Dispersive liquid-liquid micro-extraction for simultaneous preconcentration of 14 lanthanides at parts per trillion levels from groundwater and determination using a micro-flow nebulizer in inductively coupled plasma-quadrupole mass spectrometry

This paper reports on the simultaneous extraction and preconcentration of 14 lanthanide (rare earth) elements in groundwater by a dispersive liquid-liquid microextraction (DLLME) method and their determination by inductively coupled plasma quadrupole mass spectrometry (ICP-QMS). A low flow rate (200 mu L min(-1)) SeaSpray (TM) micro-flow nebulizer was used for the sample introduction. In this method, the rare earth elements are complexed with 2,6-pyridinedicarboxylic acid (2,6-PDCA) in the presence of Aliquat (R) 336 (tricaprylmethylammonium chloride), which enhanced the hydrophobicity of the ion-association complex, resulting in its improved extraction into chloroform. The extraction was carried out after adjusting the pH of the water sample to 4. The rare earth ions were back extracted from the chloroform layer with nitric acid for determination by ICP-QMS. Some effective parameters for complex formation and extraction, such as volume of extractant/disperser solvent, extraction time, pH and concentration of the chelating agent and surfactant, have been optimized. Under optimum conditions, an average preconcentration factor of 97 was obtained for 50 mL of water sample for determination by ICP-QMS. The calibration graphs were linear in the range of 1-100 ng L-1 for the 14 REEs, with limits of detection ranging from 0.05-0.55 ng L-1. The precision ranged from 1-5% R.S.D. (n = 3), when processing 50 mL aliquots of groundwater. The method has been applied to a few groundwater samples. The recoveries obtained for the rare earth elements in groundwater samples spiked to 10 ng L-1 were 92-109%.