Application of a capillary flow cell to sophisticated flow-injection systems

Demands placed on spectrophotometric detectors for use in flow-injection systems are discussed. Based on this background, it is proposed that it is worthwhile investing in high-performance detectors and other hardware to make the best use of the advantages of flow-injection methods. A novel flow cell that utilizes optical fibres to transmit light through a capillary, from and to a photometric detector, was applied in experiments that demonstrated the difference between flow systems with sample and with reagent injection through the application of gradient techniques such as electronic dilution. It is show that it is possible to extend the dynamic range of flow-injection systems using gradient techniques, but not in the case of reagent injection.In another set of experiments, the small illuminated volume of the detector cell (< 1 μl) made it possible to measure directly the segmented stream of aqueous and organic phases in flow-injection extraction without phase separation.The monitoring of a pesticide served as a basis for comparison between classical flow-injection extraction and the novel extraction without phase separation. Although the detection limit is lower in the classical technique, extraction without phase separation has the advantage of allowing simultaneous monitoring of the aqueous and organic phases and eliminating the need for the failure-prone phase separation step. A simple sorting program is introduced as an alternative data analysis scheme for the measurement of peak widths and for the evaluation of data gained from flow-injection extraction without phase separation. It proved to be much faster and considerably simpler to use than previous routines.     

Electroanalytical voltammetry in flowing solutions

A review is given of the principles, key developments and representative applications of small electrodes in flow-through electrochemical (ec) detectors having very low effective dead volume (<10 μl) for voltammetric and amperometric detection in flowing solutions. Emphasis is placed on factors contributing to high sensitivity, reliability and selectivity of ec detection as an integral part of larger analytical systems utilizing continuously flowing, unsegmented streams, e.g., flow-injection and liquid chromatographic analyses. Solid and mercury electrodes are considered under potentiostatic and potentiodynamic control. A review is given also of auxiliary chemical and photolytic derivatizations coupled to ec detection. The majority of the literature on the subject relates to liquid chromatography with electrochemical detection (l.c./ec); however, applications to these concepts to specific examples in flow-injection systems, as well as for on-line process control, should be obvious. Details of chromatographic separations and design of total analytical systems are not reviewed.     

Analysis of global components in Ganoderma using liquid chromatography system with multiple columns and detectors

In present study, a multiple columns and detectors liquid chromatography system for analysis of global components in traditional Chinese medicines was developed. The liquid chromatography system was consist of three columns, including size exclusion chromatography column, hydrophilic interaction chromatography column, and reversed phase chromato‐graphy column, and three detectors, such as diode array detector, evaporative light scattering detector, and mass spectrometry detector, based on column switching technique. The developed multiple columns and detectors liquid chromatography system was successfully applied to the analysis of global components, including macromolecular (polysaccharides), high (nucleosides and sugars)‐, and low (triterpenes)‐polarity small molecular compounds in Ganoderma, a well‐known Chinese medicinal mushroom. As a result, one macromolecular chromatographic peak was found in two Ganoderma species, 19 components were identified in Ganoderma lucidum (two sugars, three nucleosides, and 14 triterpenes), and four components (two sugars and two nucleosides) were identified in Ganoderma sinense. The developed multiple columns and detectors liquid chromatography system was helpful to understand comprehensive chemical characters in TCMs.     

Design and evaluation of a sandwich phase separator for on-line liquid/liquid extraction

Sandwich phase separators with various groove dimensions were constructed and as part of a post-column extraction detector for liquid chromatography and for a flow-injection system. The construction materials are stainless steel and PTFE; no membranes are used. The groove volumes vary between 8 and 43 μl; the dimensions of the groove are not critical. Several aqueous (acetonitrile/water and methanol/water mixtures) and organic (1,2-dichloroethane and n-heptane) phases were successfully separated by gravity as well as by wetting. Measurement of statistical second moments showed the total disperson of the extraction system to be 2.5–4 s at the optimum separation efficiency (organic flow through detector/total organic flow) of 0.3–0.4.     

