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Collect a primary field sample using one of the recommended AOAC sampling procedures
(
929.01
,
969.01
, or
992.33
) or other recognized protocol. Prepare solid materials by riffling
[Alternative B, A(
d
)] the entire 5 lb (2300 g) laboratory sample three times before further riffling
down to an approximate 100 g subsample. The entire 100 g subsample is then ground
[Alternative B, A(
e
)] to pass a Tyler-35 mesh sieve (U.S. Standard Sieve Size No. 40, 0.420 mm or
0.165 in. opening; Fisherbrand stainless steel; Fisher Scientific, Pittsburgh, PA). The ground
analytical sample is placed into a 1 quart (0.946 L) glass jar and mixed by careful rotation and
inversion. Use a coring device or spoon to transfer sufficient mass to fill a 3 ounce (90 mL) glass
jar approximately half full. For liquid materials, shake the laboratory sample vigorously to
ensure thorough mixing, and then pour sufficient volume into a 3 oz (90 mL) glass jar to fill it
approximately half full. Test portions for analyses are taken from the 3 oz (90 mL) jar after
inverting and rotating the jar to ensure contents are thoroughly mixed. Other validated sample
preparation techniques that result in a representative and homogeneous test portion are also
acceptable.
E.
Extraction (Alternative B)
Weigh a 0.5 ± 0.01 g fertilizer test portion and completely transfer to a 250 mL class A
volumetric flask. Slowly add 30 mL of deionized (or equivalent) water to each flask. Dispense
10 mL o f 4 M HCl digestion solution [Alternative B, B(
m
)] into the flask. Place flasks on a
preheated hotplate and gently boil for 15 ± 1 min. Remove individual flasks that have boiled
for 15 ± 1min and allow them to cool to room temperature (20 to 25°C). Bring flasks to
volume with deionized (or equivalent) water. Filter any test solution containing suspended
debris using P and K free filters. The final acid strength of the test solution is approximately
0.16 M HCl, so any test solutions requiring dilution should be prepared in 0.16 M HCl. Due to
a limited shelf life, all analyses should occur within 2 weeks of digestion. After repeated
heating and cooling cycles of the 250 mL volumetric flasks, check the calibration of the flasks
by adding 250 g of 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.
F.
ICP-OES Conditions
Limit the deviation from a test portion weight of 0.5 g to ±0.025 g. Also, K is sensitive to
nebulizer pressure/flow, so closely monitor the nebulizer condition, as they can deteriorate over
Candidates for 2016 Method of the Year
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