Peter B. Stockwell

Atomic Fluorescence Spectrometry: a suitable detection technique in speciation studies for arsenic, selenium, antimony and mercury

Atomic Fluorescence Spectrometry (AFS) is an ideal detection technique for speciation studies concerning hydride forming elements (mainly As, Se and Sb) and Hg. The analytical features of AFS, such as detection limits below the µg L−1 and the wide linear calibration range, up to the mg L−1, allow its application to a great variety of environmental, biological and food samples. AFS represents a suitable alternative to other atomic spectrometers commonly employed in speciation studies such as Atomic Absorption Spectrometry (AAS) and Inductively Coupled Plasma-Mass Spectrometry (ICP-MS). The instrumentation used for AFS and the design of the vapour generation and optical layouts required to sustain the full benefits of the AFS approach are also described. The present review explains and comments on the instrumental couplings of chromatographic (HPLC and GC) and non-chromatographic separations (CE) with AFS detection, with online hydride generation for the speciation of inorganic and organic compounds of As, Se and Sb, and cold vapour for Hg. Other optional intermediate steps are online photo-oxidation (UV), pyrolysis or Microwave Assisted Digestion (MAD) for non-directly reducible compounds. Many different sample types (e.g. water, soils, air, biota, food) have been analysed using these instrumental couplings with AFS detection. These are summarised and discussed.

Determination of mercury, selenium, bismuth, arsenic and antimony in human hair by microwave digestion atomic fluorescence spectrometry

A novel method for determination of Hg, Se, Bi, As and Sb based on microwave digestion followed by continuous flow vapour generation atomic fluorescence spectrometry was developed. The digestion for Hg was based on a two stage digestion involving HNO(3) and H(2)O(2), whilst for the hydride forming elements a common digestion using HCl and H(2)O(2) was found to be the most effective. The instrumentation and chemistry were optimised in order to provide the best accuracy and precision. The method detection limit for hair samples was found to be 0.2 ng g(-1) for Hg and between 2 and 10 ng g(-1) for the hydride forming elements. The atomic fluorescence detector showed excellent linearity over the concentration ranges studied with linear correlation co-efficients between 0.99984 and 0.99997. To validate the accuracy of the method a human hair certified reference material (GBW 0706) was analysed and excellent agreement with the certified value was obtained for all elements.

Determination of total mercury in hydrocarbons and natural gas condensate by atomic fluorescence spectrometry

A new and simple technique for the determination of total mercury in gas condensate was developed which eliminates the use of chemicals/additives and complicated digestion procedures. The determinations are carried out by vaporisation of the samples at 400 °C with adsorption of mercury species on a gold trap (Amasil) maintained at 200 °C. The trap is then heated at 900 °C to release metallic mercury, which is determined by atomic fluorescence spectrometry. The mercury recoveries from seven species, dimethylmercury (DMM), diethylmercury (DEM), diphenylmercury (DPM), methylmercury chloride (MMC), ethylmercury chloride (EMC), phenylmercury chloride (PMC) and mercury(II) chloride (MC) spiked individually into gas condensate were found to be in the range 80–100%. The mercury recoveries for mixtures of the seven species added in equal amounts to gas condensate were in the range 88–97%. For Conostan mercury standard added to the condensate, the recovery was 88%. The instrumental precision from 10 measurements of a toluene control was 4% RSD. For three mercury species. DEM, MC and EMC, added to condensate, the precision was between 2 and 5% RSD (n = 10). The limit of detection (3ςn–1 criterion) for the procedure was calculated to be 180 pg Hg in toluene and 270 pg in condensate. For three mercury species added to a condensate sample, the absolute detection limits were 270 pg Hg for DEM, 450 pg Hg for MC and 630 pg Hg for EMC. Total mercury measurements in five real condensate samples from two sites at different stages of production covered the range 7–50 ng ml–1 with uncertainties in the range 4–7% RSD. The total mercury concentration of two commercial heavy gas oil samples were found to be 22.2 ± 0.6 µg ml–1 with RSD 3% (n = 4) and 2.3 ± 0.1 µg ml–1 of mercury with RSD 3% (n = 7).

Development of an atomic fluorescence spectrometer for the hydride-forming elements

A novel atomic fluorescence spectrometer is reported for the determination of hydride-forming elements, principally arsenic and selenium. A miniature argon–hydrogen diffusion flame was used as the atomizer and the analyte elements were introduced as their gaseous hydrides from a fully automated continuous hydride generator. The hydrogen for the flame was chemically generated as a by-product of the sodium tetrahydroborate reduction. Excitation was achieved using a boosted-discharge hollow cathode lamp. Fluorescence wavelengths of interest were selected using an interference filter. A solar blind photomultiplier was used as the detector. The instrument is notably compact and capable of full automation by connection to a personal computer using a DIO card. Optimization of the instrumentation is described. Detection limits (3σ) of 0.10 and 0.05 µg l–1 for arsenic and selenium, respectively, are reported, as are a number of analyses of certified reference water samples, which confirm the excellent accuracy and precision of the new instrumentation.

