Other Detector Principles

Recently CCD image sensors with internal amplifieation have been developed. These devices have a readout noise of the order of one photoelectron or less [244]. They are, therefore, able to detect single photons, but not with the time resolution required for TCSPC. 

There are other single photon deteetion teehniques, e.g. devices based on superconductivity [359] and quantum dots. These teehniques have not yet led to commercially available detectors. 

Interestingly, the detection threshold of the rod cells in the human retina is close to the single photon level [427]. Most likely the sensitivity of the eye of a cat is even better. Unfortunately, most cats are not interested in serious scientific work, and it remains an open question whether they see individual photons or not. 

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The methods and means for ecological diagnostics make rapid strides among all the NDT and TD developing areas. To provide the atmosphere monitoring recently the good results were achieved in the development of surface-acoustics wave sensors (SAW), laser measuring systems, infrared detectors and systems based on other physical principles. 

Other detection principles have been applied to HPLC, e.g. conductivity, radioactivity, infra-red, and photoconductivity detectors. Such detectors are not widely used in drug analysis but can find application in special circumstances (e.g. the identification of drug metabolites arising from a radiolabelled drug by radioactivity detection). 

The other two principles that detect changes in the solvent properties are conductivity and density measurements. Conductivity as a detection principle can only be used for ionic substances. The detection range is quite high, but the detector is sensitive to changes in solvent composition and shows a baseline shift if gradient elution is applied. 

The time-of-flight (TOF) method determines the neutron energy with a resolution that is better than with any other detector. The principle of neutron TOF is... 

The various instruments used for the measurement of atomic fluorescence have been similar to each other in principle and optical design. In most studies, the source of excitation, of whatever type, has been focused on the flame the fluorescence, usually at a right angle, has been focused on the entrance slit of the monochromator. The detector in all studies has been a photomultiplier tube, the output of which has been amplified and recorded. Figure 1 is a block diagram of the apparatus used successfully in our laboratory (5) it is quite similar to one described by Winefordner... 

The RI detector is in principle used as a mass concentration detector. Other detectors can be used for this purpose as well. If the eluent is not UV-absotbing but the polymer is, and the polymer chains have no UY-absoibing parts other than the monomeric units (for example initiator fragments), a UV detector can be used. When the UV-absorbing parts of the chain are not homogeneously distributed along the polymer chains (e.g. in a copolymer), the UV detector can... 

The equipment widely used for the detection of carbohydrates in the HPLC method is the differential refractive index (RI) detector. The principle involved in this detection depends on the continuous measurement of the variation of the RIs of the mobile phase containing the samples with little or no chromophores such as carbohydrates, lipids, and other polymer compounds that do not absorb UV light. RI detection method presents high degree of reproducibility and is very convenient for the analysis of polysaccharides. However, other detectors such as evaporative light scattering detector and pulsed amperometric detector have been used for the detection of polysaccharides [100]. 

The first detector to be used for SFA was a photometer, and photometric determinations still form the vast majority of current methods. Other detectors in common use are UV spectrophotometers, used primarily for pharmaceutical compovmds and for bitterness in beer flame photometers, for potassium and sodium determination fluorimeters, used primarily for measuring low levels of determinants in the presence of interferences, such as the determination of histamine in blood, and vitamins in food extracts and ion-selective electrode and pH detectors. In principle, almost any detector with flow-through capability can be used with SFA systems, and determinations based on densitometry, thermometry, and luminescence have been published, among others. 

There s No Detector Which Is More Sensitive than a Mass Spec. This phrase touches the same misapprehension as the previous one. Sensitivity and the LoD and LoQ in mass spectrometry are not by default superior to any other detector. Under favorable conditions, like high ion formation yield and good ion transmission through the mass analyzer to the mass detector, mass spectrometers are indeed very powerful, allowing LoQs down to a femto- or even attomol level. However, in case of poorly ionizable analytes, an inappropriate ionization principle and/or perhaps not the most sensitive MS instrument design, there maybe other detection principles that are clearly in favor, for instance electrochemical or fluorescence detection. 

