Determination of N-species in soil extracts using microplate techniques

• Published in: Talanta (Oxford) Vol. 74; no. 4; pp. 648 - 654
• Main Authors: Shand, Charles A., Williams, Berwyn L., Coutts, Grace
• Format: Journal Article
• Language: English
• Published: Amsterdam Elsevier B.V 15.01.2008; Oxford Elsevier
• Subjects: Ammonium; Analytical chemistry; Chemistry; Chromatographic methods and physical methods associated with chromatography; Colourimetric analysis; Diffusion of ammonia; Exact sciences and technology; Indexing in process; Nitrate; Organic nitrogen; Other chromatographic methods; Q1; Colourimetric analysis; Ammonium; Organic nitrogen; Nitrate; Diffusion of ammonia; Water; Chemical analysis; Tetrafluoroethylene polymer; Combustion; Nitrates; Sulfuric acid; Sample preparation; Azo dye; Test method; Oxidation; Diffusion; Hydrazine; Quantitative analysis; Organic compounds; Purification; Nitrogen; Ammonia; Soils; Ion chromatography; Nutrient; Nitrogen dioxide
• DOI: 10.1016/j.talanta.2007.06.039
• ISSN: 0039-9140; 1873-3573

Colourimetric methods for the determination of NO 3 −, NH 4 + and total N in water extracts of soils using 96-well microplate techniques are described. Nitrate was determined by azo dye formation after reduction to NO 2 − using a solution of hydrazine sulphate. Ammonium in the soil extracts was purified and concentrated by diffusion as NH 3 from small volumes (750 μL) of extract treated with MgO into a H 2SO 4 collector using a double-plate, MicroResp™ method and subsequently determined by the Berthelot reaction. For the determination of total N, samples were oxidised with K 2S 2O 8 at 110 °C in a 96 × 1.1 mL polytetrafluoroethylene block with a lid that closed individual wells. The oxidised solutions were transferred to standard plates for colourimetric analysis of NO 3 −. The recovery of N, measured as NO 3 −, from NH 4NO 3 and a range of organic-N compounds was >95%. The limits of quantitation of the colourimetic assays were 0.020 mg N L −1 for NO 3 − and 0.051 mg N L −1 for NH 4 +. The methods were tested on water extracts derived from a range of 10 nutrient poor soils from Scotland. There were acceptable linear correlations between the results obtained by established methods. For soil extracts analysed by the microplate method, the relationship for NO 3 − was 1.03× result from ion chromatography + 0.0055 ( R 2 = 0.9961); for NH 4 + determined by the microplate method, the relationship was 0.9696 × result from a discrete analyser − 0.0169 ( R 2 = 0.9757) and for total N determined by oxidation in the PTFE microplate the relationship was 0.9435 × result obtained by combustion + 0.0489 ( R 2 = 0.9743). Purification of the NH 4 + in water extracts from the 10 different soils by the diffusion method did not result in any systematic difference (paired t-test, p = 0.05) between measured concentration values determined before and after diffusion.