J. Environ. Qual. (2001) 30:2206-2209

Elisa M. D’Angelo*1, J. Crutchfield 2 and M. Vandiviere1

1University of Kentucky, Soil & Water Biogeochemistry Laboratory, Department of Agronomy, N-122 Agricultural Science Building North, Lexington, KY 40546-0091.

2University of Kentucky, Analytical Services Laboratory, Department of Agronomy, S-104 Agricultural Science Building North, Lexington, KY 40546-0091


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A rapid and sensitive analysis of inorganic and organic phosphorus (P) is needed to analyze water and soil extracts at submicromolar concentrations. The proposed method, based on the complexation of malachite green with phosphomolybdate under acidic conditions, was adapted to 96-well microtiter plate format, and was tested for matrix interferences using 15 soils and common extractants, including water, KCl, CaCl2, NaOH, and HCl. Phosphorus determination was affected when CaCl2 and HCl concentrations were greater than 0.1 M and when NaOH concentration exceeded 0.4 M. Potassium chloride concentration up to 1 M did not interfere with P determination. The molar absorptivity was 46,851 M-1 cm-1 and the reagent blank absorbance was 0.069 ± 0.0044 (n=10). With a microplate pathlength of 1 cm and limiting absorbance of 0.0044, the detection limit of the proposed method was 9.4 ´ 10-8 M (0.003 mg P L-1). Recovery of added inorganic P in different types of soils and extracts ranged between 90-107% with an average of 100%. The proposed microplate method allows P to be determined rapidly and with a wide range of soil types and extracts and requires limited volume (20-200 µL). The procedure uses limited quantities (40 µL) of two stable reagents (>1 yr), and generates low amounts of hazardous waste.

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