The Detector

The detector can be considered as the soul of a HPLC system. Connected to the outlet end of the column, its role is to monitor the column effluent in real time. Detectors can be the most sophisticated and expensive component of the system. Classification of detectors is of two sorts, selective detectors which give different responses depending on the molecular structure of the sample under analysis, or universal detectors, for whom the response is similar for most compounds. Absorbance and fluorescence detectors are termed selective detectors, while the refractive index (RI) is a universal detector . The Ultraviolet-Visible (UV-Vis) detector is more selective and sensitive, being able to detect amounts as low as lO g/mL, while the RI detector s sensitivity is in the range of lO g/mL. Therefore selective detectors can be used to minimise interference from unwanted components. As for fluorescence detectors, their sensitivity is in the range of lO i g/mL for 

HPLC detectors can ahsorh in three different optical absorption regions  

There are three main classes of optical absorbance detectors  

Of these, IR detectors are not frequently used because they have limited sensitivity and suffer from significant solvent interference. Their major use has been in exclusion chromatography. 

Another important characteristic when selecting a detector is whether it is destructive or not. The commonly used optical detectors are non-destructive, thus allowing for the recovery of the compound under analysis, and its use in subsequent characterisation steps. 

The detector is devoted to the conversion of the light signal in an electric one, ideally it should be linear in the widest range possible, and present a high sensitivity and a low noise. Generally modem spectrophotometers mount photomultiplier tubes or photodiode detectors. 

The photomultiplier tubes convert the signal and, most importantly, they amplify it to such an extent that a single photomultiplier can grant a high 

It has to be noted, however, that in spectrophotometric measurements, a high sensitivity is necessary in conditions of low absorbance and, therefore, of high transmittance. This means in a simation of high levels of fight reaching the detector, as a consequence the detector must be able to operate at high intensities with very low noise, to be able to discriminate between the blank and the sample in high dilution conditions. 

The function of the detector is to produce a signal for every particle entering into it. Every detector works by using some interaction of particles with matter. Following is a list of the most common detector types. 

Gas-filled counters (ionization, proportional, Geiger-Muller counters) 

The signal at the output of most detectors is a voltage pulse, such as the one 

The ideal pulse-type counter should satisfy the following requirements  

Every particle entering the detector should produce a pulse at the exit of the counter, which is higher than the electronic noise level of the unit that accepts it (usually this unit is the preamplifier). In such a case, every particle entering the detector will be detected, and the detector efficiency, defined as the ratio of the number of particles detected to the number of particles entering the counter, will be equal to 100 percent (for more details on efficiency, see Chap. 8). 

The recombined beams are then focused on the detector. Detectors function as transducers because they convert electromagnetic radiation into electrical current. There are a number of radiation-sensitive transducers available as detectors for these instruments. One is the photomultiplier tube. These detectors operate with photocathodes that emit electrons in direct proportion to the number of photons striking the photosensitive surface and possess very large 

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Some techniques have been described that are based on the concept of flame ionization used in gas chromatography. The results are generally unsatisfactory because it is necessary to evaporate the solvent prior to introducing the mixture into the detector. 

The first requirement is a source of infrared radiation that emits all frequencies of the spectral range being studied. This polychromatic beam is analyzed by a monochromator, formerly a system of prisms, today diffraction gratings. The movement of the monochromator causes the spectrum from the source to scan across an exit slit onto the detector. This kind of spectrometer in which the range of wavelengths is swept as a function of time and monochromator movement is called the dispersive type. 

The collector contains an electrically-heated rubidium salt used as the thermionic source. During the elution of a molecule of a nitrogen compound, the nitrogen is ionized and the collection of these ions produces the signal. The detector is very sensitive but Its efficiency is variable subject to the type of nitrogen molecule, making quantification somewhat delicate. 

If there are hydrocarbons present in the formation that is being drilled, they will show in the cuttings as oil stains, and in the mud as traces of oil or gas. The gas in the mud is continuously monitored by means of a gas detector. This is often a relatively simple device detecting the total combustible gas content. The detector can be supplemented by a gas chromatograph, which analyses the composition of the gas. 

