Capillary batch injection analysis and capillary flow injection analysis with electrochemical detection: a comparative study of both methods

Capillary batch injection analysis (CBIA) and capillary flow injection analysis (CFIA) in combination with electrochemical detection as well as optical detection methods were studied and compared with respect to their performance. Despite the differences in technical equipment both techniques share the same idea of reproducible transport and washout of nanolitre samples over sensing surfaces. Thus the same electrochemical flow cell can be used for both CBIA and CFIA. The amperometric and potentiometric CBIA responses were studied under various experimental conditions in order to optimise the CBIA set-up. In particular, the density of the sample solution relative to that of the cell electrolyte had a remarkable effect on the hydrodynamic characteristics of CBIA. Dispersion in CFIA was investigated using on column UV-detection for electroosmotic flow (EOF) conditions as well as for gravity flow conditions. It is demonstrated for a 75 μm capillary that the relative band broadening of the sample plug under gravity flow is only about twice as large as under EOF. Furthermore, dispersion in a system that involves a chemical reaction between the sample and the carrier solution, namely CrO₇ ^2– and Fe²⁺ has been investigated by amperometric detection and exploited for the determination of dichromate microsamples. Received: 28 November 1997 / Revised: 23 February 1998 / Accepted: 26 February 1998     

Capillary electrophoresis and capillary flow injection analysis with electrochemical detection

Measurements by capillary flow injection analysis (CFIA) and capillary electrophoresis (CE) in conjunction with electrochemical detection are described. The detection is based on an end-column electrode arrangement. Several novel electrodes, such as a spherical gold electrode and a dual-microdisk electrode, are presented and characterized regarding their analytical utility. In order to improve the selectivity of CFIA, dual-electrode and multiple-pulse detection are studied using couples of cyanometallates or metallocenes. Capillary electrophoretic experiments with amperometric detection are performed using 50 m i.d. capillaries without any electrical-field decoupler. The practicality and analytical characteristics of this detection strategy are illustrated for the separation of serotonin and some biological precursors and metabolites of neurotransmitter substances.     

Determination of cyanide in microsamples by means of capillary flow injection analysis with amperometric detection

A new approach for determining cyanide in microsamples is described. The method is based on capillary flow injection analysis (CFIA) with amperometric detection. The sensing electrode is a silver-plated microdisk electrode, where cyanide can react under formation of a dicyanoargentate complex. A remarkably low mass detection limit of 231 fmol cyanide is obtained for an injection volume of 60 nl. The sample throughput of the CFIA-arrangement is comparable with a conventional sized FIA-system. A practical application is given by analyzing the cyanide (amygdalin) concentration in apple kernels.     

Internal standard in flow injection analysis with amperometric detection

In this work, we describe for the first time the use of the internal standard method in flow injection analysis (FIA) with amperometric detection. The method is based on the application of sequential potential pulses to the working electrode in an electrochemical flow cell. The sequence of potential pulses is selected in such a way that the analyte and internal standard compound are detected and monitored individually and independently at the same working electrode. This approach compensates for random errors associated with variations of flow rate, injection volume, ionic strength difference between standards and samples, and accidental insertion or formation of air bubbles in the carrier stream. In addition, this method can overcome the major drawback of amperometric detection using solid electrodes, which is gradual electrode passivation. To illustrate the potential of this method, the flow-injection amperometric detection of uric acid using [Fe(CN)₆]^3? as an internal standard (IS) is presented as an example.     

Amperometric biosensor for oxalate determination in urine using sequential injection analysis

An amperometric flow biosensor for oxalate determination in urine samples after enzymatic reaction with oxalate oxidase immobilized on a modified magnetic solid is described. The solid was magnetically retained on the electrode surface of an electrode modified with Fe (III)-tris-(2-thiopyridone) borate placed into a sequential injection system preceding the amperometric detector. The variables involved in the system such as flow rate, aspired volumes (modified magnetic suspension and sample) and reaction coil length were evaluated using a Taguchi parameter design. Under optimal conditions, the calibration curve of oxalate was linear between 3.0-50.0 mg·L^-1, with a limit of detection of 1.0 mg·L^-1. The repeatability for a 30.0 mg·L^-1 oxalate solution was 0.7%. The method was validated by comparing the obtained results to those provided by the spectrophotometric method; no significant differences were observed.     

