Brian A. McGaw

Phytoavailability of Cd and Zn in soil estimated by stable isotope exchange and chemical extraction

The distribution of labile Cd and Zn in two contrasting soils was investigated using isotopic exchange techniques and chemical extraction procedures. A sewage sludge amended soil from Great Billings (Northampton, UK) and an unamended soil of the Countesswells Association obtained locally (Aberdeen, UK) were used. 114Cd and 67Zn isotopes were added to a water suspension of each soil and the labile metal pool (E-value) determined from the isotope dilution. Samples were obtained at 13 time points from 1h to 50 days. For the sewage sludge amended soil, 29 μg Cd g−1 (86% of total) and 806 μg Zn g−1 (65% of total) were labile and for the Countesswells soil the value was 8.6 μg Zn g−1 (13% of total); limits of detection prevented a Cd E-value from being measured in this soil. The size of the labile metal pool was also measured by growing plants for 90 days and determining the isotopic content of the plant tissue (L-value). Thlaspi caerulescensJ. & C. Presl (alpine penny cress), a hyperaccumulator of Zn and Cd, Taraxacum officinale Weber (dandelion) and Hordeum vulgare L. (spring barley) were used. L-values were similar across species and lower than the E-values. On average the L-values were 23±0.8 μg Cd g−1 and 725±14 μg Zn g−1 for the Great Billings soil and 0.29±0.16 μg Cd g−1 and 7.3±0.3 μg Zn g−1 for the Countesswells soil. The extractable metal content of the soils was also quantified by extraction using 0.1 M NaNO3, 0.01 M CaCl2, 0.5 M NaOH, 0.43 M CH3COOH and 0.05 M EDTA at pH 7.0. Between 1.3 and 68% of the total Cd and between 1 and 50% of the total Zn in the Great Billings soil was extracted by these chemicals. For the Countesswells soil, between 6 and 83% of the total Cd and between 0.1 and 7% of the total Zn was extracted. 0.05 M EDTA and 0.43 M CH3COOH yielded the greatest concentrations for both soils but these were less than the isotopic estimates. On the whole, E-values were numerically closer to the L-values than the chemical extraction values. The use of isotopic exchange provides an alternative estimate of the labile metal pool within soils compared to existing chemical extraction procedures. No evidence was obtained that T. caerulescens is able to access metal within the soil not freely available to the other plants species. This has implications for long term remediation strategies using hyperaccumulating plant species, which are unlikely to have any impact on non-labile Cd and Zn in contaminated soil.

The comparison of capillary zone electrophoresis and atomic spectroscopy for the determination of the cation content of a standard reference material IAEA-A-11 milk powder.

Alkali and alkaline earth metals were separated and quantified by Capillary Zone Electrophoresis using a previously developed method. CZE combined with indirect detection has received considerable attention over the last few years. Methods for the determination of inorganic and organic cations and anions have been reported although few of these include quantitative application to real samples. All of these methods are characterized by high efficiencies and high capacity. Application of the method to the determination of the major cation content of an International Atomic Energy Agency (IAEA) standard reference material is described. Complete resolution and quantitation of the ions (Na(+), K(+), Ca(2+), Mg(2+)) was achieved with calibration curves, for the individual components, having correlation coefficients (r(2)) ranging from 0.996 to 0.999 and detection limits (two times the baseline noise) of 10 ppb for potassium and magnesium and of 2.5 ppb for sodium and 2 ppb for calcium were achieved. Comparable results were achieved when employing analysis by Atomic Spectroscopy. The accuracy of this method was tested by comparison with standard flame Atomic Absorption cation analysis. Statistical analysis of the instrumental results indicate that there is no evidence suggesting systematic differences between the methods. In addition, a number of potential advantages of CZE for cation analysis are discussed.

