Search

This review covers the development of measurement configurations for chemiresistors based on conducting polymers. The simplest chemiresistors are based on application of a two-electrode technique. Artifacts caused by contact resistance can be overcome by application of a four-electrode technique. Simultaneous application of the two- and four-electrode measurement configurations provides an internal control of sensor integrity. An incorporation of two additional electrodes controlling the redox state of chemosensitive polymers and connecting to the measurement electrodes through liquid or (quasi)solid electrolyte results in a six-electrode technique; an electrically driven regeneration of such sensors allows one to perform fast and completely reversible measurements.

An ion chromatographic method with on-line sample pretreatment was developed for the trace analysis of seven common anions in concentrated matrices. The pretreatment column used in this study consisted of polystyrene-divinylbenzene (PS-DVB) and multi-walled carbon nanotubes (MWCNTs). It was too hydrophobic to retain different inorganic anions, but it showed a strong affinity for organic compounds. Thus, this chromatographic system could be used to detect trace anions in organic solvents, organic acids and relevant salts. The addition of MWCNTs decreased the surface areas of stationary phases and the retention times of organic matrices were shortened. Compared with conventional column-switching technique, only a single instrument (ICS2100) was needed in this system, including a pump, a conductivity detector, an eluent generator, a six-port valve and a ten-port valve. An electrochemical self-generating suppressor (ESGS) was adopted to convert the eluent of KOH into water for the matrix elimination. Two different eluent were employed in the chromatographic system, one for separation and the other for matrix elimination. The sample pretreatment and analysis were realized simultaneously. After optimization of this system, a calibration study was conducted by preparing and analyzing eight concentrations (between 5 and 5000 μg L −1 ) of mixture standards of seven anions in deionized water. The linearity was between 0.9990 and 0.9998, and the detection limits ranged from 0.41 to 3.17 μg L −1 . A spiking study was performed on three representative organic chemicals with satisfactory recoveries between 88.1% and 118.5% when the concentrations of the matrices did not exceed 10 g L −1 .

This article is a review of the progress in application of electrochemical detections in liquid chromatography in recent 15-20 years. Based on 238 references, mostly to original research papers, it presents applications of amperometric and voltammetric detections, as well as coulometric, conductimetric and potentiometric ones. In case of those which have reached already the stage of routinely employed detections with commercially available instrumentation (amperometry, coulometry, conductometry) especially novel and original applications are presented. In case of voltammetric and potentiometric detections a ways of their improvements are showed, directed towards obtaining competitive results with other detection methods.

In this work, a method to determine the nitrogen content of nitrocellulose from gunpowders and collodions is proposed. A basic hydrolysis of nitrocellulose with 1.0% (m/v) NaOH at 150°C during 30 min was carried out for nitrocellulose from gunpowders (after its previous isolation by a protocol optimized by our research group) and from collodion samples. The concentration of nitrate and nitrite ions in the hydrolysate was determined by ion chromatography with suppression and conductimetric detection. The nitrogen content of nitrocellulose was calculated from the values of the concentration of both ions. The quantitative method was evaluated in terms of selectivity, sensitivity, robustness, limits of detection and quantification, and precision, measured as repeatability and intermediate precision. These parameters were good enough to demonstrate the validity of the method and its applicability to the determination of the nitrogen content of nitrocellulose contained in different types of gunpowders (single- and double-base gunpowders, manufactured from 1944 to 1997) and in commercial collodion samples. For gunpowders, the nitrogen content determined with the optimized method was compared with the values reported by the official label of the ammunition (obtained by a digestion/titration method) and errors, by defect, ranging from 1% to 15.2% (m/m) were calculated. The highest errors were obtained for the oldest gunpowders and could be attributed to the loss of nitro groups in the nitrocellulose molecule during aging. For collodion samples, errors could not be calculated since the real nitrogen content for these samples was not given in the label. In addition, the analysis time (2h for nitrocellulose isolation, 1.5h for nitrocellulose hydrolysis, and 0.2h for chromatographic separation) was about 10 times lower than in the digestion/titration method nowadays used for gunpowder samples.

In this work, a capillary electrophoresis (CE) procedure was developed for the simultaneous determination of a pharmaceutical drug and its counter-ion, namely labetalol hydrochloride. For this purpose, an uncoated fused-silica capillary, a low conductivity background electrolyte (BGE) and a capacitively coupled contactless conductivity detector (C4D) were employed. This detection system is highly sensitive and enables detection of inorganic as well as organic ions unlike with direct UV detection. Moreover, to be able to simultaneously analyze the cationic drug (labetalol+) and its anionic counter-ion (Cl−) in the same electrophoretic run without the need of a coated capillary, a dual-opposite end injection was performed. In this technique, the sample is hydrodynamically injected into both ends of the capillary. This method is simple and easy to perform since the different injection steps are automated by the CE software. This novel CE-C4D procedure with dual-opposite end injection has been successfully validated and applied for the analysis of chloride content in an adrenergic antagonist (labetalol hydrochloride). Thus, the hereby developed method has been shown to enable fast (analysis time

A simple flow injection (FI) conductometric system with gas diffusion separation was developed for the determination of Kjeldahl nitrogen (or proteins) in milk and chicken meat. The sample was digested according to the Kjeldahl standard method and the digest was diluted and directly injected into the donor stream consisting of 4M NaOH. In alkaline medium, ammonium was converted to ammonia, which diffused through the PTFE membrane to dissolve in an acceptor stream (water). Dissociation of ammonia caused a change in conductance of the acceptor solution, which was linearly proportional to the concentration of ammonium originally present in the injected solution. A conductometric flow through cell and an amplifier circuit was fabricated, which helped improve sensitivity of the conductometric detection system. With using a plumbing Teflon tape as a gas diffusion membrane and without thermostating control of the system, a linear calibration graph in range of 10-100mgL(-1) N-NH(4) was obtained, with detection limit of 1mgL(-1) and good precision (relative standard deviation of 0.3% for 11 replicate injections of 50mgL(-1) N-NH(4)). The developed method was validated by the standard Kjeldahl distillation/titration method for the analysis of milk and chicken meat samples. The proposed system had sample throughput of 35h(-1) and consumed much smaller amounts of chemical than the standard method (275mg vs 17.5g of NaOH per analysis, respectively).

