Response chromatographic detectors

The flame ionization detector Is the most popular of the flame-based detectors. Apart from a reduction in sensitivity compared to expectations based on gas chromatographic response factors [138] and incompatibility with the high flow rates of conventional bore columns (4-5 mm I. 0.), the flame ionization detector is every bit as easy to use in SFC as it is in gas chromatography [148,149]. It shows virtually no response to carbon dioxide, nitrous oxide and sulfur hexafluoride mobile phases but is generally incompatible with other mobile phases and mixed mobile phases containing organic modifiers except for water and formic acid, other gas chromatographic detectors that have been used in SFC include the thermionic ionization detector (148,150], ... 

The first classification is based on the nature of the detector response. Table 4.7 ranks several chromatographic detectors as specific and nonspecific. A nonspecific or universal detector responds to all solutes present in the mobile phase and this performance makes it a... 

Detection in 1C is strictly connected with the nature of eluents (composition, concentration), analytes and the sensitivity required. The ideal characteristics of a chromatographic detector are essentially the following (1) high sensitivity, (2) low cell dead volume, (3) linear relationship between concentration and signal, (4) stable and low background noise, (5) high speed of response, and (6) no signal drift. 

Several gas chromatographic detectors are sensitive to changes in the flowrate of the carrier gas. Any changes in flow rate cause the baseline to be displaced. These displacements make quantification quite difficult especially since the response of certain detectors such as thermal conductivity also changes with changes in flowrate. When an accuracy of 1% in quantitative analysis is required, the flowrate should not fluctuate more than 0.2 percent (see Chapter 4). 

The use of excess CDA in the derivatization eliminates the potential danger of unequal reaction rates of the enantiomers with the CDA, and kinetic resolution is generally not a problem. The commonly used chromatographic detectors usually produce an equal response to the diastereo-meric derivatives. 

The chromatographic detector, placed at the column exit, constantly monitors the emitted gas, and generates an electrical signal that is amplified and appears as a plot of detector response versus time,... 

The identification of the chemical forms of an element has become an important and challenging research area in environmental and biomedical studies. Two complementary techniques are necessary for trace element speciation. One provides an efficient and reliable separation procedure, and the other provides adequate detection and quantitation [4]. In its various analytical manifestations, chromatography is a powerful tool for the separation of a vast variety of chemical species. Some popular chromatographic detectors, such flame ionization (FID) and thermal conductivity (TCD) detectors are bulk-property detectors, responding to changes produced by eluates in a characteristic mobile-phase physical property [5]. These detectors are effectively universal, but they provide little specific information about the nature of the separated chemical species. Atomic spectroscopy offers the possibility of selectively detecting a wide rang of metals and nonmetals. The use of detectors responsive only to selected elements in a multicomponent mixture drastically reduces the constraints placed on the separation step, as only those components in the mixture which contain the element of interest will be detected... 

Sulphur gases in permeation devices are the usual means for calibrating the response of a gas chromatographic detector. Liquefied gases are sealed in Teflon tubes, or in glass or stainless steel tubes with Teflon "windows" on the ends. Gases permeate through the Teflon at a constant rate until the enclosed supply is exhausted. The rate of permeation is... 

During the 1970s the ECD became firmly established as the most sensitive gas chromatographic detector for some compounds. The kinetic model was described in terms of a numerical solution of the differential equations. This was assisted by the development of the constant current mode of measuring the response and the development of Ni-63 sources for the detector. The purification of the carrier gas and the further development of capillary columns improved the operation of the ECD. In addition, chemical reactions were used to make derivatives with a greater sensitivity in the ECD. Other ion molecule reactions were used to improve the sensitivity of... 

Response factor When the response of a chromatographic detector is linear, the ratio of the component concentration to the detector signal (e.g., absorbance with a UV detector). 

Gas chromatography with either sulfur chemiluminescence detection or atomic emission detection has been used for sulfur-selective detection. Selective sulfur and nitrogen gas chromatographic detectors, exemplified by the flame photometric detector (FPD) and the nitrogen-phosphorus detector (NPD), have been available for many years. However, these detectors have limited selectivity for the element over carbon, exhibit nonuniform response, and have other problems that limit their usefulness. 

Determination of solubility by headspace analysis offers several advantages over spectrophotometric techniques. First, because of the selectivity of chromatographic analysis, compound purity is not a critical factor second, absolute calibration of the gas chromatographic detector is not necessary if the response is linearly related with concentration over the range necessary for the measurements and finally, this method does not require the preparation of saturated solutions, since a partition coefficient, not a solubility, is actually measured. However, headspace methodology would probably not be applicable for determining PAH solubilities for three reasons. First, there is little data in the literature on the vapor pressures of PAHs. Second, the aqueous solubilities of most PAHs are too low to be measured by this procedure. Finally, adsorptive losses of PAHs to glass surfaces from the vapor phase would cause errors. 

A-5. Calculation of Response Factors. Detector response factors for the 12 compounds studied in this investigation were determined by injecting 23.2 fxL of standard solutions of the compounds into the liquid chromatographic system. These response factors are presented below. 

The flame ionization detectors (as well as the other flame detectors) can be used equally well with packed and capillary columns. Different considerations may apply to other detector types. The well-known classification of chromatographic detectors into the concentration-sensitive and the mass-flow-sensitive types is highly relevant in this respect. A response enhancement [108] to the mass-flow-sensitive detector types is given as... 

As mentioned before, an ideal chromatographic detector should not influence the peak shape or chromatographic resolution detector dead volume and response time are critical parameters [22]. 

Figure 1.18. Methods for calculating the linear response range for chromatographic detectors.

These expressions are for measurement of product (AO2, BO2) appearance or acceptor disappearance, respectively. Initial and final concentrations are indicated by the subscript o or f, respectively. This result is equivalent to the method of Ingold and Shaw (iS), derived for the less complex case of aromatic nitration, where no decay term (ki) for the intermediate interferes, and was used by Kopecky and Reich for the photo-oxidation, apparently without derivation (26). Previous authors followed acceptor disappearance, which is very diflBcult to measure accurately, particularly for unreactive acceptors, and is subject to severe errors if starting material is consumed by side reactions (26, 39). However, the technique has the advantage that gas-chromatograph detector calibration is not required since only concentration ratios are measured. We have used this technique also, but we found it far more accu ate to measure product appearance since highly characteristic products are formed, which in many cases distinguish the desired reaction from all side reactions. However, this method presents the difficulty that the product peroxides must be quantitatively reduced for gas chromatography, and all products must be isolated and characterized and detector response carefully calibrated. 

The response of a gas chromatographic detector to an analyte band eluting from the end of the column is the chromatographic peak. The three main characteristics that define a peak in ID GC are its size, width, and position in the chromatogram. 

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