Aminizturized two-electrode electrochemical (EC) cell was developed and was coupled on-line with an electrospray ionization Fourier transform ion cyclotron resonance mass spectrometer (ESI-FTICR MS). Electrochemistry on-line with mass spectrometry, EC/ESI-FTICR MS, of triphenylamine (TPA), which undergoes one-electron oxidation to form a radical cation (TPA(+), demonstrates a significant sensitivity enhancement compared to ESI-FTICR MS. The on-line EC cell configuration with a stainless steel ES needle as the working electrode produces the highest sensitivity in EC/ESI-MS. The results provide evidence that, during the ES ionization, electrolytic reactions occur mainly in the ES tip region, as previously predicted. The results demonstrate that ESI-MS signal suppression by tetrabutylammonium perchlorate electrolyte, which can be a problem, is minimized in EC/ESI-MS. TPA(+ dimer tetraphenylbenzidine (TPB) can be detected by EC/ESI-MS, together with TPA(+, as TPB(+ and TPB2+. The high mass resolving power of FTICR MS was exploited to identify TPB2+ dication in the presence of [TPA(+ - H(]+ ions of the same m/z, from their respective isotopic distributions. The dimer dication TPB2+ can be detected only in EC/ESI-MS.
In the investigation of the anodic oxidation of triphenylamine (TPA) by electrochemistry/particle beam mass spectrometry (EC/PB/MS), the presence of a non-volatile organic electrolyte tetrabutylammonium perchlorate (TBAP) caused a significant MS signal enhancement of the TPA(+ ion. The magnitude of the signal enhancement was ca. 35 fold and was observed in the presence of 10/1 excess of TBAP. While the mobile phase additives have been frequently used in PB/LC/MS analysis to enhance analyte signal, the use of a nonvolatile surface-active organic salt as a mobile phase additive, to enhance the signal of a neutral analyte, has not been reported. The present work demonstrates that in addition to the well-known improved mass transport of the analyte through the PB interface, the presence of TBAP also contributes to the reduced fragmentation of the analyte molecular ion, TPA((, in the EI ion source; this effect also contributes to the observed signal enhancement. This is the first report about the effect of the mobile phase additive on the EI mass spectrum of the analyte. In addition, it is demonstrated that the concentration ratio of TBAP/TPA plays a critical role in the signal enhancement process. Best linearity, sensitivity and reproducibility of the calibration curves are obtained by keeping the TBAP/TPA ratio constant near an optimum value during calibrations.
Anodic oxidation of triphenylamine (TPA) in acetonitrile was investigated by electrochemistry (EC) combined on-line with mass spectrometry (MS) through a particle beam (PB) interface, EC/PB/MS. Electrooxidation of TPA generates a TPA(.+) radical cation (m/z 245) which dimerizes to tetraphenylbenzidine (TPB, MW 488). TPB is readily oxidized to TPB.+ (m/z 488) and TPB2+ (m/z 244) at the oxidation potential of TPA. In EC/PB/MS, direct monitoring of the oxidation of TPB to TPB.+ radical cation as a function of the electrode potential was achieved via selective ion monitoring of the ion peak at m/z 488. By using the relative intensity ratio of ions at m/z 244 (TPB2+) to 245 (TPA(.+)), the formation of TPB2+ as a function of the electrode potential was also monitored, EC/PB/MS showed a maximum rate of formation of TPB.+ at +1.2 Vvs Pd, while TPB2-/- is generated at a maximum rate at +1.6 V vs Pd. The effect of spectral interference from the electron impact ionization of TPA, on EC/PB/ MS results, is also discussed. Finally, a significant signal enhancement is observed in the presence of tetrabutylammonium perchlorate (TBAP) and is reported for the first time. Compatibility of coupling of EC with MS via PB interface for EC/MS studies in nonaqueous solvents is demonstrated. The observation of significant signal enhancement in the presence of TBAP may facilitate other applications of LC/MS.