There are several important partial results. (1) Definition of quality of the CT-data in relation to the imaging task, including a model of the X-ray paths and how it is used to predict the optimal performance. (2) A model and method to determine how the information of the imaged object transfer from the detector entrance screen through the detector chain to CT... 

Information of the energy imparted to the entrance screen is then transferred through a number of conversions in the detector chain, which introduce pixel-to-pixel correlation, before it is stored as digital data. This correlation has to be considered to be able to predict absolute signal and noise levels in the stored, data the noise would otherwise be overestimated. 

Within this work [7] a method and model to determine the optical transfer function (OTF) for the detector chain without detailed knowledge of the internal detector and camera characteristics was developed. The expected value of the signal S0.2 is calculated with... 

The field distribution itself gives information about the location where the detector coil or coils should be placed. It can however be used as a basis for the calculation of defect signals... 

Figure 11 Single detector signal provided by the inspection of inner circonferential notch tube sample, and corresponding scalogram. Time axis (in s) and frequency axis (in Hz) have been scaled according to the speed of evolution of the detector in ihe tube (500 mm/s).

The detector setup consists of four 256 x 256 pixel amorphous silicon technology sensor flat panels with 0.75 x 0.75 mm pixel size, having an active area of 192 x 192 mm [5j. These sensors are radiation sensitive up to 25 MeV and therefor well suited for detecting the LINAC radiation. The four devices are mounted onto a steel Irame each having the distance of one active area size from the other. With two vertical and two horizontal movements of the frame it is possible to scan a total area of about 0.8 x 0.8 m with 1024 x 1024 pixel during four independent measurements. 

There are two principal neutron imaging techniques in NR - direct and transfer (indirect). In the former the neutron converter and the detector are simultaneously exposed in the neutron beam while in the transfer technique only the converter screen is exposed and activated by the neutrons, and transfered out of the neutron beam to subsequently expose the detector. Various types of IP can be used in both of neutron imaging techniques. 

The laminography method was developed initially for medical applications as a non-computer layer-by-layer visualization of the human body [1,2]. In this case an inclined initial X-ray beam projects an image of a specific layer of the object to the detector surface with defocusing of the other layers during a synchronous rotation of the object and the detector (Fig. 1). 

Other limitation for the spatial resolution can be found in the detector. A limited number of pixels in the camera array can be a reason for pure resolution in the case of a big field of view. For example, if field of view should be 10 by 10 nun with camera division 512x512 pixels the pixel size will be approximately 20 microns. To improve the relation of the field of view and the spatial resolution a mega-pixel sensor can be used. One more limitation for the spatial resolution is in mechanical movement (rotation) of the object, camera and source. In the case of a mechanical movement all displacements and rotations should be done with accuracy better than the spatial resolution in any tested place of the object. In the case of big-size assemblies and PCB s it is difficult to avoid vibrations, axle play and object non-planarity during testing. 

It is shown how phase contrast X-ray microtomography can be realised with a (commercial) polychromatic X-ray microfocus tomograph provided the source size and the resolution of the detector are sufficiently small and the distance between source and detector is sufficiently large. The technique opens perspectives for high resolution tomography of light objects... 

For this experiment, as well as for the microtomography ( 3.2) we used the commercial desktop microtomography system Skyscan 1072 [5], the setup of which is sketched in Figure 1. For this instrument, which is designed to study relatively large objects with a diameter up to 50 mm, the source size is 8 pm, the distance source-detector is about 50 cm and the effective resolution of the detector is about 80 pm. For this system and this object, the global effective resolution a is estimated to be of the order of 50 to 100 pm [6]. 

The GAMMASCAN 1500 HR is a combined system for two-dimensional (2D-CT) and three-dimensional (3D-CT) computed tomography, as well as digital radiography (DR). The system is equipped with two separate detector systems for the fan-beam and cone-beam CT. The sire of the objects is limited to a height of four meters, maximum diameters of 1.5 meters and a weight of up to 15 tons. The turntable which carries the test samples can be moved along and across the beam direction ( X- and Y- direction). The radiation source and the detector systems can be moved in Z- direction, both, simultaneously and independently. 

