Bo Karlberg

New interface for coupling flow-injection and capillary electrophoresis

Abstract A new interface for coupling flow injection analysis (FIA) and capillary electrophoresis (CE) has been developed. An FIA system is connected to a flow-through channel of the interface. One end of a capillary is positioned in a flow-through channel inside the interface together with a platinum electrode. The other end and a second platinum electrode are immersed in a liquid reservoir situated outside the interface. A constant high voltage is applied across the capillary. A sample plug injected in the FIA part of the system is carried by an electrolyte stream towards the interface and when it passes by the first capillary end, a minor fraction of the sample is introduced into the capillary and gets separated. A UV spectrophotometric detector is placed close to the second capillary end. The FIA-CE interface enables multiple injections so that a high sampling throughput can be achieved (up to 150 samples per hour). The interface was tested by running a standard mixture of nine common anions. The repeatability was approximately 2% (r.s.d.). Rapid, qualitative screening of samples can be performed. Real samples such as tap and rain water were also analyzed. The new interface allows mechanized sample handling prior to the CE separation step.

Extraction based on the flow-injection principle

The dispersion mechanism in flow-injection extraction systems has been investigated. The phase with the highest affinity for the tubing material forms a thin film on the wall (e.g., 0.055 mm in a 0.7-mm i.d. PTFE tube with pentanol/water at a flow velocity of 11 cm s−1). The film thickness increases linearly with increasing flow velocity and can be related to the viscosity/interfacial tension ratio in such a way that a low ratio indicates a thin film. The analyte is extracted into the film and into the adjacent segments. The film is stationary relative to the moving segments and this results in a backward transport of analyte molecules giving rise to dispersion. The thicker the film, the larger the dispersion. By decreasing the tube diameter and the flow velocity, lower dispersion results. Minimum dispersion is obtained for systems in which the phase carrying the analyte does not form the film.

Application of a contactless conductometric detector for the simultaneous determination of small anions and cations by capillary electrophoresis with dual-opposite end injection.

A contactless conductometric detection (CCD) system for capillary electrophoresis (CE) with a flexible detection cell was applied for the simultaneous determination of small anions and/or cations in rain, surface and drainage water samples. The applied frequency, the amplitude of the input signal, the electrolyte conductivity and electrode distance were found to be the most significant factors affecting the detection sensitivity. 2-(N-Morpholino)ethanesulfonic acid/histidine-based (MES/His) electrolytes were used for direct conductivity detection of anions and cations, while ammonium acetate was selected for indirect conductivity determination of alkylammonium salts. For the simultaneous separation procedure, involving dual-opposite end injection, an electrolyte consisting of 20 mM MES/His, 1.5 mM 18-crown-6 and 20 microM cetyltrimethylammonium bromide provided baseline separation of 13 anions and cations in less than 6 min. The detection limits achieved were 7-30 micrograms/l for direct conductometric detection of various common inorganic cations and anions, excluding F- (62 micrograms/l) and H2PO4- (250 micrograms/l), and 35-178 micrograms/l for indirect conductometric detection of alkyl ammonium cations. The developed electrophoretic method with conductometric detection was compared to ion chromatography.

Sample preconcentration by continuous flow extraction with a flow injection atomic absorption detection system

Abstract The flow manifold described allows automatic extraction of metal ions in aqueous samples into 4-methyl-2-penthanone with ammonium pyrrolidinedithiocarbamate as an extracting agent. The organic extract is led into the loop of an injector situated in an integrated feed system of an atomic absorption spectrometer. No dispersion of the injected organic extract plug, 110 μl, occurs in the aqueous feed stream and the resulting signal from the spectrometer is a peak. An increase in sensitivity of 15–20 is achieved for copper, nickel, lead and zinc in comparison with direct aspiration of the aqueous samples. The sampling frequency is 40 h −1 and the consumption of 4-methyl-2-pentanone is typically 0.3 ml min −1 . The detection limit for copper is about 1 μ l −1 .

Extraction based on the flow-injection principle: Part 5. Assessment with a Membrane Phase Separator for Different Organic Solvents

Abstract Several membrane phase separators have been designed and tested for use in a flow-injection extraction manifold. The membrane is sandwiched between two pieces of perspex with grooves facing the membrane. A PTFE membrane with polyethylene backing proved to be most suitable. With this type of phase separator the total dispersion in the extraction system is less than that obtained with the conventional T-piece separator. Alcohols, alkanes, chlorinated hydrocarbons and aromatic solvents pumped at a flow rate of 0.5–1.0 ml min -1 can be segmented with aqueous phase and later separated from it with a recovery of up to 95%. The organic phase passing through the detector flow cell is not contaminated by the aqueous phase to any measurable extent.