Arsenic speciation in beverages by direct injection-ion chromatography hydride generation atomic fluorescence spectrometry

The procedure developed allows the direct speciation of arsenic in these samples with good sensitivity, selectivity, precision and accuracy. Detection limits determined using the optimized conditions were found to be between 0.16 and 2.9ng ml−1 for arsenite, dimethylarsinic acid, monomethylarsonic acid and arsenate, while standard addition studies showed that the procedure is free from matrix interferences. As no certified reference materials are available for these analytes or matrices, validation was carried out by studying spike recoveries and by comparison of results with an alternative technique.

Automated Determination of Sulfide as Hydrogen Sulfide in Waste Streams By Gas-phase Molecular Absorption Spectrometry

The gas-phase molecular absorption spectrometric method for determining sulfide was applied to the determination of sulfide in waste water using a fully automated system. The instrumentation utilizes a commercially available vapour generator and an absorption cell placed in the optical path of an atomic absorption spectrometer set at 200 nm. A sulfide anti oxidant buffer (SAOB) consisting of sodium hydroxide (0.5 mol l - 1 ), sodium tetrahydroborate (0.2 mol l - 1 ) and sodium citrate (0.2 mol l - 1 ) was found to have advantages in terms of shelf-life over existing SAOB formulations. A gas–liquid separator was designed that also assisted in the transport of the hydrogen sulfide, generated by hydrochloric acid (0.6 mol l - 1 ), to the absorption cell. The limit of detection was 0.13 mg l - 1 of sulfide and the calibration was linear up to 100 mg l - 1 . Of 16 ions tested for interference, at a 50-fold excess, only copper, lead, zinc and arsenic gave serious interference problems. The method was applied to the determination of sulfide in waste waters and good agreement was found with the stabilized iodine titration procedure. Spike recoveries from the waste water ranged between 97 and 101%. Unattended operation allowed the analysis of up to 90 samples per hour.

Rapid method for the determination of total mercury in urine samples using cold vapour atomic fluorescence spectrometry

A rapid method for the determination of total mercury in urine samples is proposed. Samples are digested using a bromination procedure at room temperature. Analysis is performed using automated continuous flow vapour generation coupled to atomic fluorescence spectrometry. This approach allowed the analysis of 30 samples per hour and a limit of detection of 1 ng l-1. The analytical procedure was assessed using certified reference material NBS 2672a freeze-dried urine and two batches of Seronorm trace elements in urine samples.

Ultra-trace determination of cadmium by vapour generation atomic fluorescence spectrometry

The vapour generation of cadmium by reaction in aqueous solution with sodium tetraethylborate was performed using a conventional continuous flow reactor. This vapour generation system was successfully interfaced with atomic absorption and fluorescence instrumentation. A detection limit, with fluorescence detection, of 20 ng dm–3(3σn–1) was obtained after optimization of the chemistry using simplex routines. Interferences from transition metal ions (e.g., NiII, CuII) were observed but were attenuated by the use of citrate as a masking agent. Vapour generation atomic fluorescence spectrometry was successfully applied to the determination of cadmium in potable waters, a National Institute of Standards and Technology (NIST) Standard Reference Material (SRM) 1643c Trace Elements in H2O, and Community Bureau of Reference (BCR) Certified Reference Materials (CRMs) 144 Sewage Sludge–domestic and 145 Sewage Sludge–industrial. Full recovery of cadmium spikes was obtained from UK drinking waters. A value of 12.6 ± 0.5 ng cm–3 of cadmium was obtained for NIST SRM 1643c (certificate value = 12.2 ± 0.1 ng cm–3 of cadmium) whilst the cadmium content of BCR 144 and 145 (certificate value 3.41 ± 0.25 and 18.0 ± 1.2 µg g–1, respectively) was estimated at 3.34 ± 0.15 µg g–1(BCR 144) and 18.24 ± 0.7 µg g–1(BCR 145).

Effects of moisture on the cold vapour determination of mercury and its removal by use of membrane dryer tubes

Continuous-flow vapour generation is now a well-established sample-introduction technique for mercury and the hydride-forming elements. One of the problems associated with this technique is moisture carryover, which originates during the gas–liquid separation process. Excessive moisture carryover causes gradual loss in sensitivity and baseline drift for atomic absorption and atomic fluorescence spectrometry and inductively coupled plasma detection system. Chemical desiccants and physical moisture traps can be used to reduce moisture for short periods of time, although these may give rise to contamination and analyte losses. A more effective way to remove moisture carryover has been found by using a semi-permeable Nafion membrane dryer tube, which continuously desolvates the wet gaseous stream. This device improved the long-term stability and enhanced sensitivity. A relative standard deviation of 2% was achieved for 90 runs of a 1 µg l–1 standard, obtained over a period of 3 h.

Lead preconcentration with flow injection for flame atomic absorption spectrometry

The flow-injecton preconcentration of lead with immobilised reagents under a variety of conditions is discussed. Timed sample loading and matrix removal without passing the matrix to the nebuliser were achieved simply with one valve. Reagent consumption and calibration time were reduced by the addition of further valves. A system design incorporating control of the timing of operations by a commercial autosample is described. The effects of pH and interferent ions were examined. Water samples were analysed against aqueous standards and as standard additions solutions. For an analysis time of about 3 min a preconcentration factor of about 40 was obtained for both peak height and area measurements. Detection limits of down to 1.4 ml−1 were obtained.