Detectors source requirements

Since TIRF produces an evanescent wave of typically 80 nm depth and several tens of microns width, detection of TIRF-induced fluorescence requires a camera-based (imaging) detector. Hence, implementing TIRF on scanning FLIM systems or multiphoton FLIM systems is generally not possible. To combine it with FLIM, a nanosecond-gated or high-frequency-modulated imaging detector is required in addition to a pulsed or modulated laser source. In this chapter, the implementation with of TIRF into a frequency-domain wide-field FLIM system is described. 

Most detector systems require that the IR beam be modulated, where the source energy is adequately differentiated in the measured signal from the ambient background. One of the traditional approaches is to use some form of mechanical chopper , usually in the form of a rotating sector wheel, which modulates the beam by blocking the radiation in one or more sectors during a rotation. Note that this is not a requirement for FTIR systems where the interferometer naturally modulates the beam. 

Another limit source of uncertainty in isotope ratio measurements by mass spectrometry is the dead time of the ion detector for counting rates higher than 106cps, because a lower number of counts are usually registered than actually occur. Dead time correction of the detector is required if extreme isotope ratios are measured by channel electron multipliers and pulsed counting systems.86... 

All signal detectors are required to detect the signal against a background of noise . Therefore, the signal-to-noise ratio must be optimized or, put another way, for maximum sensitivity the noise has to be minimized. The sensitivity of any detector is determined by the noise level in the amplified output signal. In the case of a pyroelectric detector and its associated circuitry, the principal sources of noise are Johnson noise, amplifier noise and thermal fluctuations. 

A Raman spectrum is obtained by exposure of a sample to a monochromatic source of exciting photons and measurement of the frequencies of the scattered light. Because the intensity of the Raman scattered component is much lower than the Rayleigh scattered component, a highly selective monochromator and a very sensitive detector are required. 

The problems associated with the formation and detection of molecular beams have already been referred to. They are interrelated and have largely determined which reactions have been studied with this technique. The simplest method to form a beam is to collimate the effusive flow occurring from a low-pressure source, conventionally called an oven, although its temperature may be subambient. Unfortunately, this yields low beam intensities, and the velocities in the beam are thermally distributed. As a result, even for the accurate assessment of the incident-beam intensity, a highly sensitive detector is required. Moreover, the relatively low beam temperature requires that the reaction has a small threshold energy so that an appreciable proportion of the scattering is reactive. 

Detecting single nano-objects in a far-held laser spot requires carefully optimized setups, discussed in several reviews and books [3, 19]. Hereafter, we assume ideal conditions, perfect optical elements (detectors, sources, hlters, etc.), to concentrate on the fundamental limitations to signal-to-noise ratio in optical single-molecule studies. We consider three main detection techniques applied to individual small absorbers, emitting or not photoluminescence direct absorption, huorescence and photothermal contrast. 

The low power of THz sources coupled with relatively high thermal background has necessitated highly sensitive detection systems. In this respect, broadband detection based on thermal absorption is commonly used to detect low THz signal. The drawback of these detectors, however, is the fact that they require cooling units, to reduce thermal background. For pulsed THz detection, in THz time domain systems, coherent detectors are required. 

Some of the optical components (e.g., windows, lens, and mirrors) inside the detector might require cleaning or replacement after several years of use. Indicators for the need to service these optical items are low source energy or low sensitivity performance even after a new lamp has been installed. Occasionally, the monochromator might need adjustment to restore wavelength accuracy. These procedures are best performed by a factory-trained specialist. 

The radiant sensitivity of a detector - or its quantum efficiency - is one of the most important parameters for TCSPC application. Unfortunately absolute measurements of the radiant sensitivity or the quantum efficiency are extremely difficult. The problem is not only that a calibrated light source or a calibrated reference detector are required but also that extremely low light intensities have to be used. However, accurate attenuation of light by many orders of magnitude is difficult. 

Reducing the non-shot Category III noise sources of the fixed wavelength detector will require attention to Improved thermomechan-Ical detector design to reduce noise due to thermal variations Imposed on the mobile phase/flow cell and on the sample and reference photodiodes, and continued reduction In HPLC reciprocating pump flow pulsations. 

A time of flight (TOP) mass spectrometer measures the mass-dependent time it takes ions of different masses to move from the ion source to the detector. This requires that the starting time (the time at which the ions leave the ion source) is well-defined. Therefore, ions are either formed by a pulsed ionisation method (usually matrix-assisted laser desorption ionisation, or MALDI), or various kinds of rapid electric field switching are used as a gate to release the ions from the ion source in a very short time. 

The majority of detectors with radioisotope sources in current use are based on either the coaxial cylinder or asymmetric configurations, as shown in Figure 3. The low specific activity of the Ni source requires a relatively larger source area to provide a suitable background current that is easier to accommodate in these designs. Virtually all contemporary detectors employ pulse-sampling techniques to collect the thermal electrons based on the variable frequency constant... 

The term PCNAA is used when preconcentration precedes the neutron activation while if epithermal neutrons are used to excite the sample the acronym given is ENAA. The monitoring of the delayed neutrons emitted after excitation is termed DNAA. All these NAA procedures are nondestructive techniques used for characterizing solid (and in some cases also liquid) samples. However, a neutron source and a suitable detector are required and the sample can become quite radioactive after irradiation. The sensitivity of NAA techniques varies widely among different elements and sample preparation and post-irradiation methods employed. Several specific examples of NAA application for analysis of uranium in different matrices will be presented in the appropriate chapters. 

In the use of an electronic leak detector method, a 2% mixture of sulfur hexafluoride in air is used as the test gas. Detector sensitivity is approximately the same as the detector mbe method of test. The presence of the radiation source in this detector, however, requires a special... 

Many of the process and utility streams used in process facilities contain toxic gases of sufficiently high concentration that they could create a hazard if released. Therefore, gas detectors are required to alert workers that a release has occurred. Small releases of toxic gas may cause injury to personnel working directly where the leak occurs. Medium releases may affect other personnel working on the unit but not directly at the release source. Large releases may produce toxic concentrations outside the unit or outside the plant fence line. 

The combination of electrospray ionization with magnetic sector mass spectrometers proved to be less technically challenging than the interfacing of MALDI. Electrospray ionization produces ions continuously, and thus it combines easily to scanning mass analyzers such as the magnetic sector. Thus, no modifications of the mass analyzer or detector are required for electrospray ionization. However, the ions produced by electrospray must be accelerated to kilo-electron-volt energies, and this requires some innovation with the design of the ion source. One solution is to float the entire electrospray source and... 

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