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Nanostructured carbon fiber microdisk electrodes were prepared by a combination of mechanical polishing and electrolytic treatment, where the latter involved moderate oxidation of the surface, followed by a reduction. A high desnity of surface defects contributed to a high capacitance of the nanostructured electrodes. Facilitated proton transfer was observed at the nanostructured surface and was associated with cation-exchanged oxide defects. The nanostructured surfaces intercalated uric acid and adenosine and engaged in fast electron/proton transfer in the oxidation of both analytes. As a result, electrolytic treatment followed by fast-scan voltammetry determinations led to a sensitive response of both analytes in physiological buffers. The nanostructured electrodes showed remarkable stability and could be easily regenerated and resued. With long use, electrode activity decreased. Kinetic discrimination of the surface-mediated reaction of ascrobate was achived at high scan rates.
X-Ray photoelectron spectroscopy (XPS) analysis was used to verify the structure of nanosized overoxidized polypyrrole (OPPy) films that were grown on glassy carbon. The films were electrochemically grown to ca. 1.2-1.6 nm thickness from acetonitrile with tetrabutylammonium perchlorate (TBAP) and from water with adenosine triphosphate (ATP) as dopant. The films have been previously characterized by electrochemical methods, but this represents the first study of the ultrathin films by XPS. XPS analysis verifies that the key structural features of nanosized OPPy films are the same as those of much thicker films previously studied by XPS on metal substrates, where differential charging effects are much less severe. Some differences between the microstructures of OPPy and OPPy/ATP films are highlighted by the results. In addition, XPS results confirm a two-domain model of the polymer films that has been proposed from previous studies of PPy and OPPy. The results show that XPS can be used to characterize nanosized films on graphite, after correction for differential charging. Graphite represents a substrate of choice for the fabrication of permselective ultrathin membranes for biosensors from materials such as OPPy.
To obtain highly active carbon fiber electrodes (CFE) for detection of uric acid by fast scan voltammetry, electrode surfaces were modified by polishing and by electrochemical pretreatment (ECP). The effect of pretreatment parameters, such as the time of polishing, the activation potential, and composition of the physiological buffers used in pretreatment, on electrode activity in uric acid determinations was investigated. After the ECP, the background current and the capacitance of the electrode increased in the entire potential window indicating an increase in the apparent electrode area. In addition, the background current of the modified CFE was stable even after the electrode was exposed to air. Based on the experimental results we postulate that after 30 min of continous ECP in the potential window from -1.0 to 1.5 V vs. SCE, formation of nanocracks and edge nanostructures at the surface of the electrode is highly favored yielding a highly active CFE for detection of uric acid in physiological buffers.
Electrochemistry (EC) on-line with mass spectrometry (MS), with a particle beam (PB) liquid interface (EC/PB/MS), has been successfully applied to the investigation of the electrocatalytic type reaction, the oxidation of an amine, dimethylaminopyridine, by a quinone, dopamine. The investigation, of the reaction pathway, by EC/PB/MS supports catalytic type oxidation of the amine via formation of an intermediate quinone-amine adduct. EC/PB/MS allowed verification of the reaction pathway, through the identification of the intermediate, and the products of the oxidation, in close to real time, by EC/MS on-line. Oxidation of the adduct has been estimated as the limiting kinetic step in the reaction.
The performance of active graphite and carbon fiber surfaces produced by different mechanical/electrochemical methods of surface activation has been investigated in the amperometric determinations of xanthine and hypoxanthine under physiologically relevant conditions. The electrodes showed better limits of detection (LOD) when used with differential techniques with a capability of discriminating the analytical signal from the background. Square wave voltammetry and cyclic voltammetry showed the most sensitive response. Electrochemically activated carbon fiber ultramicroelectrodes showed the highest sensitivity (58 A M-1 cm(-2)) and the LOD in the 200 nM range was observed at the rough pyrolytic graphite electrodes by square wave voltammetry. The results demonstrate the feasibility of the development of new electroanalytical methods for the determination of oxypurines in biological samples.
