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  731

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Determination of Total Sulfur in Fertilizers by High Temperature Combustion: Single-Laboratory Validation J ean B ernius elementar Americas, Mt. Laurel, NJ 08054 S abine K raus Elementar Analysensysteme GmbH, Hanau, Germany S andra H ughes elementar Americas, Mt. Laurel, NJ 08054 D ominik M argraf Elementar Analysensysteme GmbH, Hanau, Germany J ames B artos and N atalie N ewlon Office of Indiana State Chemist, Purdue University, 175 S. University St, West Lafayette, IN 47907-2063 H ans -P eter S ieper Elementar Analysensysteme GmbH, Hanau, Germany

a gravimetric technique using barium to precipitate the sulfate and carbon disulfide to solubilize the elemental S. This method is labor-intensive and requires handling of hazardous materials. As an alternative technique, combustion elemental analysis is automated and is a safe technique for S determination. The required instrumentation is already in use for nitrogen (N) determination in fertilizer, AOAC Method 993.13  (3), and is familiar to laboratories making these determinations. Combustion analyzers without absorption/desorption chromatography were investigated by the Office of the Indiana State Chemist (OISC) as an alternative to the gravimetric method. Matrix and concentration sensitivities posed several analytical challenges, especially when S determination for a wide range of fertilizer products or an unknown source was required. Burn rates, scrubber types, and the need for accelerants was a function of the fertilizer type. As a result, different scan shapes and areas were generated for different products, and the most accurate data were obtained when sample and calibration sources matched. For example, a pure ammonium sulfate [(NH 4 ) 2 SO 4 ] fertilizer source has a quick burn that requires no accelerant, whereas a blended fertilizer product containing potassium-magnesium sulfate (K 2 MgO 8 S 2 ) and potassium chloride (KCl) is a slow burn that may require an accelerant and a halogen scrubber. Another requirement was the separation of samples into low and high S concentrations because low concentration required a longer cell length that could be overwhelmed by high concentration samples resulting in carryover for the subsequent samples. As a result, samples were presorted into categories based upon S source, S concentration, and matrix to best match the calibration material and concentration range. If the S source was unknown, the sample was pretested and the scan compared to scans from various fertilizer sources to determine the best calibration material and concentration. For a company or laboratory that knows the source and history of the product, presorting of samples presents no problem but can be a challenge for a laboratory testing largely unknown samples. The objective of this work was to investigate an automated, matrix-independent method using less hazardous reagents than the current regulatory procedure that allows equally simple S

Guest edited as a special report from the AOAC Agricultural Community on “Collaborations in New and Improved Methods of Analysis for Plant Food Materials” by Nancy Thiex. Corresponding author’s e-mail: j_bernius@elementar-inc.com DOI: 10.5740/jaoacint.13-385 of low S diesel fuel. Changing agronomic practices such as no or low tilling and early planting also contribute to S deficiency (1). Increased use of S-containing fertilizers has driven demand for more S determinations, including sulfate and elemental S in blended fertilizers (Hall, B., Mosaic Fertilizer, 2013, personal communication). The current AOAC Method 980.02  (2) for S in fertilizer is A single-laboratory validation study was conducted for the determination of total sulfur (S) in a variety of common, inorganic fertilizers by combustion. The procedure involves conversion of S species into SO 2 through combustion at 1150°C, absorption then desorption from a purge and trap column, followed by measurement by a thermal conductivity detector. Eleven different validation materials were selected for study, which included four commercial fertilizer products, five fertilizers from the Magruder Check Sample Program, one reagent grade product, and one certified organic reference material. S content ranged between 1.47 and 91% as sulfate, thiosulfate, and elemental and organically bound S. Determinations of check samples were performed on 3 different days with four replicates/day. Determinations for non- Magruder samples were performed on 2 different days. Recoveries ranged from 94.3 to 125.9%. ABS SL absolute SD among runs ranged from 0.038 to 0.487%. Based on the accuracy and precision demonstrated here, it is recommended that this method be collaboratively studied for the determination of total S in fertilizers. S ulfur (S) deficiency in corn and other crops is becoming more prevalent. Less S is being deposited into the soil due to reductions in power plant emissions and increased use

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