An accurate and sensitive ion chromatographic (IC) method with suppressed conductivity detection is described for determination of traces of Na, K, Mg and Ca along with nitrogen in uranium based materials. The method involves matrix separation after sample dissolution by hydrolyzing and filtering off the polyvalent cations. Transition elements interfering in the determination of Ca were removed by cation exchange cartridge containing iminodiacetate resin. Detection limits were between 2 and 6 μg L −1 and overall precision were better than ±5%. Recovery of spiked cations was in range from 96% to 104%. Results obtained were in good agreement with other methods.

A novel progesterone immunosensor using a colloidal gold-graphite-Teflon-tyrosinase composite biosensor as amperometric transducer is reported. A sequential competitive configuration between the analyte and progesterone labelled with alkaline phosphatase (AP) was used. Phenyl phosphate was employed as the AP-substrate and the enzyme reaction product, phenol, was oxidized by tyrosinase to o-quinone, which is subsequently reduced at -0.1 V at the biocomposite electrode. Variables such as the concentration of phenyl phosphate, the amount of antibody attached to the electrode surface, immersion time in a 2% BSA solution, working pH and incubation times in progesterone and AP conjugate were optimized. A linear calibration graph for progesterone was obtained between 0 and 40 ng mL(-1) with a slope value of -82.3 nA ng(-1) mL, and a detection limit of 0.43 ng mL(-1). The time needed to reach the steady-state current from the addition of phenyl phosphate was 30-40 s. These analytical characteristics improve substantially those reported for other progesterone immunosensors. A lifetime of 14 days with no need to apply any regeneration procedure was also achieved. The usefulness of the immunosensor was evaluated by determining progesterone in milk samples spiked with the analyte at 5.0 and 1.5 ng mL(-1) concentration levels. Following a very simple procedure, involving only sample dilution, mean recoveries (n=7) of 98+/-3% and 99+/-3%, respectively, were obtained.

The exploitation of digital images obtained from a CCD camera (WebCam) as a novel instrumental detection technique for titration is proposed for the first time. Named of digital image-based (DIB) titration, it also requires, as a traditional titration (for example, spectrophotometric, potentiometric, conductimetric), a discontinuity in titration curves where there is an end point, which is associated to the chemical equivalence condition. The monitored signal in the DIB titration is a RGB-based value that is calculated, for each digital image, by using a proposed procedure based on the red, green, and blue colour system. The DIB titration was applied to determine HCl and H3PO4 in aqueous solutions and total alkalinity in mineral and tap waters. Its results were compared to the spectrophotometric (SPEC) titration and, by applying the paired t-test, no statistic difference between the results of both methods was verified at the 95% confidence level. Identical standard deviations were obtained by both titrations in the determinations of HCl and H3PO4, with a slightly better precision for DIB titration in the determinations of total alkalinity. The DIB titration shows to be an efficient and promising tool for quantitative chemical analysis and, as it employs an inexpensive device (WebCam) as analytical detector, it offers an economically viable alternative to titrations that need instrumental detection.

A conductometric sensor for on-line testing of haloacetic acids has been developed based on lab-on-chip device incorporated with an integrated miniaturised liquid-handling system. The sensor utilises a molecularly imprinted polymer (MIP) synthesized by the interaction between trichloroacetic acid (TCAA) template and a functional monomer, 4-vinylpyridine (VPD), together with cross-linking polymerisation of ethylene glycol dimethacrylate (EDMA). The ability of this MIP to change its conductivity in the presence of the target molecule into the imprint cavity has been used to develop the sensor, which responds well to TCAA in a continuous flow system with relatively good linearity, although this depends on the applied frequency. Thermal influences on the resistance of the sensor were in the order of 1.45% resistance signal variation per Kelvin at 3 kHz. The sensor showed high specific sensitivity to the target analyte and a stable and reasonable signal response in a solution containing inorganic anions. The sensitivity (range 0.5–5 μg l −1 ) and selectivity achieved with standard TCAA and five other haloacetic acids (HAAs) (dichloro-, monochloro-, tribromo-, dibromo-, and monobromoacetic acid) in water was good. Minimum sample volume required is 2.5 ml and the assay time is 2 min. The sensor has successfully been applied to haloacetic acid determination in domestic and commercial drinking water samples.

