Detectors selective sensitization

Detectors. The function of the gc detector is to sense the presence of a constituent of the sample at the outlet of the column. Selectivity is the property that allows the detector to discriminate between constituents. Thus a detector selective to a particular compound type responds especially weU to compounds of that type, but not to other chemical species. The response is the signal strength generated by a given quantity of material. Sensitivity is a measure of the abiHty of the detector to register the presence of the component of interest. It is usually given as the quantity of material that can be detected having a response at twice the noise level of the detector. 

Ability to analyze unreacted monomers was dependent on detector selectivity. The UV detector was operated at 254 nm for analysis of AN/S latex solutions. Styrene is a strong UV abosrber at this wavelength while acrylonitrile has no measurable absorbance at 254 nm. Thus, the UV detector was entirely selective to monomeric styrene. The refractometer detector was sensitive to both acrylonitrile and styrene when each was present in the desired copolymer proportions (70/30). However,... 

Similarly, estimation of chemical composition of soluble polymer was also dependent on selectivity of the UV detector. Polymerized acrylonitrile has no significant UV absorbance at 230 and 254 nm. Thus, UV chromatograms were used to estimate amounts of polymerized methylacrylate and styrene In each resin system. The refractometer detector was sensitive to polymerized methylacrylate and styrene, as well as to polymerized acrylonitrile. It was therefore necessary to calculate comonomer contribution to refractometer peak areas In order to estimate concentration of polymerized acrylonitrile. This was done by obtaining a refractometer calibration for all three homopolymers. Quantity of polymerized comonomers measured by UV were then converted to equivalent refractometer peak areas. Peak areas due to polymerized acrylonitrile were then calculated by difference, and used to calculate amount of polymerized acrylonitrile. 

Detector selectivity is much more important in LC than in GC since, in general, separations must be performed with a much smaller number of theoretical plates, and for complex mixtures both column separation and detector discrimination may be equally significant in obtaining an acceptable result. Sensitivity is important for trace analysis and for compatibility with the small sizes and miniaturised detector volumes associated with microcolumns in LC. The introduction of small bore packed columns in HPLC with reduced peak volume places an even greater strain on LC detector design. It is generally desirable to have a nondestructive detector this allows coupling several detectors in series (dual... 

Gas chromatography is one of the most powerful analytical techniques available for chemical analysis. Commercially available chemiluminescence detectors for GC include the FPD, the SCD, the thermal energy analysis (TEA) detector, and nitrogen-selective detectors. Highly sensitive detectors based on chemiluminescent reactions with F2 and active nitrogen also have been developed. 

Source and detector selection are interrelated, where the output of the source is matched to the sensitivity range of the detector. However, the exact nature of the source is also dependent on the type of sample(s) under consideration, the intended optical geometry, the type of measurement technique, and the final desired performance. The bottom line is that adequate source energy must reach the detector to provide a signal-to-noise performance consistent with the required precision and reproducibility of the measurement. This assumes that the detector is well matched, optically and performance-wise, and is also capable of providing the desired performance. 

If optimum chiral separation conditions and high-efficiency sample cleanup are properly employed, the first priorities in enantioselective analysis have been achieved. The ideal detector is universal yet selective, sensitive and structurally informative. MS currently provides the closest realisation to this ideal. 

Detector selection was relatively straightforward. Because the electron capture detector (ECD) offered sensitivities for HCCP and HCBD that could not be equaled by any other GC detection system, the ECD was employed for the determination of these two... 

For trace analysis it is common to include in a derivatization step the formation of a derivative that not only aids the desired chromatographic or separation factors, but also provides sensitivity to one or another of the specific detectors available. It is common to use a halogen-substituted derivatizing reagent to provide simultaneously decreased polarity or reactivity, better chromatographic separation, and more selective, sensitive detection using electron capture. Thus, for example, trichloroacetic anhydride is used to form acetate derivatives instead of using acetic anhydride. 

Selective ion receptors represent basic units for ionic transmitters or detectors selective ion carriers correspond to ionic transducers. These units may be fitted with triggers and switches sensitive to external physical (light, electricity, heat, pressure) or chemical (other binding species, regulating sites) stimuli for connection and activation. 

Detector Principle of Operation Selectivity Sensitivity Linear Range MDQ1 Stability... 

Needs for improved measurement methods differ depending on whether one is considering low or high transmission rate materials. In the former case one needs very sensitive detectors. Selectivity is also desirable so that interferences from extraneous species can be avoided. In the case of high transmission rate materials instrumental time constants and saturation effects need to be better understood. In all cases there is a need for more convenient instruments and a better knowledge of their operating principles. 

There is no uniform operating procedure for field meters, field portable detectors, and field screening kits because different manufacturers make them in different formats. To use them correctly, we must strictly follow the manufacturer s instructions. When selecting a particular model, we need to evaluate its ruggedness, portability, selectivity, sensitivity, and reliability. To produce usable data, we must have a good understanding of the measurement mechanism, its applicability and limitations, and be concerned with the issues of field instrument calibration and maintenance. In this chapter, we will review some basic general chemistry definitions applicable to field measurements and focus on the common types of field analysis. 

In a hybrid technique the separation process and elemental detection occurs on-line. The two separate techniques must therefore be coupled by an interface. In designing the most effective coupled technique the detection system must be compatible with the separation process. The separation process in these systems is usually some form of chromatography, but the detectors normally used for chromatography lack the selectivity and sensitivity required for speciation studies. The favoured detectors for hybrid systems are the very sensitive and selective element-specific detectors. Selectivity is very useful in speciation studies because it means that only species of the element of interest will be detected, thus simplifying the sample preparation step. 

There are few methods which can measure well-defined metal fractions with sufficient sensitivity for direct use with environmental samples (approach B in Fig. 8.2). Nevertheless, this approach is necessary in the experimental determination of the distribution of compounds that are labile with respect to the time scales of the analytical method. Recent literature indicates that high-performance liquid (HPLC) and gas chromatographic (GC) based techniques may have such capabilities (Batley and Low, 1989 Chau and Wong, 1989 van Loon and Barefoot, 1992 Kitazume et al, 1993 Rottmann and Heumann, 1994 Baxter and Freeh, 1995 Szpunar-Lobinska et al, 1995 Ellis and Roberts, 1997 Vogl and Heumann, 1998). The ability to vary both the stationary and mobile phases, in conjunction with suitable detector selection (e.g. ICP-MS), provides considerable discriminatory power. HPLC is the superior method GC has the disadvantage that species normally need to be derivatised to volatile forms prior to analysis. Capillary electrophoresis also shows promise as a metal speciation tool its main advantage is the absence of potential equilibria perturbation, interactions... 

Liquid chromatography (LC) and hydrodynamic electrochemistry are, for the most part, very compatible technologies which in combination yield important advantages for a number of trace determinations. In order of decreasing importance, the three major advantages are selectivity, sensitivity, and low cost. The use of modem LC for residue determinations requires a selective detector with a rapid response time, wide dynamic range, and low... 

Selavka et al. [36] have reported that for LC, a pyrolysis chamber temperature greater than 550°C gives rise to substantial baseline noise, which severely limits the detector s sensitivity and selectivity. Thus, LC-TEA is always performed in the nitroso mode (the pyrolyser is held at 550°C). 

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