The Disc Detector

The electrochemical detector is one of the most sensitive and the most specific LC detectors available. It will respond to substances that are either oxidizable or reducible. The output from the detector results from the electron flow caused by an electrochemical reaction that takes place at the surface of an electrode. The reaction can be either oxidation or reduction and if the reaction proceeds to completion, exhausting all the reactant, then the current flow becomes zero and the total charge passed will be proportional to the total mass of material that has been reacted. This procedure is, for obvious reasons, termed coulometric detection. If, however, the electrolyte is flowing past the electrodes, e.g. the electrodes are situated in the eluent from a column, the solute, which constitutes the reactant, will be continuously replaced throughout the elution of a peak. Thus, while there is solute present between the electrodes a current will be maintained, albeit, varying in magnitude. This method of electrochemical detection, is termed amperometric detection and is, at present, virtually the only method employed in LC. It is used almost to the complete exclusion of coulometric detection. 

The various forms of electrode configuration will be discussed later but at this stage it can be said that normally, three electrodes are employed, the working electrode, the auxiliary electrode and the reference electrode. The electrochemical process is made to occur at the working electrode and the reference electrode compensates for any change in eluent 

The reaction at the surface of the electrode is extremely rapid and proceeds almost to completion. This results in the layer close to the electrode being virtually depleted of reactcmt. Consequently a concentration gradient is established between the electrode surface and the bulk of the solution. This concentration gradient results in the solute diffusing into the depleted zone at a rate determined by the concentration in the bulk solution. As the current generated at the electrode surface depends on the rate at which solute reaches the electrode, the detector exhibits a linear response with respect to solute concentration. 

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The disc detector originally developed by Dubsky [21] employed a rotating gauze disc as a carrier and a diagram of the device is shown in figure 14. It consists of a rotating disc the perimeter of which is... 

The client insisted on a test. The smoke detectors worked, and the explosive charge operated, but the cutter did not cut the rupture disc. The explosive charge could not develop enough pressure because the volume between it and the rupture disc was too great. The volume had been increased as the result of a change in design installation of a device for discharging the halon manually. 

There are two detection sensitivities that are of interest for characterizing the BioCD performance. The first is a metrology sensitivity that includes experimental noise sources such as the laser intensity fluctuations and detector noise, but does not include variability of the protein spots on the disc. The second detection sensitivity is under actual assay conditions in which antibody spot variability plays a dominant role. For both of these sensitivities, it is important to define a scale-free sensitivity that is an intrinsic property of the detection platform. 

Optical Multichannel Analyzer System (OMA 2). The OMA 2 system consisted of a PARC Model 1215 console, two PARC Model 1254 SIT detectors, two PARC Model 1216 detector controllers, and a PARC Model 1217 flexible disc drive. The SIT detector is controlled by the 1216 detector controller, which provides both power and scanning voltages and processes the signal for transmission to the OMA 2 console. The OMA 2 console performs all necessary control functions, data acquisitions, data processing and storage of spectra. The system can store 250 (500 points) spectral curves when equipped with the Model 1217 flexible disc drive. 

The disc centrifuge photosedimentometer (DCP) is an instrument used to determine the particle size and size distribution of latices, pigments and other particulates. Separation is based on Stokes Law for the sedimentation of particles in a centrifugal force field. Particles are assumed to be spherical. In the most common mode of operation the separation takes place in a cylindrical cavity of known dimensions and containing a known volume of fluid. A dilute suspension of the particles of Interest is injected onto the surface of the fluid while the disc is spinning and the particles sediment in the centrifugal force field radially outward past a detector system. The appearance time of the particle at the detector, t, in minutes is given by... 

The UV detector is in the upper left corner of the cubicle, viewing only the area inside the cubicle. The 10-pound bottle of Halon is in the opposite corner. When the flames from the torch enter the 90 viewing cone of vision of the detector inside the cubicle, the detector instantly signals a relay in its controller which closes and causes a detonator cap to rupture the disc on the Halon bottle - releasing the Halon. The Halon suppresses the flames on the torch and the unlighted torch falls harmlessly into the pan of gasoline. 

The detector cell is 3 mm long terminated at one end by a cylindrical quartz window and at the other by a plano-convex quartz lens that disperses the transmitted light over a significant portion of the light sensitive area to the photocell. The total volume of the sensor cell was about 2.5 pi. Next to the quartz windows are two stainless steel discs separated by a 3 mm length of Pyrex tube. The mobile phase enters and leaves the detector cell through radial holes in the stainless steel discs that connect to the central orifice of the disc. The stainless steel... 

Typical Response Peaks. Figure 2 shows some typical response peaks for the HPLC separation of the saturates and aromatics in a typical vacuum gas oil. The curve in the upper portion of the figure is the response for the sample using the RI detector. After the saturates peak appeared, the backflush valve was switched and the attenuation changed as indicated in the figure. The peaks were very sharp and symmetrical and appeared in less than 10-min lapsed time from the point of injection. Base line drift was minimal and the areas of the response peaks were obtained with a ball and disc integrator on the strip chart recorder. 

Other Parameters Measured Simultaneously with Bioluminescence. Measurements of several other parameters were obtained from the seawater after it had traversed the bioluminescence detector. When working on station with the bathy-photometer, which was equipped with a pressure transducer, temperature and beam transmittance were measured at depth while seawater was pumped by the submersible pump at depth to shipboard with 110 m of 2.54-cm ID hose. Sea surface temperature was obtained continuously from a probe at the intake near the sea chest. The seawater, obtained from either the sea chest or the bathyphotometer, was pumped through a Turner Designs fluorometer to measure chlorophyll fluorescence, and past a pH probe (31) and a conductivity cell when available. Samples of seawater were frozen for subsequent nutrient analysis (NO , NH4OH, P04 , and NO2). Plankton filtrates from 20 to 100 L (depending on plankton abundance) of seawater were collected from a 100-L effluent tank fitted with plankton net collection cups of 20-fxm mesh porosity. The filtrate was split, filtered onto Whatman GF/C 4.25-cm filter discs, and frozen for subsequent carbon and nitrogen determinations. The other half of the sample was preserved in 5% buffered formaldehyde solution for taxonomic analysis. 

If the disc has reasonable compression properties, it is then attached to a holder and set in motion in the dissolution medium (water, buffer, or simulated gastric fluid) we use a rotation speed of 100 rpm. A number of analytical techniques can be used to follow the dissolution process however, UV-visible spectrophotometry and HPLC with fixed or variable wavelength detectors (or diode array) appear to be the most common. The UV system employs a flow through system and does not require much attention however, if HPLC is used, then any aliquot taken should be replaced by an equal amount of solvent. The intrinsic dissolution rate is given by the slope of the linear portion of the concentration versus time curve divided by the area of the disc and has the units of mg/min cm. ... 

Another instrument called the transport detector, used for detection of lipids, proteins or carbohydrates, requires the transport of the column eluent by a moving wire disc, chain or helix. The solvent is evaporated in a furnace and the nonvolatile sample passes into a flame ionization detector (FID) which is detailed later under gas chromatography (GC) wherein FID counts amongst the major detectors. 

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