Stationary phase temperature stability

Temperature increase always helps for a kinetic optimization of separations (it does, however, require prerequisites regarding the column thermostatting and stationary phase temperature stability). 

C is not recommended and high-temperature LC has not been rigorously explored. With the advent of thermally stable phases such as zir-conia-based stationary phases, temperatures in excess of 150°C can now be utilized. Many zirconia-based phases are available, so the stability of stationary phases is less of an issue. 

The last several years have seen an enormous growth in the number and use of chiral stationary phases in liquid chromatography [742,780-791]. Some problems with the gas chromatographic approach are that the analyte must be volatile to be analyzed and larger-scale preparative separations are frequently difficult. For entropic reasons relatively high temperatures tend to minimize the stability differences between the diastereomeric complexes and racemization of the stationary phase over time may also occur. The upper temperature limit for phases such as Chirasil-Val is about 230 C and is established by the rate of racemization of the chiral centers and not by column bleed. Liquid chromatography should be s ior in the above... 

In high temperature (HT)-GC-MS and PyGC-MS experiments, special attention should be given to the stability of the column. GC columns can lose some of the stationary phase ( bleeding ) when heated up to the maximum operating temperature the thicker the stationary phase, the more column bleed may be expected. When coupled on-line to a mass spectrometer, the stationary phase may foul the ion source, which leads to rapid decay in sensitivity and detection of usually siloxane-related mass peaks at m/z 207, 281, 355, etc. HTGC-MS coupling was discussed by ref. [218]. 

Difficulties of work at high temperatures include stationary phase stability and analyte stability. However, it has been shown that the relatively short residence times in the column at high temperatures tend to minimize the effects of analyte instability... 

Aqueous eluents destroy silica gei over an extended period, of time particularly when the pH is above neutrality and/or the temperature is high. Whereas the greasy stationary phase layer protects the silica io a large extent, methods for improvement of column stability are usually needed in practice. 

The interest in HPLC at high temperatures has led to studies of the limits of various commercial stationary phases. Claessens and van Straten [97] reviewed the thermal stability of stationary phases. Teutenberg et al. [98] and Marin et al. [67] studied column stability at temperatures up to... 

The temperature stability of monolithic stationary phases based on alkyl methacrylate monomers in capillary HPLC has also been reported [103]. These columns allowed the separation time to be reduced by over 10-fold at temperatures up to 80°C. The upper-temperature limit for these columns was not reported. 

The low thermal stability and the volatility of some of the low molecular weight stationary phases restricted their general use. Therefore, thermally stable and nonvolatile polymeric chiral stationary phases were developed by coupling the diamide phase, via the amino functionality, to a statistical copolymer of dimethylsiloxane and (2-carboxypropyl)methylsiloxane of appropriate viscosity131. The fluid polymeric phase, referred to as Chirasil-Val (Table 2), exhibits excellent properties for the enantiomer separation of a variety of compound classes over a broad temperature range141142. 

The Chirasil-Metal stationary phase can be thermally immobilized on a glass surface156, thus increasing the temperature stability up to 160 °C. The immobilized Chirasil-Metal stationary phase can also be used in the SFC mode using supercritical carbon dioxide as the mobile phase156. 

In addition to providing highly selective separations, there are a multitude of other desired characteristics that a gas chromatographic stationary phase should possess. These properties include high viscosity, low surface tension allowing for wetting of the fused silica capillary wall, high thermal stability, and low vapor pressure at elevated temperatures. The stationary phase solvent should also not exhibit unusual mass transfer behavior. 

The various properties exhibited by ILs make them ideal stahonary phases in GLC. ILs exhibit a unique dual-nature selechvity that allows them to separate polar molecules like a polar stationary phase and nonpolar molecules like a nonpolar stationary phase. In addition, the combination of cations and anions can be tuned to add further selectivity for more complex separations. Viscosity, thermal stability, and surface tension are vital properties that dictate the quality and integrity of the stationary phase coating and are additional characteristics that can be controlled when custom designing and synthesizing ILs. Furthermore, thermal stability and the integrity of stationary phase film can be improved by immobilizing the IL by free radical polymerization to form stationary phases suitable for low- moderate-, and high-temperature separations. Chiral ILs have been shown to enantioresolve chiral analytes with reasonable efficiency. 

Anderson, J.L., and Armstrong, D.W., Immobilized ionic liquids as high-selectivity/high-temperature/high stability gas chromatography stationary phases. Anal. Chem., 77, 6453-6462,2005. 

The high resolution power provides for a good profile analysis of the sample with a clear pattern and minor peak overlap compared to packed column GC. Furthermore, in view of the possible toxic, synergestic or antagonistic effects of the individual PAH, it is important to quantify each compound separately. The chromatograms also demonstrate the application of 0V-73 to PAH analysis, a stationary phase similar to SE-52, but with improved temperature stability. 

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