Scintillator-fiber flow-cell radioactivity detector for liquid chromatography

Summary A new type of flow-cell radiation detector for use in liquid chromatography which is packed with aligned scintillator fibers is described. A primary advantage of the fiber packed cell is that light generated by the scintillator is absorbed to a much less extent by the fibers than by the powder scintillator used in conventional flow cells. A detection efficiency of 55% has been obtained for carbon-14 using 0.1-mm diameter hand-pulled glass fibers. Computer modeling has shown that even better results can be obtained by using smaller and more uniform diameter fibers which will allow better packing. The fiber cell also demonstrates back pressures which are a factor of 50 less than the conventional cell and much less susceptibility to absorption of compounds because of its lower surface area. Research sponsored by the Office of Energy Research, U.S. Department of Energy, under contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc.     

A fluorescence detector for high-pressure liquid chromatography

We describe a fluorescence spectrophotometer adapted with a micro quartz flow cell to record the output of modern liquid chromatographs. The optical system is double beam in that the light source variations are cancelled out by a second photomultiplier, thus enhancing the sensitivity of the technique. The emission spectra may be scanned by stopping the flow in the chromatographic column and scanning the fluorescence detector. Many specific applications have been studied: polycyclic aromatic hydrocarbons, several vitamins, porphyrins, methyl anthranilate, etc. These are studied in natural samples and it is shown that the specificity of the fluorescence detector frequently obviates the need for sample preparation. The sensivitity available with the fluorescence detector for fluorescing compounds is often much greater than is available with variable-wavelength ultraviolet spectrophotometers. We report picogram-level detectability in real samples for many of the compounds that we have studied.     

Spectroelectrochemical detector for flow-injection systems and liquid chromatography

Flow-through spectroelectrochemical detectors for flow-injection systems and liquid chromatography are described. The detectors have a rectangular flow channel with a reticulated vitreous carbon working electrode followed by an open optical window. The dead volumes of the cells are 27 μl (liquid chromatography) and 80 μl (flow injection). In situ spectral monitoring of reaction products and intermediates for compounds that are both weakly and highly absorbing is demonstrated by using o-tolidine and N,N,N′,N′-tetramethyl-p-phenylenediamine. As a detector for flow-injection systems, components in two-component mixtures can be quantified. As a detector for liquid chromatography, simultaneous absorbance and electrochemical chromatograms allow more eluting compounds to be identified and quantified. Mixtures of nitro- and chloro-phenols are used to illustrate the simultaneous profiling of spectral and redox properties.     

Multi-beam polarized photometric detector for high-performance liquid chromatography

A novel multi-beam polarized photometric detector (PPD) for high-performance liquid chromatography (HPLC) is described. By pairing a polarizing prism with a thin quartz plate as a retarder, many linear polarized beams are produced at every 1/2 wavelength of the plate, and the polarizing axes of the adjacent beams intersect each other. The addition of another prism inclining its polarizing axis by π/4 against the first one enables the simultaneous measurement of optical rotations based on the PPD at many wavelengths. The combination of these optics with a photo-diode array detector can be used to construct a modulated type polarimeter. This detector is designed to measure the optical rotation of an analyte at its absorption band. The spline function connecting the points at 1/4 wavelengths of the plate was used as a baseline to extract the PPD waves. The use of the similarity factor as a noise filter gave high sensitivity. Application of the proposed technique to an analyte carrying the Cotton absorption band provided good results.     

Novel carbon-fibre electrochemical detector for use in liquid chromatography

The preparation and application of a novel carbon-fibre flow electrode used in liquid chromatography are reported. The electrode is simple and flexible, easy to prepare and to change and is stable in organic solvents. Heat sealing of the needle holes holding the carbon fibre in the polyethylene cell tube prevents the mobile phase from leaking through the holes. A stable and repeatable response has been obtained. The detection limit for 3,5-orcinol and 2,7-dihydroxynaphthalene was ca. 1 ng.     

A sub-microlitre two-photon ionizatio detector for high-performance liquid chromatography

A simple sub-microlitre two-photon ionization detector for high-performance liquid chromatography has been constructed; it basically consists of a pulsed laser, a falling-jet cell, and a boxcar detection system. The performance of the cell has been evaluated; the cell is windowless and its volume is about 14 nl. A dye laser is applicable as excitation light-source.     