Uric acid (UA) has been under intensive investigation by electrochemists owing to its important role as a metabolite in biological fluids. One of the major problems in biological determinations of uric acid comes from electrochemical interferences such as ascorbic acid (AA), which has a similar oxidation potential, E-1/2 approximate to 200 mV versus SCE, at graphite electrodes, and is present at high concentrations in biological systems. UA undergoes a 2 H+, 2 e(-) oxidation in aqueous buffers, The oxidation product, a diimine, is an unstable intermediate with a half-life of less than 22 ms. A follow-up hydration reaction converts the diimine to an imine alcohol. Results of previous work show that UA weakly adsorbs and undergoes a fast electron transfer reaction, k(s) = 54 s(-1), at carbon fiber electrodes. These characteristics make UA an excellent candidate for fast scan voltammetric (FSV) determinations. This paper presents the results of FSV at bare carbon fiber electrodes. The results show good selectivity and sensitivity in the determination of low concentrations of UA in the presence of high concentrations of AA, By increasing the scan rate above 500 V s(-1), voltammograms of UA in the presence of AA can be resolved because of the kinetic differences in the response of the two anions, without the need for a permselective film on the electrode. Results are also presented that demonstrate an effective way to reach a stable background current at bare carbon fiber electrodes, which is required in FSV because the signal from the analyte is smaller than the electrochemical signal from the background current. Signal-to-noise ratios at bare carbon fiber electrodes in FSV are improved, because the high temporal resolution in fast scan methods allows the acquisition of a large number of scans that can be signal averaged in a short period of time. In addition, large signals can be measured because the voltammetric peak current increases with increase in scan rate.
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A new electrochemical flow-through cell has been developed for on-line electrochemistry/mass spectrometry (EC/MS) that allows easy access to the working electrode for resurfacing or replacement and that can withstand back pressures of similar to 2000 psi. Simple off-line hydrodynamic electrochemical methods have been developed to determine the cell conversion efficiency. The methods can be used to determine the number of moles of analyte transferred from the cell to the LC/MS interface and thereby estimate the sensitivity in on-line EC/MS. The results that have been obtained show that the mobile-phase composition may influence the cell conversion efficiency. The highest conversion efficiency in the flow-through cell(similar to 32%) was obtained using 90/10 CH3OH/H2O as the mobile phase, while the lowest conversion efficiency (similar to 1.8%) was observed with an aqueous mobile phase. Using different mobile phases, the electrochemical now-through cell was tested in thermospray and particle beam (PB) EC/LC/MS. Conversion efficiencies obtained with the cell on-line in EC/PB/MS are in agreement with the efficiency determined by off-line hydrodynamic electrochemical methods. New insights into nucleophilic reactions of a quinone formed in the electrochemical oxidation of dopamine were obtained online by EC/PB/MS using the new cell.
Selectivity and sensitivity in the determination of low concentrations of dopamine (cation) in the presence of large excess of ascorbic acid (anion), and in the determination of low concentrations of uric acid in the presence of ascorbic acid, has been investigated by fast scan voltammetry at bare electrodes. In spite of similar reaction potentials of dopamine and ascorbic acid, selectivity to dopamine can be substantially improved at a bare electrode by pushing the scan rate up to 10000 V/s. Determination of dopamine in the presence of 1000 fold excess of ascorbic acid is possible at high scan rates because of the differences in the electrochemical kinetics of the two probes. In addition, fast scan voltammetry at bare electrodes can achieve improved selectivity to uric acid (anion) in the presence of ascorbic acid (anion). The effect of the method of electrode surface treatment on response (peak separation and peak current in voltammetry) to dopamine and uric acid is also discussed. Signal-to-noise ratios at bare electrodes can be improved in fast scan methods because of the high temporal resolution of the method which allows acquisition of a large number of scans which can be signal averaged in a short period of time. In addition, because voltammetric peak current increases with scan rate, large signals can be measured. The limitations of fast scan voltammetry at bare electrodes are also discussed, such as the requirement for accurate background subtraction and the limited digital resolution which results from high background currents.
Ultrathin overoxidized polypyrrole (OPPy) films templated with adenosine, inosine and adenosine 5'-triphosphate (ATP), grown on glassy carbon (GC), have been characterized. High sensitivity for ruthenium hexaamine, uric acid and adenine at the ultrathin him electrodes indicates that the interactions with the film rather than in-film transport controlled the response of the him electrodes in slow scan voltammetry. The high sensitivity at the film electrodes allowed the analysis of the effect of templating on the probe-film interactions. The results show that templating with purines changes the selectivity and sensitivity of OPPy films. It is demonstrated that templating with ATP, via electrostatic interactions of ATP with the positively charged PPy, has a most significant effect on the response of the OPPy films, and the ATP-templated films show high selectivity to adenine. The sensitivity for adenosine is significantly enhanced at OPPy films considering that the response is not apparent at the bare glassy carbon.
