Determination of Cr(III) and Cr(VI) at Sub-ppb Levels in Water with Solid-Phase Extraction/Metal Furnace Atomic Absorption Spectrometry

A simple method using solid-phase extraction combined with metal furnace atomic absorption spectrometry was developed for the determination of Cr(III) and Cr(VI) at sub-ppb levels in water. A 500-ml water sample was adjusted to pH 3 with nitric acid and then passed through an iminodiacetate extraction disk placed on a cation-exchange extraction disk at a flow rate of 20 - 40 ml min-1 for concentrating Cr(III). The filtrate was adjusted to pH 10 with aqueous ammonia and then passed through an anion-exchange extraction disk at a flow rate of 2 ml min-1 for concentrating Cr(VI). The Cr(III) and Cr(VI) collected were eluted with 40 ml of 3 moll-1 nitric acid for Cr(III) and 40 ml of 1 gl-1 diphenylcarbazide solution for Cr(VI). Each eluate was diluted to 50 ml with deionized water and injected into a U-type tungsten board on the metal furnace. The calibration curves of Cr(III) and Cr(VI) showed good linearity in the range of 0.1 - 0.5 ng. The detection limits corresponding to three times the standard deviation (n = 5) of blank values were 8.1 pg for both Cr(III) and Cr(VI). The analytical value of total Cr (Cr(III) + Cr(VI)) in certified reference material of river water (JSAC 0302-3) was in good agreement with the reference value. The recovery test for 0.50 μg (1.00 μgl-1) of Cr(III) and Cr(VI) added to 500 ml of the water samples showed sufficient values (98.1 - 106%), except for river water sampled downstream due to relatively higher CODMn value. The relative standard deviations (n = 5) were less than 5% for both Cr(III) and Cr(VI).