Detectors performance characteristics

In order to eliminate some of these variables in comparing detector performance characteristics, it is usual to use a parameter described as specific detectivity (D ) and defined as ... 

Important performance characteristics of UV/Vis detectors are sensitivity, linearity, and band dispersion. These are controlled by design of the optics and the flow cell—more specifically by spectral bandpass, stray light characteristics, and the volume and path length of the flow cell. 

Residue chemists would be most interested in comparable confidence bands or confidence bandwidths. These values become a performance characteristic for any detection system. The values indicate the precision of not only the prepared standards but also the precision of the overall operating detection system. It is ultimately envisioned that a given system of a separation column in a chromatograph with a certain type of detector should give bands of standardized values. If a chemist finds he has not met... 

Manufacturers publish their product s performance characteristics as specifications, which are often used by the customer for comparison during the selection process. Table 1 shows the specifications of an Agilent 1100 Series Quaternary Pump, which is quite representative of other high-end analytical pumps. Note pulsation is particularly detrimental to the performance of flow-sensitive detectors (e.g., mass spectrometer, refractive index detector). Differences in dwell volumes and composition accuracy between HPLC systems might cause problems during method transfers. 

Two parameters have been demonstrated to be fundamental in calculating the performance characteristics of a continuous analyser, the lag phase and the half-wash time they afford a correlation between the approach to steady state, fraction of steady state reached in a given time and the interaction between samples. The half-wash time (Wy ) is the time for the detector response to change from any value to half that value, the lag phase L is defined in the ensuing discussion. 

Therefore, specific information is required on the characteristics of detectors to allow one to be selected for a particular application. In many cases, however, major performance characteristics of detectors such as noise, sensitivity, response, and linearity, are not presented in a standard format by suppliers. To complicate matters further, there are no published reference values for many of the properties utilized by different detectors for most analytes. Therefore, to determine whether a particular detector is adequate for a particular application, a similar analysis in the pertinent literature has to be found. Ultimately, the analyst will often have to test the detector under consideration on the analyte of interest itself. 

With the advent of advanced characterization techniques such as multiple detector liquid exclusion chromatography and - C Fourier transform nuclear magnetic resonance spectroscopy, the study of structure/property relationships in polymers has become technically feasible (l -(5). Understanding the relationship between structure and properties alone does not always allow for the solution of problems encountered in commercial polymer synthesis. Certain processes, of which emulsion polymerization is one, are controlled by variables which exert a large influence on polymer infrastructure (sequence distribution, tacticity, branching, enchainment) and hence properties. In addition, because the emulsion polymerization takes place in an heterophase system and because the product is an aqueous dispersion, it is important to understand which performance characteristics are influended by the colloidal state, (i.e., particle size and size distribution) and which by the polymer infrastructure. 

A critical part of the electron spectrometer is the detector which registers the energy-analysed electrons. Channeltrons or channelplates are very convenient detectors, and they will now be discussed with respect to their performance characteristics, the use of channelplates as position-sensitive detectors and their detection efficiencies. 

Mass spectrometry is the most universal of detectors because it detects most organic compounds and is highly selective when selective ionization techniques are employed. Off-line LC-MS where the analyte is collected, concentrated, and analyzed by mass spectrometry is a relatively common practice in the pharmaceutical industry. When the compound in the collected fraction is unstable, on-line LC-MS techniques are preferred. There are eight LC-MS interfaces that have been reviewed [128] and their performance characteristics tabulated (Table 5.2). 

Unfortunately (alas, not uncommonly), the significance of the optical multichannel detector has as yet escaped most potential users, who erroneously consider it a mere curiosity still emerging from its embryonic stage. This manuscript is another attempt to demonstrate the maturity and viability of the optical-multichannel detection approach. No longer is the technique a novelty toyed with by a handful of curious instrumentalists, but rather a readily available scientific tool whose performance characteristics and spectrometric applicability are relatively well understood. 

The following are some general guidelines on the placement of gas detectors. The manufacturers of detectors should be consulted for recommendations on placement of their detectors, since they are more familiar with their performance characteristics and capabilities. At the same time the system designer must do his/her own research to ensure the equipment being installed will deliver the required results. 

TABLE 1 Summary of performance characteristics of several reported GaN photoconductive detectors. 

Detectors. A detector capable of continuously monitoring effluent from the column is essential for efficient HPLC analyses. Considerations in connection with detector performance include absolute and relative sensitivity, drift characteristics, noise, linearity, specificity, and band spreading resulting from detector design. The selection of a proper detector is essential for successful analysis, both from the standpoint of sensitivity and elimination of effects of interfering compounds (specificity). 

Table 1 Performance Characteristics of the Spray Impact Detector Mobile Phase, Boiled Distilled Water...

The ultimate in selectivity in HPLC detection is seen with the use of mass-spectrometric detection, and for many applications this could be seen as the ideal detection method. However, more mundane considerations such as size of the instrumentation and limited budgets combine to reduce HPLC-MS to a relatively small number of applications which most effectively exploit its unique properties. When such practical constraints are taken into account, the real detector coimected to the HPLC system usually turns out to be a device that is a compromise, and its performance characteristics need to be taken into account during the development of many analyses just as much as the performance of the column or any other component of the HPLC system. For example, lack of detection selectivity may require extra method development to completely resolve an interfering peak, or lack of sensitivity could force the inclusion of an extraction-concentration step in an analytical method to achieve detectable levels of analyte. 

Even though no detector meets the entire criteria, many commercially available instruments, come very close to ideal with respect to performance characteristics. 

The convenient photon detectors discussed in the previous section cannot be used to measure infrared radiation because photons of these frequencies lack the energy to cause photoemission of electrons as a consequence, thermal detectors must be used. Unfortunately, the performance characteristics of thermal detectors are much inferior to those of phototubes, photomultiplier tubes, silicon diodes, and photovoltaic cells. 

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