UV detection is used in most chiral analysis by HPLC and other liquid chromatographic modalities. However, some other detectors, such as conductivity, fluorescent and refractive index types, are also used. The choice of detector depends on the properties of the racemic compound to be resolved [41, 144]. Chiroptical detectors, which are based on the principle of polarimetry [145] or circular dichroism [146, 147], are also available. The enantiomer (+)- or (—)-notation is determined by these detectors. Some organochlorine pesticides are not UV-sensitive, and hence they are difficult to detect in liquid chromatography. The detection of these types of pollutant can be achieved by using a mass spectrometry (MS) detector, and therefore LC-MS instruments are now being put on the market for routine use [148, 149]. 

In order to evaluate a detector for use in liquid chromatography (LC), accurate performance criteria or specifications must be provided by the manufacturers. This is necessary to assess the pertinence of a given detector for a particular chromatographic separation, and/or to permit a rational comparison with other detectors. More important, such performance criteria will allow the optimum column to be designed to achieve a particular separation for which the detector is to be used. It follows that the performance data provided for each detector must be presented in a standard form and given in standard units which will be consistent between detectors that function on widely different principles. The principal characteristics of a detector that will fulfill these requirements are given as follows. 

While most GC determinations are performed with solute quantities between 10 and 10 , certain selective detectors can reach down to the 10 -g levels, representing some of the most sensitive measurement techniques available to the chemist. Some GC detection principles are based on the measurement of certain transport properties of the solutes (e.g., thermal conductivity or optical properties), while other detectors are transducers, measuring ultimately some product of a solute molecule (e.g., gas-phase ionization products). The latter detectors are destructive to the solutes. 

Oxygen Transport. The most widely used methods for measuring oxygen transport are based upon the Ox-Tran instmment (Modem Controls, Inc.). Several models exist, but they all work on the same principle. The most common apphcation is to measure the permeabihty of a film sample. Typically, oxygen is introduced on one side of the film, and nitrogen gas sweeps the other side of the film to a coulometric detector. The detector measures the rate that oxygen comes through the film. The detector response at steady state can easily be converted to At (eq. 1). Simple... 

Water Transport. Two methods of measuring water-vapor transmission rates (WVTR) ate commonly used. The newer method uses a Permatran-W (Modem Controls, Inc.). In this method a film sample is clamped over a saturated salt solution, which generates the desired humidity. Dry air sweeps past the other side of the film and past an infrared detector, which measures the water concentration in the gas. For a caUbrated flow rate of air, the rate of water addition can be calculated from the observed concentration in the sweep gas. From the steady-state rate, the WVTR can be calculated. In principle, the diffusion coefficient could be deterrnined by the method outlined in the previous section. However, only the steady-state region of the response is serviceable. Many different salt solutions can be used to make measurements at selected humidity differences however, in practice,... 

Such effects principally cannot be observed in multi band detectors such as a UV diode array detector or a Fourier transform infrared (FTIR) detector because all wavelengths are measured under the same geometry. For all other types of detectors, in principle, it is not possible to totally remove these effects of the laminar flow. Experiments and theoretical calculations show (8) that these disturbances can only be diminished by lowering the concentration gradient per volume unit in the effluent, which means that larger column diameters are essential for multiple detection or that narrow-bore columns are unsuitable for detector combinations. Disregarding these limitations can lead to serious misinterpretations of GPC results of multiple detector measurements. Such effects are a justification for thick columns of 8-10 mm diameter. 

This is a method which is very attractive in principle and which has been applied to yield approximate barriers for a number of molecules. There are, however, difficulties in its use. In the first place, it is not easy to measure the intensities of microwave lines with accuracy. There are unsolved problems of saturation, reflections in the wave guide, and variation of detector efficiency with frequency which are presumably reponsible for the fact that measurements made with ordinary wave guide spectrometers are not very reproducible. In addition, both the spectral lines may be split into components by tunnelling from one potential minimum to another and this splitting, even though it is not resolved, can alter the apparent intensity. Furthermore, it is often difficult to find pairs of lines such that neither is obscured by Stark lobes from the other. 

Because of the commercial availability of interferometers that have a repeatability around 1 nm peak-to-valley (P-V) at any pixel location in the detector, the discussion will be limited to the use of interferometric tests but the principles apply to any type of optical test device. Using this 1 nm repeatability as a benchmark, it will be easy to demonstrate where some of the other testing problems occur long before we hit the repeatability benchmark. 

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