AOACRIFeedsFertilzerMethods-2017Awards

918 T hiex : J ournal of aoaC i nTernaTional V ol . 99, n o . 4, 2016

Table 2015.18D . Monitor the rinse time and buffer concentration closely, because they are sensitive to change (1). ICP-OES instruments differ in their design and options, so minor adjustment to the conditions listed in Table 2015.18D may be necessary; however, any adjustments to these conditions must be performance based and validated. Special attention should be paid to the recovery of P in fertilizer concentrates or fertilizers containing ≥40% P 2 O 5 , because these materials pose the greatest need for optimal instrument performance. Several variables exist in the instrument software for data reporting, including units, test portion weight, test solution volume, and dilution factor. The calibration standards are prepared as micrograms per milliliter P and K, and the final fertilizer results are reported as percentage P 2 O 5 and K 2 O, which requires the following two calculations, respectively: P 2 O 5 , % = [P × (250/W) × 142/(31.0 × 2)]/10000 where P is the ICP-OES P reading in micrograms per milliliter, 250 is the final volume in milliliters, W is the test portion weight in grams, 142 is the FW of P 2 O 5 , 31.0 is the FW of P, 2 is the mole ratio of P 2 O 5 to P, and 10000 is the conversion of percentage to micrograms per milliliter; and K 2 O, % = [K × (250/W) × 94.2/(39.1 × 2)]/10000 where K is the ICP-OES K reading in micrograms per milliliter, 250 is the final volume in milliliters, W is the test portion weight in grams, 94.2 is the FW of K 2 O, 39.1 is the FW of K, 2 is the mole ratio of K 2 O to K, and 10 000 is the conversion of percentage to micrograms per milliliter. Alternatively, the standards can be entered as equivalent theoretical percentages of P 2 O 5 and K 2 O in solution values, listed in Tables 2015.18A and 2015.18B . When empirical calibration [ see Alternative A , section D(d) ] is used, conversion of the percentage P 2 O 5 in the certified or consensus material to milligrams per liter P in the calibration solution is obtained by using the following equation: P, g mL P O 10,000 W 250 31.0 2 142 2 5 ( ) ( ) ( ) µ = % × × × × where P, μg/mL is the P concentration in the extracted standard solution; % P 2 O 5 is the certified or consensus value, 10000 is the conversion of percentage to micrograms per milliliter, W is the test portion weight in grams, 250 is the final volume in milliliters, 31 is the FW of P, 2 is the mole ratio of P 2 O 5 /P, and 142 is the FW of P 2 O 5 . Relative to other AOAC Methods ( 960.03 , 978.01 , and 993.01 ), the ICP-OES method can produce lower P recoveries and/or greater data variability (http://www .magruderchecksample.org). Critical factors and common error sources are included here. For P, three issues are critical: addressing matrix challenges, implementing robust plasma conditions, and utilizing proper standards. Carbon in the citrate and EDTA will reduce the plasma efficiency, so it must be addressed. Diluting the matrix by using a smaller sample pump H. Calculations I. Comments

and mix by careful rotation and inversion. For liquid materials, shake the laboratory sample vigorously to thoroughly mix. Invert and rotate the container again (for solid materials) or shake (for liquids) immediately before selecting a test portion. Other validated sample preparation techniques that result in a representative test portion are also acceptable. When the analytical sample is split or the mass is reduced for any reason, the splitting process should be validated to not introduce unintended sampling error. Weigh a ~0.5 g test portion to the nearest 0.01 g ( see Alternative A , section E ) and completely transfer to a 250 mL wide-mouth class A volumetric flask. Dispense 100 mL 65 ± 2°C preheated citrate–EDTA extraction solution [ see Alternative A , section C(m) ] into each flask and insert a rubber stopper. Shake test solutions in a 65 ± 2°C preheated water bath set to approximately 200 reciprocations/min for 60 ± 1 min, remove from the water bath, allow to cool to room temperature (20–25°C), dilute to volume with deionized (or equivalent) water, stopper, and mix. Filter any test solution containing suspended debris using P- and K-free filters. Due to a very limited shelf life, analyze test solutions within 16 h of extraction. After repeated heating and cooling cycles of the 250 mL volumetric flasks, check the calibration of the flasks by adding 250 g deionized (or equivalent) water and verify that the volume is at the meniscus. When a flask loses calibration, either use the corrected volume established by water weight, or discard it. Table 2015.18D. Final ICP-OES conditions used for citrate–EDTA-soluble P and K validation Factor Setting Power, kW 1.45 Plasma flow, L/min 19.5 Auxiliary flow, L/min 2.25 Nebulizer pressure, L/min 0.7 Nebulizer type Seaspray Spray chamber Cyclonic Sample pump tube Black/black a Buffer/internal standard pump tube Gray/gray a CsCl ionic buffer concn, M 0.018 Internal standard and concn, μg/mL 10 Buffer matrix 4% nitric acid Exposure length, s 10 No. of exposures 3 Rinse time, s 35 Total analysis time, min 2 a   An orange/white sample pump tube and a red/red buffer/internal  standard pump tube provide approximately the same dilution factor, but  use less volume of solution. Ensure that a sufficiently large waste pump  tube is used to prevent flooding of the spray chamber. F. Extraction

G. ICP-OES Conditions

The optimal instrument conditions identified during method validation of citrate–EDTA-soluble P and K are listed in