Detectors universal

Another classification of detector is the bulk-property detector, one that measures a change in some overall property of the system of mobile phase plus sample. The most commonly used bulk-property detector is the refractive-index (RI) detector. The RI detector, the closest thing to a universal detector in lc, monitors the difference between the refractive index of the effluent from the column and pure solvent. These detectors are not very good for detection of materials at low concentrations. Moreover, they are sensitive to fluctuations in temperature. 

In practice a few iodine crystals are usually placed on the bottom of a dry, closed trough chamber. After the chamber has become saturated with violet iodine vapor the solvent-free plates are placed in the chamber for 30 s to a few minutes. The iodine vapor condenses on the TLC layers and is enriched in the chromatogram zones. Iodine vapor is a universal detector, there are examples of its application for all types of substances, e.g. amino acids, indoles, alkaloids, steroids, psychoactive substances, lipids (a tabular compilation would be too voluminous to include in this section). 

The conductance detector is a universal detector for ionic species and is widely used in ion chromatography (see Section 7.4). 

The FID has wide applicability, being a very nearly universal detector for gas chromatography of organic compounds, and this, coupled with its high sensitivity, stability, fast response and wide linear response range ( — 107), has made it the most popular detector in current use.70... 

One great advantage of GC is the variety of detectors that are available. These include universal detectors, such as flame ionization detectors and selective detectors, such as flame photometric and thermionic detectors. The most generally useful detectors, excluding the mass spectrometer are described in the following sections. 

Thermal Conductivity Detector In the thermal conductivity detector (TCD), the temperature of a hot filament changes when the analyte dilutes the carrier gas. With a constant flow of helium carrier gas, the filament temperature will remain constant, but as compounds with different thermal conductivities elute, the different gas compositions cause heat to be conducted away from the filament at different rates, which in turn causes a change in the filament temperature and electrical resistance. The TCD is truly a universal detector and can detect water, air, hydrogen, carbon monoxide, nitrogen, sulfur dioxide, and many other compounds. For most organic molecules, the sensitivity of the TCD detector is low compared to that of the FID, but for the compounds for which the FID produces little or no signal, the TCD detector is a good alternative. 

Universal detector An alternative term for a general detector. 

In the pharmaceutical industry, it is essential to produce pure drug substance, suitable for human consumption, in a cost-effective manner. The purity of a drug substance can be checked by separation techniques such as GC, TLC, and HPLC. Both techniques tend to be more sensitive and specific than spectroscopic methods. HPLC has an advantage over GC as an analytical technique, since analytes need be neither volatile nor extremely stable to elevated temperatures. Highly accurate, almost universal detectors, such as... 

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... 

Hyphenation of chromatographic separation techniques (SFC, HPLC, SEC) with NMR spectroscopy as a universal detector is one of the most powerful and time-saving new methods for separation and structural elucidation of unknown compounds and molecular compositions of mixtures [171]. Most of the routinely used NMR flow-cells have detection volumes between 40... 

Increasing reliance on mass spectrometry as the universal detector for GC has not solved all the problems of additive identification. Isomer identification is impossible (except for REMPI technology), but is hardly an issue in additive analysis. 

SFC-SFC is more suitable than LC-LC for quantitation purposes, in view of the lack of a suitable mass-sensitive, universal detector in LC. Group quantitation can be achieved by FID. The ideal SFC-SFC system would consist of a short (10-30 cm) packed-capillary primary column, interfaced to a long (5-10m) open-tubular column, but such a combination is difficult to realise, due to the different flow-rates required for each column type. Coupled SFC-SFC is often configured with a solute concentration device prior to valve switching on to the SFC. The main approaches to this concentration stage are the use of absorbent material or cryofocusing. Davies el at. [924] first introduced two-dimensional cSFC (cSFC-cSFC), and its use has been reported [925,926]. 

Coupled LC-LC can separate high-boiling petroleum residues into groups of saturates, olefins, aromatics and polar compounds. However, the lack of a suitable mass-sensitive, universal detector in LC makes quantitation difficult SFC-SFC is more suitable for this purpose. Applications of multidimensional HPLC in food analysis are dominated by off-line techniques. MDHPLC has been exploited in trace component analysis (e.g. vitamin assays), in which an adequate separation for quantitation cannot be achieved on a single column [972]. LC-LC-GC-FID was used for the selective isolation of some key components among the irradiation-induced olefinic degradation products in food, e.g. dienes and trienes [946],... 

After passing through the column, the separated solutes are sensed by an in-line detector. The output of the detector is an electrical signal, the variation of which is displayed on a potentiometric recorder, a computing integrator or a vdu screen. Most of the popular detectors in hplc are selective devices, which means that they may not respond to all of the solutes that are present in a mixture. At present there is no universal detector for hplc that can compare with the sensitivity and performance of the flame ionisation detector used in gas chromatography. Some solutes are not easy to detect in hplc, and have to be converted into a detectable form after they emerge from the column. This approach is called post-column derivatisation. 

These detectors sense the difference in refractive index between the column eluent and a reference stream of pure mobile phase. They are the closest thing in hplc to a universal detector, as any solute can be detected as long as there is a difference in ri between the solute and the mobile phase. 

Trathnigg, B., Kollroser, M.J. (1997). Liquid chromatography of polyethers using universal detectors V. Quantitative aspects in the analysis of low-molecular mass poly(ethylene glycols) and their derivatives by reversed-phase HPLC with an evaporative light scattering detector. J. Chromatogr. A 768, 223-238. 

Consider one small molecule, phenylalanine. It is an essential amino acid in our diet and is important in protein synthesis (a component of protein), as well as a precursor to tyrosine and neurotransmitters. Phenylalanine is one of several amino acids that are measured in a variety of clinical methods, which include immunoassay, fluorometry, high performance liquid chromatography (HPLC see Section 4.1.2) and most recently MS/MS (see Chapter 3). Historically, screening labs utilized immunoassays or fluorimetric analysis. Diagnostic metabolic labs used the amino acid analyzer, which was a form of HPLC. Most recently, the tandem mass spectrometer has been used extensively in screening labs to analyze amino acids or in diagnostic labs as a universal detector for GC and LC techniques. Why did MS/MS replace older technological systems The answer to this question lies in the power of mass spectrometer. 

The refractive index is another very rapid analytical method for determining purity and identity of a substance. It is uniquely useful for quality control monitoring of raw materials and finished products. Many students encounter a refractometer first as a universal detector for high-performance liquid chromatography. 

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