Multiple detectors

Powder diffraction studies with neutrons are perfonned both at nuclear reactors and at spallation sources. In both cases a cylindrical sample is observed by multiple detectors or, in some cases, by a curved, position-sensitive detector. In a powder diffractometer at a reactor, collimators and detectors at many different 20 angles are scaimed over small angular ranges to fill in the pattern. At a spallation source, pulses of neutrons of different wavelengdis strike the sample at different times and detectors at different angles see the entire powder pattern, also at different times. These slightly displaced patterns are then time focused , either by electronic hardware or by software in the subsequent data analysis. 

The arrangement for a single coincidence measurement can be expanded to a multiple detector arrangement. If, for example, it is necessary to measure coincidence rates over an angular range, detectors can be placed at the angles of... 

Such effects principally cannot be observed in multi band detectors such as a UV diode array detector or a Fourier transform infrared (FTIR) detector because all wavelengths are measured under the same geometry. For all other types of detectors, in principle, it is not possible to totally remove these effects of the laminar flow. Experiments and theoretical calculations show (8) that these disturbances can only be diminished by lowering the concentration gradient per volume unit in the effluent, which means that larger column diameters are essential for multiple detection or that narrow-bore columns are unsuitable for detector combinations. Disregarding these limitations can lead to serious misinterpretations of GPC results of multiple detector measurements. Such effects are a justification for thick columns of 8-10 mm diameter. 

New and multiple detectors are part of the solution to this problem. However, according to the most recent published developments in this area Q5,20, alone they have not been able to provide an answer. As will be seen in the following sections, obtaining the desired fractionation by GPC, before multiple detection, offers hope of a general solution. 

Garcia-Rubio, L.H., MacGregor, J.F., Hamielec, A.E., "Copolymer Analysis Using GPC with Multiple Detectors , presented at the Symposium on Recent Developments in Size Exclusion Chromatography , 178th ACS National Meeting, Washington, D.C., September 9-14, 1979. 

Meehan, E. and O Donohue, S., Characterization of block copolymers using size exclusion chromatography with multiple detectors, in Chromatographic Characterization of Polymers, Hyphenated and Multidimensional Techniques, Provder, T., Barth, H. G., and Urban, M. W., Eds., American Chemical Society, Washington, D.C., 1995, chap. 18. 

HPLC-PDA-MS) are already being used. Although HPLC-NMR-MS provides a very powerful approach for compositional and structural analysis, it by no means represents the limit of what is possible in terms of hyphenation. On-line extraction and the attachment of multiple detectors (e.g. IR, F) make the technique even more powerful. Other analytical laboratories such as TG-DTA-DSC-FTIR, TD-CT/Py/GC-MS/FTIR and HPLC-UV/NMR/IR/MS have been put to work, but do not represent practical solutions for routine polymer/additive analysis. 

The above limitations are implementation dependent and no intrinsic limitations. The throughput of TCSPC can for instance be improved by the use of multiple detectors, and multiple TCSPC boards [42]. The photon-economy of TGSPC could be optimized somewhat by increasing the number of gates. 

AA and ICP instruments can be equipped with multiple detectors that allow for analysis of more than one element at a time [3,25-29], Recently, an ICP instrument that used microwaves and air or nitrogen as the supporting gas has been made commercially available. Because argon gas is relatively expensive, microwave-based instruments should prove to be significantly less expensive to operate. 

The concurrent identification and quantification of organic impurities is a principal use of liquid chromatography in the pharmaceutical industry. However, the application of liquid chromatography to this task highlights a weakness of this technique when compared to gas chromatography specifically, the lack of a universal detector. Great strides have been made to create detectors and hyphenated techniques to address these problems. However, multiple detectors and analytical procedures may be necessary to accurately and specifically identify and quantify the impurities in complex systems. 

With the aid of this prototype, an adequate scanning unit -interfaced to the TEM -scans with pre-determined step size resolution- a part or whole of ED pattern against a fixed detector. Electron beam is deflected by means of deflector coils in the TEM which are situated after the sample. Fixed detector can be either a combination of a scintillator and a photomultiplier or a Faraday cage (one or multiple). Detector is fitted at the bottom of the TEM column, but can also be adapted in the 35 mm port, if the port below the TEM column is occupied by e g. a CCD camera (see fig. 1). 

