Detectors requirements

In order to accurately describe narrow peaks and not cause peak distortion in fast HPLC, detector parameters such as sampling rate and digital filtering must 

Once the chromatographic separation on the column has been conducted, the composition of the eluent at the column end must be determined using a detector. In all HPLC detectors, the eluent flows through a measuring cell where the change of a physical or chemical property with elution time is detected. The most important parameter of the detector is sensitivity, which is influenced by the noise and baseline drift, the absolute detection limit of the detector, the linearity, the detector volume (band broadening), and the effects of pressure, temperature and flow (pulsation, gas bubbles). 

In gradient elution, detection requires quantitative measurement of the sample components in an eluent whose composition and, hence, physical properties alter in the course of the analysis. The detection problem in gradient elution can be solved by either using a selective detector sensitive to a property of only the solute or stripping off the solvent with subsequent measurement of non-volatile residues. 

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Calibration. Wavelength calibration to measure the dispersion relation between wavelength and position on the detector requires illumination... 

The simplest fluorescence measurement is that of intensity of emission, and most on-line detectors are restricted to this capability. Fluorescence, however, has been used to measure a number of molecular properties. Shifts in the fluorescence spectrum may indicate changes in the hydrophobicity of the fluorophore environment. The lifetime of a fluorescent state is often related to the mobility of the fluorophore. If a polarized light source is used, the emitted light may retain some degree of polarization. If the molecular rotation is far faster than the lifetime of the excited state, all polarization will be lost. If rotation is slow, however, some polarization may be retained. The polarization can be related to the rate of macromolecular tumbling, which, in turn, is related to the molecular size. Time-resolved and polarized fluorescence detectors require special excitation systems and highly sensitive detection systems and have not been commonly adapted for on-line use. 

As GC is not only used as a separation medium but also as an analytical technique detection has an important function. Even if the column tolerates high-solute levels, detector requirements may determine the best injection technique or they may dictate adding a sample dilution step before injection to bring injected quantities within the optimal operating range. GC instruments accommodate an extremely wide range of solute concentrations. Minimum and maximum solute... 

The basic requirements for the CBMS II are to reliably detect and identify with sufficient sensitivity and selectivity both CWA and BWA in point detection and reconnaissance missions, in order to be deployable in wheeled reconnaissance vehicles and be operable by nontechnical personnel wearing, at the extreme, MOPP IV protective gear. Contrary to the usual practice for a military detector system, the CBMS II does not have its own requirements document. Instead, the requirements and specifications for the CBMS II are based on the detector requirements of the host platforms in which it will be deployed. These requirements are described in terms of performance, as opposed to the usual practice of being enumerated in volumes of detailed specifications. As is usual for a complex multiyear program, the requirements changed over the course of the CBMS II program as the requirements for the host platforms evolved. 

In very large reactor plants, the need exists to monitor neutron flux in various portions of the core on a continuous basis. This allows for quick detection of instability in any section of the core. This need brought about the development of the self-powered neutron detector that is small, inexpensive, and rugged enough to withstand the in-core environment. The self-powered neutron detector requires no voltage supply for operation. Figure 29 illustrates a simplified drawing of a self-powered neutron detector. 

The sensitivity to a flame can be affected by deposits of IR and UV absorbing materials on the lens if not frequently maintained. The IR channel can be blinded by ice particles on the lens. While the UV channel can be blinded by oil and grease on the lens. Smoke and some chemical vapors will cause reduced sensitivity to flames. UV/IR detectors require a flickering flame to achieve an IR signal input. The ratio type will lock out when an intense signal source such as arc welding or high steady state IR source is very nearby. 

Three different detection methods (gas chromatography with electron capture, mass spectrometric and atomic emission detectors) have been compared for the determination of polychlorobiphenyls in highly contaminated marine sediments [74], Only atomic emission detection in the chlorine-selective mode provided excellent polychlorobiphenyl profiles without interferences. However, the lower sensitivity of the atomic emission detector, compared to the other two detectors required a 10 to 20g sample size for most analyses. 

A third type of detector, required for some environmental and biomedical applications, is the electron capture detector (ECD). This detector is especially useful for large halogenated hydrocarbon molecules since it is the only one that has an acceptable sensitivity for such molecules. Thus, it finds special utility in the analysis of halogenated pesticide residues found in environmental and biomedical samples. 

Gas Chromatographic Analysis. We used temperature programmed glass capillary gas chromatography to separate PCB residues. Use of an electron capture detector required an efficaceous sample cleanup for isomer quantitation (27). These combined techniques offered enhanced separations and enabled us to identify and quantitate individual PCB constituents (jL> 27). Schwartz (27) separated more than 100 constituents from a 1 1 1 1 mixture of Aroclors 1242, 1248, 1254, and 1260. 

