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

F. CALCULATIONS

1. Calibration Curve Perform a least squares (log fit) analysis of the electrode response vs. standard concentration. This may be achieved using a laboratory calculator’s statistical program or via spreadsheet. A separate calibration curve must be set up for each electrode used. The correlation coefficient for the standard curve must be greater than 0.995 before proceeding. Calibrations must be checked every 2 hours. The calibration is considered stable if standard check is within 2% of the initial mV reading. If the difference in the mV readings exceed this limit, the electrodes must be cleaned, and the calibration procedure repeated. 2. Sample Calculation The ion-selective electrode measures the fluoride activity of a solution. This fluoride activity (mV response) is made useful by comparing this activity to known concentrations of fluoride standards (calibration curve). Using this curve, the fluoride concentration of samples measured under the same conditions is predicted. The concentration of the diluted sample (predicted from the calibration curve) is then calculated back to the original undiluted concentration using the following calculations: = − × Where: F = Fluoride concentration in undiluted sample X = tare weight of beaker Y = Sample weight

Z = weight of beaker, sample, and dilution agents (acid, buffer) C = Sample concentration predicted from calibration curve

Example: Infant formula RTF Tare weight:

3.2569 g

Sample: 40.1113 g Tare + Sample + dilution 105.5809 g Concentration from curve 0.028 ppm 105.5809

− 3.2569 40.1113 × 0.028 = 0.071

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