Forced-flow liquid chromatography with a coulometric detector

A tubular platinum electrode packed with small chips of platinum is described which was shown to function with 100% electrolytic efficiency. Application of the electrode is described for the coulometric determination of electroactive species in the effluent from a liquid chromatograph. Cu and Fe were determined in a series of standards. An average relative deviation of less than 2 ppt was obtained for analysis of samples containing 5 mug of Cu(II) and 5 mug of Fe(III).     

Universal on-line detector for high-performance liquid chromatography via magneto-optical rotation

Detecting changes in magneto-optical rotation is useful as a universal on-line detector for high-performance liquid chromatography. Such apparatus is similar to a polarimeter except for the external magnetic field on a flow cell. Two modulation modes suitable for the magneto-optical rotation detector are discussed. Use of a semiconductor laser provides better sensitivity than a He-Ne laser. The detection limit is 0.006% (w/w) for polyethylene glycol 20000 in a 20-mul injection.     

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.     

Novel inverse liquid chromatography detector configuration for studying solid-liquid adsorption

Inverse liquid chromatography (ILC) is a physicochemical chromatographic technique for studying solid-liquid interactions and surface properties of unknown solid materials. However, such studies are dependent on the ability to self pack the sample powder into a LC column in a reliable and robust manner. In practise, this is often difficult and anomalous chromatograms can be encountered quite frequently. Results obtained with a novel LC system configuration utilising both pre and post-column UV-vis detectors for ILC studies are reported here which greatly assist in our attempts to interpret such chromatograms. The operational issues for using a two detector system are discussed including the use of both detectors in the linear concentration regime; with the choice of an optimal detector wavelength being the most critical factor. The two detector system was demonstrated to be capable of verifying reversible adsorption processes, as well as assisting in the interpretation of complex or problematic chromatograms. This paper demonstrates that the two detector ILC system can offer a number of practical benefits in the study of complex chromatographic phenomena.     

A highly sensitive laser two-photon ionization detector for liquid chromatography

A highly sensitive detector for liquid chromatography using the laser two-photon ionization technique is described. Detection limits for several aromatic molecules, such as pyrene and chrysene, with a nitrogen laser (337 nm) are in the range 5–30 pg, which is considerably below the limits attainable with a commercial u.v. absorption detector.     

A differential photoacoustic mercury detector

A sensitive instrument for the detection of mercury is described. The difference in mercury concentration between two identical flow-through silica cells is measured by means of the photoacoustic effect. The cells are illuminated laterally by a 20-W germicidal mercury discharge lamp. A microphone in each cell detects pressure changes produced when mercury vapour absorbs resonance radiation from the lamp and releases the energy to nitrogen flowing in the partially closed cells, as heat derived from quenched fluorescence. A differential microphone connection discriminates against ambient noise and contributes to a detection limit of less than 0.02 ng of mercury with a linear response up to 70 ng. Variables such as gold collector design and heating rate, inlet filters and traps, changes in carrier gas flow, carrier gas composition, lamp operating current and modulation frequency which all affect instrument performance are discussed. Photochemical loss of mercury can be avoided if dry nitrogen, carbon dioxide, or air is used as the carrier gas provided contamination by avid vapours and water is prevented by a calcium oxide trap at the inlet.     

Laser-induced fluorescence detector for liquid chromatography: applications to protein analysis

A laser-induced fluorescence detector for liquid chromatography was developed. This detector was assessed by utilizing it in conjunction with gel filtration chromatography. Using the 488 nm line of an argon ion laser for excitation and monitoring the emitted fluorescence centering at 535 nm, the limit of detection of fluorescein was 580 fM. Bovine serum albumin labeled with fluorescein was detected at a concentration of 500 fM.     

