AOAC 2018 Methods

(n) Water .—Millipore grade.

(o) Ammonium hydroxide solution .—25% in water (VWR International GmbH, Darmstadt, Germany). (p) Solution 1 .—0.6 g sodium hydroxide dissolved in 100 mL methanol. (q) Solution 2 .—0.25 g sodium hydroxide dissolved in 100 mL methanol. (r) Solution 3 .—Aqueous sodium bromide solution (600 g/L) acidified with 6.5 mL/L ortho -phosphoric acid. 1.2 mL of this solution should just acidify 700 µL of solution 1 beyond the neutral point. If this is unsuccessful after freshly preparing the solutions, the acid solution must be adjusted by dilution or by adding more ortho -phosphoric acid to solution 1. (s) Solution 4 .—Aqueous sodium bromide solution (600 g/L) acidified with 2.7 mL/L ortho -phosphoric acid. 1.2 mL of this solution should just acidify 700 µL of solution 2 beyond the neutral point. If this is unsuccessful after freshly preparing the solutions, the acid solution must be adjusted by dilution or by adding more ortho -phosphoric acid to solution 2. (t) Solution 5 .—Saturated aqueous sodium sulphate solution (supernatant above sediment). (u) Solution 6 .—Aqueous sodium bromide solution (approx. 600 g/L). (v) Solution 7 .—Phenylboronic acid in diethyl ether: approx. 5 mg/mL. Phenylboronic acid may vary significantly in reactivity between different producers or different lots. Therefore, it may be necessary to adapt the concentration of solution 7 in a way that excess reagent allows for sufficient analyte derivatization, but does not exceed the capability of the GC injection system. Note: As glassware containing sodium hydroxide might be unstable, solutions 1 and 2 should not be stored in glass vessels. Plastic vessels, e.g., made from polypropylene, are more suitable. From practical experience, solutions 1 and 2 are stable for at least for 3 months, and solutions 3 to 7 are stable for at least 1 year. D. Stock and Working Solutions In general, reference substances and isotope-labelled standards are weighed in their solid form and diluted to prepare stock solutions in methanol (free analytes) or toluene (bound analytes) at concentrations of 1 mg/mL or 100 µg/mL. These concentrations are always based on the core analytes 2- MCPD, 3-MCPD and glycidol. This means that increased molecular weight as a result of isotope labelling or esterification has to be taken into account by proportionally higher weights. Mixed working solutions are prepared from these stock solutions by gravimetric control, and contain either the free analytes dissolved in methanol or the bound analytes dissolved in toluene. Stock and working solutions are stable for at least one year. (a) Working solution 1 .—2-MCPD and 3-MCPD, 10 µg/mL of each in methanol (b) Working solution 1d .—d 5 -2-MCPD and d 5 -3-MCPD, 10 µg/mL of each MCPD equivalent in methanol (c) Working solution 2 .—2-MCPD and 3-MCPD, 1.0 µg/mL of each in methanol (d) Working solution 2d .—d 5 -2-MCPD and d 5 -3-MCPD, 1.0 µg/mL of each MCPD equivalent in methanol