Stable isotope dilution—mass spectrometry for determining total selenium levels in plants, soils and sewage sludges

Quantitation of selenium in plants, soils and sludges was achieved by isotope dilution-mass spectrometry using a benchtop instrument. Samples for analysis were spiked with (76)Se isotope solution. Plant material was digested on a heating block at 150 degrees C using a mixture of nitric acid and hydrogen peroxide. Selenium in soils and sludges was released by treatment with nitric acid followed by digestion with nitric and hydrofluoric acids. Selenium in the digests was reduced to Se(IV) with hydrochloric acid and derivatised with nitro-1, 2-phenylenediamine to 5-nitropiazselenol. Analysis by gas chromatography-mass spectrometry using selected ion monitoring was validated using certified reference materials (CRMs) and gave results within the certified range with a low standard deviation. The CRMs plant (Chinese cabbage leaves) and soil (Chinese soil) were found to contain (+/-95% confidence limits) 0.091(+/-0.007) mug g(-1) and 1.67(+/- 0.04) mug g(-1)Se respectively. The certified values were 0.083(+/-0.008) mug g(-1) and 1.56(+/-0.12) mug g(-1) respectively. The selenium content of four different freely drained acid Scottish soils under grasslands was in the range 0.5-0.8 mug g(-1) air-dried soil. Sewage sludges were found to contain measurably more selenium than the soils, and samples of three sludges taken from sites in the UK contained between 1.1 and 3.5 mug g(-1) dry matter.

Sequential extraction of selenium from four Scottish soils and a sewage sludge

Abstract A sequential extraction procedure was used to assess the availability of selenium (Se) from four Scottish soils which may receive sewage sludge and a sewage sludge that may be applied to agricultural land. The procedure identified two exchangeable (available) and three non‐exchangeable Se fractions. The exchangeable fractions were defined as soluble (0.25M KCl) and ligand exchangeable (0.1M KH2PO4). The non‐exchangeable fractions were defined as acid‐extractable (4M HCl), oxidizable (dissolution in KClO3 and concentrated HNO3), and residual (dissolution in HF and HNO3). The fraction of Se defined as acid‐extractable is not readily available for plant uptake, but has the potential to become available through chemical and microbial mobilization. The total Se was also determined for each soil and the sewage sludge by a one step digestion procedure using HNO3 and HF acids. The determination of Se in the individual fractions was performed by isotope dilution‐mass spectrometry using 76Se. The total Se ...

Determination of Cd and Zn by isotope dilution-thermal ionisation mass spectrometry using a sequential analysis procedure

Isotope dilution-thermal ionisation mass spectrometry (ID-TIMS) was used to examine the certified Cd and Zn content of 4 Certified Reference Materials (CRMs); 2 soils: GBW07401 and GBW07405, 1 plant CRM060 and an animal tissue SRM1566a. The CRMs were chosen to be of contrasting origin and Cd:Zn content. Three digestion procedures were compared: (i) an open tube aqua regia procedure (ii) microwave digestion using Teflon bombs and (iii) hydrofluoric acid (HF) digestion using PTFE bombs. The Cd and Zn levels obtained using ID-TIMS all fell within the published certified range for the CRMs. This was the case regardless of the digestion procedure used, although HF digestion tended to yield marginally higher levels than the other procedures and in one instance, Cd in GBW07401, was significantly different (P<0.05) from the certified range. A filament loading procedure was developed, to allow sequential analysis of Cd and Zn on the same single filament during thermal ionisation mass spectrometry analysis. The sequential analysis technique was evaluated to ensure that Zn did not fractionate during Cd analysis and there was no inter-element interference. No marked difference in the precision and accuracy of the isotope ratio measurements were obtained from sequential element analyses on the same filament when compared to individual element analyses for a range of standard solutions or for sample digests. The most efficient procedure in terms of costs and productivity for future work of this kind would be a combination of microwave digestion and sequential analysis of Cd and Zn on the same filament.

Tracer studies of nutrient bioavailability using ‘naturally enriched’ 13C-labelled substrates

The stable carbon isotope 13 C is now used widely as a tracer in metabolic and clinical studies. Very low levels of isotope incorporation can be measured with great precision by modern isotope-ratio mass spectrometers. This allows the study of ‘naturally enriched’ foodstuffs formed as a result of isotope fractionation during photosynthesis; such materials are now being used to measure body tissue/store turnover and the bioavailability of nutrients in humans.