Detector function

The pressure sensitivity of a detector will be one of the factors that determines the long term noise and thus can be very important. It is usually measured as the change in detector output for unit change in sensor-cell pressure. Pressure sensitivity and flow sensitivity are to some extent interdependent, subject to the manner in which the detector functions. The UV detector, the fluorescence detector and the electrical... 

Chromatograms demonstrating the simultaneous use of all three detector functions are shown in figure 22. It is seen that the anthracene is clearly picked out from the mixture of aromatics by the fluorescence detector and the chloride ion, not shown at all by the UV adsorption or fluorescence detectors, clearly shown by the electrical conductivity detector. 

However, it is not the simultaneous use of all detector functions that makes this detector so useful. The real advantage of the trifunctional detector is that it allows the analyst a choice of the three most useful detector functions in one detecting system. Furthermore, any of the three functions can be chosen at the touch of a switch and without any changes in hardware. 

Chromatograms Demonstrating the Simultaneous Monitoring of a Mixture by All Three Detector Functions... 

An example of the use of the three individual detector functions in the analyses of three quite different types of sample is shown in figure 23 and demonstrates this versatility. 

As mentioned previously, this detector has been claimed to be universal, i.e., it has been reported that the response of the ELSD is not a function of the nature of the solute. Although this is not, strictly speaking, true, the detector is almost universal. The detector functions by nebulizing column effluent into droplets, which are evaporated in a heated gas stream. A droplet of evaporate, containing some solvent, remains. Light of a wavelength considerably smaller than the evaporate particles, which are 5 to 15 pm in size,161... 

Bulk property detectors function by measuring some bulk physical property of the mobile phase, e.g., thermal conductivity or refractive index. As a bulk property is being measured, the detector responses are very susceptible to changes in the mobile phase composition or temperature these devices cannot be used for gradient elution in LC. They are also very sensitive to the operating conditions of the chromatograph (pressure, flow-rate) [31]. Detectors such as TCD, while approaching universality in detection, suffer from limited sensitivity and inability to characterise eluate species. 

Point Combustible Gas Detectors (IR) are used to indicate the presence of gas at a particular location (e.g., in a congested area of the planter in small ducts.) IR technology has proven to be more reliable than catalytic bead detectors. The point detector functions in the same manner as the open path detector, by comparing absorbed and reference frequencies of IR light. The main difference between these and open path type is that the path length of the point type is short (3 inches) and is kept within the confines of the instrument. 

Lacourse, W. R. (2001). Electrochemical detectors Functional group analysis. Enantiomer 6 141-152. 

Multiple-point fluorescent deteclion has been proposed to enhance detection sensitivity. This method is based on the use of a detector function, such as the Shah function. The time-domain signals were first detected, and they were converted into a frequency-domain plot by Fourier transformation. Therefore, this technique was dubbed Shah convolution Fourier transform detection (SCOFT). As a comparison, the single-detection point time-domain response is commonly known as the electropherogram [698,699,701]. 

Instead of realizing the Shah detector function using the multiple slits, multiple excitation points were created. This was achieved by monolithically integrating multiple planar waveguide beam splitters on a fluidic channel. In this way, 128 excitation points were created on the fluid channel for multipoint fluorescent detection [413]. 

The isolation of an antibody is an extremely useful first step toward understanding the function of the protein to which it binds. scFvs derived from phage antibody libraries have been used in immunofluorescence, immunoprecipitation, fluorescence-activated cell sorting, Western blotting, and inhibition of function studies, both in vivo, in tissue culture cells, and in vitro. In this sense, they can essentially be used in the same way as conventional hybridoma-derived antibodies. They have the advantage, however, that the genes for the variable regions are cloned simultaneously with selection. This allows the fusion of functional elements, such as dimerization domains, effector or detector functions to selected scFvs [38], the re-creation of complete... 

From the above consideration one can furthermore infer that the detector functions gA are always of the form... 

Here. .. depends on the W-kernels introduced above, but in general stands for terms of the form cross-section, appropriately weighted by factors [1 — cosl(0)], l = 1, 2,. .., if the efficiency of exchange of quantity A in a collision depends on the scattering angle 0 in this way (as, e.g., in case of elastic neutral particle - ion collisions, see Sect. 2.2.4). In case of inelastic collision processes a is simply the total cross-section, denoted weighting exponent l = 1, 2,... is possible) or a1. In principle the detector functions qa must be obtained by numerical integration and tabulated for the parameters of the relevant distribution functions fa. 

The FID was a standard detector used in gas chromatography and the output was processed in the normal manner. The detector functioned well in respect to operating independent of the nature of the mobile phase but the sensitivity realized was disappointing, being little better than the average refractive index detector viz. 5 x lO g/ml. The poor... 

Flames certainly represent one of the easiest and most eflBcient ways to build detectors. Therefore, it may come as a surprise to some that fiames are not really necessary. This is to say, they are not indispensable —they can be replaced by another source of energy without abandoning the basic detector function. 

There are several commercial companies producing alkali flame detectors. In one version, two flames are stacked, one above the other. The lower, plain hydrogen-air flame bums the sample the combustion products are swept into the second flame which is doped with a sodium salt deposited on an electrically heated wire (31). The upper detector functions as alkali flame detector. Another modification uses a detector jet tip formed from fused salt the flame bums in contact with the salt surface (15, 32, 33, 34, 35). A third form uses an alkali-doped porous metal (36) or a platinum capillary filled with potassium hydroxide and carbon (37). When the capillary is heated to 900°—1000°, the carbon causes the grain boimdaries in the platinum to become enlarged, allowing alkali to diffuse slowly through it. The above remarks should be considered illustrative of detectors on the market and by no means comprehensive. Commercial firms, of course, must add the limitations of the patent situation to the diflBculties encountered in constructing these detectors. In fact, some detectors seem to be constructed more for the patent lawyer than for the analyst (38). 

The detector functions in the following manner. The heated alkali bead emits electrons by thermionic emission, which are collected at the anode, providing a base current across the electrode system. When a solute that contains nitrogen or phosphorus is eluted, the partially combusted nitrogen and phosphorus materials are adsorbed on the surface of the bead. This adsorbed material reduces the work function of the surface and, as a consequence, the emission of electrons is increased, which increases the current collected at the anode. The sensitivity of the NPD is very high and only about an order of magnitude less than that of the electron capture detector g/mL for phosphorus... 

---