Flow-injection interface for on-line coupling solid-phase extraction and X-ray fluorescence measurements

A flow-injection (FI) manifold including a solid-phase extraction (SPE) mini-column has been coupled on-line to an energy-dispersive X-ray fluorescence detector (EDXRFD) by locating a lab-modified 18-microL flow-cell, 10-mm path length, connected to the SPE column by PTFE tubing of 0.5 mm i.d., in the X-ray spectrometer chamber. The optical window of the flow-cell was adjusted and fixed to the X-ray irradiation zone of the spectrometer. Two PTFE tubes connected the flow-cell to the FI device and were introduced into the spectrometer chamber by a small orifice without distortion nor modification of the instrument. The SPE-EDXRFD coupling was tested for Pb and Cd aqueous solutions using Dowex 50 cation-exchange resin as sorbent, and flushing the eluate through the flow-cell for monitoring. The LODs and LOQs thus obtained were 1 and 3.2 microg for Pb and 1.8 and 4.8 microg for Cd, respectively; values which allow using the approach for the analysis of waste water by injecting 20 mL of sample into the FI manifold. The linear dynamic ranges are a function of the sample volume circulated through the mini-column. For a sample volume of 20 mL the ranges are between 1 and 4000 microg for Pb and between 1.8 and 2000 microg for Cd. The method was validated by the standard addition method using ground-water samples. The SPE-EDXRFD coupling enables to carry out the study of those variables influencing the SPE process--namely, the effect of the sample volume flushed through the column, concentration of analytes in the sample, amount of resin packed, breakthrough volume of the resin, elution profiles, sample pH and retention and elution flow-rates--in an automatic, cheap, fast and precise way.