Gold fiber thioctic acid (TA) monolayer (thickness of similar to 8 Angstrom) ultramicroelectrodes (UMEs) have been fabricated and characterized. Gold fibers were acid etched in order to produce monolayers that can act as high-quantity permselective membranes. The selectivity and sensitivity of the TA monolayer UMEs compares favorably to that of TA monolayers prepared on Au vacuum deposited on large (similar to 1 cm(2) area) single-crystal silicon wafers. The results obtained here illustrate selectivity of the monolayer electrodes to electroinactive electrolyte anions, which participate indirectly in the electrode reaction, in addition to the previously established selectivity to electroactive cations and anions. The permselectivity of the monolayer to electrolyte anions can result in lower than expected steady-state currents at the monolayer UMEs, at the reaction potentials of fast-reacting analytes, and at high overpotentials for kinetically slow probes. Application of TA UMEs to amperometric pH sensing is also described in the determination of the pK(a) of TA at the monolayer surface. The method is compared with other methods of monolayer pK(a) measurements.
The relationship between apparent diffusion coefficients, D', of electroactive probe and the microstructure of an oil-in-water anionic microemulsion, sodium dodecyl sulfate (SDS)/dodecane/1-pentanol/0.1M NaClaq has been investigated. Three probes, ferrocene (Fc), Fe(CN)(6)(3-) and methyl viologen (MV(2+)) allowed the effect of different microstructure and interactions on the D'-values to be investigated. The results indicate that the droplet as well as bicontinuous microemulsion structures can be identified and point to the effects of cross-phase exchange, partitioning, and multiple binding on the D'-values.
In this paper a simple current transducer is described which can measure pA currents at low scan rates in conjunction with a commercial potentiostat such as the one supplied by Bioanalytical Systems (BAS), and which will allow fast-scan voltammetry measurements up to 4000 V/s in conjunction with a waveform generator and a data acquisition system.
The effect of monolayer structure and solution composition on electrochemical response at ultrathin monolayers formed by self-assembly has been investigated. The monolayers studied consisted of thioctic acid (1,2-dithiolane-3-pentanoic acid) and 1-hexanethiol, where different degrees of surface hydrophobicity were obtained through the coassembly of these two molecules. On the more hydrophilic thioctic acid monolayers, fast kinetics and high electrochemical sensitivity were obtained for hydrophilic probes Fe(CN)6(3-) and Ru(NN3)6(3+). Permselectivity at the hydrophilic monolayer electrodes could be achieved by controlling the extent of dissociation of the monolayer COOH head groups. As the hydrophobicity of the film increased with the coadsorption of hexanethiol, the kinetics of the hydrophilic probes became slower. For more hydrophobic catecholamines and quinone probes, slower kinetics (lower sensitivity) were consistently observed at the monolayer electrodes, independent of the monolayer composition. A simple modified Stern model is proposed to represent the relationship between the probe response, the monolayer structure, and the electrolyte composition. The role of the Donnan potential in controlling film permselectivity is addressed, and the roles of monolayer stability and substrate quality on the monolayer electrode response are discussed.
The modification of vitreous C electrodes (Jaramillo et al., Anal. Chem., 1993, 65, 3441) and rough pyrolytic graphite electrodes (0.05-0.06 cm2) with deoxycholic acid (I) for the determination of cytochrome c (II) was investigated. All measurements were made by cyclic voltammetry (CV) using SCE reference electrodes and Pt auxiliary electrodes. Interaction of I with the electrode surfaces was demonstrated by analysis of 0.5mM-dopamine in 0.5M-phosphate buffer of pH 8 containing 5mM-I. Modification with I eliminated the irreversible surface changes which occur during determination of II at graphite electrodes, and the sensitivity and reproducibility of the responses to I were improved. These CV experiments were performed using 100.mu.M-II and 1mM-I in 100mM-phosphate buffer of pH 8, with a scan rate of 0.1 V/s. For determination of II at vitreous C electrodes by CV at 0.1 V/s using 35mM-phosphate buffer of pH 8 containing I at a constant I to II ratio of 12:1, the calibration graphs were linear for 9-23.mu.M-II; the detection limit was .apprx.9.mu.M. No RSD are given. The interaction of II with graphite in the presence of I is discussed.
