A new method was developed in this work to account for inorganic Sb species interconversion during soil sample preparation and subsequent separation steps. The Sb(III) and Sb(V) concentrations at each investigated step in the analytical procedure were determined using species specific spikes (121Sb(III) with 81.18% and 123Sb(V) with 74.04% enrichment). The spiking of these enriched isotopes species solutions was done separately before soil sample extraction and before HPLC separation. Simply by subtracting the final concentration of each species done by on-line isotope dilution (ID) from its concentration at different stages of the analytical procedure done by species specific ID, the influence of each step on species transformation can be estimated. After optimization, the extraction procedure for inorganic Sb species 6% Sb(III) (1.3 RSD, n = 3) and 43.2% Sb(V) (2.9% RSD, n = 3) as percent of total Sb were detected in the examined soil sample using online ID. Using the above described methodology we found that there was no reduction of Sb(V) to Sb(III) during sample preparation or species separation. While about 9.3% of extractable Sb (4.6% of total) was converted from Sb(III) to Sb(V) during the extraction step, no conversion during HPLC separation step was observed. By compensating for Sb(III) transformation during the sample preparation step; the extractable Sb(III) and Sb(V) as percent of total Sb yielded 10.6% and 38.7%, respectively.
Antimony and its compounds are considered to be priority pollutants by Environmental Protection Agency of the United States (USEPA) and European Union (EU) [
Current measurement procedures like On-line coupling of liquid chromatography with ICP-MS can provide selective, sensitive and rapid measurements for inorganic metallic speciation analysis, but it doesn’t necessarily guarantee accurate analytical results. Independent of the other effects, they are also presented in elemental speciation analysis, varying compositions in chromatographic peaks influence the ionization efficiency in the plasma and, therefore, hinder exact external calibration curve [
However, the sample preparation is the most challengeable step of the analytical speciation procedures preventing that reliable enough speciation results can be achieved routinely. This statement refers specially to the speciation of elements in the solid samples where the major difficulty is not the quantification itself but the quantitative extraction of those compounds from the complex solid sample matrix without degradation or species transformation [
Speciated isotope dilution mass spectrometry (SIDMS) takes a unique approach to speciated analysis that differs from traditional methods. Traditional speciation methods inherently measure the species after species conversion has occurred. SIDMS has been developed to address the correction for the species conversion. In SIDMS, each species is labeled with a different isotope-enriched spike in the corresponding species form. Thus, the interconversion that occur after spiking is traceable and can be corrected. While SIDMS maintains the advantages of IDMS, it is capable of correcting the degradation of the species or the interconversion between the species [
The soil sample examined in this work consisted of 20 subsamples and represented a road section between 50 - 100 m. The subsamples were taken from the soil surface (0 - 5 cm) near the edge of a highway (Vienna/A23) [
A standard Agilent Technologies HP4500 quadruple inductively coupled plasma mass spectrometry (QICP-MS) system was used with Babington for Sb speciation and a cooled quartz glass spray chamber was used. For the chromatographic module a quaternary HP 1050 series pump was used. Separation of Sb(III) and Sb(V) was carried out on a Hamilton PRP-X100 anion exchange column (150 mm × 4.6 mm i.d.) using 10 mmol·L−1 EDTA, 1 mmol·L−1 phthalic acid at pH 4.5 as a mobile phase of a f1ow rate of mL·min−1. Samples and standards were injected using an autosampler (HP 1050 Series) with a l00 µL injection loop. The chromatographic system was interfaced with an ICP-MS instrument using 250 mm of PEEK tubing (0.54 mm i.d.) to connect the column to the nebulizer. On-line isotope dilution was preformed by a continuous addition (0.5 mL of min−1) of the enriched spike was achieved by joining the spike solution line, pumped by a peristaltic pump equipped with a silicone tube (1.02 mm i.d.), with the HPLC effluent (1 mL·min−1) via a three-way valve. The mixed solution passed directly to the nebulizer of the ICP-MS. The “time-resolved analysis” mode of the ICP-MS software was used for the data acquisition. The integration times of Sb isotopes were 0.9 s per mass.
