Choosing the Detector

The choice of the detectors is dictated by the required time resolution, speetral range and sensitivity, tolerable dark count rate, deteetor area, available count rates, ruggedness, and possible budgetary constraints. The paragraphs below attempt to 

For multiexponential decays the situation is more complex and depends on the ratio of the lifetimes and the ratio of the amplitude coefficients. If the ratio of the lifetimes to eaeh other is on the order of 10 and the ratio of the amplitudes close to 1, the eomponents ean easily be resolved even if the short one is hidden within the IRF. Lifetimes eomponents eloser than 1 1.5 are generally hard to resolve. The situation beeomes almost hopeless if two components shorter than the IRF width have to be resolved. Therefore, the detector should be faster than at least the second fastest deeay eomponent. Moreover, to resolve multiexponential decays it is helpful to have a clean IRF without secondary peaks and bumps. Complex decay functions are a strong argument for using an MCP PMT. 

A crucial point of detector selection is whether or not an accurate IRF can be recorded in the given optical system. IRF recording is often a problem in micro-seopes or other systems that use the same beam path for excitation and detection. Reflection and scattering makes it difficult to record an accurate IRF in these systems. In two-photon microscopes the detector may not even be sensitive at the laser wavelength, or the laser wavelength may be blocked by filters. If an accurate IRF is not available, lifetimes much shorter than the detector IRF cannot be reliably deconvoluted. The rule of thumb is to use a detector with an IRF width shorter than the shortest lifetime to be measured. 

Another point to be considered is the pulse width of the light source and the pulse dispersion in the optical system. Multimode fibres or fibre bundles used at high NA can easily add a few hundred ps to the IRF widths. It is, of course, not necessary to use a detector that has an IRF width shorter than 30-50% of the pulse dispersion of the optical system. 

The most common cathode types for PMTs are the bialkali, the multialkali, the extended multialkali, and the GaAs and GaAsP cathodes. Typical curves of the cathode radiant sensitivity are given in Fig. 6.16, page 230. The selection of the cathode is often a tradeoff between red and NIR sensitivity and dark count rate. 

---

So, it is necessary to assess the site risk and prime objective, then to identify the gas to be detected, and finally, based on requirement, choose the detector taking into account the financial aspect. 

One important component of the detector resolution is a function of n (this is discussed in Chapter 6) and, other things being equal, one would choose the detector material with a low 8 so as to maximize n. 

Again an approach that chooses the displacement a to maximize the probability of obtaining the received data appears reasonable. This is illustrated in Fig. 4, where the two possible locations of the positioning of the pdf in Fig. 4(a) is clearly more consistent with the observed data than Fig. 4(b). Let us assume an ideal detector, one with a large number of pixels and with the only noise resulting from the discrete nature of photons. The probability of D photons in a pixel is given by... 

Electrochemically active compounds can be evaluated using a potentiometer to generate a cyclic voltammogram for the analyte. Cyclic voltammetry will allow the analyst to determine whether the compound can be oxidized or reduced, to choose the appropriate potential to use in the electrochemical detector, and to establish whether oxidation or reduction is irreversible. Irreversible oxidation or reduction of the analyte could be predictive of problems with electrode poisoning and reduced sensitivity of the electrochemical detector over time. Turberg et al. used EC detection at an applied potential of -1-600 mV to analyze for ractopamine. 

Figure 2.3. A. Mass spectrometer consisting of an ionization source, a mass analyzer and an ion detector. The mass analyzer shown is a time-of -flight (TOF) mass spectrometer. Mass-to-charge (m/z) ratios are determined hy measuring the amount of time it takes an ion to reach the detector. B. Tandem mass spectrometer consisting of an ion source, a first mass analyzer, a collision cell, a second mass analyzer and a detector. The first mass analyzer is used to choose a particular peptide ion to send to the collision cell where the peptide is fragmented. The mass of the spectrum of fragments is determined in the second mass analyzer and is diagnostic of the amino acid sequence of the peptide. Figure adapted from Yates III (2000).

