On-line coupling flow injection microcolumn separation and preconcentration to electrothermal atomic absorption spectrometry for determination of (ultra)trace selenite and selenate in water

A flow injection manifold with an air-segmented and air-transported operational sequence for on-line coupling of microcolumn separation and preconcentration to electro-thermal atomic absorption spectrometry (ETAAS) was developed for the determination of (ultra)trace selenite and selenate in water. The determination of selenite was achieved by selective reaction with pyrrolidine dithiocarbamate (PDC), sorption of the resultant Se-PDC compound onto a conical microcolumn (10.2 microL) packed with RP C18 sorbent, elution with ethanol, and detection by ETAAS. The concentration of selenate was obtained as the difference between the concentrations of selenite after and before prereduction of selenate to selenite. With the developed manifold and operation sequence,the dispersion during elution and eluate transport and the eluent volume required for complete elution of the sorbed analyte were minimized. As a result, the sorbed analyte was quantitatively eluted from the column with only 26 microL of ethanol, and all the eluate was automatically introduced into the graphite tube by an air flow without the need of preheating the graphite tube or precise timing. Pretreatment of the graphite tube with iridium as a long-term "permanent" modifier effectively prevented analyte loss arising from the high volatility of the Se-PDC compound and greatly improved the precision, sensitivity, and detection limit. One thermal pretreatment of the graphite tube with injection of 150 microgram of iridium made possible at least 200 repetitive atomization cycles. With a preconcentration time of 180 s and a sample flow rate of 1.4 mL min(-1), an enhancement factor of 112 was achieved in comparison with direct injection of 30 microL of aqueous solution. The detection limit (3s) was 4.5 ng L(-1)Se. The RSD (n = 7) was 3.8% at 20 ng L(-1)Se. The concentrations of selenite and selenate determined in synthetic aqueous mixtures were in good agreement with the expected values. The recoveries for selenite from spiked seawater samples ranged from 98 to 102%. The concentrations of selenite in several seawater reference materials obtained with simple aqueous standard solutions for calibration agreed well with the certified and information values, respectively. In addition, the developed method was successfully applied to the certification of selenite and selenate in water.