A Milli-Q (Millipore) purified water (18.2 MW·cm−1) was used for the preparation of all solutions. Extraction solutions were prepared from citric acid monohydrate (99.5% - 100.5% mm−1, Merck). The HPLC eluents were prepared with adequate concentrations by dissolving appropriate amounts phthalic acid (≥99.5% mm−1, Fluka) and EDTA disodium salt (99.0% - 101.0% mm−1, Baker). The pHs of these eluents were adjusted by adding ammonia solution (25%, Merck) or nitric acid (200 mmol·L−1). The resulting solutions were de-gassed before using. natSb(V) standard solution with natural isotopic abundance was prepared from antimony(V) oxide (99.995% mm−1, Aldrich). Sb(V) stock solution (1 g » 10 mg Sb) enriched with (94.2%) in 123Sb was obtained as antimony oxide (Sb2O5) [lot number H3451] from Euriso-top France. Procedure describes antimony oxide as source material for 123Sb(V) and natSb(V) illustrated elsewhere [
100 μL of extraction solution was injected in the HPLC using the autosampler. From the measured intensities of the corresponding isotopes 121Sb and 123Sb the isotope ratio R123/121(t) for every measured time was calculated. The isotope ratio chromatograms were converted into mass flow chromatograms by calculating the absolute Sb amount for each R123/121(t) on the chromatogram by applying on-line isotope described on our previous work [
Species-specific isotope dilution procedure which was used to determine the concentrations of inorganic Sb separately in two stages of the speciation analysis procedure is explained below:
1) Before extraction step 500 mg of a representative soil sample were weighed to the nearest 0.01 mg into a 10 mL screwed-cap polystyrene tube. This sample was spiked with 1.00 g of a calibrated isotope-enriched species 121Sb(III) 0.93 mg·g−1 or 123Sb(V) 2.91 mg·g−1 to quantify Sb(III) and Sb(V) at the spiking stage separately. Then soil and the spike solution were shaken manually and the suspension was left for 10 min in a refrigerator at less than 4˚C. The spike species and the original species were extracted using 10 ml of 100 mmol·L−1 citric acid solution using an ultrasonic bath (power 120 W) for 45 min where equilibration can be achieved. The solution was then centrifuged at a speed of 4000 rpm for 5 min. and filtered using a 0.45 μm syringe filter. The filtrate was stored in polystyrene vials in a refrigerator at a temperature ≤ 4˚C until the analysis was possible. Before analysis 3.0 g of spiked soil extract was diluted to 5.0 g using Milli-Q water. Then 100 μL of the diluted extraction solution was injected in the HPLC using autosampler. From the measured intensities of the corresponding isotopes 121Sb and 123Sb the isotope ratios R121/123 for Sb(III) and R123/121 for Sb(V) were calculated from the measured of the peak area ratios after mass bias corrections. In order to correct for mass bias a soil extract obtained in the same way as the spiked soil portion was used before each measurement where the soil extract was assumed to have a natural Sb isotopic composition. The corrected isotope ratios Sb(III)121/123 calculated from the chromatogram obtained for spiked soil sample with enriched 121Sb(III) were used to calculate Sb(III) concentration at this step. While Sb(V) concentration was calculated using the corrected isotope ratio of Sb(V)123/121 obtained from another portion of sample spiked with 123Sb(V). Calculations for element species concentrations are illustrated elsewhere [
2) After extraction and before HPLC separation Soil extraction was carried out in the same way as mentioned above for the spiked sample portions. 3.0 g of soil extract was spiked with 1.0 g of calibrated isotopeenriched species 121Sb(III) 0.93 mg·g−1 or 1.0 g of 123Sb(V) 2.91 mg·g−1 to quantify Sb(III) and Sb(V) and the spiked soil extract was diluted with highly purified water to 5.0 g. Then the determinations of species were done in a similar way to that used in case of the spiked soil portions (described above).