Universal detector, example

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 selectivity universal detectors exhibit a response to every sample (except the mobile phase), whatever the chemical species. Selective detectors respond to samples that exhibit a physical or chemical property (UV absorbance, for example). In this mode the mobile phase should not interfere. 

Several researchers have combined the separating power of supercritical fluid chromatography (SFC) with more informative spectroscopic detectors. For example, Pinkston et. al. combined SFC with a quadrupole mass spectrometer operated in the chemical ionization mode to analyze poly(dimethylsiloxanes) and derivatized oligosaccharides (7). Fourier Transform infrared spectroscopy (FTIR) provides a nondestructive universal detector and can be interfaced to SFC. Taylor has successfully employed supercritical fluid extraction (SFE)/SFC with FTIR dectection to examine propellants (8). SFC was shown to be superior over conventional gas or liquid chromatographic methods. Furthermore, SFE was reported to have several advantages over conventional liquid solvent extraction (8). Griffiths has published several... 

The detector converts a change in the column effluent into an electrical signal that is recorded by the data system. Detectors are classified as selective or universal depending on the property measured. Selective (solute property) detectors, such as fluorescence detectors, measure a physical or chemical property that is characteristic of the solute(s) in the mixture only those components which possess that characteristic will be detected. Universal (bulk property) detectors measure a physical property of the eluent. Thus, with refractive index (RI) detectors, for example, all the solutes which possess a refractive index different from that of the eluent will be detected. Selective detectors tend to be more sensitive than universal detectors, and they are much more widely used. Universal detectors are more commonly used in preparative chromatography, where a universal response is desired and sample size is large. 

Traditionally, the TCD has been recognized as a universal detector, but one of limited sensitivity. Several manufacturers have invested in miniaturization of the TCD, with improvements in sensitivity, resulting in lower detection limits. For example, Varian sells a multichannel GC (CP-4900 Micro GC) incorporating a MEMS-based TCD with the unrivaled sensitivity of about one ppm. 

Detectors may be classified on the basis of selectivity. A universal detector responds to all compounds in the mobile phase except carrier gas. A selective detector responds only to a related group of substances, and a specific detector responds to a single chemical compound. Most common GC detectors fall into the selective designation. Examples include flame ionization detector (FID), ECD, flame photometric detector (FPD), and thermoionic ionization detector. The common GC detector that has a truly universal response is the thermal conductivity detector (TCD). Mass spectrometer is another commercial detector with either universal or quasi-universal response capabilities. 

Universal detectors which can detect any kind of com x>und find a quantitative use only in GC. Examples are the thermal conductivity detector and the gas density balance (see Section 15.3.4.2). Most GC detectors are mass flow sensitive, which means that the output signal is directly proportional to the mass flow of a compound through the detector. The signal from such detectors is normally obtained from a direct effect of the detection system on the eluted compound, eg, ionization or transfer to an excited state. 

Besides the universal detector systems, for example electron capture, flame ionisation and thermal conductivity usually coupled with gas chromatographic columns, various other detectors are now being used to provide specific information. For example, the gas chromatograph/mass spectrometer couple has been used for structure elucidation of the separated fractions. The mechanics of this hybrid technique have been described by Message (1984). Other techniques used to detect the metal and/or metalloid constituents include inductively coupled plasma spectrometry and atomic absorption spectrometry. Ebdon et al. (1986) have reviewed this mode of application. The type and mode of combination of the detectors depend on the ingenuity of the investigator. Krull and Driscoll (1984) have reviewed the use of multiple detectors in gas chromatography. 

Universal detectors respond to all components. The thermal conductivity detector (TCD) is an example of this, as most eluates cause a change in thermal conductivity. 

The UV-Visible detector is the universal detector used in analytical and preparative CCC. It does not destroy solutes. It is used to detect organic molecules with a chromophore moiety or mineral species after formation of a complex (for instance, the rare earth elements with Arsenazo III ). Several problems can occur in direct UV detection, as has already been described by Oka and Ito 1) carryover of the stationary phase due to improper choice of operating conditions, with appearance of stationary phase droplets in the effluent of the column 2) overloading of the sample, vibrations, or fluctuations of the revolution speed 3) turbidity of the mobile phase due to difference in temperature between the column and the detection cell or 4) gas bubbling after reduction of effluent pressure. Some of these problems can be solved by optimization of the operating conditions, better control of the temperature of the mobile phase, and addition of some length of capillary tubing or a narrow-bore tube at the outlet of the column before the detector to stabilize the effluent flow and to prevent bubble formation. The problem of stationary phase carryover (especially encountered with hydrodynamic mode CCC devices) can be solved by the addition between the column outlet and UV detector of a solvent that is miscible with both stationary and mobile phases and that allows one to obtain a monophasic liquid in the cell of the detector (a common example is isopropanol). 

