A. Izquierdo

Continuous microwave assisted pervaporation/atomic fluorescence detection : An approach for speciation in solid samples

Abstract A method for the speciation of mercury in solid samples based on microwave assisted pervaporation-atomic fluorescence detection in a continuous flow system has been developed. The sample is placed in a laboratory-made pervaporation cell, and either SnCl 2 (for Hg(NO 3 ) 2 ) or oxidising solution followed by SnCl 2 (for PhHgAc) are injected into the sample via a syringe. The cell is then placed inside a microwave device so that the microwaves are focused only on the sample chamber. After application of the microwaves, the flow of gas or liquid in the upper acceptor chamber is started and the Hg driven to the detector via a gas-liquid separator. The method features a linear range from 10 to 500 μg kg −1 ( r 2 = 0.9999) and has a sampling frequency of 10–12 h −1 . Acceptable results were obtained for the speciation of mercury in a certified reference material

Speciation of inorganic selenium using flow injection hydride generation atomic fluorescence spectrometry

Two methods based on flow injection atomic fluorescence have been developed in order to speciate Se as SeIV–SeVI. Both methods use hydride generation of SeIV with atomic fluorescence detection as the derivatization–detection step. In the first method, two sample plugs are injected simultaneously in series, so that the first plug passes straight to the detector to determine SeIV. The second plug passes through a focused microwave device where SeVI is reduced to SeIV prior to its conversion into the hydride. The SeVI content is then given as the difference between the two results. In the second method a mini-column is used to retain both the Se species; SeIV and SeVI are then eluted sequentially with formic and hydrochloric acids, respectively. The columns can be used for preconcentration of the analytes as well as for sampling. Both methods show exceptional sensitivity [limits of detection (3s) of 0.04 µg l–1] and wide linear range up to 50 µg l–1, with excellent linearity (r2= 0.999), and reproducibility (relative standard deviation <5%). The methods have been used in an intercomparison exercise for the Measurements and Testing Programme and have been applied to the determination of the analytes in tap water and haemodialysis samples.

Use of focused microwaves for expeditive shortening of sample pre-treatment: digestion and reduction procedures prior to selenium speciation as selenium(IV) or selenium(VI)

Digestion programmes for different types of samples (milk, sausage and lyophilized pig kidney certified reference material) have been developed using a focused microwave digestor prior to the determination of selenium. The digestion step does not alter the oxidation state of the analyte; thus, the speciation of the target analyte is also achieved with the help of the microwave digestor, which facilitates a subsequent on-line reduction step in a flow injection (FI) manifold before determination by FI cathodic stripping voltammetry. Excellent recoveries for the spiked and certified values were obtained using digestion programmes which varied from 20–45 min. The on-line microwave procedure for the reduction of SeVI affords good sensitivity and linearity (3–120 µg l–1, r= 0.994) of the over-all determination method, and dramatically reduces the time necessary for SeVI reduction.

Flow-injection anodic stripping voltammetry at a gold electrode for selenium(IV) determination

Abstract A continuous method for the determination of selenium based on flow-injection anodic stripping voltammetry is proposed. After the deposition/preconcentration of Se(IV) on a gold working electrode at an applied voltage of — 0.4 V, an anodic scan from —0.4 to 1.16 V was applied, whereby the selenium was stripped at a potential of 0.912 V. The current produced was proportional to the Se(lV) concentration in the sample. Off-line reduction of Se(VI) to Se(IV) gave the original Se(IV) + Se(VI) concentration so that the Se(VI) concentration could be calculated as the difference between the two results. Interfering divalent cations were removed by on-line separation by use of a cation exchange column prior to the injection valve. The method has a linear determination range between 5 and 100 ng ml−1 with a correlation coefficient, r2, of 0.9969 and an R.S.D.

Supercritical fluid extraction of carbamate pesticides from soils and cereals

SummaryA method for the supercritical fluid extraction of carbamate pesticides (propoxur, aminocarb, carbaryl and methiocarb) from soil and cereal samples using CO2 is proposed. Extractions were at 378 bar and 54 °C. Analytes were determined in the extracts by HPLC with fluorescence detection after post-column derivatization. Recoveries from spiked soil ranged between 39.6 and 91.7%, depending on analyte and soil components. Lowest recoveries were from sandy soils. Aminocarb could not be recovered from any soil using CO2. Recovery of aminocarb from diatomaceous earth was improved by adding methanol to the extraction cell prior to SFE, but the effect was not observed in soil samples. Recoveries for propoxur and aminocarb from spiked wheat were about 75%, and only between 30–50% for aminocarb from corn and oats, and carbaryl from wheat. Fat was coextracted using CO2 and retained in the trap together with the analytes, however, appropriate rinsing solvent allowed on-line clean-up of the extract.