Flow-rate dependence of post-column reaction chromatographic detectors and optimization of reactor length for slow chemical reactions

In high-performance liquid chromatography, use of any post-column reactor invariably involves a compromise between the conditions needed to obtain complete reaction and avoidance of excessive dispersion by band broadening in the reactor. Flow rate and the reactor geometry interact to establish the final chromatographic performance. Based on the flow-rate dependence of the peak area and peak height, post-column detectors constitute a distinct class of detectors which differ from mass-flow and concentrations-sensitive detectors such as the flame ionization and absorbance detector, respectively. The concept of reactor length optimization is developed for first-order chemical reactions in a post-column detector. The findings are applicable to both chromatographic and flow-injection systems.     

An electrochemical detector with a dropping mercury electrode for high-performance liquid chromatography

The electrochemical flow-through cell described has an active volume of less than 1 μl, and incorporates a dropping mercury electrode with drop times of about 0.05 s. Its performance as a detector for high-performance liquid chromatography is assessed for p-nitrophenol and nitrobenzene. The detection limits are 4–5 ng; the relative standard deviation of the peak height is better than 5% for the range 5–150 ng. The dependence of the response on flow rate, mercury pressure and drop time is described.     

Analysis of gaseous samples by flow injection

A flow-injection configuration based on the repeated change of the flow direction is proposed for the direct analysis of gaseous samples. The sample is injected into a liquid carrier-reagent stream and the repeat passage of the liquid zone close to the gas-liquid interface nearest to the detector is monitored to obtain an absorbance-time multi-peak recording. The system has been applied to the determination of nitrogen dioxide in gaseous samples for the range 0.5–100 μl ml^?1, with a relative standard deviation of 4–5% and a sampling frequency of 60 h^?1. A modification of the injection system allows preconcentration steps to be implemented.     

Simultaneous determination by iterative spectrophotometric detection in a closed flow system

A flow-injection configuration based on a closed flow system which includes a single spectrophotometric detector and allows iterative detection by passage of the reacting plug n times through the same detector is described. The information obtained can be used in the simultaneous determination of species by kinetic methods. The example given is the simultaneous determiantion of iron(III) and cobalt(II) via the EGTA/PAR ligand displacement reaction.     

Flow-through calcium-selective electrode: application in flow-injection analysis and ion chromatography

The use of solid contact flow-through calcium-selective electrodes as potentiometric detectors in flow-injection analysis and non-suppressed ion chromatography is discussed. Owing to the high selectivity of the membrane electrode based on tetratolyl-m-xylylenediphosphine dioxide, it can be used to monitor trace amounts of calcium ions in the presence of a 100-fold excess of alkali metal, ammonium and magnesium ions. The detection limit is about 1 × 10^?6 M. The composition and thickncss of the calcium selective membrane influence both the detection limit and selectivity of the electrode. The sensitivity of this potentiometric detector in ion chromatography relative to alkaline earth and heavy metals is significantly higher than that of a commercial conductivity detector.     

A thin-layer contactless conductivity cell for detection in flowing liquids

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 × 10⁻⁶ to 1.5 × 10⁻² S m⁻¹, corresponding to the KCl concentration range from 0.5 to 1000 μM, the limit of detection equals 3.5 × 10⁻⁶ S m⁻¹ (0.2 μM KCl), the detection repeatability, expressed in terms of the relative standard deviation, amounts to 1.13% and the detection volume is 0.6 μL. 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.     

Glass capillary gas chromatography with simultaneous flame ionization (FID) and Hall® element-specific (HECD) detection

Two glass capillary gas chromatographic systems were equipped with inert effluent splitters which allowed simultaneous data acquisition using nonspecific and element-specific detectors. Simultaneous detection was achieved using the nonspecific flame ionization detector (FID) and the Hall^® electrolytic conductivity detector (HECD) operated in either the sulfur-or the nitrogen-specific mode. Typical application of the simultaneous detection system as applied to analysis of petroleum residues is briefly described. The Hall electrolytic conductivity detector can be made element specific for halogen-, sulfur-, or nitrogen-containing compounds. Simultaneous detection enhances the information yield from a single sample injection and proves to be a powerful complementary technique when used with computerized gas chromatography/mass spectrometry.     

