AOAC ERP Fertilizers - December 2017

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

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

( i )  Sulfanilamide .—Sigma-Aldrich (St. Louis, MO) as sample additive.

Table 1. Operating conditions Combustion furnance temp., ° C

1150

Reduction furnace temp., ° C

850

Test Sample Preparation

Oxygen pressure, bar

2.0–2.2

( a )  Source of materials .—For method validation experiments, S-containing materials were obtained from commercial sources, the Magruder Check Sample Program, and previously analyzed fertilizer samples provided by the OISC and Kentucky Division of Regulatory Services (Lexington, KY). The materials were chosen to represent a range of S concentrations and matrixes that make up 90% of fertilizer tonnage worldwide (Hall, B., Mosaic Fertilizer, personal communication, 2013). The concentration of the validation materials ranged from 1.47 to 91% S and included sulfate, thiosulfate, and blended elemental S and sulfate- containing fertilizers. To obtain materials for Magruder Check Sample Program go to www.magruderchecksample.org. ( b )  Sampling and sample preparation .—The liquid ammonium thiosulfate sample was collected using AOAC Method 969.01  (4). A field inspector collected approximately 500 mL sample from a tap point on a liquid bulk storage tank. After shaking the plastic field container well, approximately 50 mLwas poured into a clean 100 mL glass jar that served as the laboratory test portion. The remaining sample was retained for result confirmation or dispute resolution. The solid elemental S sample was collected using AOAC Method 929.01  (5). Multiple cores were taken from a bulk sample resulting in approximately a 2.3 kg subsample. The subsample was reduced further with a Carpco (Jacksonville, FL) SS16-25 gated riffler. Approximately 200 g sample was finely ground using a Retsch (Newtown, PA) ZM200 grinder and a 1 mm screen and then poured into a clean 100 mL glass jar. The glass jar was rotated multiple times to ensure sample uniformity before testing. An unground portion was retained for result confirmation or dispute resolution. All samples were stored at room temperature in restricted access, locked storage. The Magruder samples were used as prepared by the Check Sample Program supplier. All of these samples were stored at room temperature in sealed plastic pouches.

Helium pressure, mbar

1200–1250

Method for oxygen dosing

Sulf1, Sulf2, blank according to instrument software

determination in single species and complex blended fertilizer products.

Experimental

Principle The sample is wrapped in tin foil and dropped into the analyzer through a blank-free helium-purged ball valve. Oxygen is jet injected over the sample at 1150°C. Separation of combustion gases is performed using a thermal desorption purge and trap chromatographic method. Combustion gas components CO 2 , H 2 O, and SO 2 are adsorbed on three specific columns. N is not adsorbed and flows directly to the thermal conductivity detector (TCD). Based on the detector reading, gas components are released sequentially from their individual adsorption/desorption columns. Time of analysis for C, N, and S is 8 min/sample. S analysis is not available as a single analyte. ( a )  Analytical balance .—Resolution to 0.01 mg (Mettler- Toledo, Columbus, OH). ( b )  vario MACRO cube analyzer .—Elementar Analysensysteme GmbH (Hanau, Germany). ( c )  Tin foil and tin capsule sample containers .—Elementar Analysensysteme GmbH. ( d )  Sample hand press for powdered samples .—Elementar Analysensysteme GmbH. ( e )  Sample sealing press for liquid samples.— Elementar Analysensysteme GmbH. ) granulate .—Supplied by the instrument manufacturer (Elementar Analysensysteme GmbH). ( d )  Copper wires .—Supplied by the instrument manufacturer (Elementar Analysensysteme GmbH). ( e )  Corundum balls .—Supplied by the instrument manufacturer (Elementar Analysensysteme GmbH). ( f )  Quartz wool .—Supplied by the instrument manufacturer (Elementar Analysensysteme GmbH). ( g )  Silver wool .—Supplied by the instrument manufacturer (Elementar Analysensysteme GmbH). ( h )  Sicapent ® with indicator .—Supplied by the instrument manufacturer (Elementar Analysensysteme GmbH). Apparatus Reagents ( a )  Helium .—99.995% purity (Praxair, Danbury, CT). ( b )  Oxygen .—99.995% purity (Praxair). ( c )  Tungsten (VI) oxide (WO 3

Calibration Standard AOAC Research Institute Expert Review Panel Use Only Factory calibration of the combustion analyzer was tested daily using 99% purity sulfanilamide. Drift correction was applied by the instrument software. Operating Procedure

The elements C , H, N, and S bound in the sample burn to form CO 2 , H 2 O, N 2 , NO X , SO 2 , and SO 3 . Halogen bound in the sample reacts to form volatile halogen compounds within the combustion tube. WO 3 granulate delivers additional oxygen to prevent the formation of nonvolatile sulfates and to bind alkali and alkaline earth elements. The carrier gas flow transfers the gaseous combustion products into the reduction tube, where copper contact completely reduces NO X to N 2 . SO 3 is reduced to SO 2 . Volatile halogen compounds are bound on silver wool. H 2 O contained in the reaction gas mixture is bound in an absorption tube filled with Sicapent. N 2 gas bypasses the adsorption/desorption columns and passes directly to the TCD. When the N 2 peak comes down to baseline,