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356 T hiex : J ournal of AOAC I nternational V ol . 99, N o . 2, 2016

V = Volume (in mL) of respective extract collected. W = Total unground test portion weight in g. Calculations. —(An example calculation is provided in ref. 1.)

      

      

1 g 1000 mg

×

×

AC , mg L x ex ( )

V, mL 1000 mL

=

×

%AR

100

x ( ) ex

  

  

W, g A , g ×

t ( )

100 g

G. Expression of Results

Results for each extraction are presented as cumulative percentage of total nutrient. Extraction 1 (7 days) is considered water-soluble and not an SRF. However, slowly available water-soluble materials (low-MW urea formaldehydes and methylene ureas) may be present. These materials can be analyzed directly from Extract 1. Graphing release plots. —Plot the cumulative % of analyte (nutrient) released on the y -axis versus days of extraction on the x -axis as in Figure 2015.15B . (The example calculations are provided in ref. 1.) Alternative B: Accelerated 74 h Extraction at 25–60°C (a)  Covered water bath capable of maintaining a temperature of up to 60°C for extended periods. Ensure the mean temperature in the system is 50.0, 55.0, or 60.0 ± 1.0°C by monitoring incoming and exit temperatures to the manifold at comparable locations. Before Extractions 2–4 begin, it is necessary to preheat the bath several degrees ( see Extraction section below) above the desired temperature to account for initial heat exchange and temperature equilibration with manifold and columns. The bath should be stabilized at the desired temperature within 10 min. (b)  Reversible peristaltic pump capable of delivering 4.0 (±0.1) mL/min continuously for 54 h. Pump heads capable of using 16–40 tubes are used for an 8–20 column apparatus, respectively (Lsmatc ® No. 78006–00; Cole-Parmer, Vernon Hills, IL). (c)  Extraction apparatus consisting of two parts (illustrated in Figures 2015.15C – F ). Example equipment with sources can be found as a parts list in Appendix A of ref. 2 available on the J. AOAC Int . web site. H. Apparatus

Figure 2015.15C. Extraction apparatus with eight jacketed chromatography columns.

(d)  Vertical jacketed chromatography columns enclosing inner column of 2.5 × 30 cm (e.g., No. 5821–24, filter removed, with Teflon adapter No. 5838–51; Ace Glass, Vineland, NJ). PTFE rods (6 mm × 15 cm) should be used to avoid channeling of air or caking. Assure all fittings attaching column, pump tubes, and transfer tubing are secure to avoid leaks. Standardize the length of tubing for each column (typically about 75 cm). Example equipment with sources can be found as a parts list in Appendix A of ref. 2 available on the J. AOAC Int . web site. (e)  Constant temperature water circulation manifold and pump system capable of maintaining adequate (minimum 4 L/min) flow and stable temperature for each column. Insulation is typically required to maintain a stable temperature. Two inline, symmetrically placed thermometers (Figure 2015.15E ) are used to monitor temperature to input and outflow of manifolds. Attach roll clamps and flow monitors to column manifold tubing to ensure balanced flows and uniform temperatures. Example equipment with sources can be found as a parts list in Appendix A of ref. 2 available on the J. AOAC Int . web site. (f)  Solvent/extract reservoirs [500 mL volumetric flasks (e.g., Cat. No. 28100–500; Kimball Chase Life Science, Vinland, NJ)] with three-hole stoppers and properly placed rigid tubing attached to transfer tubing and to pump ( see Figure 2015.15E ). Ensure return tube remains approximately 2 cm from the bottom of the flask to prevent pickup of any precipitates.

Figure 2015.15B. Example release plot showing % N released over 180 days.

Figure 2015.15D. Schematic diagram of water manifold used in the extraction apparatus.