Ross et al. analysed samples of soil leachates from laboratory columns and of soil pore water from field porous cup lysimeters for aluminium by atomic absorption spectrometry under two sets of instrumental conditions. Aluminium standards were prepared and calibration curves used for the colorimetric quantification of aluminium. Nitrate in the sample solution is reduced to ammonia by titanous sulphate and the ammonia is then released from the solution and diffused and absorbed onto a nylon square impregnated with dilute sulphuric acid. The nylon is then put into a solution which colours quantitatively when ammonia is present and a spectrophotometer is used to measure the colour. The absorption signal for arsenic is dependent on the oxidation state of the element prior to the hydride-generation stage. The determination of cadmium by graphite furnace atomic absorption spectrometry is especially difficult because cadmium is a volatile element, and matrix constituents cannot be removed by charring without a loss of cadmium.