Detector manufacture

Every three years, representatives of all of the major observatories and every detector manufacturer gather to exchange information on the state-of-the art. The proceedings from the past two workshops, held in 2002 (Hawaii) and 1999 (Germany), capture the most recent developments in optical and infrared detectors ... 

In order to assess the accuracy of the present method, we compared it with two other methods. One was the Track Etch detector manufactured by the Terradex Corp. (type SF). Simultaneous measurements with our detectors and the Terradex detectors in 207 locations were made over 10 months. The correlation coefficient between radon concentrations derived from these methods was 0.875, but the mean value by the Terradex method was about twice that by our detectors. The other method used was the passive integrated detector using activated charcoal which is in a canister (Iwata, 1986). After 24 hour exposure, the amount of radon absorbed in the charcoal was measured with Nal (Tl) scintillation counter. The method was calibrated with the grab sampling method using activated charcoal in the coolant and cross-calibrated with other methods. Measurements for comparison with the bare track detector were made in 57 indoor locations. The correlation coefficient between the results by the two methods was 0.323. In the case of comparisons in five locations where frequent measurements with the charcoal method were made or where the radon concentration was approximately constant, the correlation coefficient was 0.996 and mean value by the charcoal method was higher by only 12% than that by the present method. 

The NO + 03 chemiluminescent reaction [Reactions (1-3)] is utilized in two commercially available GC detectors, the TEA detector, manufactured by Thermal Electric Corporation (Saddle Brook, NJ), and two nitrogen-selective detectors, manufactured by Thermal Electric Corporation and Antek Instruments, respectively. The TEA detector provides a highly sensitive and selective means of analyzing samples for A-nitrosamines, many of which are known carcinogens. These compounds can be found in such diverse matrices as foods, cosmetics, tobacco products, and environmental samples of soil and water. The TEA detector can also be used to quantify nitroaromatics. This class of compounds includes many explosives and various reactive intermediates used in the chemical industry [121]. Several nitroaromatics are known carcinogens, and are found as environmental contaminants. They have been repeatedly identified in organic aerosol particles, formed from the reaction of polycyclic aromatic hydrocarbons with atmospheric nitric acid at the particle surface [122-124], The TEA detector is extremely selective, which aids analyses in complex matrices, but also severely limits the number of potential applications for the detector [125-127],... 

Isoprene, the most abundant hydrocarbon emitted to the atmosphere by plants, can also be measured using ozone chemiluminescence. As discussed above, al-kenes react with ozone to produce formaldehyde in its A2 electronic state, in addition to several other chemiluminescent products. In a fast isoprene detector manufactured by Hills Scientific (Boulder, CO), the chemiluminescence is detected using a blue-sensitive PMT to maximize the sensitivity for isoprene detec-... 

Where detectors manufactured outside the United States are used, the location and spacing recommendations of the AHJ should be followed. The manufacturer s recommended location and spacing distance should also be taken into account. 

Note that US-based detector manufacturers are usuaiiy reiuctant to provide technicai information and schemes of their instruments (e.g. types of components, assembiy performance). By exception. Dr. Dukhin has kindiy aiiowed part of the description of the DT100 acoustic spectrometer [100] to be inciuded in this section. 

It is not unreasonable to expect the detector manufacturer to specify their products in units that are most useful to their customers. It is therefore recommended that detector sensitivities be given not only in the basic units of measurement but also in g/ml of a readily available solute. The solute chosen should be one that often occurs in mixtures with which the detector will be frequently used for analysis. Unfortunately, instrument manufacturers are not reputed to listen favorably to such simple suggestions and it is likely the analyst will need to measure the detector sensitivity experimentally. A simple procedure for measuring detector sensitivity will be given later in this chapter. 

The noise level of detectors that are particularly susceptible to variations in column pressure or flow rate (e.g. the katherometer and the refractive index detector) are often measured under static conditions (i.e. no flow of mobile phase). Such specifications are not really useful, as the analyst can never use the detector without a column flow. It could be argued that the manufacturer of the detector should not be held responsible for the precise control of the mobile phase, beitmay a gas flow controller or a solvent pump. However, all mobile phase delivery systems show some variation in flow rates (and consequently pressure) and it is the responsibility of the detector manufacturer to design devices that are as insensitive to pressure and flow changes as possible. 

A diagram of the sensor of the evaporative light scattering detector manufactured by Polymer Laboratories is shown in figure 8. The eluent is atomized in a stream of nitrogen and the finely divided spray passes down a heated chamber to evaporate the solvent. The removal of the solvent converts the stream of droplets into a stream of particles... 

Surveys, presented at one of the ICH EWG meetings (30), support the fact that the VIS detector used by most laboratories is the one shown in Figure 1, the photometric one. Reference to detector manufacturer s catalogs will also support this statement, if they have identified some of their products as applicable to pharmaceutical photostability testing. 

In many HPLC detectors the column eluent flows through a cell within which some physicochemical interaction with the solutes takes place. Exceptions to this include mass spectrometric detectors where the eluent has to be vaporised before introduction into the vacuum system, or the evaporative mass detector, where again the eluent is heated and vaporised before undergoing analysis by light scattering. Often a number of flow cell options are offered by detector manufacturers, and this reflects the effect of the detection volume on the detected peak. The total peak variance, a, is the sum of all the variance contributions. 

