Interference removal in the organic trace-level analysis of aqueous environmental samples by on-line liquid chromatographic preconcentration techniques with two precolumns

Abstract

On-line sample handling is used to enrich trace organic compounds in order to isolate and preconcentrate them prior to their liquid chromatographic separation. The main difficulty is that aqueous samples are complex matrices and therefore many interferences are also preconcentrated, giving rise to a high background. It is shown that matrix interferences are not a problem for determination at the ppb (μg/l) level in natural waters, but have to be removed to reach the 0.1 ppb level in drinking water (EEC requirement for many pesticides). Preconcentration is then optimized in terms of interference removal: two precolumns are coupled in series for the preconcentration step, the first acting as an interferent filter and the second trapping the analytes of interest. The methodology is different whether a specific sorbent for the analytes exists or not. Applications to the determination of moderately and rather polar herbicides (chlorotriazines and phenylureas) are presented. When no selective sorbent can be found, it is possible to optimize the fractionation between many rather apolar interferences and analytes by coupling two non-selective materials. Determination of polar herbicides is solved by coupling two precolumns, one packed with an alkylsilica (C 18 or C 8) and the other with a styrene—divinylbenzene copolymer (PRP-1). The choice of the n-alkylsilica (carbon loading and chain length) is then important and is directly related to the solute polarity because the solutes must be slightly retained to have recoveries as high as possible on the PRP-1 precolumn. When a selective sorbent such as an ion exchanger can be selected, inorganic and organic interferences are efficiently removed by a two-step preconcentration and a first PRP-1 precolumn. Detection limits are therefore below 0.1 μg/l.