Ideal detector

As with MID on MS, the more wavelengths that are simultaneously monitored, the greater is the likelihood of valid identification. Another analytical technique is the formation of derivatives which are fluorescent or absorb UV radiation at unique wavelengths. The compound of interest may be derivatized and injected onto the HPLC system the column separates the reactants and then passes them through the detector. The compound may also be derivatized "post column" as done by amino acid analyzers. The de-rivatizing reactant is metered to mix with the column effluent and is then sent to the detector. Ideally, only the derivatized products should be detectable. 

The preconcentrator efficiency 17P accounts for several factors associated with the preconcentrator performance. These factors include explosive passing through the preconcentrator during the adsorption (collection) phase, explosive loss due to adsorption on cool internal surfaces, and molecular decomposition associated with the desorption phase in the preconcentrator. The value for tjp ranges from 0 (no explosive delivered to the detector) to 1 (entire explosive sample delivered to the detector). Ideally, tjp would be 1. 

Quadrupole instrument made up of the source, the focusing lenses, the quadrupole cylindrical rods and the detector. Ideally, the rods should be hyperbolic. Reproduced (modified) from Kienitz H., Massenspektrometrie, Verlag Chemie, Weinheim,... 

Corrosive or reactive substances do not come into contact with the filaments, which makes the detector ideally suitable for the detection of such substances. 

Fig. 7. Schematic diagram of the events during a two-pulse Hahn echo sequence. Re and Im refer to the so-called real and imaginary NMR signal components, that is, the two channels of the quadrature phase-sensitive detector. Ideally, 0, = 90° and 02 1 0°, with phase cycling of and 02- dashed regions of the NMR signals following the pulses represent the deadtime of the receiver. From Ranee and Byrd (1983).

It was found that that in the case of soft beta and X-ray radiation the IPs behave as an ideal gas counter with the 100% absorption efficiency if they are exposed in the middle of exposure range ( 10 to 10 photons/ pixel area) and that the relative uncertainty in measured intensity is determined primarily by the quantum fluctuations of the incident radiation (1). The thermal neutron absorption efficiency of the present available Gd doped IP-Neutron Detectors (IP-NDs) was found to be 53% and 69%, depending on the thicknes of the doped phosphor layer ( 85pm and 135 pm respectively). No substantial deviation in the IP response with the spatial variation over the surface of the IP was found, when irradiated by the homogeneous field of X-rays or neutrons and deviations were dominated by the incident radiation statistics (1). 

The first detector for optical spectroscopy was the human eye, which, of course, is limited both by its accuracy and its limited sensitivity to electromagnetic radiation. Modern detectors use a sensitive transducer to convert a signal consisting of photons into an easily measured electrical signal. Ideally the detector s signal, S, should be a linear function of the electromagnetic radiation s power, P,... 

Selecting the Voltammetric Technique The choice of which voltammetric technique to use depends on the sample s characteristics, including the analyte s expected concentration and the location of the sample. Amperometry is best suited for use as a detector in flow systems or as a selective sensor for the rapid analysis of a single analyte. The portability of amperometric sensors, which are similar to po-tentiometric sensors, make them ideal for field studies. 

The final part of a gas chromatograph is the detector. The ideal detector has several desirable features, including low detection limits, a linear response over a wide range of solute concentrations (which makes quantitative work easier), responsiveness to all solutes or selectivity for a specific class of solutes, and an insensitivity to changes in flow rate or temperature. 

A further important property of the two instruments concerns the nature of any ion sources used with them. Magnetic-sector instruments work best with a continuous ion beam produced with an electron ionization or chemical ionization source. Sources that produce pulses of ions, such as with laser desorption or radioactive (Californium) sources, are not compatible with the need for a continuous beam. However, these pulsed sources are ideal for the TOF analyzer because, in such a system, ions of all m/z values must begin their flight to the ion detector at the same instant in... 

Ideal Performance and Cooling Requirements. Eree carriers can be excited by the thermal motion of the crystal lattice (phonons) as well as by photon absorption. These thermally excited carriers determine the magnitude of the dark current,/ and constitute a source of noise that defines the limit of the minimum radiation flux that can be detected. The dark carrier concentration is temperature dependent and decreases exponentially with reciprocal temperature at a rate that is determined by the magnitude of or E for intrinsic or extrinsic material, respectively. Therefore, usually it is necessary to operate infrared photon detectors at reduced temperatures to achieve high sensitivity. The smaller the value of E or E, the lower the temperature must be. 

When equation 12 is vaUd, the detector is said to be a background-limited infrared photodetector (BLIP). When this is the case, attempts often are made to improve D by cold shielding which reduces ( ). The ideal D is shown in Figure 3 as a function of wavelength with background photon flux as a parameter. The line of termination in the lower left corner represents TN values for a 180° (27T) detector field of view, 300 K ambient background... 

Fig. 3. Ideal photon detector sensitivity as a function of cutoff wavelength. Lower background flux generates less photon-induced noise giving higher sensitivity. The sensitivity limit for the condition of 300 K background temperature and hemispherical (27T) field of view is shown.

Eig. 18. Microbolometer (a) array portion showing pixels on a 50-pm pitch. Each pixel is coimected to a readout amplifier in the supporting siUcon IC chip, (b) Detector having a 35 x 40 pm active area. The serpentine arms give excellent thermal isolation and the low mass results in a 10-ms response time, ideal... 

The reduction of dimensions also reduces volumes which are accessible to the detector. Thus, detection principles related to geometric dimensions of the detector cell ai e not ideally suited for coupling to microsystems, whereas surface sensitive principles, such as electrochemical methods or optical methods utilizing the evanescent field of a waveguide, or methods which can be focussed on a small amount of liquid, such as electrochemiluminescence (ECE), ai e better suited. This is why electrochemiluminescence detectors ai e combined to microsystems. Moreover ECE has found wide applications in biochemistry because of its high sensitivity, relatively simplicity and feasibility under mild conditions. 

The Mattauch-Herzoggeometry (Fig. 3.20) enables detection of several masses simultaneously and is, therefore, ideal for scanning instruments [3.49]. Up to five detectors are adjusted mechanically to locations in the detection plane, and thus to masses of interest. Because of this it is possible to detect, e. g., all isotopes of one element simultaneously in a certain mass range. Also fast, sensitive, and precise measurements of the distributions of different isotopes are feasible. This enables calculation of isotope ratios of small particles visible in the image. The only commercial instrument of this type (Cameca Nanosims 50) uses an ion gun of coaxial optical design, and secondary ion extraction the lateral resolution is 50 nm. 

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