water. The determination
of elements by ICP-MS and ion chromatography was performed
as described in a previous study (Mesko et al., 2010). All
vessels were cleaned with 6 mL of concentrated HNO3 under
microwave heating at 1000 W for 10 min and 0 W for 20 min for
cooling. Glass and quartz material were soaked in 1.4 mol L1
HNO3 for 24 h and further washed with water before use.
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