AOAC ERP Fertilizers - March 2019

3/14/2019

AOAC EXPERT REVIEW PANEL FOR FERTILIZERS

• Method “updates” – Weight change and/or Empirical Calibration • Recap: High C/organic solvent and high P “strain” the plasma’s efficiency, which can result in a low bias for the  higher Phosphate fertilizer products  (note this is less of a problem for lower P guarantees < 40% P 2 O 5 ) • Original method: • Higher power  setting > 1400 W • On‐line dilution with smaller sample pump tube and larger internal standard pump tube • New: • Weigh higher guarantees at a lower weight • For guarantees > 40% P 2 O 5 and/or K 2 O, weigh sample at 0.4 g  rather than 0.5 g • Empirical calibration • The need for this may be based upon introduction and/or plasma efficiency and instrument type • Perform high P calibration with fertilizer extracts  rather than “pure” standards • Make sure fertilizer calibration source(s) is accurate: Magruder Check Sample Program? UPDATE ON ADDITIONAL VALIDATIONS AND NEXT STEPS FOR THE METHOD 

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AOAC EXPERT REVIEW PANEL FOR FERTILIZERS

UPDATE ON ADDITIONAL VALIDATIONS AND NEXT STEPS FOR THE METHOD

• Method “updates” – Empirical Calibration • Helps compensate for reduction of signal  or sensitivity with higher P concentrations • Additional options with Empirical • Could fit a quadratic? • Could split curve into segments? • Could use weight fitting • Down side: • Need to make fertilizer based  calibration standards  fresh every time • shelf‐life ~ 16 hours • Multiple options lend to potential for  higher reproducibility or inter‐lab  variability of data

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