Identification and Determination of the Major Constituents in the Traditional Uighur Medicinal Plant Saussurea involucrata by LC-DAD-MS

A liquid chromatography coupled with diode array detector and electrospray ionization mass spectrometry method was developed for the simultaneous analysis of the major constituents in Saussurea involucrata (SI). A comprehensive validation of the developed method was conducted, and the unique properties of the present method were confirmed by analyzing 11 SI samples. Seventeen compounds including phenolic acids, flavonoids, and lignanoids were identified by online ESI-MS and by comparison with known data in the literature and standard compounds; of these, eight were simultaneously quantified by LC-DAD. All linear regressions were acquired with R ² > 0.99, and the limits of detection ranged from 0.85 to 3.03 ng. Repeatability was evaluated by intra- and inter-day assays; the relative standard deviation (RSD) value was within 4.95%. Recovery studies for the quantified compounds were found to be within the range 97.31–101.17% with an RSD of less than 2.18%. Overall, the present hyphenation procedure is highly efficient and reliable, and hence suitable for qualitative and quantitative analysis of a large number of samples.     

Dual Electrochemical Detection of Biogenic Amine Metabolites in Micro High-Performance Liquid Chromatography

Abstract

The dual electrochemical detectors for ordinary and micro high-performance liquid chromatography were briefly reviewed.

The electrochemical behaviors of biogenic amine metabolites were studied by cyclic semi-differential and semi-integral voltammetry with a glassy carbon working electrode. It was found that the electrochemical reactions of many biogenic amine metabolites are quasi-reversible. The dual electrochemical detector based on thin-layer electrolytic cell with two working electrodes (anode and cathode) in series configuration was tested for selective detection of biogenic amine metabolites on their electrochemical quasi-reversibility. The detector was successfully utilized for the simultaneous determination of 3, 4-dihydroxyphenylacetic acid, homovanillic acid and 5-hydroxyindole-3-acetic acid in human urine directly injected by micro high-performance liquid chromatography.     

Atomic spectrometric detectors for flow-injection analysis

The distinction between liquid chromatography and flow injection analysis is discussed in terms of the underlying concepts, the performance characteristics and the hardware involved. For the last aspect, attention is focused on the role of the detector and the development of spectroscopic detectors for these techniques is discussed. The limitations of atomic spectrometric detectors are discussed for the three most widely used techniques, flame atomic absorption spectrometry, plasma spectrometry and electrothermal atomisation atomic absorption spectrometry, and the recent literature concerned with the use of these techniques for chromatographic detectors is summarized. The use of flow-injection methods to extend the capabilities of the techniques, particularly as far as detection limits and matrix interferences are concerned, is discussed. These topics are illustrated by a detailed review of relevant papers from 1988 and 1989. It is concluded that there is a considerable sustained research effort in this field.     

Simultaneous use of fluorescence,ultraviolet, and electrochemical detectors in high performance liquid chromatography-separation and identification of phenolic antioxidants and related compounds

Isocratic conditions are described for the separation, identification and quantitation of selected phenolic antioxidants and related compounds by high performance liquid chromatography on a reverse phase microparticulate ODS column. Mobile phases containing 0.05 M lithium perchlorate in aqueous methanol are used. Three detection systems: ultraviolet; fluorescence; and electrochemical detectors are connected in tandem. The use of these three detection systems in sequence as additional means for separation, identification, and quantitation is demonstrated. Other advantages of the system include speed of analyses and improved detection. The ? values and detector responses at 2 settings per detector for 12 compounds are reported.     