Sequential speciation of selenium by flow injection cathodic stripping voltammetry

A semi-automatic continuous method for the determination of Se(IV) based on flow-injection cathodic stripping voltammetry (FICSV) is reported. The flow injection approach incorporates a thin mercury film on glassy carbon as the working electrode, on which Se(IV) is deposited at an applied potential of 0.0 V. A cathodic scan (from 0.0 to −0.9 V) is applied and the Se is stripped at −0.54 V, providing a current intensity proportional to the Se(IV) concentration in the sample. This method features a linear determination range between 0.5 and 30 ng/ml (r2=0.998, RSD=3.6%). The non-interference levels (foreign species to analyte ratio) are 2.5:1 for Cu(II), 7.5:1 for Pb(II), 35:1 for Cd(II), 250:1 for Zn(II) and 500:1 for Fe(III). After developing the method for Se(IV), the speciation of this element has been performed by sequential injection of the dissolved sample into a carrier which may or may not have been previously reduced off-line thus determining the sum (Se(IV)+Se(VI)) or only Se(IV), respectively. The method has been applied to selenium speciation in water samples.

Speciation with unsegmented continuous systems

Speciation applications of the flow-injection technique are discussed in relation to the hydrodynamic manifold components that are adapted for this purpose. Alterations include double halting of the propulsion system, coupling of injection valves, modifications of the transport-reaction zone, use of switching valves, and merging and splitting points. Detector remodelling, the use of sensors and special detectors, and data processing methodologies are also reviewed in connection with the flow-injection/chromatography coupling. Finally, foreseeable trends and desirable advances are outlined.

Bioremediation of an area contaminated by a fuel spill.

In order to decontaminate a large area of restricted access contaminated by a fuel spill, laboratory and field studies were developed in two steps: (a) monitoring of the laboratory experiment on bacterial growth under aerobic and anaerobic conditions with and without addition of nutrients; and (b) use of the best conditions obtained in (a) for the decontamination of the soil. A hydraulic barrier was installed both to clean the aquifer and to avoid migration of hydrocarbons as a consequence of their solution in the groundwater and subsequent displacement. The objective was to create an ideal environment for the treatment of the affected area that favoured the growth of the indigenous bacteria (Pseudomonas and Arthrobacter) that biodegrade the hydrocarbons. Monitoring of the changes in the total concentration of petroleum hydrocarbons in the soil subjected to bacterial action was performed by gas chromatography. In a field study, the progress of biodegradation of hydrocarbons was evaluated in situ by changes in subsurface CO2/O2 levels by means of an analyser equipped with an infrared detector. Biostimulation and oxygen were the most influential factors for the biodegradation of the hydrocarbons. The use of bioventing of the soil was shown as an excellent technology to promote in situ bioremediation of the polluted area.

Fully automated robotic method for the screening of polychlorinated biphenyls in used mineral oils

A screening method for the determination of polychlorinated biphenyls (PCBs) in waste mineral oils based on dechlorination by sodium and potentiometric measurement is proposed. The method is fully automated by a robotic station in which the robot performs the weighing of the sample, adds the reagents, develops a liquid–liquid extraction and obtains the chloride concentrations using a chloride-selective electrode. The data are acquired and treated by the computer. The method affords a detection limit of 1 µg g–1 chloride and a precision, expressed as relative standard deviation, of 3.2% which corresponds to a detection limit of 12 µg g–1 for Aroclor 1242. This detection limit is far below the regulatory limit of 50 µg g–1 which establishes whether or not a waste oil is contaminated by PCBs. The sample throughput is 12 h–1.

Lyophilization: a useful approach to the automation of analytical processes?

An overview of the state-of-the-art in the use of lyophilization for the pretreatment of samples and standards prior to their storage and/or preconcentration is presented. The different analytical applications of this process are dealt with according to the type of material (reagent, standard, samples) and matrix involved.