The primary beam profile is reasonably measured during adjustment of the optics just before the beam stop is inserted. If overexposure of the detector can be avoided by choosing a short exposure interval this method is to be favored. Instead, attenuation of the primary beam by an absorber must be considered. 

The Scatchard formalism can of course be applied to the binding of any small molecule to any biomacromolecule, such as the binding of a substrate or inhibitor to an enzyme, or the binding of a metal ion to an apoprotein. In receptor research, the determination of Kd typically requires labeling of the substrate by radioactivity or by fluorescence. However, we might just as well choose paramagnetism as the label, and this then makes the EPR spectrometer the detector for the determination of binding equilibria. The Scatchard plot in Equation 13.4 has two experimental observables [L] and [RL], and so we must find ways to determine these quantities from EPR spectra. 

For each of the analytical problems below, choose the following characteristics of a gas chromatograph that will solve the problem packed or capillary column, inlet, column dimensions and detector. Justify your choice of each. You may need to use additional sources from the bibliography or references. 

When choosing the appropriate detector for an experiment, it is important to look at its basic parameters. As we will see, some of these parameters are defined with respect to noise. Even in the absence of incident light, detectors generate output signals that are usually randomly distributed in intensity and time. These signals are denoted by noise. The basic parameters of a detector are as follows ... 

It should be noted here that in specifying the rules for the first probe (phenols), it became clear that rules for choosing the column and mobile phase interact significantly with detector rules. 0.1% acetic acid works well as a competing acid additive in terms of chromatography of the phenols. However, carboxylate ions are known to quench the fluorescence of phenols. Thus, if one were to use a fluorescence detector for trace phenol detection, an alternative competing acid, such as 0.1% phosphoric acid should be substituted. It was decided that mobile phase/detector interaction rules would be the first detector rules to be added to the knowledge base. 

Another consideration when choosing a detector is whether it is important to preserve the separated analytes, either for use or for further analysis. Some methods, such as evaporative laser scattering detection and mass spectrometry, destroy the sample during the measurement. Other methods, such as fluorescence or radiochemical detection, may require chemical labeling of the analytes ... 

A final point about factors. They need not be continuous random variables. A factor might be the detector used on a gas chromatograph, with values flame ionization or electron capture. The effect of changing the factor no longer has quite the same interpretation, but it can be optimized— in this case simply by choosing the best detector. 

The next step is to choose a detector. Do you need information about everything in the sample or do you want a detector that is specific for a particular element or a particular class of compounds ... 

An ultraviolet detector using a flow cell such as that. in Figure 25-19 is the most common HPLC detector, because many solutes absorb ultraviolet light. Simple systems employ the intense 254-nm emission of a mercury lamp. More versatile instruments have deuterium, xenon, or tungsten lamps and a monochromator, so you can choose the optimum ultraviolet... 

As in gas chromatography (Section 24-5), the first steps in method development are to (1) determine the goal of the analysis, (2) select a method of sample preparation to ensure a clean sample, and (3) choose a detector that allows you to observe the desired analytes in the mixture. The remainder of method development described in the following sections assumes that steps 1 through 3 have been carried out. 

We choose conditions so that analyte is focused into narrow bands at the start of the capillary by a process called stacking. Without stacking, if you inject a zone with a length of 5 mm, no analyte band can be narrower than 5 mm when it reaches the detector. 

Several points had to be taken into consideration when choosing the mobile phase. First, the mobile phase had to dissolve the silver nitrate properly while at the same time being sufficiently nonpolar for the elution of saturated TGs. Second, the mobile phase had to be inert to silver ions so that no reaction would take place. Third, the refractive index of the mobile phase had to be different from that of the TGs, since TGs were to be detected using a refractive index detector. For these reasons, mobile phases such as propionitrile, acetonitrile, methanol, and 2-propanol were tested. The mobile phase that gave the best separation results was methanol-2-propanol (3 1 v/v) with dissolved silver nitrate. 

---