More detailed information on the reaction course of the three-step functionalization of an HHPA polyesteramide resin was obtained by functionalization with benzoic acid, after the polycondensation step. The reaction proceedings were monitored by multiple detector GPC (UV/RI) and titration. In the UV detector of the GPC the absorption of the aromatic acid is very dominant over the ahphatic resin itself, so that differentiation between free benzoic acid and benzoate-functionalized resin is feasible. It was observed that the titrated total acid concentration decreased much slower than the amount of free benzoic acid (Fig. 11). 

Most standard infrared spectrometers currently operating in the mid-IR spectral region feature single element detectors. Exceptions are analyzers that are equipped with multiple detectors dedicated to single wave-... 

Smoke detectors are primarily used where smoldering fires can be expected and where electrical equipment is located indoors. Examples of their use are in offices and sleeping quarters, computer rooms, control rooms, electrical switchgear rooms, etc. Their response is typically faster than that of heat detection devices. Smoke detectors are more susceptible to false alarms and usually multiple detectors are required to be in alarm before an extinguishing system is activated. 

In the first section, the mechanisms involved in size exclusion chromatography are discussed this is an area where additional understanding and clarification still are needed. Data treatment with respect to statistical reliability of the data along with corrections for instrumental broadening is still a valid concern. Instrumental advances in the automation of multiple detectors and the developm.ent of a pressure-programmed, controlled-flow supercritical fluid chromatograph are presented. 

Automated Data Analysis System for a Gel Permeation Chromatograph with Multiple Detectors... 

Other analysis methods dependent on multiple detectors can be implemented using this automated system. Two methods under development are the use of a continuous viscometer detector with a refractive index detector to yield absolute molecular weight and branching, utilizing the universal calibration curve concept (4), and the use of a UV or IR detector with the refractive index detector to measure compositional distribution as a function of molecular weight. 

High efficiency columns currently available for gel permeation chromatography of small molecules and oligomers provide high speed separations. The use of multiple detectors provides additional information which facilitates characterization and determination of the separated species. 

Some of these applications, including those requiring the use of multiple detectors, are presented here. 

High-efficiency gel permeation chromatography offers a very useful technique for the characterization and analysis of oligomers and small molecules particularly when multiple detector systems are used. 

T. Provder, "An Automated Data Analysis System for A Waters Model 150C ALC/GPC System with Multiple Detectors", This Volume. 

After passing through the magnetic field, the separated ions are collected in ion detectors, where the input is converted into an electrical impulse, which is then fed into an amplifier. The use of multiple detectors to simultaneously integrate the ion currents was introduced by Nier et al. (1947). The advantage of the simultaneous measurement with two separate amplifiers is that relative fluctuations of the ion currents as a function of time are the same for all m/e beams. Each detector channel is fitted with a high ohmic resistor appropriate for the mean natural abundance of the ion current of interest. 

The second approach uses multiple detectors (Figure 3.13), allowing a double extrapolation to zero concentration and zero angle with the data forming what is called a Zimm plot (Figure 3.14). The extrapolation to zero angle corrects for finite particle size effects. The radius of gyration, related to polymer shape and size, can also be determined from this plot. The second extrapolation to zero concentration corrects for concentration factors. The intercepts of both plots are equal to /M . 

The concentration of the polymer molecules eluting from SEC columns is continuously monitored by a detector. The most widely used detector in SEC is the differential reftactometer (DRI), which measures the difference in refractive index between solvent and solute. Other detectors commonly used for SEC are functional group detectors ultraviolet (UV) and infrared (IR), and absolute molecular weight detectors low angle laser light scattering (LALLS) and in-line continuous viscometers. Applications of these detectors to SEC analysis will be discussed later in the Multiple Detectors Section. Other detectors also being used are the densimeter (11-19) and the mass detector (20-23). 

Attenpts to Analyze Complex Polymers Using SEC Detector Technology. For linear copolymers, multiple detectors and, more recently, diode array UV/vis spectrophotometers have been used in attempts to overccxne the above analysis problems. The basic idea is to provide more than one detector response so that the polymer concentration and the number of properties will together equal the number of detector responses (Figure 4). This provides the same number of equations as the number of unitnowns (5,6). 

The automation of the SEC/Viscometer detector system is achieved by interfacing the DRI and viscosity detectors to a microcomputer for real time data acquisition. The raw data subsequently are transferred to a minicomputer (DEC PDP-11/44) for storage and data reduction. The automated data acquisition and analysis system for the size exclusion chromatograph with multiple detectors used in this study have been described previously.(23)... 

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