Because a mass spectrometer is a universal detector, MIMS offers the advantage that unexpected, short-lived reaction intermediates can often be spectrally identified during kinetic studies. A -chloromethyUmine (C1N=CH2) is such an example intermediate that was found to play an important role in the formation of the DBF CNCl due to the reaction of chlorine and glycine [17] (ct Table 27.1). The direct observation and identification of C1N=CH2 by MIMS greatly simplified the process of elucidating the reaction pathway for CNCl formation. 

Some detectors respond to almost every compound in the column effluent. These detectors are called general or universal detectors. Flame ionization detectors (FIDs) and thermal conductivity detectors (TCDs) are examples of... 

The evaporative light-scattering detector (ELSD), sometimes wrongly called a mass detector, is influenced by many variables. Schulz and Engelhardt [28] showed that its specific response in addition to the analyte s molecular size also depends on the linear flow rate of the mobile phase consequently, this detector is mass sensitive. Furthermore, they found that the ELSD behaves in certain situations more like a selective and not a universal detector. For example, it can detect only those solutes that are nonvolatile under... 

FTDI. The authors remarked that the instrumental advances and the associated open-source project recently reported can spread out the use of this universal detector for food analysis. Furthermore, the inherent portability of the C D system could make it an ideal instrument for on-site food testing. One example that combines creativity and the instrumental simplicity of this home-made conductivity detection is the analysis of monoalkyl carbonates in carbonated alcoholic beverages (sparkling wine, beer, and mixed drinks) [151]. By using two detectors, the authors not only demonstrated the presence of monoethyl carbonate (MFC) in the selected drinks but also proposed that the low pH values were responsible for the larger concentrations of MEC observed in lager beer and mixtures of rum and cola. In addition, the possibility to control not only a C D detector but also a series of valves using open-source software (Arduino) has been recently demonstrated [152]. 

Refractive index detectors also have several applications in lipid analysis. They are "universal" detectors, but lack sensitivity, require isocratic elution conditions and are sensitive to minor fluctuations in temperature. Their main value is probably in small-scale preparative applications, say with 1-2 mg of a lipid extract. For example, a refractive index detector was utilized with a column (4.6 x 250 mm) of Ultrasil Si (5 micron silica gel) and isocratic elution with isooctane-tetrahydrofuran-formic acid (90 10 0.5 by volume) to separate most of the common simple lipid classes encountered in animal tissue extracts, such as those of liver [304]. Cholesterol esters, triacylglycerols and cholesterol were each resolved and gave symmetrical peaks. 

Several variations on the basic technique described above have been reported. For example, the absorbance values between 4000 cm and the detector cutoff can be integrated. Since all compounds give rise to a signal when this approach is used, the GC/FT-IR system switches from being a selective detector to being a universal detector. In an alternative approach, the chromatogram is simply constructed from the value of the largest absorbance measured in each spectmm. GC/FT-IR interfaces used in this manner would also be considered universal detectors. 

There are several types of detectors that can be coupled to a gas chromatograph. One example is the flame ionization detector (FID), where, as the name implies, the effluent flows through a flame as it leaves the column, which generates ions as one of the products. These ions are then detected via an electrical current monitor. When coupled with suitable pre-concentration of the analyte, GC-FID can approach compound detection sensitivities as low as a few pptv. However, the FID works best for compounds such as hydrocarbons and is therefore not a universal detector. Other types of well-known GC detectors include the electron capture detector (BCD) and the thermal conductivity detector (TCD), and as with FID these alternatives also have their strengths and weaknesses but we will not discuss these in this book. 

The electronic block, which includes block of the analysis and registration and control system engines, and block of the source-receiver of acoustic oscillations are universal for any installations of this type. As the source-receiver of acoustic oscillations the ultrasonic flow detector is usually use. It s, as a rule, the serial devices for example y/f2-12. The electronic block contains the microprocessor device or PC, device of the power supply and management of engines... 

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