Full Paper Evaluation of Liquid- and Solid-Contact, Pb 2þ -Selective Polymer-Membrane Electrodes for Soil Analysis Christina M. McGraw , a Tanja Radu, b Aleksandar Radu, b * Dermot Diamond b a Department of Chemistry, University of Otago, Dunedin, New Zealand b Adaptive Sensors Group, National Centre for Sensor Research, School of Chemical Sciences, Dublin City University, Dublin, Ireland *e-mail: Aleksandar.Radu@dcu.ie Received: September 23, 2007 Accepted: October 24, 2007 Abstract Pb 2þ -selective electrodes (ISEs) have been developed for the analysis of soil. Recent advances have extended the previous achievable limits of detection of ion-selective electrodes (ISEs) to submicromolar levels, opening up possibilities for new applications in environmental analysis. The simple construction of ISE)s, their low cost and low powerdemand,coupledwithimproveddetectionlimitsmaketheelectrodeswellsuitedforenvironmentalanalysis.In thispaper,thedevelopmentanduseofbothliquid-andsolid-contactISEsforthedetectionofPb 2þ inarangeofsoil samplesfromabandonedminingsitesisdescribed.Solid-contactelectrodesweremadeusingpoly(3-octylthipophene) as the internal mixed conductor layer. In addition, three techniques for the extraction of analyte from soil were studied in order to optimize the extraction of metal ions using techniques and solvents compatible with portable devices. Soil Pb 2þ levels between 30 and 29,100 mg/kg were measured with the ISEs and were shown to be in good agreement with reference AAS measurements. Issues arising from the current definition and interpretation of ISE detection limits are also discussed. Keywords: Lead, Ion selective electrode, Environmental monitoring, Polymeric membranes, Soil analysis, Detection limit DOI: 10.1002/elan.200704068 Dedicated to Professor Ernç Pretsch on the Occasion of His Retirement from ETH Zürich 1. Introduction Themostcommonmethodsforthedeterminationofleadin soil are atomic absorption spectroscopy (AAS), X-ray fluorescence (XRF), inductively coupled plasma-atomic emissionspectroscopy(ICP-AES)andinductivelycoupled plasma-mass spectroscopy (ICP-MS). AAS, ICP-MS and ICP-AESarelaboratory-basedmethodsthatofferreliable andsensitiveresultsafterdigestionandextractionofmetals fromthesoilsamples,whereasXRFcanbeusedtoanalyze themetalcontentofsoilsinsituwithouttheneedforsample collectionordigestion[1]. While AAS, ICP-AES, and XRFare well-characterized methods, the cost of ownership per unit is prohibitive for large-scale deployments which are increasingly necessary for monitoring the status of the environment in real time over large geographical regions. ISEs are much more attractive for this type of distributed sensing, as they are relatively simple to use, have low unit costs, are easily miniaturized and have very low power requirements, pre- requisites that are vital for wide-scale sensor network deployments [2]. Furthermore, the last decade has seen a significantincreaseintheunderstandingofthemechanisms thatdictatetheresponseandthelimitofdetection(LOD)of ISEs[3],whichuntilrecently,wasregardedasafundamental obstacletotheirapplicationtotraceenvironmentalanalysis. The emergence of new ISE experimental protocols that allowthedetectionofanalytesattracelevels(ppbandppt) [4,5]hasopenedupthepossibilityoftheiruseinenviron- mentalanalysis.Examplesoflow-LOD ISEprotocolsbeing appliedtoenvironmentalmonitoringhavebeguntoappear in the literature, such as the measurement of lead [6] and copper[7]indrinkingwater,thedeterminationofcesiumin environmental waters [8] and monitoring the biouptake of lead [9] and cadmium [10]. Miniaturized ISEs with nano- particlelabelshavealsorecentlybeenemployedasbiosen- sors[11]. With these recent improvements in LODs, it is now possibletoapplybothliquidcontact(LC)andsolidcontact (SC) electrodes to the analysis of environmental samples. This paper describes the fabrication of LC and SC lead- selectiveISEsandtheirapplicationtosoilsamplescollected near Silvermines, Co. Tipperary, Ireland. Centuries of mining near Silvermines has lead to pockets of soil with highlevelsofheavymetalssuchaslead,cadmium,mercury, zinc, arsenic, and copper. Although average lead values in Irishsoilsareca.30mg/kg,agovernmentalreportshowed that about 20% of agricultural soil samples collected near 340 Electroanalysis 20, 2008,No.3,340–346 # 2008 WILEY-VCH Verlag GmbH&Co. KGaA, Weinheim