Amperometric detection for capillary flow injection analysis

Ultrasmall-volume measurements of oxidizable compounds have been accomplished by coupling a capillary flow injection system with amperometric detection. Remarkably low (femtomole) mass detection limits result from the combination of nanoliter sample volume and the inherent sensitivity of the wall-jet detector. A substantial economy of reagent consumption and disposal accrues from the operation of the nl/min flow regime. Variables influencing the physical dispersion in the capillary flow injection system, including capillary length, sample volume or flow rate, are explored and optimized.     

Electrochemically assisted injection – a new approach for hyphenation of electrochemistry with capillary-based separation systems

A new concept based on the electrochemical conversion of analyte species during the injection into capillary flow systems is presented. This approach is termed electrochemically assisted injection (EAI). In a specially designed injection cell containing the analyte solution a conversion efficiency of about 83% can be achieved. Potassium octacyanotungstate(IV) served as a model compound for the analytical characterisation of EAI applying capillary flow injection analysis with double-pulse amperometric detection. Capillary electrophoresis experiments were performed using EAI to study the electrochemical oxidation of various ferrocene derivatives in acetonitrile solution. The electropherograms recorded with UV detection show separated signals for the ferrocene compounds and their oxidation products. The migration behaviour and the stability of ferrocenium cations and other reaction products were investigated.     

Electroosmotic flow-balanced isotachophoretic stacking with continuous electrokinetic injection for the concentration of anions in high conductivity samples

An EOF counter-balanced ITP boundary has been used to stack anions from high conductivity samples during continuous electrokinetic injection of the sample. In a polystyrenesulfonate/poly(diallyldimethylammonium chloride) polyelectrolyte coated capillary, the time at which the ITP boundary exited the capillary could be prolonged by balancing the movement of the boundary with the EOF. Using a bis-tris-propane electrolyte, the ITP boundary was removed from the capillary within 7 min, while when using triethanolamine the ITP boundary was still at 30% of the capillary after 2 h of injection. Using these systems, the sensitivity of a mixture of simple organic acids in 100 mM Cl⁻ was improved by 700–800-fold using bis-tris-propane with a whole-capillary injection of the sample and 5 min of electrokinetic injection at +28 kV, and 1100–1300-fold using triethanolamine and 60 min of electrokinetic injection under the same conditions. The potential of the method to be applicable to high conductivity samples was demonstrated by stacking a whole capillary filled with urine spiked with naphthalenedisulfonic acid, with limits of detection 450 times lower than those achievable with a normal hydrodynamic injection.     

Batch‐injection versus Flow‐injection Analysis Using Screen‐printed Electrodes: Determination of Ciprofloxacin in Pharmaceutical Formulations

This article highlights the potential use of multi‐walled carbon‐nanotube modified screen‐printed electrodes (SPEs) for the amperometric sensing of ciprofloxacin and compares the association of batch‐injection analysis (BIA) and flow‐injection analysis (FIA) with amperometric detection. Both analytical systems provided precise (RSD<5 %) and sensitive determination of ciprofloxacin (LOD<0.1 μmol L^?1) within wide linear range (up to 200 μmol L^?1). Accuracy of both methods was attested by recovery values (93–107 %) and comparison with capillary electrophoresis. The BIA system is completely portable (especially due to association with SPEs) and provided faster analyses (130 h^?1) and more sensitive detection than the FIA system due to the higher flow rates of injection.     

Construction and Application of Flow Enzymatic Biosensor Based of Silver Solid Amalgam Electrode for Determination of Sarcosine

In this paper, the flow amperometric enzymatic biosensor based on polished silver solid amalgam electrode for determination of sarcosine in model sample under flow injection analysis conditions is presented. The biosensor works on principle of electrochemical detection of oxygen decrease during enzymatic reaction which is directly proportional to the concentration of sarcosine in sample. The whole preparation process takes about 3 h. The RSD of repeatability of 10 consecutive measurements is 1.6 % (c_sarcosine=1.0×10^?4 mol dm^?3). Under optimal conditions the calibration dependence was linear in the range 7.5×10^?6–5.0×10^?4 mol dm^?3 and limit of detection was 2.0×10^?6 mol dm^?3.     