Acid–base properties of an organophosphorous silica-based ion-exchanger with covalently bonded complexone groups— N- propylaminodi(methylphosphonic) acid (AdPA-SiO 2 ) were studied. By comparison of AdPA-SiO 2 chemical analysis and data of pH and conductometric titration it was demonstrated that the composition of immobilised layer differ from sample to sample and might consist of 60–88% aminodi(methylphosphonic) acid and 12–40% of residual aminopropyl groups. Regardless of the multifunctional nature of AdPA-SiO 2 interfacial layer, its acid–base properties can be predicted. All studied AdPA-SiO 2 samples have the properties of strong multiprotic H 4 L acids with pK 1 2 ionisation in neutral and weak-basic media. Linear correlation was found for pK 2 and amine-to-acid conversion degree ( ω ) with pK 2 = 8.8 − 4.0 ω . It allows us to determine the conditions of AdPA ionisation for all AdPA-SiO 2 samples. The established correlation can be used to check the quality (the degree of interfacial layer heterogeneity) of AdPA-SiO 2 . The final measurable ionisation constant for AdPA-SiO 2 , pK 3 = 7.9 ± 0.6 is 2-orders of magnitude higher then acidity constant for AdPA homogeneous analogue. It seems that such reduction is caused by increase of electro static interaction between movable counter-ions and negatively charged surface groups. Conductometric study of AdPA-SiO 2 also suggests firm fixation of counter-ions in the interfacial layer.

Polyhexamethylene biguanide hydrochloride (PHMB-HCl) and chlorhexidine digluconate (CHG) are cationic disinfectants widely used in aqueous personal products such as eye drops, lotions and creams. The antiseptics belong to the biguanide group, which are cationic substances well known for their effective action against microbial infection. In this work, capillary electrophoresis combined with contactless conductivity detection was used to determinate these substances in ophthalmic solutions. Statistical analysis of the results showed satisfactory precision ranging from 3 to 6% in peak area. The detection limits were determined to be 4 μg/mL for PHMB and 0.4 μg/mL for CHG. It is also demonstrated that the high sensitivity allows the method to be used for in vitro perfusion studies with rabbit cornea.

Critical micelle concentration (CMC) of sodium dodecyl sulphate (SDS), lithium perfluorooctanesulfonate (LPFOS), hexadecyltrimethylammonium bromide (HTAB), tetradecyltrimethylammonium bromide (TTAB), and sodium cholate (SC), surfactants commonly used as pseudostationary phases in micellar electrokinetic chromatography (MEKC), have been determined by means of three different methods: MEKC, spectrophotometry, and conductometry. Determinations have been performed in water, and also in different concentrations of phosphate buffer at pH 7.0. CMC values ranging from 8.08 (water) to 1.99 (50 mM phosphate buffer) mM for SDS, from 7.16 (water) to 2,81 (30 mM phosphate buffer) mM for LPFOS, from 3.77 (water) to 1.93 (20 mM phosphate buffer) mM for TTAB, from 0.91 (water) to ∼0.34 (20 mM phosphate buffer) for HTAB, and around 13 mM (20 mM phosphate buffer) for SC, are obtained. The effect of the electrolyte concentration on the CMC, as well as the linear relationship between the electrolyte counter-ion concentration and the CMC are discussed. This linear relationship provides an easy way for users to estimate the CMC of a MEKC system, at a given electrolyte concentration. A comparison between experimental methods, as well as a discussion about the suitability of a given method for the determination of the CMC for a given surfactant system is also provided.

In this research, new electrodes were prepared by incorporating a new calix[4]arene derivative into a plasticized poly(vinyl chloride) matrix. Calibration plots with Nernstian slopes (29.9 ± 1.1 mV/decade) for vanadyl ion were observed over a linear range of about four decades of concentration (1.0 × 10 −5 to 1.0 × 10 −1 mol dm −3 , at 25 °C). This electrode revealed a lower limit of detection of 3.9 × 10 −6 mol dm −3 . Conductometric data showed the relatively strong interaction between calix[4]arene and vanadyl ions. The results show that this electrode can be used in acetonitril and methanol media until 10% (v/v) concentration without interference. It has a short response time and can be used for more than two months without any considerable divergence in the potentials. The influence of membrane composition, the pH of the test solution, and the interfering ions on the electrode performance was investigated. The effect of temperature on the electrode response showed that the temperature higher than 50 °C deteriorates the electrode performance. The isothermal temperature coefficient of this electrode amounted to 0.0015 V °C −1 . The results of application show that the electrode can be used successfully in present Cr 3+ and Fe 3+ .

Suppressed micro-bore ion chromatographic methods were developed for the analysis of monobromo- (MBAA), dibromo- (DBAA), monochloro- (MCAA), dichloro- (DCAA), trichloro- (TCAA), chlorodifluoro- (CDFAA) and trifluoroacetic acids (TFAA) in drinking water. Two methods were developed and optimised, one using a Dionex AS11HC (250 × 2 mm) column and the second using a Dionex AS16 (250 × 2 mm) column. Both methods employed electrolytically generated gradients of potassium hydroxide, combined with suppressed conductivity detection. Linearity, repeatability and limits of detection were determined for both methods. For trace determinations of haloacetic acids (HAAs) in real drinking water samples, combined preconcentration and matrix elimination was necessary and was investigated using solid phase extraction (SPE). Selectivity, percent recovery, capacity and reproducibility studies were carried out on LiChrolut EN cartridges. Under optimised conditions, average recoveries for MCAA, DCAA, TCAA, MBAA and CDFAA spiked in dinking water samples ranged from 62 to 88%, with an optimum preconcentration factor of 25. The reproducibility of recovery data for the above HAAs was found to range from 12 to 29% based upon six repeat recovery experiments. Excess residual levels of chloride and sulphate where reduced using Maxi-Clean IC-Ba and IC-Ag extraction cartridges. The optimised procedure was applied to drinking water samples and accuracy determined using standard addition. The combined SPE/IC method proved linear for MCAA, MBAA, DCAA, CDFAA, DBAA and TCAA (R2 ranging from 0.967 to >0.999, n = 4).

