The performance of some liquid phase flow-through detectors

Miniaturization of flow-through analytical systems such as flow injection analysis and liquid chromatography is necessary to achieve higher rates of sample throughput and smaller absolute detection limits. During this miniaturization, however, it is essential to preserve the concentration detection limits of the devices used. Some principles for liquid phase flow-through detection, u.v.-visible absorption photometry, fluorimetry and electrochemical detection, are discussed from this point of view. It appears that, although technical difficulties will be large, there are good prospects of detecting concentrations of about 1 ng ml^?1, while keeping the peak broaadening down to values of about 1 nl.     

Presentation of differential laser-induced fluorimetry as a reference measurement procedure for determination of total uranium content in ores and similar matrices

The metrological principle of ‘differential technique in laser-induced fluorimetry’ analysis is discussed and recommended as a reference measurement procedure for determination of total uranium content in ores and similar matrices. The estimated relative expanded uncertainty values obtained for uranium content in standard IAEA samples are, S 1, 0.04 g/kg, S 2, 0.06 g/kg, S 3, 0.04 g/kg, and for S 4, 0.10 g/kg, respectively. These low uncertainty values obtained for uranium show high metrological quality of differential technique. This reference measurement procedure guarantees the quality of an analytical result (accuracy, high precision, reliability, comparability, and traceability). Laser-induced fluorimetry will be useful for the analysis of uranium in ores, certification of reference materials, borehole core assay, and other diverse applications in nuclear fuel cycle. Differential technique in spectrophotometry/laser fluorimetry has inherent high metrological quality. In principle, laser-induced fluorimetry is an ideal technique for the very accurate determination of uranium by the use of appropriate fluorescence-enhancing reagents and methodology depending upon the concentration of uranium and sample matrices.     

Immobilized enzyme kinetics analyzed by flow-through microfluorimetry : Resorufin-β-d-galactopyranoside as a New Fluorogenic Substrate for β-Galactosidase

Bioreactor kinetics depend on the dispersion of catalytic parameters within the catalyst particle population. In the conventional determination, reactor performance is assessed from the total turnover rate. In the proposed method the dispersion of turnover rates in reacting immobilized enzyme gel spheres of a continuously stirred tank reactor is evaluated. This more informative method is based on flow-through microfluorimetry and is exemplified with β-galactosidase immobilized on Sepharose 4B, with resorufin-β-d-galactopyranoside as a new fluorogenic substrate. By use of sieved gel fractions, effectiveness factors and Damköhler numbers determined in individual beads can be correlated with integral turnover rates of the reactor.     

Rapid method for measuring plasma low-density lipoprotein turnover using high-performance gel exclusion chromatography

High-performance gel exclusion chromatography using flow-through radioactivity monitoring was employed for rapid measurement of low-density lipoprotein (LDL) turnover. Iodinated LDL was injected into five fasting rabbits. Serial blood samples were obtained and small aliquots (0.1-0.2 ml) were injected into the chromatographic system using 0.2 M Tris-acetate buffer (pH 7.0). Radioactivity in lipoproteins was measured by an on-line gamma counter (Flo-One) attached to the high-performance liquid chromatograph and in a regular gamma counter after collecting the fractions. Sequential ultracentrifugation was also used to separate lipoproteins, and the radioactivity in each fraction was measured. The Flo-One method was faster; however, the efficiency of Flo-One varied with the amount of radioactivity and therefore it was necessary to inject the same amount of radioactivity to get comparable values.     

