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This study into the SPLITS{Sampling Partitioning (Liquid) Irregularities in Turbid Solutions}effect is part of the Aqua- STEW (Water Quality Surveillance Techniques for Early Warning by Tensiographic Sensors) 5th Framework European project. The paper reports new experimental measurements into this effect using the fibre drop analyser. Capillary hydrodynamic fractionation has been previously been observed, but the drop analyser technique provides improved experimental characterisation of the effect. A description of the optical tensiograph system used is given with details of the five strategies designed to minimize this SPLITS effect. It has been observed that the sampling of the solution by a stepper-pump to the instrument drophead, leads to irregular concentrations in the delivery tubing and thence the drophead. The measured tensiotrace variations in successive microlitre samples delivered from the capillary have provided an insightful experimental approach for the study of this important effect. It has been found that the best approach for the minimization of these unwanted concentration irregularities is using high-speed aspiration. The paper ends with discussion on the general relevance of this SPLITS effect to other chemical techniques that must sample turbid solutions and analyses the specific issues posed for on-line water monitoring systems.

The drop analyser, also termed the tensiograph, is an optical fibre-based instrument system for monitoring liquids. A comprehensive assessment of the drop analyser used as a UV–visible spectrophotometer has been undertaken employing both experimental and theoretical studies. A model of the tensiograph signal (tensiotrace) has been developed using a ray-tracing approach to accurately predict the form of the tensiotrace as an aid to drop spectroscopy. An analytical equation is derived for quantitative drop spectroscopy and the form of the equation has been experimentally tested. The equation applies to both the case of a growing drop and the situation in which the drop volume is held stationary. Measurements on both stationary and moving drops are of practical value. Modelling has been used to compute the average path length of the coupled light in the drop to give a result that compares favourably with values obtained from experimental measurements. An optimized method has been identified for quantitative drop spectroscopy measurements. Results from UV–visible studies on both pollutants in water and pharmaceuticals demonstrate the utility of this approach. Two key matters relating to the practicalities of drop spectroscopy are then discussed. Some experimental studies have been made to ascertain the practical limit in analyte concentration above which variations in transmitted light from the drop shape variations result. Here, tabulated information on a representative range of liquid types has been provided as a guide to optimized spectroscopic drop analysis. Secondly, the handling of micro-volume samples is discussed. The paper concludes with a brief evaluation of the usefulness of this drop spectroscopy approach, but specifically points to the importance of drop spectroscopy for nanoscience applications.