Ultrathin overoxidized polypyrrole films were coated on different substrates (graphite and Pt) and the cyclic voltammetric behaviour of the resulting electrodes were studied using 10mM-dopamine and ascorbic acid as analytes. Carbon electrodes were modified by applying a mixture of 20mM-pyrrole/0.1M-tetrabutylammonium perchlorate in acetonitrile to the electrode surface and applying a potential of +0.95 V vs. a Ag wire for electropolymerizaion. After rinsing with H2O, the electrode was transferred to 0.5M-phosphate buffer of pH 7 and overoxidized at +0.95 V vs. SCE. The procedure was repeated several times. Platinum ultramicroelectrodes were modified by immersion for 5 min in 20mM-pyrrole in acetonitrile, followed by rinsing and electropolymerization in 0.5M-phosphate buffer of pH 7 at +0.95 V vs. SCE. Excellent permselectivity against anions and excellent selectivity toward cations was achieved. The permselectivity of films produced by the two methods pyrrole and pyrrole-2-carboxylic acid are discussed and compared. The ultramicroelectrodes were suitable for applications to in vivo investigations.
An electrochemical analyser fitted with a vitreous carbon or rough pyrolytic graphite working electrode, a Pt auxiliary electrode and a reference SCE was used for cyclic voltammetry of dopamine and 3,4-dihydroxyphenylacetic acid (DOPAC) in phosphate buffer media of pH 7 with and without added 10mM-SDS and 3mM-hexadecyltrimethylammonium bromide (I), respectively. The effect of I was examined by adjusting the buffer concentration between 0.05 and 0.5M and added 16mM-MgCl2 was used to evaluate the effect of multivalent cations. Although interactions between the catechol derivatives and the electrode surface were significantly affected by I and by electrolyte charge, the surfactant/surface and surfactant/analyte interactions were not, indicating the ionic buffer effect of the surfactants. For the vitreous carbon electrode, the response to DOPAC in the presence of I was best at low I. The results confirmed the value of surfactants in providing a renewable surface environment favourable to the analysis of biological compounds having hydrophilic and hydrophobic moieties.
An expression correlating the experimental E1/2 value with the aq. formal potential, the diffusion coefficient and the partitioning constants for a micro-heterogeneous (e.g., micellar) system (cf. Georges and Desmettre, Electrochim. Acta, 1984, 29, 521) was examined experimentally by using ferrocene and methylviologen as model compounds in micro-emulsions comprising 0.1M-NaCl, dodecane, SDS and pentanol in a range of proportions. Cyclic voltammetric and chronocoulometric measurements were carried out with use of a vitreous carbon working electrode, a Pt-wire auxiliary electrode and a SCE. Methylviologen soln. was deaerated with N2 before analysis. Shifts of E1/2 from the aq. formal potential depended on the relative hydrophobicities of the oxidized and reduced forms of the analyte and the theoretical expression was validated for their prediction. Partitioning constants and diffusion coeffients depended on the composition of the micro-emulsion; suitable choice of this composition allowed the E1/2 value to be shifted to a region suitable for analysis.
Carbon fibre ultramicro-electrodes were used at scan rates of .ltoreq.11 000 V/s to investigate the fast redox reaction of Ru(NH3)6(3+/4+) (I) and Fe(CN)6(3.minus./4.minus.) (II) in aq. soln. The ultramicro-electrodes were prepared from 7 .mu.m diam. carbon fibres using a freshly cut surface for each measurement. Measurements were made on a homemade instrument comprising a two-electrode potentiostat (carbon fibre and SCE electrodes) and an online positive current feedback iR compensation circuit. The interference of the background current was eliminated by background subtracting. The standard reaction rate constants of I and II at the carbon fibre in aq. soln. were 1.22 .+-. 0.07 (n = 15) and 0.13 .+-. 0.02 (n = 12) cm/s, respectively. Uric acid which undergoes a 2H + 2e.minus. redox reaction in aq. soln. with a rapid follow up reaction was also investigated. Uric acid was weakly adsorbed onto the carbon fibre surface and therefore high scan rates were necessary for kinetic measurement to avoid interference from diffusion current.