Flow Injection Analysis of Maleic Hydrazide Using an Electrochemical Sensor Based on Palladium‐Dispersed Carbon Paste Electrode

A new analytical methodology for the electrochemical detection of the herbicide maleic hydrazide (3,6‐dihydroxypyridazine) by flow injection analysis is presented. This method is supported by the novel application of a palladium‐dispersed carbon paste electrode as an amperometric sensor for this herbicide. Maleic hydrazide shows anodic electrochemical activity on carbon‐based electrodes (glassy carbon or carbon paste electrodes) in all the pH range. This electrochemical activity is enhanced using metal‐dispersed carbon paste electrodes, especially at Pd‐dispersed CPE which displays good oxidation signals at 690 mV (0.050 M phosphate buffer pH 7.0), 140 mV lower than at unmodified electrodes. Under the optimized conditions, the electroanalytical performance of Pd‐dispersed CPE in flow injection analysis was excellent, with good reproducibility (RSD 3.3%) and a wide linear range (1.9×10^?7 to 1.0×10^?4 mol L^?1). A detection limit of 1.4×10^?8 mol L^?1 (0.14 ng maleic hydrazide) was obtained for a sample loop of 100 μL at a fixed potential of 700 mV in 0.050 M phosphate buffer solution at pH 7.0 and a flow rate of 2.0 mL min^?1. The proposed method was applied for the maleic hydrazide detection in natural drinking water samples.     

Application of a Square‐Wave Potential Program for Time‐Dependent Amperometric Detection in Capillary Electrophoresis

Use of a square‐wave potential program for time‐dependent amperometric detection of analyte zones in capillary electrophoresis (CE) is described. Electrochemical detection for CE requires that the separation field be isolated from that of the electrochemical detection. This is generally done by physically separating the CE separation field from that of the detection. By applying a time variant potential program to the detection electrode, the detector current has a time dependence that can be used to help isolate the electrochemical detection current from that of the separation. When using a 20 μm inner‐diameter capillary, we find that a square‐wave potential program decreases the RMS baseline current from 4.5×10^?10 A, found with a constant potential amperometric detection, to 1.1×10^?10 A when using a square‐wave potential program. With a 75 μm inner‐diameter capillary, the improvement is even more dramatic, from 2.3×10^?9 A with amperometric detection to 2.06×10^?10 A when using a 1 Hz square‐wave potential program. When not using the time‐dependent detection with the 75 μm capillary, the analyte zones were beneath the S/N for the system and not detected. With the square‐wave potential program and time‐dependent detection, however, the analyte zones for an electrokinetic injection of 200 μM solution of 2,3‐dihydroxybenzoic acid were observed with the 75 μm inner‐diameter capillary. The improvement in the ability to discriminate the analytical signal from the background found experimentally is consistent with modeling studies.     

On-line sequential preconcentration of inorganic anions by anion-selective exhaustive injection and base-stacking in capillary zone electrophoresis

A sequential electrostacking method based on anion-selective exhaustive injection (ASEI) and base-stacking (BS) is presented for the preconcentration and determination of inorganic anions by capillary zone electrophoresis (CZE) in this paper. Tetradecyltrimethylammonium bromide as an electroosmotic flow (EOF) modifier was added into the buffer to suppress EOF of the capillary. Firstly, a water plug was hydrodynamically injected into the capillary. During ASEI under negative high voltage, the sample anions migrated quickly towards the boundary between the water plug and buffer in the capillary. Then an alkaline zone was injected electrokinetically to concentrate the anions further. With the sequential electrostacking method, the preconcentration factor of (0.8-1.3) x 10(5) was obtained compared with the conventionally electrokinetic injection and the relative standard deviation of peak area was 3.3-5.3% (n = 5). The detection limits of ASEI-BS-CZE for six inorganic anions were 6-14 ng/L. The proposed method has been adopted to analyze six anions in cigarette samples successfully.     