The potentiometric response properties of several PVC-based membrane electrodes using phthalocyanine complexes of aluminum (AlPc), nickel (NiPc) and copper (CuPc) as anion carriers, toward thiosalicylic acid (TSA) were investigated. The influences of lipophilic ionic additives (cationic and anionic) and the pH of the buffered solutions were used for the interpretation of the mechanism of the potentiometric response of sensors. The sensitivity, linear range, detection limit, and potentiometric selectivity of the membrane sensors show a considerable dependence on the nature of central metal of the ionophore. The membrane electrodes based on AlPc demonstrate sub-Nernstian responses toward TSA over the range of 0.01 to 1×10 −5 M. In the case of NiPc and CuPc as ionophores and in the presence of trioctylmethyl ammonium (TOMA + ) as a cationic additive, a Nernstian response could be established in a range of 4 orders of magnitudes of TSA concentration (0.01 to 1×10 −6 M). The results of potentiometric investigations revealed that from thermodynamic point of view, the axial coordination of thiosalicylate with the central metal of NiPc and CuPc is more efficient with respect to AlPc. This preference in response to TSA was discussed on the basis of the softness nature of NiPc and CuPc and more affinity for coordination with the thiolate group of thiosalicylate as a soft anion. These potentiometric sensors manifest prominent advantages of high selectivity for TSA over the various inorganic and organic anions, fast response times and micromolar detection limits and can be used over a wide pH range of 4.0–8.0. The prepared electrodes based on NiPc and CuPc were successfully applied in the potentiometric titration of sub-milimolar quantities of Hg 2+ in aqueous solutions and very good recovery results were obtained in these measurements. The results of complexometric studies between Hg 2+ and TSA using electrodes based on NiPc and CuPc as indicator electrodes were compared with conductometric measurements.

In this paper an attempt is made to model the alkalimetric titration trend (pH Analytical measurements of carboxylic acids, amino acids, inorganic anions and metals were executed to know the concentration of a series of acid–base active reactants (pH

This work shows developing conductometric sensor based on molecularly imprinted polymer (MIP) for the screening of complex mixture of haloacetic acids (HAAs) in drinking water. The recognition of the HAAs was achieved by trichloroacetic acid (TCAA)-imprinted polymers synthesised from the copolymerization of 4-vinylpyridine (4-VPD) and ethylene glycol dimethacrylate (EDMA) in the presence of the TCAA template in acetonitrile, either by bulk polymerization (BP) method or by a multi-step swelling polymerization (MSP) method. TCAA-imprinted polymer of both methods was tested for re-binding with the template and its analogs. It was found that these polymers could bind selectively to the template molecule and HAA derivatives. HAA measurements were carried out by the application of the polyvinyl chloride membrane fabricated with TCAA-imprinted polymer on conductometric sensors. The technological parameters (operating frequency, membrane composition, ionic strength and medium pH) for the sensors were identified and optimised in respect to the response to TCAA, using sensor fabricating with BP-based MIP as a model. The selectivity of the sensors constructed with MIPs made by either that of the two imprinting methods was also investigated, which the influence of the method of imprinting on the binding strength and selectivity of the recognition element embedded in sensor was observed. The sensors showed high sensitivity and selectivity for the response toward TCAA, the sensor modified with MSP-based MIP being better. In addition, the sensors, particularly when was constructed with MSP-based MIP exhibited good cross-reactivities with a wide range of HAAs, which is useful for the screening of the group of HAA usually present in chlorinated water in complex mixtures. Thus, the sensor modified with MSP-based MIP was chosen for analytical application. The calibration of this sensor was determined, showing the good linear graphs (R 2 > 0.970) for HAAs over the concentration range of 25–1000g/l and the detection limit of each HAA in the range 0.2–5.0g/l. Moreover, the results in real analysis of the sensor indicate the simplicity and reliability of the method. The present work demonstrated that the sensor based on TCAA-imprinted polymer is a fast and sensitive screening method of HAAs in drinking water. © 2003 Elsevier B.V. All rights reserved.

A method is described for the conductimetric determination of CO 2 . The proposed system was constructed with a pervaporator chamber coupled to a flow system comprised of a six-way solenoid valve, a conductivity cell, a conductivimeter and a PC microcomputer for data acquisition and external controls. An amount of 2 g of dry soil samples were directly weighted in the donor compartment of the pervaporation chamber. The CO 2 was determined from an analytical curve within 5.0 x 10 -3 -5.0 x 10 -2 mol kg -1 CaCO 3 . This curve was constructed with the matrix matching approach and presented a linear regression of y = 13.58 + 584.77r (r = 0.9962). Soils with different lime amounts were analyzed by the proposed methodology and with the official method, that uses the calcium difference between the total, extracted with HCl 1 mol l -1 and exchangeable, extracted with the KCl 1 mol l -1 After paired t-test application, no significant difference between the methods was found at the 95% probability level. The proposed method presented sampling rate of 17 h -1 , detection limit of 8.0 x 10 -4 mol kg -1 CaCO 3 , and R.S.D. < 3%. The use of the pervaporator allowed the in situ soil pretreatment for the residual lime determination.

A new design of a dual, UV photometric - contactless conductometric detector is described. The separation capillary with an optical window created is pressed onto two semitubular electrodes, 3 mm wide and 2 mm apart. The electrodes form the detection cell of the contactless conductometric detector. An optical fiber, placed in the gap between the conductometric electrodes, brings radiation from the source. The radiation that passes through the separation capillary is recorded by a large-area photodiode. The optical fiber and the photodiode operate the photometric cell which is between the conductometric electrodes. The detector thus permits simultaneous photometric and conductometric detection in the same place of the capillary, while exchanging of the separation capillary is easy and without effect on the detector geometry and performance.