Rods of metallographically polished vitreous carbon and of roughened pyrolytic graphite were used in cyclic voltammetric measurements of dopamine (I) and 3,4-dihydroxyphenylacetic acid (II) in 0.5M-phosphate buffer (pH 7) and in soln. containing 10mM-SDS or 3mM-tetradecyl-, dodecyl- or hexadecyltrimethylammonium bromide. The electrochemical kinetics of both analytes were slow at the unmodified vitreous carbon electrode, and adsorption was a problem at the roughened pyrolytic graphite electrode. In the presence of cationic surfactant, response to II at the vitreous carbon was enhanced and kinetics were improved; anionic surfactant improved the reproducibility for I and decreased the response to II. Adsorption at pyrolytic graphite was lessened by surfactants, thereby improving reproducibility. A surface model for surfactants acting as spacers at either electrode is proposed.
Electrodes were prepared by vacuum deposition of Au (2000 .angstrom.) on to a Si wafer which was pre-cleaned in a heated soln. of 30% H2O2 - conc. H2SO4 (1:4). The wafer was cut into .simeq.1-cm2 pieces and a Cu wire was connected to the Au with Ag epoxy. The connecting point and the Si side of the electrode were insulated with epoxy. Before use, the electrode was cleaned with the above cleaning soln. and rinsed with H2O. The self-assembled monolayer was prepared by immmersing the electrode for 24 h in ethanolic 0.1% thioctic acid soln. The resulting self-assembled monolayer electrode (SME) was characterized to determine the quality of packing and the effect of soln. pH on response. The electrode was permeable and stable. The charge of the monolayer controls the response to species in soln. and the Au substrate plays an important role in the response of the SME.
Electrodes were prepared from tetrathiafulvalene tetracyanoquinodimethane in polymer paste (McKenna et al., Ibid., 1988, 206, 75) and high-pressure pellet (Freund and Brajter-Toth, Anal. Chem., 1989, 61, 1048) form and from rough-surfaced pyrolytic graphite. They were used in conjunction with a Pt wire auxiliary electrode and a reference SCE in cyclic voltammetric and chronocoulometric studies carried out at 24.degree. on a Bioanalytical Systems BAS-100 analyser. Measured background currents and electroactive area fractions are tabulated. Ferrocyanide, quinol and 2,6-diaminopurin-8-ol (I) were used as probe substances; the supporting electrolyte was 0.1M-KCl [for Fe(CN)64.minus.] or 0.5M-phosphate buffer of pH 7.0. Detection limits were lowest on the polymer paste electrode for Fe(CN)64.minus. and quinol (1.mu.M in each instance); for I, detection limits on the graphite and polymer paste electrodes were 0.57 and 1.04.mu.M, respectively. Linear dynamic ranges are also reported. The contributions of adsorption and electrochemical kinetics to the background current and the sensitivity are discussed.
Films of polymerized pyrrole were prepared potentiostatically from monomer soln. on vitreous-carbon and rough pyrolytic carbon electrodes as described previously (Ibid., 1991, 63, 622). They were then oxidized in aq. soln. at +1.0 V vs. SCE, thereby introducing carbonyl groups into the polymer. The modified electrodes were used in cyclic voltammetry with a Pt auxiliary electrode and a SCE. Measurements were made on 5mM-Ru(NH3)6Cl3, paraquat, 3,4-dihydroxyphenylacetic acid, K3Fe(CN)6 and dopamine in 0.5M-phosphate buffer of pH 7. Differences in diffusion coeff. of these species were used to draw conclusions about the film structure and the relative extents of electrostatic and hydrophobic interactions in the film. The behaviour of the oxidized polymer was also compared with that of Nafion.
This review examines how the cited technique can be used to directly monitor reactants, short-lived intermediates and products of electrochemical reactions as a function of electrode potential. (39 references).
