Solid phase extraction of trace elements

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On-line preconcentration and determination of copper, lead and chromium(VI) using unloaded polyurethane foam packed column by flame atomic absorption spectrometry in natural waters and biological samples.

Removal and preconcentration of lead (II) and other heavy metals from water by alumina adsorbents developed by surface-adsorbed-dithizone

The speed and the efficiency of sample preparation have been improved over the past few years by microwave techniques. This article reviews the literature dealing with the application of on-line microwave sample-preparation techniques, which is based on the coupling of microwave-assisted extraction, reaction, digestion, speciation or desorption to other sample-preparation or detection processes through appropriate interfaces. Compared with the off-line method, the on-line technique is more advantageous with respect to time for analysis, solvent consumption and sample contamination.

Application of on-line microwave sample-preparation techniques

Wet digestion methods

A flow injection system incorporating a micro-column of activated alumina was used in conjunction with flame atomic absorption spectrometry for the pre-concentration and determination of lead in drinking waters. The analyte was deposited on the alumina micro-column (basic form of alumina) during the sampling stage and elution was effected by injection of nitric acid (250 µl, 2 M). The procedure was successfully applied to a range of water samples and the accuracy was assessed through recovery experiments, the analysis of a certified reference water and by independent analyses by inductively coupled plasma mass spectrometry. The limit of detection based on a sample volume of 25 ml (sampling time 5 min at 5 ml min–1) was 0.36 µg l–1 and the relative standard deviations at 40 and 4 µg l–1 were 1.4 and 12%, respectively.

On-line pre-concentration and determination of lead in potable water by flow injection atomic absorption spectrometry

A flow through microwave digestion device has been developed for the determination of Cd, Cr, Mn, Ni and Pb in soil by aqua regia extraction. This device differs from existing commercially available devices as it uses a double pumping action to replace the back pressure regulator traditionally used to achieve internal pressurisation. An acid front has also been included to overcome problems associated with the dilution of acid in samples due to dispersion. Recoveries between 95 and 105% of certified values were achieved with standard deviations of less than 4% for certified reference soil (BCR 143R). A sample throughput of 6 samples per hour was achieved in the optimised system. The performance of the device was tested by digesting real soil samples ground through a 250 microns sieve and slurried without the use of surfactants. A comparison of analytical performance for analysing real samples was made between the microwave flow method and a thermal method.

Development of a wide bore flow through microwave digestion device for the determination of trace metals in soil

The microwave heating synthesis of (phthalocyaninato)bis(chloro)silicon(IV) prepared from diiminoisoindolene and silicon tetrachloride in quinoline has been shown to be rapid (5 min reaction time compared to 30 min with thermal heating) and results in a high yield (91% compared to 71% using thermal heating). A modified microwave ashing furnace was used to heat the reaction mixture. The high yield has led to a reduction in the purification time to 1 h (compared to 4 h or more using conventional heating).

A high-yield microwave heating method for the preparation of (phthalocyaninato)bis(chloro)silicon(IV)

An effective decontamination procedure was developed using 
thermogravimetric data obtained from TFM and PFA vessels that were 
contaminated with aqua regia. This procedure allows one set of 
vessels to be used without contamination of subsequent samples. The vessels 
are decontaminated at 140 °C in a vacuum oven at a pressure of 6 mmHg. 
It has been demonstrated that if the decontamination procedure is carried 
out, 100% recoveries of silver are possible in TFM [a copolymer of 
tetrafluoroethylene and a small amount of the perfluoro(propyl vinyl 
ether)] and PFA (a perfluoroalkoxy resin) microwave digestion vessels that 
have been previously used in conjunction with aqua regia. This 
study illustrated how chlorine-based species present in aqua regia 
can enter microwave digestion vessels constructed from TFM and PFA due to 
permeation processes. It was demonstrated using thermogravimetric analysis 
that this process is reversible and that chlorine-containing species can 
later leave PFA and TFM when the vessels are re-heated in a microwave oven 
and thus contaminate subsequent digests. The study used silver to indicate 
the release of chlorine-containing species from previously contaminated 
vessels. In this study sufficient chlorine-containing species were released 
back into a vessel to precipitate completely 0.1 mg of silver in a nitric 
acid digest.

Philip Riby

Papers

On-line pre-concentration and determination of lead in potable water by flow injection atomic absorption spectrometry

Development of a wide bore flow through microwave digestion device for the determination of trace metals in soil

A high-yield microwave heating method for the preparation of (phthalocyaninato)bis(chloro)silicon(IV)

The minimisation of aqua regia cross contamination in PFA and TFM microwave digestion vessels using silver as an indicator