Due to the pulsed radiation output of the LINAC the detectors and the detector electronics have to handle very high counting rates in very short periods. Therefore the detectors have to work in a mode, where the detector output is integrated for one or several beam pulses. For that purpose the crystals are coupled to photo- diodes. Their currents are read out and analysed by the electronic board, which has been developed for this special application. 

In case of some samples besides the cross sectional CT-slice also a projectional image is of interest. In these cases the test mode Digital Radiography (DR) is applied. In the DR-mode the object is not turned, but scanned horizontally and vertically. Again the very high dynamic of the detector and the mechanical accuracy of the complete system are of large benefit to the image quality. 

Therefore it is reasonable to prepare already the data acquisition for a three dimensional evaluation in cone-beam-technique by means of two-dimensional detectors. The system is already prepared to integrate a second detector- system for this purpose. An array of up to four flat panel detectors is foreseen. The detector- elements are based on amorphous silicon. Because of the high photon energy and the high dose rates special attention was necessary to protect the read-out electronics. Details of the detector arrangement and the software for reconstruction, visualisation and comparison between the CT results and CAD data are part of a separate paper during this conference [2]. 

Although direct coupling of a camera to a scintillator can give acceptable results one of its major drawback is the degradation of the quantum noise mainly related to the low transmission of the optics. The following schematics summarizes the particles flux (photons and electrons) across the different stages of the detector ... 

Due to the conversion process an absorbed photon give rise to less than one electron generated in the CCD. This phenomenon, also called a "quantum sink" shows that the detector is degrading the S/N ratio of the image. The quality of an image being mainly limited by the quantum noise of the absorbed gamma this effect is very important. 

In addition, the mirrors are adjustable, so that unimportant areas can be ignored. Light re-emmited from the surfaee is detected, and the detector signal is transmitted to a computer programmed with acceptable deviation levels for comparison with a reference component. Tolerance levels can vary for different areas of the same test piece they may, for example, be higher on a ground section than on adjacent unmachined areas. 

As the dimensions of the speckle grain on the detectors is about 1,7 im, 4 times lesser than the pixel dimensions (for D = 350 mm illuminated area = 6,3mm x 2 mm magnification CCD = 0,05), the halo results not very clear. 

The method is based on the international standard ISO 4053/IV. A small amount of the radioactive tracer is injected instantaneously into the flare gas flow through e.g. a valve, representing the only physical interference with the process. Radiation detectors are mounted outside the pipe and the variation of tracer concentration with time is recorded as the tracer moves with the gas stream and passes by the detectors. A control, supply and data registration unit including PC is used for on site data treatment... 

In the case, where all 3 phases are present, the detector measurements reveal the amounts of tracers in each phase and the position of the boundaries between the phases The cross section area of each phase is calculated fi-om the latter. From this the tracer concentrations and hence the volume flows of the 3 phases are calculated. 

The uppermost curve shows the response from the detector mounted on the heat exchanger... 

Tajima and co-workers [108] determined the surface excess of sodium dode-cyl sulfate by means of the radioactivity method, using tritiated surfactant of specific activity 9.16 Ci/mol. The area of solution exposed to the detector was 37.50 cm. In a particular experiment, it was found that with 1.0 x 10" Af surfactant the surface count rate was 17.0 x 10 counts per minute. Separate calibration showed that of this count was 14.5 X 10 came from underlying solution, the rest being surface excess. It was also determined that the counting efficiency for surface material was 1.1%. Calculate F for this solution. 

Fig. XVII-5. Schematic detector response in a determination of nitrogen adsorption and desorption. A flow of He and N2 is passed through the sample until the detector reading is constant the sample is then cooled in a liquid nitrogen bath. For desorption, the bath is removed. (From Ref. 28. Reprinted with permission from John Wiley Sons, copyright 1995.)...

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