Determination of isoprostanes in urine samples from Alzheimer patients using porous graphitic carbon liquid chromatography-tandem mass spectrometry

F2-isoprostanes (F2-iPs) comprise four classes of isomers produced non-enzymatically by free radical attack on arachidonic acid, a component of the cell membrane. This paper describes a new method for the quantification of F2-isoprostanes in urine samples from thoroughly diagnosed Alzheimers disease (AD) patients. The sample pretreatment consisted of liquid extraction of 900 microl urine with diethyl ether, its subsequent evaporation, and finally, reconstitution in 50 microl water. Of this, 20 microl was injected into a HPLC system with a 15 mm x 1 mm porous graphitic carbon column coupled to a triple quadrupole mass spectrometer running in negative electrospray ionization mode. The F2-isoprostanes were separated in 15 min using a linear solvent gradient comprising water, methanol, acetonitrile and ammonium hydroxide at a pH of 9.5. The average recovery obtained was approximately 75%. The limit of detection (3S/N) was calculated for iPF2alpha-III to be 0.7 pg injected on column, corresponding to 0.1 nM. The average level of iPF2alpha was 241 +/- 163 pg/mg creatinine in the urine samples from AD patients (average +/- standard deviation). The corresponding control values were 216 +/- 101 pg/mg creatinine, i.e. no statistically significant difference was noticed. No correlation pattern specific to Alzheimers disease was revealed by principal component analysis of the isoprostane peaks obtained either. The results from this study support earlier findings that levels of peripheral isoprostanes are not increased in patients with Alzheimers disease.

On-line coupling of gas diffusion to capillary electrophoresis

On-line gas diffusion has been coupled to a capillary electrophoresis system (CE) via a specially designed interface. The sample is merged with a modifying solution, e.g., a strong acid, in a flow system to transform the analytes of interest into their respective gaseous forms. These transformed, gaseous analytes permeate through a PTFE membrane into an acceptor stream comprising of a tris-buffer. The continuously flowing acceptor stream is led into an injector forming an integrated part of a flow injection analysis (FIA) system. The sample receiving carrier stream in the FIA system, a chromate buffer, brings the sample, 50 mul, to the FIA-CE interface into which one end of a separation capillary has been inserted. A small portion of the injected sample enters the capillary (electrokinetic injection) and separation takes place. A UV detector is placed at the other capillary end and a run potential of 25 kV is applied to two platinum electrodes positioned in the flow system. Multiple sample injections can be performed in one uninterrupted electrophoretic run. A typical sampling frequency is 15 h(-1); each run may result in quantitation of at least five anions. The overall repeatability is in the range 1.8-3.6% (RSD). The technique has been applied to the analysis of real samples such as soft drinks, vinegar and wine. Selective discrimination of anions which are unable to form volatile species is accomplished. No off-line sample pre-treatment is needed.

Determination of nitrate in municipal waste water by UV spectroscopy

Abstract A method is proposed in which diode-array UV-visible spectroscopy and multivariate data analysis are applied to determine nitrate in municipal waste water. No filtering of the samples is required and no reagents are added. The working range for the nitrate determination is 0.5–13.7 mg/l (0.008–0.22 mM). The relative standard deviation was found to be 3.4%. Other constituents, notably total phosphorus, total nitrogen, ammonium nitrogen and iron can be determined simultaneously with the same method. The developed method can also be used to classify samples. The described method concept is well suited for in-line monitoring.

Flow analysis based on a pulsed flow of solution: theory, instrumentation and applications.

The increased demands placed on solution propulsion by programmed flow systems, such as sequential injection analysis, lab-on-value technology, bead injection and multi-commutation, has highlighted the inability of many conventional pumps to generate a smooth, consistent flow. A number of researchers have examined ways to overcome the inadvertent, uncontrolled pulsation caused by the mechanical aciton of peristaltic pumps. In contrast, we have developed instruments that exploit the characteristics of a reproducible pulsed flow of solution. In this paper, we discuss our instrumental approaches and some applications that have benefited from the use of a reproducible pulsed flow rather than the traditional linear flow approach. To place our approach in the context of the continuously developing field of flow analysis, an overview of other programmed flow systems is also presented.

INTERFACING OF FLOW INJECTION PRE-TREATMENT SYSTEMS WITH CAPILLARY ELECTROPHORESIS

Abstract This article reviews the interfacing of flow injection analysis (FIA) with capillary electrophoresis (CE). Various sample pre-treatment procedures such as dialysis and gas diffusion can be fully automated as can the application of the electrokinetic injection principle for introduction of the treated sample into the CE system using a specially designed interface. The practical applicability of the FIA–CE technique has been demonstrated by the determination of small anions in, for instance, water, juice and milk samples. Sample throughput rates can reach 170/h and the repeatability is typically 2% (R.S.D.).