OMB Meeting Book - Jan. 11, 2018

J audzems : J ournal of AOAC I nternational V ol . 99, N o . 4, 2016  1115

Put the wash solution [ D ( f )] in the washing position if an auto sampler used. Ensure that the volume of wash solution is adequate. Under continuous stirring and without touching the electrode, titrate the sample solution automatically with 0.1M standardized silver nitrate solution [ C ( g ) or D ( a )] up to the end potential. The consumption of 0.1 M of silver nitrate solution [ C ( g ) or D ( a )] should be recorded automatically and can be read from the titrator software or documented in the titrator operating records. For manual titration, using a buret, add 0.1 M standardized silver nitrate solution [ C ( g ) or D ( a )] until the end potential has nearly been met. Continue to titrate slowly until the end point is met, as observed by the two small additions (about 0.05 mL) of silver nitrate solution. See Figures 201603A and 2016.03B . (c) Special case: determination of very low amounts of chloride. —When determining low chloride concentrations such as found in desalted whey powder, for greater precision, it is preferable to use a standardized 0.025 mol/L AgNO 3 [ D ( g )] solution for the titration. (d) Blank test: determination of reagent background content of chloride .—Perform a blank test using reagents, substituting water [ C ( a )] for the sample portion. The titrant consumption of the blank test obtained at the end point should be less than 0.05 mL when using the 0.1 M standardized silver nitrate, and less than 0.2 mL when using the 0.025 M standardized silver nitrate. Otherwise, check the reagents and water involved into the procedures and then perform the blank test again until the criterion is achieved.

to room temperature and dilute to volume with water. Filter using a 0.45 μm membrane disposable syringe before use.

E. Sample Preparation

(a) Milk product, infant formula, and adult/pediatric nutritional. —Mix well to ensure that sample is homogeneous. (b) Powder samples. —Reconstitute by dissolving 25 g powder sample in 200 g warm water (40°C).

F. Extraction

For high-protein samples requiring additional protein precipitation beyond that accomplished by addition of nitric acid solution, perform steps F ( a – c ), below. Otherwise, begin with step F ( d ). (a) Weigh an appropriate aliquot of RTF or reconstituted powder (e.g., 25 g, accurate to 0.1 mg) into a 50 mL centrifuge tube. Note : For samples with a high chloride content, weigh a smaller test portion, e.g., 5 g reconstituted or RTF product. (b) Transfer 2.5 mL precipitating solution I [ D ( c )] and 2.5 mL precipitating solution II [ D ( d )] into the tube. Dilute to 50 mL with water. Mix well. If foam impacts the constant volume, then one or two drops of defoaming agent [ C ( l )] should be added. (c) Centrifuge at 12000 × g for 5 min at 4°C, and then equilibrate to room temperature. (d) Accurately transfer either 10 mL supernatant from steps F ( a – c ) or weigh an appropriate aliquot of RTF or reconstituted powder (e.g., 25 g, accurate to 0.1 mg). Note : For samples with a high chloride content, weigh a smaller test portion, e.g., 5 g reconstituted or RTF product. Into a 120 mL sample beaker or autosampler cup, add 5 mL nitric acid solution [ D ( e )] and 50 mL water before titration. Add a magnetic stirring rod (if the titrator does not have a built-in rod stirrer). Place the autosampler cup or beaker onto a magnetic stirrer and stir until dissolved or finely suspended. (e) The pH of the test solution must be below 1.5. If in doubt, check pH by means of a pH meter and, if necessary, add a little more nitric acid solution [ D ( e )]. (a) Check and maintenance of the combined silver electrode .—Rinse electrode with deionized water and wipe before use. Renew the electrolyte with 1 M KNO 3 [ D ( i )] periodically per the manufacturer’s recommendations. If fat sticks to the electrodes during a series of analyses, then eliminate it by briefly immersing the electrode in acetone. The silver electrode must be stored in 1 M KNO 3 [ D ( i )] after appropriate cleaning. Note : Instead of the combined silver electrode, separate silver and reference electrodes may also be used. (b) Titration .—Connect the combined silver electrode to the titration apparatus according to the manufacturer’s indications. Ensure that the titration vessels are correctly placed on the autosampler and that there are enough reagents: both nitric acid solution {if added automatically [ D ( e )]} and 0.1 M AgNO 3 [ C ( g ) or D ( a )]. If no autosampler is available, then place the sample solutions manually under the titration equipment. G. Instrument Operating Conditions

H. System Suitability Test

Perform a system suitability test prior to use. (a) Transfer 5 mL NaCl solution [ C ( h ) or D ( b )] into a 120 mL sample beaker. If 0.025 M AgNO 3 titrant is required, then use 1 mL NaCl solution.

Figure 2016.03A. Automatic titration end point recognition using the dynamic titration mode on a Methohm Titrodo 905 titrator. U[mV], voltage of Ag electrode detected during titration; V[mV], volume of consumption of the standardized AgNO 3 titrant during titration; ERC, first derivative of the titration curve drawn by voltage of electrode versus volume of titrant consumption.

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