A conductometric urea biosensor based on sol–gel-immobilized urease on a screen-printed inter-digitated array (IDA) electrode has been developed. The screen-printed thick-film IDA electrode has proven to be an excellent conductometric transducer in which the admittance signal was dominated by the conductance signal and the resulting dynamic range was wide enough to be employed for the construction of conductometric urea biosensor. Urease was immobilized by a conventional sol–gel process using tetramethyl orthosilicate as a precursor. The sol–gel-derived urea biosensor showed a reasonably wide linear dynamic range 0.03–2.5 mM in 5.0 mM imidazole–HCl buffer at pH 7.5 (detection limit = 30 μM, signal : noise = 2), thus enabling it to be used to monitor urea in a diluted serum sample with the differential measurement format consisting of an active IDA electrode with sol–gel-immobilized urease and a control IDA electrode with sol–gel-immobilized bovine serum albumin. The urea biosensor exhibited good sensor-to-sensor reproducibility (4.4%) and storage stability (63% of its original activity retained after 25 days). The coupling of the sol–gel immobilization method and screen-printing technology can offer mass production of a durable urea biosensor.

The influence of the ligand and metal ion concentrations on the conductimetric behavior of humic-like laurel extract–Cu(II) mixtures are analyzed by conductimetric titrations. The results obtained show that the extracts can be considered as polyelectrolytic molecules in the presence of heavy cations, forming complexes whose stability (related to the slope of a conductimetric excess function versus added Cu(II) plots) is low when compared to stronger ligands such as humic acids. The influence of ligand concentration and degree of neutralization is discussed.

An enzyme biosensor specific for hydrogen peroxide was developed using a new conductometric transducer based on tetra-tert-butyl copper phthalocyanine (ttb-CuPc) thin films and horseradish peroxidase as sensitive element. This analytical system is based on detection of molecular iodine produced as a result of the oxidation of the iodide ions by hydrogen peroxide in the presence of horseradish peroxidase. For the detection of the peroxidase-initiated reaction the ability of the ttb-CuPc thin film to change its conductivity in response to the appearance of molecular iodine is used. To minimise the interfering effect of the aqueous electrolyte on the conductometric response of the ttb-CuPc thin film itself, gold interdigitated electrodes bearing ttb-CuPc layer were covered with a hydrophobic gas-permeable membrane. Thermally evaporated calixarene or plasma polymerised hexamethyldisiloxane was used as a gas-permeable membrane material. In order to assess the optimum sensor technology as well as the operating regime, impedance spectroscopy data were analysed. For biosensor creation horseradish peroxidase was deposited on the sensitive part of the electrodes in a cross-linked bovine serum albumin matrix. The possibility of hydrogen peroxide detection with the biosensor proposed in the range 5–300 μM was demonstrated. The operational stability of biosensor was at least 7 h and the relative standard deviation did not exceed 10%. When stored at +4°C the sensor response was stable for more than 90 days. The dependencies of the sensor response on pH, buffer and NaCl concentrations were investigated.

Conductometric detectors employed in microseparation methods are designed to primarily match the separation criteria. Consequently, their geometry is seldom optimal from the point of view of conductometry. An unsuitable detection cell geometry is manifested in the dependence of the conductometric cell constant on the frequency of the AC signal applied to the electrodes. Variations in the cell constant value cause changes in the measured signal even if the solution conductivity is constant. By varying the frequency of the AC signal it is thus possible to find an optimal frequency at which the highest detection sensitivity is attained. The optimum frequency depends both on the cell geometry and on the conductivity of the mobile phase solution.

An unmodified silica gel (Super Micro Beads Silica Gel B-5: SMBSG B-5) was used for the ion chromatographic (IC) separation of common mono- and divalent cations (Na + , NH + 4 , K + , Mg 2+ and Ca 2+ ) in various natural water samples with strongly acidic eluents. When using 1 mM oxalic acid as the eluent, the SMBSG B-5 silica gel acted as a cation-exchange stationary phase and incomplete separation of these cations was achieved. Therefore, the effect of 18-crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) added to 1 mM oxalic acid as the eluent was investigated for the achievement of complete separation of these cations. As a result, when using 1 mM oxalic acid/3 mM 18-crown-6 as the eluent and a 15 cm SMBSG B-5 column, highly sensitive and excellent separation for these cations was achieved in 15 min. The proposed IC method was successfully applied to the determination of these cations in various natural water samples.

A glucose conductometric biosensor has been fabricated by immobilization of glucose oxidase on the sensitive area of thin-film interdigitated electrodes. The properties of the glucose conductometric biosensor with potassium hexacyanoferrate(III) as an electron acceptor have been studied. The use of potassium hexacyanoferrate(III) as an oxidizing agent in the biocatalytic oxidation of glucose increased acidification of the solution inside the enzymatic layer at glucose concentrations above 2 mM. The dynamic range of the biosensor response extended up to 20 mM glucose concentration in the presence of 160 mM potassium hexacyanoferrate(III) in a sample solution. Further increase of the hexacyanoferrate(III) concentration only resulted in a somewhat higher sensitivity of the biosensor. The sigmoidal form of the calibration curve may be explained by the competition between oxygen and potassium hexacyanoferrate(III) as oxidizing agents. It was shown that, at potassium hexacyanoferrate(III) concentrations greater than 40 mM, the measurements must be performed at a low voltage frequency (1–100 Hz), because at higher frequencies (1 kHz and more) no response of the biosensor to glucose was observed.