Differential ion-selective membrane electrode-based potentiometric gas-sensing cells with enhanced gas sensitivity

A novel design for devising static and automated flow-through type potentiometric gas-sensing systems with enhanced response slopes is described. The approach involves the use of two working gas-sensing electrode half-cells in a differential measurement arrangement. One of the half-cells employs a pH-sensitive polymeric membrane electrode to sense pH changes from diffusing analyte gas within a suitable inner electrolyte solution housed behind an outer gas-permeable membrane. The second working half-cell is fabricated with an anion- or cation-selective membrane electrode that responds selectively to the conjugate acid or base ionic form of the analyte gas trapped within an inner buffer solution housed behind a similar gas-permeable membrane. When the two internal solutions of the half-cells are in electrolytic contact, the differential response of the resulting gas-sensing scheme is significantly enhanced compared with the response of a conventional single working electrode/reference electrode type gas cell. For the model analyte gas ammonia, response slopes observed for both static and flow-through measurement schemes approach the 118 mV decade^?1 predicted by theory. To demonstrate its analytical utility, the flow-through arrangement was used to determine ammonia-N concentrations in bioreactor media with good correlation with conventional electrode and enzymatic methods. The prospects of fabricating similar differential detection arrangements for CO₂, NO₂ and SO₂ are discussed.     

激光诱导荧光光谱分析进展

本文对激光诱导荧光光谱分析的研究现状和发展趋势进行了综述。着重介绍了激光诱导原子荧光光谱、激光诱导分子荧光光谱、激光低温光谱,时间分辨光谱和激光诱导荧光光谱与其它分析技术的联用研究。     

Fiber-Optic time-resolved fluorimetry for immunoassays

Rare-earth metal chelates used as fluorescent labels for immunoassays possess extremely long fluorescence lifetimes and permit the elective use of time-resolved detection. This is very important in fiber-optic fluorimetry, a technique that ordinarily exhibits large signal backgrounds from back-scattered radiation. With time-resolved detection to reject the back-scattered radiation, the limit of detection for Eu 2-naphthoyltrifluoroacetonate is 10(-12)M, nearly three orders of magnitude lower than for the fiber-optic measurement of the most common fluorescent label, fluorescein isothiocyanate. Commercially available reagents labeled with an europium chelate are used to demonstrate the potential utility of time-resolved fluorimetry in fiber-optic immunoassays. Rabbit immunoglobulin G (IgG) is covalently bonded to the distal end of quartz optical fibers prior to exposure to anti-rabbit IgG labelled with europium chelate. The limit of detection for the assay is approximately 0.1 mug/ml.     

Determination of cortisol in human plasma by reversed-phase high-performance liquid chromatography

A method is described for the measurement of cortisol in human plasma using 45% aqueous methanol eluent on a 120 mm x 4.5 mm I.D. Hypersil octadecylsilane column with UV detection at 239 nm after a simple dichloromethane extraction and evaporation with a prednisone internal standard. The sample preparation time and chromatography time are each about 15 min and linear correlations have been obtained with plasma samples assayed by the Mattingly fluorimetric technique and a commercial-kit competitive protein binding method. Concentration down to 30 nmol/l may be measured and the method can be used when fluorimetry is invalidated by interference, particularly from spironolactone.     

A potentiometric detector for following pH shifts in liquid chromatography

A potentiometric detector suitable for measurement of pH shifts caused by cation-exchange phenomena in reverse-phase liquid chromatography is described. A capillary flow-through glass pH electrode and a small reference electrode are included in a design that produces reliable results in the acid pH range. The detector is calibrated in the steady-state and dynamic modes with pH reference solutions in water and water-methanol mixtures.     

Kontinuierliche katalytisch-kinetische bestimmung von silber und sulfid im p.p.m.-bereich unter verwendung einer durchflusszelle

The determination of silver and sulphide in the p.p.m. range by a catalytickinetic flow method is described. A flow-through cell is used as the mixing and reaction vessel as well as for measurement. The determination of silver is based on its catalytic action on the oxidation of sulphanilic acid by potassium per-sulphate in the presence of 2,2'-bipyridine. Sulphide retards the catalytic activity of silver and so can be determined indirectly. Interfering ions are studied.     