To detect EE from interply failure, the newly-formed fracture surfaces have to be exposed to the detector, requiring a change of sample and testing configuration. Such a study is currently underway, with unidirectional composites made from Hercules AU (untreated) and AS4 (surface treated) graphite fibers. 

In addition to an intense source and a well matched monochromator/mirror system, detectors require optimisation. For protein XAS, it is now well established that fluorescence detection is the preferred mode of detection Multi-detector... 

The most general purpose detector for open tubular chromatography is a mass spectrometer. Flame ionization is probably the most popular detector, but it mainly responds to hydrocarbons and Table 24-5 shows that it is not as sensitive as electron capture, nitrogen-phosphorus, or chemiluminescence detectors. The flame ionization detector requires the sample to contain SlO ppm of each analyte for split injection. The thermal conductivity detector responds to all classes of compounds, but it is not sensitive enough for high-resolution, narrow-bore, open tubular columns. 

Mass Spectrometry. The use of a quadrupole mass spectrometer as a GC detector for nonmethane hydrocarbon analysis has come of age in recent years. Development of capillary columns with low carrier gas flows has greatly facilitated the interfacing of the GC and mass spectrometer (MS). The entire capillary column effluent can be dumped directly into the MS ion source to maximize system sensitivity. GC-MS detection limits are compound-specific but in most cases are similar to those of the flame ionization detector. Quantitation with a mass spectrometer as detector requires individual species calibration curves. However, the NMOC response pattern as represented by a GC-MS total ion chromatogram is usually very similar to the equivalent FID chromatogram. Consequently, the MS detector can... 

All detectors must be thermostatted at temperatures that are sufficient to keep sample and column bleed from condensing. Thermal conductivity and electron capture detectors require very accurate temperature control. If the TCD or ECD block temperatures are allowed to drift, so will the detector baseline. The sensitivity of the TCD increases with an increase in temperature differential between the block and the filaments. For this reason it is ad-... 

All of the fat-soluble vitamins, including provitamin carotenoids, exhibit some form of electrochemical activity. Both amperometry and coulometry have been applied to electrochemical detection. In amperometric detectors, only a small proportion (usually <20%) of the electroactive solute is reduced or oxidized at the surface of a glassy carbon or similar nonporous electrode in coulometric detectors, the solute is completely reduced or oxidized within the pores of a graphite electrode. The operation of an electrochemical detector requires a semiaqueous or alcoholic mobile phase to support the electrolyte needed to conduct a current. This restricts its use to reverse-phase HPLC (but not NARP) unless the electrolyte is added postcolumn. Electrochemical detection is incompatible with NARP chromatography, because the mobile phase is insufficiently polar to dissolve the electrolyte. A stringent requirement for electrochemical detection is that the solvent delivery system be virtually pulse-free. 

Recently, the word nano has become a trend in science and technology and some of us think that it is the new generation but, as mentioned above, its root is about 22 years old. NLC and NCE are gaining importance day by day. They are very useful and effective tools for samples of low quantities or having low concentrations of the analytes. Columns of low internal diameter are ideal for use in NLC and NCE, especially with detectors requiring very low flow rates, such as electrospray liquid chromatography/mass spectroscopy (LC/MS). Besides, these columns offer high sensitivity due to their low... 

Good hyphenation of detectors requiring mobile phase flow. 

Operation of photo-conductive detectors requires current bias. A steady bias current produces a standing DC output. The bias pedestal is difficult to compensate for accurately for each detector of an imager. An imager designed to be operated by a current bias which is cyclic in time and has a waveform and frequency to produce zero net average signal both in dark and in presence of radiation of uniform intensity is presented in GB-A-2128019. 

The ultimate selectivity of gas chromatography is determined by the detector. The most selective detectors are spectroscopic, such as Fourier-Transform Infrared or Mass Spectrometer. Automated systems can employ chemometric algorithms to discriminate unresolved chromatographic peaks. These combinations are expensive and require significant computer support. As such, they are more likely to be used in a laboratory for confirmation. Efforts to convert this approach to field units are still under development. The MiniCAMS described above, based on a FPD is a reliable monitor but requires 3-5 min to make a determination. Gas chromatographs also require a source of purified gas for operation and the flame detector requires additional hydrogen and air for operation. This device will have the fewest false positives and the most... 

The use of an ATR accessory normally requires a Mercury-Cadmium-Telluride (MCT) detector to achieve enough sensitivity. The detector requires liquid nitrogen cooling, which limits the on-site usage of this method. 

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