A tetrathiafulvalene - tetracyanoquinodimethane polymer paste electrode was constructed by the procedure described by McKenna et al. (Ibid., 1988, 206, 75). The electrode was mounted in a flow cell and used as the detector in a flow-injection system. The performance of the system was assessed by studying the electrochemical behaviour of quinol, dopamine and ascorbic acid; the results were compared with those obtained with active and inactive vitreous-carbon electrodes. The sensitivity of the electrodes depended on the formation of an active electrode area and on the kinetics of the analytes. The dependence of the response on flow rate was also investigated.
Coating glassy carbon or rough pyrolytic graphite substrates with polypyrrole films results in durable electrodes that do not require re-surfacing between samples. The response of the films can be tailored by the choice of substrate and its treatment. The high background currents typical of polypyrrole electrodes are eliminated, which enhances the analytical usefulness of these surfaces. The permselective response of the films and their ability to retain electroactive counter-ions is discussed.
The modification was made by oxidizing the film deposited on a vitreous-carbon electrode between +0.90 and .minus.0.30 V vs. SCE in 0.5M-phosphate buffer supporting electrolyte. The film produced was electronically non-conductive but ionically conductive and had ion selectivity based on anion exclusion. The effective pH within the film was controlled by the choice of the oxidative supporting electrolyte. The effect of this pH on the rate of dopamine oxidation in a neutral supporting electrolyte was demonstrated.
Electrochemical activation was carried out with a coulometric cell online with the chromatograph. The column (25 cm .times. 4.6 mm) was packed with OD5 (5 .mu.m), with 0.005M-Na 1-heptanesulphonate - 0.175M-acetic acid - 10% methanol as the micellar mobile phase (pH 4.88) and detection at 280 nm. The analytical column was protected with a column (10 cm .times. 4.6 mm) of Adsorbosil silica (200 to 425 mesh). In the potential range 0.4 to 0.8 V vs. Pd, intermediate purine-6-sulphenic acid was detected together with purine-6-sulphinic acid and 6-mercaptopurine disulphide. At potentials >0.8 V, purine-6-sulphonic acid was detected and the oxidation of 9-mercaptopurine was completed. Enzymic activation of oxidation by horseradish peroxidase was similar to electrochemical oxidation at 0.8 V.
The electrochemical behaviour of NADH, xanthine, uric acid, mercaptopurine, 6-thioxanthine, dopamine and ascorbic acid was studied at the organic metal-like polymer paste electrodes based on tetrathiafulvalene - and N-methylphenazene - tetracyanoquinodimethan. The reactivity of small biological molecules at polymer paste electrodes was generally lower than at most active graphite electrodes. Detection limits were .simeq.10.mu.M and the upper limit of rectilinearity was typically 1mM for the compounds studied.
Under unsatd. enzyme conditions the electrode (prep. described) has rectilinear responses for purine (I) and hypoxanthine (II) from 16 to 640.mu.M and for xanthine from 12.mu.M to 1.7mM in 0.5M-phosphate buffer, pH 8.0. The response in blood plasma is rectilinear for I and II between 0.1 and 1.1mM with slopes of .simeq.2.45 .mu.A per 1mM change in concn.
The capacity for FeII of a graphite electrode modified by coating with the cited polymer was evaluated by cyclic voltammetry as a function of pH. The quaternized N atoms of the 4-vinylpyridine - cross-linking agent groups in the film were the principal sites of FeII complexation. The modified electrode was used for determination of FeII by pre-concentration on the electrode, then measuring the current at the oxidation potential of the complex in metal-free soln. The response was rectilinear in the range 10 to 100nM.
The preferential adsorption of allopurinol {1H-pyrazolo[3,4-d]pyrimidin-4-ol} at rough pyrolytic-graphite electrodes (2.25 mm2 area) allowed the simultaneous determination, by differential pulse polarography, of 2,6-diaminopurine and 2,6-diaminopurin-8-ol in phosphate buffer medium (pH 7). The optimum parameters were pulse amplitude 40 mV, sweep rate 5 mV s-1 and timer 0.3 s. A platinum foil was used as counter-electrode. The calibration graphs were rectilinear for 1.mu.M to 0.5mM for both compounds, and the coeff. of variation of peak current was .simeq.10%. The electrodes were polished before each analysis.
