Theory, chromatography temperatures

The peak recorded in a chromatogram represents the distribution of molecules in a band as it elutes from the column, the overall broadness being conveniently m sured in terms of the width of the peak. A number of independent factors such as sample-injector and detector characteristics, temperature and column retention processes, contribute to the dispersion of molecules in a band and band broadening. The cumulative effect of small variations in these factors is described in statistical terms as the variance, cr, in the elution process. Classical chromatography theory considers that the separation process takes place by a succession of equilibrium steps, the more steps in a column the greater the column efficiency with less band broadening (variance) occurring, therefore... 

See alsa Carbohydrates Starch. Derivatization of Analytes. Extraction Solvent Extraction Principles. Fluorescence Food Applications. Gas Chromatography High-Temperature Techniques. Lipids Fatty Acids Polar Lipids. Liquid Chromatography Food Applications. Mass Spectrometry Food Applications. Quality Assurance Quality Control Internal Standards. Sample Handling Comminution of Samples. Sampling Theory. 

So far the plate theory has been used to examine first-order effects in chromatography. However, it can also be used in a number of other interesting ways to investigate second-order effects in both the chromatographic system itself and in ancillary apparatus such as the detector. The plate theory will now be used to examine the temperature effects that result from solute distribution between two phases. This theoretical treatment not only provides information on the thermal effects that occur in a column per se, but also gives further examples of the use of the plate theory to examine dynamic distribution systems and the different ways that it can be employed. 

Davis, J.M., Pompe, M., Samuel, C. (2000). Justification of statistical overlap theory in programmed temperature gas chromatography thermodynamic origin of random distribution of retention times. Anal. Chem. 72, 5700-5713. 

Dyeing applications, tetrahydrofurfuryl alcohol in, 12 279 Dyeing theory, 26 394-395 Dyeing transition temperature, 9 159 Dye intermediates, 9 265-298 chemistry, 9 266-291 classification, 9 265-266 economic aspects, 9 293-295 equipment and manufacture, 9 291-293 health and safety factors, 9 295-298 obtained by nitration, 9 2 7 It obtained by sulfonation, 23 525 unit processes, 9 269-283 Dye lasers, 74 702-705 23 144 output characteristics of, 74 705 Dye-ligand affinity chromatography, 6 402 Dye liquor, 9 163 Dye manufacturing... 

From both theory and experimental evidence, raising the temperature by 10°C decreases the retention time by about 20% in isocratic chromatography and decreases the backpressure by 10% to 20% because of a reduction in the viscosity of the mobile phase. This can help to overcome the instrument limitations associated with running shorter columns packed with smaller particles, i.e., the pressure limitations of current HPLC systems. However, since the majority of reversed-phase columns available are silica-based, operating at temperatures above... 

Van Deemter rate theory analychem A theory that the sample phase in gas chromatography flows continuously, not stepwise. van dam tar rat. the a re ) van der Waals adsorption physchem Adsorption in which the cohesion between gas and solid arises from van der Waals forces. van dar, w6lz ad.sorp shan ) van der Waals attraction See van der Waals force. van dar, w6lz a.trak shan ) van der Waals covolume physchem The constant b in the van der Waals equation, which is approximately four times the volume of an atom of the gas in question multiplied by Avogadro s number. van dar, w6lz ko val yam ) van der Waals equation phys chem An empirical equation of state which takes into account the finite size of the molecules and the attractive forces between them p = RT/(v — b) - (a/v ). where p is the pressure, v is the volume per mole, T is the absolute temperature, R is the gas constant, and a and b are constants. van dar, w6lz i,kwa-zhan ... 

In a chromatographic separation procedure the parameters of the chromatographic system (stationary phase, flow, temperature, etc.) have to be selected respectively optimized with respect to some criterion (resolution, time, etc.). In gas chromatography retention data series are published and used for the sttidy of solvent/solute interaction, prediction of the retention behaviour, activity coefficients, and other relevant information usable for optimization and classification. Several clKmometrk techniques of data anal s have been employed, e.g. PCA, numerical taxonomic methods, information theory, and j ttern recognition. 

The air oxidation of 2-methylpropene to methacrolein was investigated at atmospheric pressure and temperatures ranging between 200° and 460°C. over pumice-supported copper oxide catalyst in the presence of selenium dioxide in an integral isothermal flow reactor. The reaction products were analyzed quantitatively by gas chromatography, and the effects of several process variables on conversion and yield were determined. The experimental results are explained by the electron theory of catalysis on semiconductors, and a reaction mechanism is proposed. It is postulated that while at low selenium-copper ratios, the rate-determining step in the oxidation of 2-methylpropene to methacrolein is a p-type, it is n-type at higher ratios. 

The mechanism of separation on polar bonded phases is not clear. Due to their limited proliferation, no theories have been developed solely to descibe this particular form of liquid chromatography. Instead, descriptions from other fields may be applied. If polar bonded phases are used in combination with more polar mobile phases in the reversed phase mode, then the same rules may be applied as in RPLC (section 3.2.2.1) to describe the effects of the mobile phase, the temperature, the pH, etc.. 

Theory of Programmed Temperature Gas Chromatography The Prediction of Optimum Parameters, J. C. Giddings, in Gas Chromatography, Academic Press, New York, 1962, Chapter V. 

The n-heptane, isobutane and n-butane studies give strong support therefore to Antonik and Lucquin s theory. It should be noted, however, that attempts by gas—liquid chromatography to identify 2-methylbut-2-yl hydroperoxide in the products of the oxidation of 2-methylbutane at 320—460 °C failed [139] and that Hoare and Lill [140] have recently put forward evidence which suggests that in systems in which acetaldehyde is formed methyl hydroperoxide becomes more important than peracetic acid as the temperature approaches 350 °C, the methyl hydroperoxide being formed as a result of the methyl radicals produced in reaction (37). 

To illustrate the approach to the theory of retention and band broadening, we examine some aspects of programmed temperature gas chromatography. ... 

In theory, GC retention times should be useful for identifying components in mixtures. In fact, however, the applicability of such data is limited by the number of variables that must be controlled to obtain reproducible results. Nevertheless, gas chromatography provides an excellent means of confirming the presence or absence of a suspected compound in a mixture, provided that an authentic sample of the substance is available. No new peaks in the chromatogram of the mixture should appear on addition of the known compound, and enhancement of an exi.st-ing peak should be obseiwed. The evidence is particularly convincing if the effect can be duplicated on different columns and at different temperatures. On the other hand, because a chromatogram provides but a single piece of information about each species in a mixture (the retention time), the application of the technique to the qualitative analysis of complex samples of unknown composition is limited. 

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