AOAC ERP Fertilizers - December 2017

OMAMAN-24 A /Single Laboratory Validation Expert Review Panel for Fertilizers September 2015

734  B ernius et al . : J ournal of AOAC I nternational V ol . 97, N o . 3, 2014

Table 2. Source of validation materials No. Sample Expected S, %

Source of material

Source/comments

1 2 3 4 5 6 7 8 9

2011-05 2011-08 2011-06 2011-01 2011-07

1.56 6.39

Magruder Magruder Magruder Magruder Magruder

Processed poultry manure complex forms of S Turf fertilizer blend, both elemental and sulfate S

11.95 13.22

Lawn and garden blend, all sulfate S Lawn and garden blend, all sulfate S

7.76

Homogenous agriculture fertilizer, 5% sulfate and 5% elemental S

)SO 4

24 90

USP Fisher Scientific

Homogenous material, used in alkaline soils, source all sulfate S

(NH 4

S 12-508 Cysteine

OISC

Raw sulfur recovered from oil refining, all elemental S

26.72

Sigma-Aldrich

Analytical grade,organic form S

Sulpomag 12-1773

22 26 18

OISC OISC

Langbenite, mined natural mineral fertilizer/all sulfate S

) 2

SO 3

12-591

Liquid fertilizer

10 11

(NH 4

K 2

SO 4

KY12289

KY Division of Regulatory Services

Homogenous material/all sulfate S

changed from high to low levels. Factors with extreme difference between high and low levels were noted. Magruder 2012-10 was the only matrix used in the trial. This material was selected as the most recent material available at the time of the trial. Source of S in Magruder 2012-10 is ammonium sulfate with an expected S value of 5.48%. The squared differences of each parameter were determined according to the Youden and Steiner Statistical Manual of the AOAC (7) and listed based on their value (Table 5). Accuracy Table 3 shows the comparison of the elementar vario Macro CUBE method to the known values for S. Recovery for all materials averaged between 94.29 and 125.94% S. Average recovery for S was 101.07%. Bias for S ranged between –1.082 and 2.013%. Recoveries for Samples 1–3 appear to be low, but they were within the error range for the current accepted method. At 125.94%, recovery of Sample 5 Magruder 2011-07 ran higher than expected but was within the range of laboratories that reported results to the program. All Magruder check sample S results of the proposed method agreed with those obtained Discussion

four replicates/day. Within-run repeatability and among-run repeatability of the method was expressed as the SD. Within-run repeatability includes variability associated with the instrument. Among-run repeatability includes all variables associated with the method (Table 4). ( d )  LOD and LOQ.— As suggested in the AOAC guideline (6) for single-laboratory validation (SLV), the LOD was calculated as the blank value plus three times the SD of the blank. The LOQ was calculated as the blank value plus 10 times the SD of the blank. LOQ is also referred to as “limit of determination.” LOD and LOQ of the proposed method were 47 and 106 µg S, respectively. For comparison purposes, the lowest absolute amount of S in the validation materials was in Sample 1. When we analyzed 50 mg of Sample 1 (with 1.56% S), the absolute S value was 780 µg, which exceeded the LOQ of the method. ( e )  Ruggedness trial.— A ruggedness trial with only eight determinations was used to explore the effects of the seven most important factors as described in the AOAC requirements for SLVs (6): no blank measurement, improper WO 3 addition, incorrect oxygen dosing, not wrapping sample in tin, improper combustion temperature, high and low sample weights, and broken oxygen dosing device. Each factor was assigned with reasonable high and low values. The effect of each factor was determined by the difference in S content when a factor was

Recovery, % AOAC Research Institute Expert Review Panel Use Only

Table 3. Accuracy (bias and recovery) determined by comparison to validation materials S content Sample % S Grand average % abs SD Grand average a % S Proposed method % abs Repeatability Bias

1 2 3 4 5 6 7 8 9

1.56 6.39

0.12 0.38 0.51 0.68 2.50

1.471 6.202 11.742 13.265

0.022 0.159 0.118 0.207 0.082 0.087 0.246 0.320 0.543 0.198

–0.089 –0.188 –0.208

94.29 97.05 98.25

11.95 13.22

0.045 2.013

100.34 125.94

7.76

9.773

24 90

Not known Not known Not known Not known Not known

23.964

–0.036

99.85

91

1.000 0.097 0.044

101.11 100.36 100.20

26.72

26.817 22.044 24.918

22 26

10

–1.082

95.83

11

18

Not known

17.755

0.138

–0.245

98.63

a  % abs = Absolute SD within runs.