Analytical methods and quality assurance

Abstract

The development of strategies to improve the efficiency of nutrient uptake by crops and to minimise leaching to surface and drainage water needs to be considered at various scales: catchment, field, soil and crop type. Therefore, methodological approaches for routine sample preparation and analysis are crucial to agricultural and environmental policies aimed at reducing nutrient losses. While the official way of analysing water is time consuming and requires a laboratory with standard equipment, the ion‐selective electrodes (ISE) technique presents a method with the advantages of precision, speed and portability. Analysis of water by ISE has become an accepted practice for the detection of certain elements and this technique could replace standard testing currently being used. Accuracy and reproducibility of NO₃ ^‐, Cl^‐, Ca⁺², K⁺ and Na⁺ ion‐selective electrodes were achieved by comparison with standard methods. The apparatus used consists of a sensor unit, which contains multielement ISE's, and a mesurement unit for operational and data storage functions. Samples of water, obtained from runoff and drainage of agricultural catchments, ground water, were selected from different locations to represent a wide range of ion concentrations. Using the ISE measurements values varied from 0.42 to 83.5 (mg L^‐1) for NO₃ ^‐, 3.89 to 241 for Cl^‐, 1.3 to 76.4 for Ca²⁺, 0.12 to 131 for K⁺ and 0.63 to 59 for Na⁺. The investigation confirms the good correlation between standard and ISE methods for almost all ions with correlation coefficients of 0.97 for NO₃ ^‐, 0.99 for Cl^‐, 0.96 for Ca²⁺, 0.98 for K⁺. Only Na⁺ coefficient is below 0.9. Variations in the slope and intercept of the regression equations were slight or small for NO₃ ^‐ and Ca²⁺, and larger for Cl^‐, K⁺ and Na⁺.