Preparation of a Reference Material for the Determination of Hexavalent Chromium in Tap Water.

We developed a reference material (RM) for the determination of hexavalent chromium (Cr(VI)) in tap water. The tap water RM was prepared by adding a Cr(VI) standard solution to the raw material without acidification, i.e., under the original pH conditions of 7.6, because the decrease in the concentration of Cr(VI) was observed when the tap water had been adjusted to pH 1 with HNO3. The prepared tap-water RM (2 L) was packed in 10 fluororesin (PFA) bottles with an inside plug (200 mL each). Each PFA bottle (Cr(VI)-containing tap water) was sealed in a reclosable poly bag and then stored at 5°C in a refrigerator. The tap water RM had a Cr(VI) concentration of 51 µg L-1. The concentration of Cr(VI) was determined by diphenylcarbazide absorptiometry using a 100-mm quartz cell. The detection limit of Cr(VI) in the sample solution corresponding to three-times the standard deviation (n = 5) of blank values was 0.51 µg L-1. The homogeneity of Cr(VI) in the tap water RM was evaluated by an analysis of the variance after the Cochran test. There was no significant difference between the within-bottle and between-bottle variances of the analytical results, indicating that the tap water RM was sufficiently homogeneous. The stability of Cr(VI) in the tap water RM was investigated by monitoring the Cr(VI) concentration over a period of 6 months. The slope of the regression line of the Cr(VI) concentration versus the storage time did not significantly differ from zero, indicating that the tap water RM was stable for 6 months. The concentrations (50 - 51 µg L-1) of Cr(VI) in the tap water RM were in good agreement with the total chromium concentrations (50 - 51 µg L-1) obtained by atomic absorption spectrometry.

Speciation of inorganic arsenic in drinking water by wavelength-dispersive X-ray fluorescence spectrometry after in situ preconcentration with miniature solid-phase extraction disks.

A rapid and simple method using wavelength-dispersive X-ray fluorescence (WDXRF) spectrometry after in situ solid-phase extraction (SPE) was developed for the speciation and evaluation of the concentration of inorganic arsenic (As) in drinking water. The method involves the simultaneous collection of As(III) and As(V) using 13 mm ϕ SPE miniature disks. The removal of Pb(2+) from the sample water was first conducted to avoid the overlapping PbLα and AsKα spectra on the XRF spectrum. To this end, a 50 mL aqueous sample (pH 5-9) was passed through an iminodiacetate chelating disk. The filtrate was adjusted to pH 2-3 with HCl, and then ammonium pyrrolidine dithiocarbamate solution was added. The solution was passed through a hydrophilic polytetrafluoroethylene filter placed on a Zr and Ca loaded cation-exchange disk at a flow rate of 12.5 mL min(-1) to separate As(III)-pyrrolidine dithiocarbamate complex and As(V). Each SPE disk was affixed to an acrylic plate using adhesive cellophane tape, and then examined by WDXRF spectrometry. The detection limits of As(III) and As(V) were 0.8 and 0.6 µg L(-1), respectively. The proposed method was successfully applied to screening for As speciation and concentration evaluation in spring water and well water.

Determination of diphenylarsinic acid, phenylarsonic acid and inorganic arsenic in drinking water by graphite-furnace atomic-absorption spectrometry after simultaneous separation and preconcentration with solid-phase extraction disks.

A simple method of graphite-furnace atomic-absorption spectrometry (GFAAS) after solid-phase extraction (SPE) was developed for the determination of diphenylarsinic acid (DPAA), phenylarsonic acid (PAA), and inorganic arsenic (iAs) in drinking water. This method involves the simultaneous collection of DPAA, PAA, and iAs using three stacked SPE disks, i.e., an Empore SDB-XD disk (the upper layer), an activated carbon disk (the middle layer), and a Cation-SR disk loaded with Zr and Ca (ZrCa-CED; the lower layer). A 200-mL aqueous sample was adjusted to pH 3 with nitric acid and passed through the SPE disks at a flow rate of 15 mL min(-1), to concentrate DPAA on the SDB-XD disk, PAA on the activated carbon disk, and iAs on the ZrCa-CED. The As compounds were eluted from the disks with 10 mL of ethanol containing 0.5 mol L(-1) ammonia solution for DPAA, 20 mL of 1 mol L(-1) ammonia solution for PAA, and 20 mL of 6 mol L(-1) hydrochloric acid for iAs. The eluates of DPAA, PAA, and iAs were diluted to 20, 25, and 25 mL, respectively, with deionized water, and then analyzed by GFAAS. The detection limits of As (three-times the standard deviation (n = 3) of the blank values) were 0.13 and 0.16 µg L(-1) at enrichment factors of 10 and 8, respectively, using a 200-mL water sample. Spike tests with 2 µg (10 µg L(-1)) of DPAA, PAA, and iAs in 200 mL of tap water and bottled drinking water showed good recoveries (96.1-103.8%).

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 mol l(-1) nitric acid for Cr(III) and 40 ml of 1 g l(-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 µg l(-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 COD(Mn) value. The relative standard deviations (n = 5) were less than 5% for both Cr(III) and Cr(VI).