Fiber optic multi-channel protein detector for use in preparative continuous annular chromatography

On-line coupling continuous-flow liquid membrane extraction (CFLME) with HPLC, a novel automatic system was developed for the determination of sulfonylurea herbicides in water. After an automatic trace-enrichment process by CFLME, which is the combination of continuous flow liquid-liquid extraction and support liquid membrane (SLM) extraction, the target analytes were concentrated in 50 microl of 0.2 M Na2CO3-NaHCO3 (pH 10.0) buffer. The concentrated sample solutions were injected directly onto a C18 analytical column with a valve, and detected at 240 nm with a diode array detector. Metsulfuron methyl (MSM), and DPX-A 7881 were baseline separated with a mobile phase consisting of methanol and 67 mM KH2PO4-Na2HPO4 (pH 5.91) buffer (45+55, v+v) at a flow-rate of 1.0 ml min(-1). With an enrichment time of 10 min and enrichment sample volume of 20 ml, the enrichment factors and detection limits are 100 and 0.05 microg l(-1) for MSM, and 96 and 0.1 microg l(-1) for DPX-A 7881, respectively. The linear range and precision (RSD) are 0.1-50 microg l(-1) and 7.0% for MSM, and 0.2-50 microg l(-1) and 9.2% for DPX-A 7881, respectively. This proposed method was applied to determine MSM and DPX-A 7881 in seawater, tap water, and bottled mineral water with spiked recoveries in the range of 83-95% for MSM and 88-100% for DPX-A 7881, respectively.     

Sol-gel carbon composite electrode as an amperometric detector for liquid chromatography

The performance characteristics of an electrochemical detector for liquid chromatography based on a sol-gel carbon composite working electrode in a wall-jet configuration are described. The new detector combines the versatility of sol-gel processes with several favorable characteristics, including fast electron-transfer kinetics, mechanical rigidity and renewability. Factors influencing the amperometric response are explored and optimized. Detection limits of 58-170 pg are reported for various neurotransmitters. Repetitive injections yield peak heights with relative standard deviations of 2.6-3.7%. The prospects of using sol-gel derived electrochemical detectors are discussed.     

A phosphorus-sensitive molecular emission cavity analysis detector for high-performance liquid chromatography

Molecular emission cavity analysis is adapted to monitor organophosphorus compounds emerging from both normal-phase and reversed-phase columns. Reproducibility of detection was ca. 3% (using peak area measurements) and sub-microgram quantities were easily detectable. The detector consists of a water-cooled duralumin disc with 40 cavities drilled into its circumference. Eluent is collected in the cavities and rotated stepwise into a H₂—N₂—air flame where the HPO emission is monitored at 528 nm. During rotation, the bulk mobile phase is completely evaporated. Examples are given of separations of phosphate esters, phosphonates, pyrophosphonates, phosphonic acids and pesticides. The detector also operates as a sulphur-selective detector at 384 nm.     

Band broadening in a wall-jet type electrochemical detector for liquid chromatography

Band broadening in a large volume wall-jet cell electrochemical detector for liquid chromatography has been studied. Two regimes of stable operation were found which correlated well with the formation of smooth flow patterns. Smooth flow is also found to be important for avoiding erroneous results caused by backmixing.     

Electrochemical detector with a 20 nl volume for micro-columns liquid chromatography

Summary A flow-through two electrode wall-jet cell with a platinum measuring electrode and a cell volume of 20 nl has been designed and evaluated. It has been used to detect phenols by reversed phase liquid chromatography using short micro bore columns. The linear dynamic range between the measured current and the concentration is greater than 10³ (1.5×10^–7–5×10^–4 mol/l) and the minimum analyzable amount was found to be 10 pg for pyrocatechol. A negligible broadening in the detector permits the use of micro columns down to 0.5 mm internal diameter, packed with 5 m particles, without any substantial distortion of eluted zones.Presented at the 14th International Symposium on Chromatography London, September, 1982     

A thin-layer electrochemical detector coated with nafion film for liquid chromatography

The selectivity and analytical application of a thin-layer electrochemical detector comprised of glassy carbon electrode coated with Nafion film were investigated. As a result of the ion-exchange characteristics of the Nafion polymer, the selectivity and stability were improved greatly. The coated electrode has a good response only for cations with the same sensitivity as an uncoated (bare) electrode, but not for anions, and the response for neutral molecules is decreased three-fold. The diffusion of electroactive compounds in Nafion film is discussed based on the results of flow injection experiments. The peak current at the coated electrode was independent of the flow-rate of the mobile phase. Electrode poisoning due to protein adsorption was minimized. The use of 30% methanol or 10% acetonitrile in the mobile phase did not affect the performance of the coated electrode. Various analytes having three kinds of charge state, i.e., anionic, cationic and neutral, were tested. Liquid chromatography with electrochemical detection of ascorbic acid, norepinephrine, epinephrine, dopamine and uric acid was demonstrated.