Coaxial flow‐gating interface for capillary electrophoresis

A coaxial flow‐gating interface is described in which the separation capillary passes through the sampling capillary. Continuous flow of the sample solution flowing out of the sampling capillary is directed away from the injection end of the separation capillary by counter‐current flow of the gating solution. During the injection, the flow of the gating solution is interrupted, so that a plug of solution is formed at the inlet into the separation capillary, from which the sample is hydrodynamically injected. Flow‐gating interfaces are originally designed for on‐line connection of capillary electrophoresis with analytical flow‐through methods. The basic properties of the described coaxial flow‐gating interface were obtained in a simplified arrangement in which a syringe pump with sample solution has substituted analytical flow‐through method. Under the optimized conditions, the properties of the tested interface were determined by separation of K⁺, Ba²⁺, Na⁺, Mg²⁺ and Li⁺ ions in aqueous solution at equimolar concentrations of 50 μM. The repeatability of the migration times and peak areas evaluated for K⁺, Ba²⁺ and Li⁺ ions and expressed as relative standard deviation did not exceed 1.4%. The interface was used to determine lithium in mineral water and taurine in an energy drink.     

Dual opposite injection electrokinetic chromatography: nonionic microemulsion pseudostationary phase and novel approach to electrokinetic sampling bias

Dual opposite injection capillary electrophoresis (DOI-CE) is a family of CE techniques in which the sample is introduced into both ends of the capillary. For the analysis of compounds with widely varying pKa values using a voltage-driven separation scheme, DOI-CE is superior to conventional CE with sample introduction at only one end of the capillary due to DOI-CE's broader elution window. To enhance the DOI-CE separation, a running buffer with a microemulsion system was developed. Since DOI-CE works best under conditions of low electroosmotic flow (EOF), the suppression of EOF via the addition of a multiply charged cation (e.g., Zn2+) to the buffer was investigated, and was found to suppress the EOF effectively at moderate concentrations (2.5-10 mM). Three different dual opposite injection modes were studied: simultaneous electrokinetic injection, sequential electrokinetic injection, and sequential hydrodynamic injection. The injection bias in the first two electrokinetic injection modes was compared with the sequential hydrodynamic injection. Corrections in the bias of the electrokinetic injections were discussed, and an improved approach was suggested. Finally, the effect of the relative concentration of the multiply charged cation in the sample plug and running buffer on the peak shape of co-electroosmotic and counter-electroosmotic ions was examined, and found to be much more influential on the latter.     

Flow injection analysis with detection at mercury drop electrode in the presence of surfactants

The surfactant presence in a sample may cause distortion of the flow injection peak obtained with amperometric detection at a mercury drop electrode. Distortion depends on detection parameters and flow system operating parameters and in some cases it may be eliminated by their careful optimization. Introduction of fumed silica into the carrier solution allows the dynamic removal of surfactant during the sample passage through the mixing coil. In the case of higher surfactant concentration in the sample, addition of fumed silica directly to the sample may be effective. Examples with amperometric detection as well as anodic stripping and adsorptive stripping voltammetric detection are described.     

Combination of flow injection with capillary electrophoresis: 8. Miniaturized capillary electrophoresis system with flow injection sample introduction and electrogenerated chemiluminescence detection