This chapter provides an overview of modern HPLC equipment, including the operating principles and trends of pumps, injectors, detectors, data systems, and specialized applications systems. System dwell volume and instrumental bandwidth are discussed, with their impacts on shorter and smaller diameter column applications. The most important performance characteristics are flow precision and compositional accuracy for the pump, sampling precision and carryover for the autosampler, and sensitivity for the detector. Manufacturers and selection criteria for HPLC equipment are reviewed. 

The best energy resolution is obtained with silicon surface-barrier detectors. Most detector manufacturers quote the resolution obtained for the 5.486-MeV alphas of A typical spectrum obtained with a detector having 25 mm ... 

The radiation sensitive depleted layer is available in various thicknesses, < 5 mm, enough to stop electrons of 2.2 MeV, p of 32 MeV, and a of 120 MeV. A typical silicon surface barrier detector for a-spectroscopy has a sensitive area of 300 mm, 300 fim depletion depth, 20 keV FWHM (full width at half maximum) and operates at 100 V reverse bias. The resolving time is about 10 s. Special "rugged" detectors are available which have an acid resistant Si02 surface layer permitting cleaning and contact with liquids. Detailed information for detector selection is available from various detector manufacturers. 

Only for particular molecules, e.g. ammonia because of its strong lines in the 20-40 GHz region, or water at 22 GHz because there is no other line until 183 GHz, would spectral considerations force the worker to lower frequencies. The 20-40 GHz band is also attractive, however, because of the cheap sources and low-noise semiconductor detectors, manufactured for movement sensors and short-path wireless links. The projected automobile collision-avoidance radar systems will make cheaper sources and detectors available for the 60-70 GHz region within the next few years. The 60 GHz across-office circuits for wireless data links could provide useful narrow-band sources for oxygen determination. The 35 GHz and 94 GHz close-range radar bands provide a useful reservoir of components and sources for the potential manufacturer of MMW spectrometers. 

The GC detector is the last major instrument component to discuss. The GC detector appears in Fig. 4.7 as the box to which the column outlet is connected. Evolution in GC detector technology has been as great as any other component of the gas chromatograph during the past 40 years. Among all GC detectors, the photoionization (PID), electrolytic conductivity (EICD), electron-capture (ECD), and mass selective detector (MSD) (or quadrupole mass filter) have been the most important to TEQA. The fact that an environmental contaminant can be measured in some cases down to concentration levels of parts per trillion (ppt) is a direct tribute to the success of these very sensitive GC detectors and to advances in electronic amplifier design. GC detectors manufactured during the packed column era were found to be compatible with WCOTs. In some cases, makeup gas must be introduced, such as for the ECD. Before we discuss these GC detectors and their importance to TEQA, let us list the most common commercially available GC detectors and then classify these detectors from several points of view. 

This table can also serve as a crude guide to help us think about the placement of smoke detectors and carbon monoxide detectors in the home. Smoke detectors should be placed on ceihngs since, although smoke is relatively heavy in term of its constituents, it will always be very hot, which wiU always produce a relatively low density. Hot air rises is the rule of thumb in this matter. Carbon monoxide detectors, on the other hand, can be placed at any location since the CO has the same density as air. Leaking CO will likely be slightly warmer than ambient air since it is the product of some incomplete combustion but mixing and convection effects will not allow it to pool at the ceiling. CO detector manufacturers recommend eye level placement of CO detectors more to keep them away from pets and children than for any reasons related to density. 

If we go to the opposite size extreme and consider the same interactions in a very small detector - defined as one so small that only one interaction can take place within it -a different picture emerges (Figure 2.9). While the very large detector referred to above is entirely hypothetical, the very small detector now being discussed is not too different from the small planar detectors manufactured for the measurement of low-energy gamma and X-radiation and the necessarily small room-temperature semiconductor detectors that will be discussed in Chapter 3 Section 3.2.5. Again, we can consider various interaction histories for the three modes of interaction. 

It is not unknown for Dewars and cryostats to spring a leak. If this happens, the loss of vacuum results in a loss of thermal insulation. The increase in evaporation rate will then lead to complete loss of liquid nitrogen over a short time and warm-up of the detector. If a detector is allowed to warm up while the bias supply is connected, damage will be caused to the preamplifier. To prevent this, every detector system should have a temperature sensor and some means of switching off the bias if the detector warms up, whatever the failure - mechanical or human. Most bias supplies suitable for germanium spectrometry now have the appropriate cut-out built in and the detector manufacturers now routinely provide a temperature sensor. However, these systems do not work unless the user bothers to connect them together ... 

DOPING In the context of detector manufacture, the insertion of impurity atoms into a semiconductor material lattice in order to control its properties. 

In attempts to overcome the abovementioned risks, detector manufacturers have incorporated several functions to permit the choice between fast-response mode and high-sensitivity mode (preconcentration mode), or between the fast-response mode and the GC mode (less prone to false alarms). 

In a pulsed flame photometric detector (PFPD), the combustion of hydrocarbon molecules is fast and irreversible, and heteroatom species such as S2, HPO, and HNO emit light after the flame is extinguished and thus under cooler temperatures. Consequently, their respective emissions can be electronically gated and separated from the hydrocarbon emission. Thus, PFPD can provide selectivity against hydrocarbon interference during detection analysis. PFPD sensitivity was reported to be superior to FPD. Moreover, N and As could be also detected. The PFPD is currently available for use in benchtop instruments, such as the MINICAMS from O. I. Analytical and other GC detector manufacturers. 

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