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

d) Add approximately 2 mL of concentrated hydrochloric acid. Stir on magnetic stirrer 10 minutes to mix the sample. Adjust speed of stirring so that the sample is well mixed without splattering. e) Add approximately 50 mL of the sodium citrate buffer to the sample and swirl to mix. Remove stir bar, rinsing well into cup. f) Allow sample to sit for a minimum of five minutes. Record the weight of the cup and diluted sample to 0.0001 g. 2. Standard and Sample Measurements Note: Samples and standards must be at the same temperature before analysis. The magnetic stirrer will generate enough heat to change the temperature of the solution and affect the potential reading. Place insulating material, such as Styrofoam, between the beaker and the stirrer to prevent heat transfer. Samples are stirred to reduce the electrode response time. The optimum stirring rate is fast enough to mix the sample but not create a vortex. Air bubbles that adhere to the Lanthanum Fluoride crystal will affect the stability of the reading. Two calibration curves are used with this method. Separate calibration curves must be set up for each electrode used. a) Working from highest to lowest concentration, pour the standard into a plastic beaker or sample cup, add a Teflon stir bar, and place on the magnetic stirrer. Immerse the electrode pair into the solution. Gently shake or tap the electrode if needed to remove any bubbles clinging to the bottom of the electrode. b) Program the meter for automatic readings using the strict endpoint criteria. If this feature is not available, equilibrate the electrodes for a minimum of 5 minutes (set a timer.) c) Record the mV response. d) Rinse the electrode pair with laboratory water, and blot dry with a Kimwipe or equivalent wiper. e) Repeat steps (a-e) for each standard. f) Calculate the calibration curve statistics and check against acceptance criteria (see F.1.) before analyzing any samples. 3. Samples are analyzed in a similar manner. If a sample reading falls outside the calibration curve by more than 2 mV, the sample must be prepped again using a larger or smaller sample size so that the sample potential falls within the curve limit.

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