The combined flow injection (FI)-capillary electrophoresis (CE) system was further exploited by coupling to an electrogenerated chemiluminescence (ECL) detection system. A low-cost miniaturized CE system was developed on a chip platform to provide easy interface both with FI sample introduction and with ECL detection. A falling-drop interface was employed to perform FI split-flow sample introduction while achieving electrical isolation from the CE high voltage. A plexiglas reservoir at the capillary outlet served as both the reaction and detection cell for the ECL reaction, with Ru(bpy)₃²⁺ reagent continuously flowing through the cell. An optical fiber was positioned within the reservoir close to the capillary outlet for transferring the ECL emission to the PMT. The relative positions of the capillary outlet, working electrode and optical fiber as well as reagent renewal flow-rate were optimized to achieve both good sensitivity and separation efficiency under non-interrupted sampling conditions, involving large numbers of samples. An on-column joint often used in other works for isolating the ECL detection system from the CE separation voltage was not found necessary. The performance of the system was illustrated by the baseline separation of proline, valine and phenylalanine with a high throughput of 50 h⁻¹ and plate height of 14 μm for proline under 147 V cm⁻¹ field strength. Detection limits (3σ) were 1.2, 50 and 25 μM and peak height precisions were 1.4, 5.4 and 4.3% R.S.D. (n=9) for proline, valine and phenylalanine, respectively.     

A capillary electrophoresis chip with hydrodynamic sample injection for measurements from a continuous sample flow

A microchip-based capillary electrophoresis device supported by a microfluidic network made of poly(dimethylsiloxane), used for measuring target analytes from a continuous sample flow, is presented. The microsystem was fabricated by means of replica molding in combination with standard microfabrication technologies, resulting in microfluidic components and an electrochemical detector. A new hydrodynamic sample injection procedure is introduced, and the maximum number of consecutive measurements that can be made with a poly(dimethylsiloxane) capillary electrophoresis chip with amperometric detection is investigated with respect to reproducibility. The device features a high degree of functional integration, so the benefits associated with miniaturized analysis systems apply to it.     

Flow injection analysis with inductively coupled plasma-atomic emission spectroscopy: critical comparison of conventional pneumatic, ultrasonic and direct injection nebulization

The analytical figures of merit observed under flow injection analysis (FIA) conditions of a direct injection nebulizer (DIN) interfaced to an inductively coupled plasma-atomic emission spectroscopy (ICP-AES) facility were found to be comparable to or better than conventional pneumatic nebulization in terms of limits of detection, reproducibility and interelement effects. The DIN offered clog-free operation and part per billion limits of detection for 30μl sample injection volumes and carrier stream consumption rates in the range of 100–200μl min ⁻¹. The relative detection limits observed were generally comparable to those obtained for: (a) FIA introduction of 200μl or continuous sample introduction into a conventional cross flow nebulizer; and (b) FIA introduction of 500μl or continuous sample introduction into an ultrasonic nebulizer. Absolute and relative detection limits were comparable to or within the range of values reported for electrothermal vaporization-ICP-AES and comparable or superior to those reported for the graphite cup, direct insertion-ICP-AES. The reported absolute detection limits for the graphite-rod direct insertion approach ranged from comparable values to superior by a factor of 30. At the normal compromise observation height (20 mm), the interelement effects, to the extent they were observable, were comparable in magnitude for both the DIN and conventional cross-flow pneumatic nebulizer.     

Solenoid Micro‐pumps: A New Tool for Sample Introduction in Batch Injection Analysis Systems with Electrochemical Detection

This work presents the use of solenoid micro‐pumps as a new strategy for sample introduction in batch‐injection analysis (BIA). The volume of solution dispensed on each pulse of the solenoid micro‐pump (μL) is used as fixed and reproducible injection volume for BIA. In this system, the injection steps are possible in stopped flow mode resulting in low background noise levels, which would not be possible under continuous flow conditions and using solenoid micro‐pumps. As a proof‐of‐concept, amperometric and square‐wave voltammetric (SWV) determination of dopamine was demonstrated as well as anodic‐stripping voltammetry (ASV) of metals. The micro‐pump provided injections of 14 μL of solution per pulse at 512 μL s⁻¹ over the electrode during electrochemical measurement. Moreover, fast injections of analyte or electrolyte were programmed during deposition or conditioning steps of ASV for analyte preconcentration or electrode cleaning. The proposed system improved limits of detection and sensitivity (2‐fold), precision and sample throughput in comparison with traditional BIA due to enhanced mass transfer and consequent reduced dispersion of analyte, and possible control of injections without analyst intervention. This work opens new possibilities of applications of the BIA system, including on‐line sample treatment (derivatization or dilution steps).