The dissociation constants of several acids (perchloric, hydrochloric, phosphoric, acetic and benzoic acids) and of some sodium salts (chloride, acetate and benzoate) have been conductometrically determined in tetrahydrofuran/water mixtures up to a 90% of tetrahydrofuran in volume. The results demonstrate that conductometry can be successfully applied to determine the dissociation constants of salts and moderately weak and strong acids in the studied mixtures. The dissociation constants of the acids and some bases taken from the literature have been fitted to solvent composition through a previously derived equation, which is based on a preferential solvation model. The fitting parameters obtained allow calculation of the dissociation constant for any solvent composition inside the applicability solvent composition range. From the pK value, the pH of any buffered solution, such as those used in liquid chromatography, can be calculated for the particular tetrahydrofuran/ water composition of interest. Appreciable ion-pairing for sodium salts and strong acids has been observed for tetrahydrofuran contents higher than 60% in volume. Therefore, the accurate calculation of the pH values of buffers in tetrahydrofuran-rich solutions must take into account the pK values of the acid and salt.

The limiting molar conductances and dissociation constants of perchloric, hydrochloric, picric and benzoic acids as well as their tetrabutylammonium salts have been determined conductometrically in propan-2-ol/water mixtures in the whole composition range at 25 °C.The variation of Walden product (conductance times viscosity) with solvent composition shows a maximum for all the acids studied at the mole fraction of propan-2-ol of about 0.1, whereas the Walden product of the salts shows a constant value irrespective of the solvent composition. The pK of dissociation of all the studied electrolytes fits very well a general equation developed previously to relate the dissociation constants with the compositions in binary solvent mixtures. The determined parameters of this equation for each one of the studied electrolytes allow the interpolation of the pK value at any solvent composition in propan-2-ol/water mixtures.

Urea-sensitive enzymatic conductometric biosensors based on interdigitated gold electrodes were prepared by cross-linking urease with bovine serum albumin in a saturated glutaraldehyde vapour on the sensor chips. Additional Nafion or PVP (poly (4-vinylpyridine-co-styrene)) membranes were deposited on top of the urea microsensor by a spin-coating process. The main analytical characteristics (dynamic range, buffer concentration sensitivity) of these urea-sensitive microconductometric devices were not significantly modified by the use of Nafion additional membranes. Better results were obtained for sensors coated with PVP additional membranes: their operational storage stability and reproducibility are promising for further applications, especially in the biomedical field.

Theory and characteristics of the surface acoustic wave (SAW)/impedance sensor system in electrolyte solution are reported. This kind of sensor was constructed by a 62-MHz SAW resonator and a pair of conductive electrodes in series. By using the circuit network analysis theory, two oscillation equations were derived and used to explain the frequency-shift response to solution conductance. Results showed that the sensor possesses high sensitivity to the solution conductivity and can be applied to conductivity measurements in solution. Its sensitivity and accuracy could be improved in the presence of a large amount of unreacting foreign electrolytes. The sensor was successfully applied to four types of titration (acid-base, precipitation, oxidation-reduction and compleximetric titration) by monitoring the frequency-shift response. Factors influencing the frequencimetric titration curve were also investigated. Analytes at levels down to 1 × 10 −5 M could be determined by the proposed titration technique. The total acidity in several food samples was determined and the results were consistent with those obtained by conventional methods.

Severe adverse side effects have recently been reported after co-administration of the drugs, ampicillin and clozapine, to a schizophrenic patient. In the present work, conductimetric results indicate that ampicillin and clozapine form a 1:2 charge transfer complex which tends to associate together in water. In contrast, a charge transfer complex with a stoichiometric ratio of 2 moles ampicillin to 1 mole clozapine is obtained when the conductimetric experiment is carried out using acetonitrile as a solvent. Also, the complex appears to dissociate easily in the non-aqueous solution. Nevertheless, in both systems, unexpectedly high capacitance readings were obtained, indicating an adsorption phenomenon of the drugs on electrode surfaces. Further, conductimetric results obtained in water and with Sonification yields a 1:1 ampicillin to clozapine mole ratio. Such results were used to infer the formation of micelles in aqueous solutions. The observed change in stoichiometric ratio of the ampicillin-clozapine complex in aqueous and non-aqueous systems was then used to suggest the action of the drugs on the surface of biomembranes.

A versatile and fully automated system for conductometric titrations has been developed. The system is based on a personal computer (PC) and is able to control up to three simultaneous titrations and it is coupled with a robotic arm used to enhance the processes with the possibility of addition of solids. The appropriate software has been developed in order to control automatically the different phases of the operation. Different types and modes of titrant addition are available as well as different measuring options. The system has been tested and two titration examples proving its usefulness in conductometric studies of electrolyte solutions are presented.

A cyano-bonded phase column together with indirect conductivity detection are used for the liquid Chromatographic determination of the basic drugs benzhexol hydrochloride and ethambutol hydrochloride in tablets as alternatives to existing pharmacopoeia methods. Optimisation of the experimental conditions, linear detection range, and precision of the proposed method have been studied. The results by the proposed method were checked against pharmacopoeia methods. The method is successfully applied to analyse different brands of commercially available tablets. Problems encountered in the determination of these drugs by pharmacopoeia methods are discussed.