Comparative study of a thin-layer cell for analysis of stationary and streaming solutions

A comparative study with a home-built Kissinger-type twin-electrode thin-layer cell has been made, with the cell either filled with quiescent solutions or used as a flow-through detector. In both modes the cell was polarized by a slow linear potential sweep. The current-voltage curves recorded showed a limiting current region in both cases, in spite of the different types of transport process (steady-state diffusion caused by the redox cycling effect of the counter-electrode in the quiescent solution and convective diffusion in the flowing solution). The sensitivities and also the lower detection limits were found to be nearly the same for the two different modes of operation of the same cell, for which an explanation is given. The main analytical advantage of the thin-layer twin-electrode cell filled with quiescent solution is that only a few mul of analyte solution are necessary for the measurement. It was proved experimentally that this type of cell, used as a flow-through detector, gives the expected limiting current, but this current is dependent on the cube root of the linear flow-rate, in agreement with Weber's theoretical prediction, rather than on the square root of the flow-rate (commonly quoted in the literature).     

Constant-energy synchronous fluorescence for reduction of raman scatter interference

Constant-energy synchronous fluorimetry is described as a means of reducing solvent Raman scatter interference on the fluorescence spectrum of trace concentrations of organic molecules. This technique involves maintaining a constant energy difference between the excitation and emission wavelengths. Spectra are compared from conventional fluorimetry, conventional (constant-wavelength) synchronous fluorimetry, and constant-energy synchronous fluorimetry.     

Thin-layer separation and low-temperature luminescence measurement of mixtures of carcinogens

Low-temperature fluorescence and phosphorescence characteristics of a number of polynuclear aromatic hydrocarbons are reported. Complex mixtures of hydrocarbons are studied to determine the selectivity of low-temperature luminescence measurement. Low-temperature fluorimetry is applied to measure several hydrocarbon carcinogens after ethanolic extraction from thin-layer chromatograms. The method described permits determination of ca. 0.1 μg of most of the potent carcinogens.     

Application of laser-induced optical fiber fluorimetry to the analysis of ultralow level of uranium

A laser-induced optical fiber fluorimetry has been reported for the analysis of ultralow level of uranium. The fluorescence spectrometer includes five major components: a pulsed nitrogen laser, optical fibers, an optrode, a detector, and a boxcar. The fluorescence intensity of uranyl ions is linear with respect to the concentration of uranium. The detection limit of uranium in 1M phosphoric acid is 24 ppb. This technique can be used for the remote, on-line measurement of low level uranium.     

Frequency Response Method for Measuring Mass Transfer Rates in Adsorbents via Pressure Perturbation

A new apparatus for the measurement of equilibria and dynamics for gas-phase adsorption systems is utilized to examine the adsorption of carbon dioxide on BPL activated carbon. The apparatus has a flow-through configuration. For dynamics, with constant inlet flow, pressure within the adsorbent-containing section is varied sinusoidally, and the time-dependent outlet flow rate is measured to determine an amplitude ratio and phase lag. Studies are made of temperature effects and particle size effects. Results are compared with several mathematical models. Frequency response data show that the BPL system follows surface (or micropore) diffusion kinetics. The rate of adsorption for the activated carbon is found to be only weakly dependent on the bulk particle size.     

Biosensoric flow-through method for the determination of cysteine sulfoxides

Cysteine sulfoxides are known for their canceroprotective and antioxidant potential and have been detected in several plants, mainly those belonging to the leek family (Alliaceae). A number of these plants are used as herbal remedies, spices, or vegetables, including garlic (Allium sativum), onion (Allium cepa) and leek (Allium porrum). Determination of the quality and pharmacological activity of garlic-containing herbal remedies produced from garlic powder is by measurement of their content of the cysteine sulfoxide alliin. A convenient method allowing reliable routine estimation of the alliin levels present in the fresh plant material through to the final remedy is urgently required. The newly developed flow-through method described in this paper was designed in order to fulfill this demand. In contrast to conventional HPLC methods, neither a pre-column derivatization nor a chromatographic separation are required allowing a high throughput of samples. This technique is based on immobilized alliinase (EC 4.4.1.4), which was combined with an ammonia gas electrode mounted in a miniaturized flow-through cell. The lower detection limit obtained for alliin was 3.7×10⁻⁷ M alliin. High specificity for alliin was demonstrated for the electrode and a variety of garlic samples including garlic powders, dry extracts and garlic tablets were analyzed successfully. Results obtained with the new method showed a good correlation with those obtained with conventional HPLC methods. In conclusion, the flow-through method provides a valuable method for quality control of garlic preparations and screening plant materials for their cysteine sulfoxide-related health potential.     

