Operational options to reduce matrix effects in liquid chromatography-electrospray ionisation-mass spectrometry analysis of aqueous environmental samples

Matrix effects like signal enhancement or suppression can severely compromise quantitative analysis of environmental samples with liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS). Several operational options were studied to reduce such matrix effects in the determination of polar organic trace contaminants from water, like non-steroidal anti-inflammatory drugs, among them ibuprofen, diclofenac and naproxen, lipid regulators like bezafibrate and clofibric acid and industrial chemicals (2-substituted benzothiazoles). A step-wise removal of organic matrix from a wastewater sample by ultrafiltration showed that the majority of matrix effects in that sample was due to low molecular weight compounds <1 kDa. For such wastewaters samples size-exclusion, as in restricted access material (RAM), is not a useful clean-up strategy. Reducing the eluent flow entering the ESI interface by post-column splitting increased instrumental sensitivity and reduced matrix effects. The flow optimum was analyte-dependent and ranged from 20 to 100 μL/min. Sensitivity in the positive ion mode increased up to nine-fold upon flow-reduction for some analytes detected in the positive ion mode. At low flow rates matrix effects are reduced by 45-60% on average. If moderate matrix effects occurred, post-column splitting may allow obtaining reliable quantitative data even with external calibration.