A conductimetric method was developed for the monitoring of thermodynamically-controlled enzyme peptide synthesis. It was applied to the synthesis of Nα-benzyloxycarbonylalanylphenylalaninamide, in the presence of Pseudomonas aeruginosa elastase. A linear continuous decrease of conductance could be demonstrated in the first minutes of the reaction, allowing a precise measurement of the initial rate of synthesis. Using independent calibration graphs, the conductimetric method correlated well with liquid chromatography, the most often applied technique for enzymatic peptide synthesis monitoring. Conductimetry may therefore be considered as a promising technique for quantitative measurements of such reactions.

A liquid chromatographic method using indirect conductimetric detection is proposed for the determination of low levels of organic compounds, which does not require any special functional characteristics of the analyte. The signal detected is proportional to the molar concentration of the analyte and independent of its nature. The detector response is linearly dependent on analyte concentrations over at least three orders of magnitude. The basis of the detection is to create a conducting background, which will decrease on elution of the organic compounds. The theory of the method is discussed, with special reference to the quantitative displacement of the conducting species of the mobile phase from the column by the analyte on sample injection. The proposed method has been applied to study the chromatographic behaviour of twenty-one amino acids, where a 5 -μm Econosil CN column was used as the stationary phase with a mixture of water-acetonitrile-tetrahydrofuran (70:20:3) containing 1 mM perchloric acid or trichloroacetic acid as the mobile phase. The proposed method allows as little as 10 ng of each amino acid to be determined.

A conductimetric method was applied to assay Streptococcus pyogenes pyroglutamyl peptidase (E.C. 3.4.8.11) activity using pyroglutamylalanine or pyroglutamyltyrosine. A spectrophotometric comparison using pyroglutamyl-β-naphthylamide indicated that the sensitivity was much increased when using conductimetry. The use of a C-protected dipeptide such as pyroglutamylalanineamide also increased the sensitivity. Kinetic parameters have been measured for both dipeptides, with KM values of 0.34 mM and 0.47 mM, respectively, for the alanine and tyrosine derivatives.

The reactions between iodine and methyl-, ethyl-, n-propyl- and n-butylamines were investigated conductimetrically and spectrophotometrically in acetonitrile. The mole ratios of iodine to the amines titrated are 1:2, 2:2, 3:2 and 4:2, respectively, indicating strongly the formation of anionic species of the types I−, I−3, I−5 and I−7 and cationic species of the type [R3N-I-NR3]+. Although the conductivities of the primary amines do not show a regular order, the secondary and tertiary amines show orders at least at the first end-points. The order for secondary amines is Me2NH>Et2NH>n-Pr2NH>n-Bu2NH and for tertiary amines n-Bu3N>Et3N>Me3N. The end-points of the conductimetric titrations are fairly sharp and can be used in the determination of these amines.

A new type of conductometric probe based on a molecularly imprinted membrane (MIM) for the detection of salbutamol has been designed and fabricated. The probe consists of two parallel screen-printed electrodes (SPE). One of the SPEs was coated with a molecularly imprinted membrane using salbutamol as the template, and the other was modified with a non-molecularly imprinted membrane (N-MIM). Measurements of salbutamol were conducted after the conductometric probe had been connected to a commercial portable conductometer. Multi-sample or successive detections could be easily accomplished by replacing the one-off SPE coated with the salbutamol molecularly imprinted membrane with a new one. The conductometric response of the sensor to the concentration of salbutamol displayed a linear correlation over a range from 50 to 280 nM, with a detection limit of 13.5 nM. The recoveries reached 92.1-98.3% based on pig urine samples. In addition, the sensor based on this new type of probe demonstrated high sensitivity and selectivity for salbutamol.

An accurate and reliable analytical method, based on ion chromatography and suppressed conductivity detection, has been developed and validated for the quantitative determination of sulfites in fresh meats and shrimps. The chromatographic separation was accomplished by using an anion-exchange column eluted with sodium carbonate and sodium hydroxide. The optimized step-change elution, followed by column re-equilibration at the initial mobile phase composition, guaranteed a good resolution even toward endogenous interfering peaks, and an excellent retention time repeatability (1.1%, n = 6). Good results in terms of sample extract stability, recovery efficiency were achieved with an extraction solvent mixture based on sodium hydroxide, fructose and EDTA. The method validation, performed by an in-house model according to Decision 657/2002/EC and Regulation 882/2004/EC, provided excellent results with respect to linearity (correlation coefficient up to 0.9998), limits of detection and quantification (2.7 and 8.2 mg kg−1, respectively, expressed as SO2), expanded measurement uncertainty (below 10%), recovery values (ranging from 85% to 92%) and repeatability (down to 8%), demonstrating the conformity of the proposed method with the European directives. Finally, by major changes ruggedness studies, the method applicability to the quantitative analysis of cow hamburger, pork and horse sausage, and shrimps was demonstrated.

Dissociation constants of a series of substituted benzoic acids and their tetrabutylammonium salts were determined in tetrahydrofuran by potentiometric and conductimetric methods. A comparison of the pKa values of the acids with those obtained in other amphiprotic and aprotic media showed the better resolution of acid strength in tetrahydrofuran.

Based on the linear relationship between the conductivity of an aqueous solution and the piezoelectric quartz sensor frequency, it was derived that under certain experimental conditions the frequency shift should show a linear dependence on concentration for common acids and aspirin. This verified experimentally. A calibration model for the simultaneous determintaion of aspirin (ASA) and salicylic acid (SA) was established. The usefulness of the technique was evaluated by the analysis of mixtures of known composition using a P-matrix calibration method. The average relative errors were 0.81% for ASA and 1.63% for SA.