Development of a pulsed-laser,fiber-optic-based fluorimeter: determination of fluorescence decay times of polycyclic aromatic hydrocarbons in sea water

A fiber-optic-based system for remote measurement of time-resolved fluorescence emission spectra is described and characterized. A pulsed nitrogen laser is used to induce fluorescence and a time-gated, one-dimensional photodiode array is used to measure the decay of the fluorescence emission spectra. The results compare favorably with reported values for well characterized compounds having fluorescence decay times in the range 4–50 ns. The potential of using time-resolved fluorimetry (TRF) over fiber-optic cables as a means of improving the specificity of remote fluorescence determinations of spectrally similar polycyclic aromatic hydrocarbons in sea water is demonstrated.     

Advances in technologies for the measurement of uranium in diverse matrices

An overview of the advances in technologies, which can be used in the field as well as in a laboratory for the measurement of uranium in diverse matrices like, waters, minerals, mineralized rocks, and other beneficiation products for its exploration and processing industries is presented. Laser based technologies, ion chromatography, microsample X-ray analysis method followed by energy dispersive X-ray fluorescence technique (MXA-EDXRF), sensors for electrochemical detection followed by cyclic voltammogram and alpha liquid scintillation counting techniques are the most promising techniques. Among these techniques, laser fluorimetry/spectrofluorimetry, in particular, is the technique of choice because of its high performance qualification (PQ), inherent sensitivity, simplicity, cost effectiveness, minimum generation of analytical waste, rapidity, easy calibration and operation. It also fulfills the basic essential requirements of reliability, applicability and practicability (RAPs) for the analysis of uranium in solution of diverse matrices in entire nuclear fuel cycle. A very extensive range of uranium concentrations may be covered. Laser fluorimetry is suitable for direct determination of uranium in natural water systems within the μg L⁻¹ and mg L⁻¹ range while differential technique in laser fluorimetry (DT-LIF) is suitable for mineralized rocks and concentrates independent of matrix effects (uranium in samples containing >0.01% uranium). The most interesting feature of TRLIF is its capability of performing speciation of complexes directly in solution as well as remote determination via fiber optics and optrode. Future trend and advances in lasers, miniaturization and automation via flow injection analysis (FIA) has been discussed.     

A novel flow-through fluorescence optosensor for the sensitive determination of tetracycline

A flow-through fluorescence optosensor with Sephadex G-50 microbeads as solid support is developed for the sensitive determination of tetracycline (TC). The fluorescent TC derivative encapsulated in CTAB micelle structures is retained onto the surface of the microbeads packed into a fluorescent flow cell in a flow system, followed by measurement of the native fluorescence of the TC derivative on the bead surface. The retained TC derivative is easily stripped off with DI water from the bead surface by breaking-down the micelle structure. This offers a convenient and effective way for the regeneration of the used solid support with DI water as a carrier. Under the optimal conditions, a linear calibration graph is obtained within a range of 3-500 μg L⁻¹, along with a detection limit of 1.0 μg L⁻¹. The present solid surface fluorescence optosensor provides a 22-fold improvement on the detection sensitivity for TC in comparison with that derived by fluorescence detection in aqueous medium. The feasibility of this flow-through fluorescence optosensor is evaluated by analyzing TC in a commercial drug tablet and surface water samples.