Applications chromatographic detectors

In addition to these principal requirements, there are further characteristics that affect the performance and applicability of analysers and detectors. The signal should depend as little as possible on the liquid flow-rate and temperature. With continuous monitors, the sensor should be selective for the test component. On the other hand, with chromatographic detectors, the sensor should respond to all eluted components with the same sensitivity. 

The use of various sorbents and desorption systems permits (often necessitates) gas chromatographic detectors other than the most commonly used flame detector or electron capture detector. The use of a selective detector can greatly simplify a difficult analytical problem by reducing interferences and background peaks. Many applications are discussed in the literature (21,22,23,24). 

Contrary to potentiometric methods that operate under null current conditions, other electrochemical methods impose an external energy source on the sample to induce chemical reactions that would not otherwise spontaneously occur. It is thus possible to measure all sorts of ions and organic compounds that can either be reduced or oxidised electrochemically. Polarography, the best known of voltammetric methods, is still a competitive technique for certain determinations, even though it is outclassed in its present form. It is sometimes an alternative to atomic absorption methods. A second group of methods, such as coulometry, is based on constant current. Electrochemical sensors and their use as chromatographic detectors open new areas of application for this arsenal of techniques. 

The use of polymer-coated acoustic sensors as chromatographic detectors (GIX, HPLC) has also been demonstrated [1,43,218]. In such applications, a lack of selectivity fcH a given analyte is actually beneficial, since the function of the coated sensor is to detect each and every species passing the detector after preseparation by the chromatographic column (see Chapter 6). 

Supercritical fluid chromatography has some of the same characteristics of both HPLC and gas chromatography (GC). Packed column SFC uses the same column technology as HPLC, and when used with binary or tertiary solvents, has a broad range of applicability [1]. This range is much broader than GC, because compounds need not be volatile or thermally stable. As in GC, SFC can be coupled to most modern chromatographic detectors, such as element-specific detectors. These detectors are often very selective for... 

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. 

Another broad field of applications involves continuous coulometric titrators which are employed in process stream analyzers. In these the generating current is continuously adjusted to maintain a small excess of electrogenerated titrant to react with material in the incoming liquid or gaseous sample stream. The level of generating current is a measure of the instantaneous concentration of the titrated substance (29, 30), Coulometric titration methods have also been used in chromatographic detectors and for determination of homogeneous reaction rates (31). 

Working Principle and Application Range of Selected Chromatographic Detectors... 

The general description mass spectrometer covers a variety of instruments that vary in size, cost, versatility, performance and operational complexity, but all find specific applications appropriate to their design. The common use of mass spectrometers as chromatographic detectors has resulted in a significant market for low cost instruments with modest performance characteristics that are easy to operate, robust, and require little bench space. Instruments in this category will be the focus of the following sections. 

The application of GC for the introduction of complex samples into field asymmetric IMS (FAIMS) and differential mobility spectrometry (DMS) instruments is also used extensively. Fast capillary chromatography in which relatively simple mixtures can be separated in less than a second provides a rapid separation-and-introduction method for DMS. One specific advantage of FAIMS (or DMS) as a chromatographic detector is that both positive and negative ions can be monitored simultaneously from the GC effluent. Figure 3.10 provides a schematic of a typical capillary GC/DMS instrument in which SPME is used to inject semivolatile compounds into the capillary column with DMS detection. 

This book is concerned with the unique benefits of mass spectrometry for trace level quantitative analysis, so no detailed discussion of other chromatographic detectors is included except for occasional comparisons. However, some general aspects applicable to all detectors are discussed here, with special emphasis on how these generalizations apply to mass spectrometry (see also Section 6.2.3). 

The flame ionization detector (FID) is the most popular detector because of its high sensitivity and wide linear dynamic range [1]. The FID is an ionization detector that exhibits a nearly universal response to all organic compounds. The sensitivity, stability, excellent linear range, ease of operation and maintenance, along with wide applicability and low cost has made this detector the most popular gas chromatographic detector in use today [2]. 

This requirement of small anal3d e amoimts for the utilization of fixed voltage titration cells is fulfilled, e.g., in chromatography applications. With the setup given in Fig. 7, it could be shown that the application of simple and miniaturized SE titration cells can be a successful way to calibration-free and easy to use chromatographic detectors for the measurement of oxygen as well as a huge... 

Progress in this area has been considerable in recent years and hopefully some non precious metal catalyst will be used in fuel cells in the near future. On the other hand, the stability of metallophthalocyanines makes them appropriate for applications in various fields such as chemical catalysis (such as the MEROX process for the sweetening of oils), dye stuffs, coloring for plastics and metal surfaces, sensors, chromatographic detectors, photoconducting agents, and so on. These complexes are also used for photobiology and photodynamic cancer therapy, electrochemical... 

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