The design is described of a thin-layer contactless conductivity detector suitable for liquid chromatography and flow-injection analysis. Its principal analytical parameters have been determined using a potassium chloride solution: the linear dynamic range extends from 7.5 x 10(-6) to 1.5 x 10(-2) S m(-1), corresponding to the KCl concentration range from 0.5 to 1000 microM, the limit of detection equals 3.5 x 10(-6) S m(-1) (0.2 microM KCl), the detection repeatability, expressed in terms of the relative standard deviation, amounts to 1.13% and the detection volume is 0.6 microL. The detector was applied to detection of ionic compounds, benzoic, lactic and octanesulfonic acids, and sodium capronate, after their separation by liquid chromatography in a Biospher PSI 100C 18 columns using a 60% aqueous acetonitrile mobile phase. The frequency characteristics of the detector are reasonably theoretically described on the basis of a simple model which is commonly used in the field of contactless impedance detectors.

The formation of mononuclear and polynuclear aqueous silver complexes of 4-substituted-1,5-dimethyl-1,2,4-triazolium-3-thiolates was determined by titrations and then analyzed with a Monte-Carlo-based algorithm developed to fit a model for competitive equilibria to titration data. The apparent stability constants, βnm, for AgnLn+m complex formation (β11, β12, β13, β23, β34) are tabulated for various thiolates, L, at 23° C and at an ionic strength μ = 0.1 M in KNO3. These Ag+-triazolium thiolate complexes form with a strength that is intermediate between those of the strong Ag+-thiosulfate complexes and the weaker Ag+-thione complexes. The polynuclear Ag2L2+3 complex dominates in ligand-rich solutions of the triazolium thiolates, in contrast to the mononuclear Ag(S2O3)3−2 complex, which predominates over the other complexes in thiosulfate-rich solutions.

A piezoelectric detector was used to determine end-points of redox titrations of iron(II) with potassium dichromate and permanganate. Iron(II) at concentrations down to 1 × 10−5 mol l−1 can be determined. Factors influencing the frequency shift-volume titration curve were investigated.

The suitability of capillary electrophoresis (CE) with capacitively coupled contactless conductivity detection (C(4)D) for the direct determination of uric acid in human plasma and urine was investigated. It was found that a careful optimization of the buffer composition and pH was necessary to achieve selective determination in the complex sample matrices. An electrolyte solution consisting of 10mM 2-morpholinoethanesulfonic acid (MES), 10mM histidine and 0.1mM hexadecyltrimethylammonium bromide (CTAB), pH 6.0, was finally found suitable for use as running buffer for both sample matrices. The limit of detection (3 S/N) was determined as 3.3 microM. The linearity of the response was tested for the range between 10 and 500 microM and a correlation coefficient of 0.9996 was obtained. Intra- and inter-day variabilities were >10%. Quantitative analysis of urine and plasma samples showed a good correlation with the routine enzymatic method currently used at the University Hospital of Basel.

This review explores the question whether chemometrics methods enhance the performance of electroanalytical methods. Electroanalysis has long benefited from the well-established techniques such as potentiometric titrations, polarography and voltammetry, and the more novel ones such as electronic tongues and noses, which have enlarged the scope of applications. The electroanalytical methods have been improved with the application of chemometrics for simultaneous quantitative prediction of analytes or qualitative resolution of complex overlapping responses. Typical methods include partial least squares (PLS), artificial neural networks (ANNs), and multiple curve resolution methods (MCR-ALS, N-PLS and PARAFAC). This review aims to provide the practising analyst with a broad guide to electroanalytical applications supported by chemometrics. In this context, after a general consideration of the use of a number of electroanalytical techniques with the aid of chemometrics methods, several overviews follow with each one focusing on an important field of application such as food, pharmaceuticals, pesticides and the environment. The growth of chemometrics in conjunction with electronic tongue and nose sensors is highlighted, and this is followed by an overview of the use of chemometrics for the resolution of complicated profiles for qualitative identification of analytes, especially with the use of the MCR-ALS methodology. Finally, the performance of electroanalytical methods is compared with that of some spectrophotometric procedures on the basis of figures-of-merit. This showed that electroanalytical methods can perform as well as the spectrophotometric ones. PLS-1 appears to be the method of practical choice if the %relative prediction error of approximately +/-10% is acceptable.

A new dual detection system for microchip is reported. Both fluorescence detector (FD) and contactless conductivity detector (CCD) were combined together and integrated on a microfluidic chip. They shared a common detection position and responded simultaneously. A blue light-emitting diode was used as excitation source and a small planar photodiode was used to collect the emitted fluorescence in fluorescence detection, which made the device more compact and portable. The coupling of the fluorescence and contactless conductivity modes at the same position of a single separation channel enhanced the detection characterization of sample and offered simultaneous detection information of both fluorescent and charged specimen. The detection conditions of the system were optimized. K(+), Na(+), fluorescein sodium, fluorescein isothiocyanate (FITC) and FITC-labeled amino acids were used to evaluate the performance of the dual detection system. The limits of detection (LOD) of FD for fluorescein Na(+), FITC, FITC-labeled arginine (Arg), glycine (Gly) and phenylalanine (Phe) were 0.02micromolL(-1), 0.05micromolL(-1), 0.16micromolL(-1), 0.15micromolL(-1), 0.12micromolL(-1) respectively, and the limits of detection (LOD) of CCD achieved 0.58micromolL(-1) and 0.39micromolL(-1) for K(+) and Na(+) respectively.