Seeking high-sensitivity flow injection analysis

The use of flow-injection systems for biotechnology assays is attrctive but the reduced sensitivity of such systems over their manual descrete-sample counterparts has limited their use. Model systems are studied to develop the concepts needed for the design of high-sensitivity flow-injection systems for the determination of macromolecules. Both open- and packed-tube systems were examined. Initial hydrodynamic studies were done with dye; then selected designs were tested for protein determinations based on a bromocresol green assay. The packed-bed systems provided better mixing, but dispersion was about the same as that of the open-tube systems and back-pressures were much higher. This work indicates that combinations of narrow (i.d. ? 0.5 mm) and short (? 10 cm) open tubes, small-volume flow cells (8 μl) and slow flow rates (? 0.05 ml min^-1) give better sensitivity.     

Simultaneous determination of free and EDTA-complexed copper ions by flame atomic absorption spectrometry with an ion-exchange flow-injection system

A simple method is described for the simultaneous determination of free and complexed copper ions in a flow-injection system comprising ion-exchange and flame atomic absorption spectrometry. Sampling rates for 400-μl samples were 90 h^?1. Typical relative standard deviations for the simultaneous determinations were 1.6% for the complexed metal (0.50 μg ml^?1) and 1.0% for the free metal (0.20 μg ml^?1).     

LC�CMS Analysis of Sertraline and Its Active Metabolite in Human Serum Using a Silica Column with a Non-Aqueous Polar Mobile Phase

A sensitive liquid chromatography?Cmass spectrometry method for the simultaneous determination of sertraline (SER) and its major metabolite norsertraline (NOR) from serum was developed and validated in the context of a pharmacokinetic study in pregnant women. The separations were achieved on a silica column with a non-aqueous polar mobile phase consisting of acetonitrile, methanol and ammonium acetate at a flow rate of 0.5 mL min^?1. The concentrations were measured using a single quadruple mass spectroscopic detector supplied with atmospheric pressure ionization electrospray. Sample preparation consisted of a simple liquid?Cliquid procedure. The detector was set in selective ion mode for each compound of interest, 306 m/z for SER and 275 m/z for NOR. Calibration curves were generated by least square linear regression for concentration of 5?C160 ng mL^?1 for SER and from 10 to 320 ng mL^?1 for NOR. The curves for both compounds of interest were linear, with correlation coefficients r ² ?? 0.999.     

Flow-injection determination of organic contaminants in water using an ultraviolet-mediated titanium dioxide film reactor

A novel reactor for use in the flow-injection determination of organic solutes in water was constructed by first immobilizing a thin film of titanium dioxide onto the inner surface of a length of Teflon tubing then wrapping the treated tubing around a near-UV illuminating source. The reactor was installed after the injector port of the flow system. Organic compounds injected in the flowing stream were oxidized photocatalytically to carbon dioxide, which was subsequently monitored by a conductivity detector. To optimize the sensitivity of detection, a number of reactor parameters, such as temperature, configuration and catalyst loading, were studied. The technique was applied to alcohols, formaldehyde, ethylene oxide and single-cell algae. The detection limit is 1 × 10^?9 M methanol (in 20 μl).     

A novel RP-HPLC refractive index detector method development and validation for determination of trace-level alcohols (un-sulfated) in sodium lauryl sulfate raw material

The main aim of the study is to develop and validate a simple and rapid liquid chromatographic analytical method for simultaneous determination of trace level of un-sulfated alcohol impurities in sodium lauryl sulfate using high-performance liquid chromatography with a refractive index detector. The chromatographic separation was achieved using flow rate of 3.0 ml/min with a Waters Symmetry C₁₈ (150 × 4.6 mm, 5 μm) column and 50°C as a column temperature. The mobile phase comprised milliQ water and acetonitrile in the ratio of 30:70 v/v respectively. The detection was performed using a refractive index detector at a sensitivity of 64. The resolutions among n-decanol, n-dodecanol, n-tetradecanol, n-hexadecanol, and n-heptadecanol were found to be >4. Regression analysis showed a correlation coefficient for the stated compounds of >0.999. The validated HPLC method was utilized effectively for the estimation of un-sulfated alcohols in quality control labs for commercial release of sodium lauryl sulfate.