Abstract: Method for detecting a basic target species in a fuel composition, by (i) adding a spectroscopically active indicator which is capable of reacting with the target species, and (ii) detecting the spectroscopic response (suitably a colour change) of the fuel composition to the presence of the indicator. The method may be used to detect an additive such as a detergent or dispersant additive, in particular in an automotive gasoline or diesel fuel or a lubricating oil. Also provided is a kit for carrying out the method, in particular in the field. The indicator may be a phenolphthalein indicator such as tetrabromophenolphthalein ethyl ester (HTBPE).

Abstract: A Fischer-Tropsch derived fuel component is provided in a fuel composition to alter the spectroscopic response of the composition to the presence of a spectroscopically active indicator. The Fischer-Tropsch derived component may be used to increase the detectability of the indicator, and/or to increase the resolution of an analytical test subsequently carried out on the composition. Also provided is a method for investigating a property of a fuel composition, by adding a spectroscopically active indicator and detecting a subsequent spectroscopic response in the composition, wherein the composition contains a Fischer-Tropsch derived fuel component. Further provided is a fuel composition containing a spectroscopically active indicator and a Fischer-Tropsch derived fuel component. The spectroscopic response may be a visible response such as a colour change.