AOAC ERP Fertilizers - December 2017

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

B ernius et al .: J ournal of AOAC I nternational V ol . 97, N o . 3, 2014  733

the adsorption column is heated to CO 2

and

then SO 2

desorption

material. Reference materials can include a fine chemical of known S content such as sulfanilamide, a Magruder check sample, or a National Institute of Standards and Technology reference material. ( d ) Perform replicate analysis on at least 10% of the samples, or every sample if homogeneity is difficult to achieve. Replicate results should be within 10–15% of the mean value of the replicates. This method was validated in a single laboratory by a single analyst. Validation experiments were conducted using 11 materials including five from the Magruder Fertilizer Check Sample Program, four previously analyzed fertilizer samples from the OISC and Kentucky Division of Regulatory Services, and two commercial S-containing materials. For the Magruder samples, the grand average of results, after outlier removal, obtained for each analyte/method combination on a check sample material was considered a consensus value. The consensus values were used in the absence of reference materials to validate the method. Participating laboratories submitted their results to the Association of American Plant Food Control Officials Magruder Program, and the grand average (minus outliers) for each analyte/method combination was publicly posted. Label values for ammonium sulfate U.S. Pharmacopeia grade and O-acetyl-l-serine cysteine were used. Previously determined S concentrations were used for ammonium thiosulfate 12-1591, sulpomag 12-1773, and elemental S 12-0508 as determined by the OISC. Previously determined S concentration was used for potassium sulfate 12289 as determined by the Kentucky Division of Regulatory Services (Table 2). ( a )  Linearity and range.— The instrument was calibrated according to the manufacturer’s instruction utilizing a total of 32 different sulfanilamide (purity ≥99%) and phenylalanine (purity ≥99%) samples of known weight. The calibration covered 19.73–11190.62 µg absolute element content for S. For a 30 mg sample, this correlates to 660 ppm to 37.3% S. The calibration functions for S content are best described on the basis of fourth- order polynomial functions. SD of the method for S was 11.73 µg. ( b )  Accuracy.— Matrix effects were investigated using three sulfanilic acid samples with an average sample weight of 30 mg. A recovery of >99% was achieved. The accuracy of the method was determined by comparing results to the accepted published values of the check samples, previously analyzed samples, and label values. The accepted value of the check sample (usually called the grand average) was calculated from results submitted to the respective check sample program managers from the dozens of laboratories participating, using statistical packages established by the Magruder Program. The difference between the grand average or previously reported results and the results obtained by the proposed method was reported as both a recovery and a bias, where a negative value indicates the method average is lower than the grand average and a positive number indicates the method average is higher than the grand average. Recovery and bias were calculated for each validation material as compared to the published available S grand averages (Table 3). (c )  Precision.— Each Magruder check sample validation material was analyzed on 3 different days with four replicates/ day. Other materials were analyzed on 2 different days with Validation of the Method

and H 2

O adsorption columns are bypassed

temperatures. The CO 2

via two valves. An absorption tube removes traces of H 2 is desorbed and enters the detector with the carrier gas. A fan cools the adsorption column, making it ready for the next analysis. The software reports C, N, and S but since C is not of interest, analysts can disregard C results. N results may be of interest. The N method conforms to AOAC Method 993.13 (3). Caution : During operation, the furnaces, the valve area, and the quartz glass bridge inside the instrument reach very high temperatures (in excess of 1000°C). Even after switching off the instrument, these components stay hot for long periods of time. Serious burns can occur if working inappropriately inside the instrument. ( a ) Weigh 26–50 mg samples into tin foil cones or tin capsules. Add between 260 and 500 mg WO 3 combustion aid for a 10:1 ratio of aid to sample. Seal with appropriate sealing device. ( b ) Turn on the vario MACRO cube analyzer. Conduct a leak check. Ensure combustion, reduction, and drying tubes do not need replacement. When furnace temperatures are up to required values, analysis may begin. ( c ) Load carousel with start-up samples: three blanks, three run in or conditioning samples using sulfanilamide, and three standard samples using 15 mg sulfanilamide. Include a sulfanilamide standard sample after every 10 samples and as the last sample in the batch to check for drift in the calibration. ( d ) Analyze standards and samples while monitoring instrument conditions for proper gas flows, restrictions, leaks, and consumable reagent life. O from the gas stream. SO 2 Calibration is performed by analyzing samples of nonhygroscopic fine chemicals with known concentrations (standard samples) in which the measured peak areas are correlated with the corresponding absolute element content. In a standard sample, the following parameters are known: element concentration (content, %) and weight. Absolute element content is computed according to the following equation: A = WC/100 where A= absolute element content in mg, W = sample weight in mg, and C = element concentration in percentage. ( a ) Blank determination should be performed before daily operation and any time a reagent or carrier gas tank is replaced. ( b ) Drift correction using 15 mg sulfanilamide standard should be performed before each day’s analyses. If the drift correction or daily factor exceeds 0.9 or 1.1, maintenance must be performed to check for leaks or that reagents in the combustion or reduction tubes are not spent. ( c ) Include one reference material with each batch of 30 samples. Results should be within limits specified for reference Quality Control Operating Conditions See Table 1. Calculations

AOAC Research Institute Expert Review Panel Use Only