AOAC SPIFAN Nutrients ERP (December 7, 2022)

For ERP/Working Group Use Only DO NOT DISTRIBUTE Fluor-04 (SLV) Resubmission FOR ERP USE ONLY DO NOT DISTRIBUTE

citrate also preferentially complexes with other polyvalent anions such as calcium, aluminum, and magnesium, which can bind to fluoride. Each sample was mixed well by pouring from one cup to another repeatedly for a minimum of 10 times, splitting equally into two cups for analysis. A Mettler Toledo (MT) SevenExcellence S470 ion meter was used to collect all data. Two fluoride electrodes were used: Electrode #1 was from Mettler (perfectION™ combination fluoride electrode) and Electrode #2 was from Thermo Scientific (Orion Fluoride combination electrode 9609BNWP). A pH probe (Mettler InLab pH electrode) was used alongside the Mettler fluoride electrode to measure pH and sample temperature. A temperature probe (Mettler PT1000) was used with the Orion electrode so that temperature was measured in both sample cups. All readings for each sample on both electrode configurations were taken simultaneously. The meter was programmed for automatic readings employing a “strict” endpoint criterion, meaning that the reading was taken when the value varied < 0.03 mV during the last 8 seconds or 0.08 mV during the last 20 seconds. All samples were stirred during data acquisition at a rate that was fast enough to mix the samples without creating a vortex. Styrofoam was placed on the stir plates to prevent heat transfer to the samples. The testing schematic is illustrated in Figure 1.

Figure 1: Experimental schematic

Calibration standards were prepared the same as the samples and measured daily on each electrode (see Figure 2 ). A least squares (log fit) analysis of the electrode response versus the theoretical standard concentration of two curves (low: 0.02 – 0.1 µg / 100 g; high: 0.1 -2.0 µg / 100 g) was performed. The correlation coefficient of every calibration curve generated in the SLV was greater than 0.995. Each electrode had a separate calibration curve. Calibrations were checked every two hours by measuring the 0.1 ppm (10 µg / 100 g) standard. This standard was chosen as it is included in both the high and the low standard curves. The readings were all within ± 2 mV of the initial reading, indicating a stable curve. Both high and low curves were

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