Molecule-specific detector

The determination of organic selenium compounds is done preferably by GC coupled to element-or molecule-specific detectors, such as GC-AED or molecular mass spectrometric detection (GC-MS).240 In this case, ICP-MS detection does not yield the improvement in sensitivity otherwise seen, which is due to spectral interferences. Dietz et al.241 have compared the analytical figures of merit of three detector systems for GC (AED, atomic fluorescence spectroscopy (AFS), and ICP-MS), arriving at the conclusion that GC-AED is the most sensitive and most practical... 

The test for triazine herbicides is based on the gas-chromatographic detection of undecomposed active substances with the assistance of various molecule-specific detectors. This method combines high sensitivity and good selectivity. The active substances which may be determined using this... 

Chromosorb W (AW, DMCS). The arsines are separated on this column and passed into a mass spectrometer operated in the electron impact mode (70 eV), with the ion source at 180 C. Ions with m/z of 76 and 78 for AsHa, 90 for CH3ASH2, 90 for (CH3)2AsH, and 103 and 120 for (CH3)3As are selectively monitored. The peaks at these m/z values are the most intense in the mass spectra of the arsines. The mass spectrometer used in the selective-ion monitoring mode serves as an element- and molecule-specific detector. This system allows arsenite to be determined by pH-controlled reduction (pH 5). Total inorganic and reducible methylarsenic compounds are reduced at pH 1 (eqn. 1) ... 

The reason for such difficulties is the GPC mechanism itself. We do not separate by molar mass but by the size of the solvated molecules. Different solvation of chemical unlike molecules results in breaking the M sequence of the calibration curve this becomes visible especially in the low molar mass range. Sometimes such difficulties can be circumvented if a specific detector is used, e.g., if the sample absorbs in the ultraviolet (UV) range and the disturbing peaks are UV transparent. 

Detectors are composed of a sensor and associated electronics. Design and performance of any detector depends heavily on the column and chromatographic system with which it is associated. Because of the complexity of many mixtures analysed and the limitation in regard to resolution, despite the use of high-resolution capillary columns and multicolumn systems, specific detectors are frequently necessary to gain selectivity and simplify the separation system. Many detectors have been developed with sensitivities toward specific elements or certain functional groups in molecules. Those detectors that exhibit the highest sensitivity are often very specific in response, e.g. the electron capture detector in GC or the fluorescence detector in LC. Because... 

Certain SEC applications solicit specific experimental conditions. The most common reason is the limited sample solubility. In this case, special solvents or increased temperature are inavoid-able. A possibility to improve sample solubility and quality of eluent offer multicomponent solvents (Sections 16.2.2 and 16.8.2). The selectivity of polymer separation by SEC drops with the deteriorating eluent quality due to decreasing differences in the hydrodynamic volume of macromolecules with different molar masses. The system peaks appear on the chromatograms obtained with mixed eluents due to preferential solvation of sample molecules (Sections 16.3.2 and 16.3.3). The multicomponent eluents may create system peaks also as a result of the (preferential) sorption of their components within column packing [144,145]. The extent of preferential sorption is often sensitive toward pressure variations [69,70,146-149]. Even if the specific detectors are used, which do not see the eluent composition changes, it is necessary to discriminate the bulk sample solvent from the SEC separated macromolecules otherwise the determined molecular characteristics can be affected. This is especially important if the analyzed polymer contains a tail of fractions possessing lower molar masses (Sections 16.4.4 and 16.4.5). 

Analytical methods used to identify monomers have improved significantly from those that quantify whole classes of compounds, such as amino acids, peptides, proteins, and primary amines (Undefriend et al., 1972) or carbohydrate-like compounds (Johnson and Sieburth, 1977) to ones that are molecule-specific (Table II). Most of these methods are based on combining chromatography techniques that can separate complex mixtures of molecules with highly sensitive detectors that can approach the nanomolar or picomolar range. Monomers are usually present at low concentrations, so... 

The simultaneous use of a combination of a universal detector (FID) and a specific detector to monitor the effluent of a column can provide useful information about the properties of functional groupsand substituents in a molecule. 

The use of nitrogen- or sulfur-specific detectors for GC enables small quantities of nitrogen- or sulfur-containing molecules to be detected. 

This detector can function at more than 400 °C and is not destructive since the ionization is reversible and affects only a small fraction of the molecules of each compound passing through. ECD and PID detectors are examples of class-specific detectors often used in trace environmental analysis. 

An important and rapidly expanding detector group is that of the derivatization devices in which the sample components are reacted or interacted with appropriate substance so that the specific groups, e.g., chromophores, are introduced into their molecules and, consequently, they can be detected by the specific detectors (e.g. by photometers). Turbidimeters, in which the precipitate is formed by the controlled addition of a nonsolvent for the sample to the effluent, also belong to the group of derivatization detectors. Naturally, the nephelometers measuring turbidity can also... 

Infrared Spectroscopy. The infrared (IR) region of the electromagnetic spectrum between 2.5 and 25 micrometers has proven valuable for the identification and quantification of gaseous molecular species. When infrared radiation passes through a gas, adsorption of radiation occurs at specific wavelengths that are characteristic of the vibrational structure of the gas molecules. Infrared detectors are used in mobile detectors to detect blister and nerve agent... 

The technique is illustrated by Fig. 8.29. A collimated supersonic beam of argon, which contains Na atoms and Na2 molecules, is crossed perpendicularly with a noble gas beam [1077]. Molecules of Na2 in the level W J ) that have been scattered by the angle d- are monitored by a quantum-state-specific detector. It consists of a cw dye laser focused into a spot behind the aperture A and tuned to the transition, 7 v p 7j>, and an optical system of a mirror and lens, which collects the LIF through an optical fiber bundle onto a photomultiplier. The entire detector can be turned around the scattering center. 

Pore sizes of columns for SEC must match the sizes of dissolved macromolecules. The column also must reliably sort the analyzed macromolecules with lowest molar mass from the molecules of initial solvent and the low-molecular substances present in sample. The latter substances - that is various polymer additives, as well as humidity and gases, mainly oxygen dissolved in sample solution - are recognized with the non-specific detector such as differential refractometer, and the resulting peaks could inferfere with the peaks of sample. 

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