Programmed retention

H.L. Yin and Y.L. Sun, The achievement of reproducible temperature programmed retention indices in gas chromatography when using different columns and detectors, Chromatographia 29, 39-43 (1990). 

The GC separation of 2,4-dinitrophenylhydrazones has been studied by a number of workers [52-55], Non-polar stationary phases of the SE-30 and SF-96 type were utilized for this purpose at temperatures of 200—250°C, mostly with temperature programming. Retention indices of the 2,4-DNPHs of some carbonyl compounds on OV-type stationary phases are presented in Table 5.5. Using columns with a higher separation efficiency, some 2,4-DNPHs provide two peaks. The discussion of whether these artifacts are caused by thermal decomposition or syn-anti isomerization of the derivatives seems to favour the latter. The ratio of the areas of the peaks of the two derivatives depends on the polar-... 

The approach described by Arey and co-workers [54], which uses n-alkanes as reference solutes in both dimensions, is acceptable for nonpolar stationary phases, since the difference between the temperature-programmed retention index and the isothermal retention index for terpenes does not generally exceed 1% [49]. However, there is a marked temperature dependence of retention indices on polar polyethylene glycol stationary phase columns [55], so this approach may not be suitable for the "reversed polarity" column GCxGC arrangement that is sometimes employed for essential oil analysis. 

C. T. Peng, Prediction of retention indices. VI Isothermal and temperature-programmed retention indices, methylene value, functionality constant, electronic and steric effect,/ Chromatogr., A, 2010,1217,3683-... 

These mathematical models enable prediction of isothermal or temperature-programmed retention times with very good accuracy, and so chromatographers can estimate the effects of changing conditions on peak elution sufficiently well to provide a good basis for optimization. These models do not take into account any of the band-broadening processes that determine peak shapes, and therefore alone they cannot predict peak resolution, Trennzahl or separation number, or any other measurement of chromatographic quality. 

Song, C Lai, W.C. Reddy, K.M. Wei, B. Temperature-Programmed Retention Indices for GC and GC-MS of Hydrocarbon Fuels and Simulated Distillation GC of Heavy Oils In Analytical Advances for Hydrocarbon Research, C.S. Hsu (Ed.), Kluwer Academic/Plenum Publishers New York, 2003. 

Lai, W.-C. and Song, C. Temperature-Programmed Retention Indices for GC and GC-MS Analysis of Coal- and Petroleum-derived Liquid Fuels. Fuel, 1995,74 (10), 1436-1451. Edwards, T., System Drivers for High Heat Sink Fuels. Am. Chem. Soc. Div. Petrol. 

Berngard, A., Colmsjo, A., and Wrangskog, K. (1994) Prediction of temperature-programmed retention indexes for polynuclear aromatic hydrocarbons in gas chromatography. 

For a SIM (selected ion monitoring) analysis the mass spectrometer is programmed in such a way that a certain number of compounds in a defined retention time range are detected. These SIM segments are switched at the programmed retention time so that a list of target compounds can be worked... 

Van Den Dool, H. and P.D. Kratz (1963), Generalization of the retention index system including linear temperature programmed gas-liquid partition chromatography . J. Chromatogr, Vol. 11, p. 463. 

The enrichment program followed in the United States is (/) the enrichment of flour, bread, and degerminated and white rice using thiamin [59-43-8] C 2H y N O S, riboflavin [83-88-5] C2yH2QN4Na02P, niacin [59-67-6] CgH N02, and iron [7439-89-6]-, (2) the retention or restoration of thiamin, riboflavin, niacin, and iron in processed food cereals (J) the addition of vitamin D [67-97-0] to milk, fluid skimmed milk, and nonfat dry milk (4) the addition of vitamin A [68-26-8], C2qH2qO, to margarine, fluid skimmed milk, and nonfat dry milk (5) the addition of iodine [7553-56-2] to table salt and (6) the addition of fluoride [16984-48-8] to areas in which the water supply has a low fluoride content (74). 

Figure 3. Curves Relating Elution Time to Flow Program Rate for Solutes having Different Retention Volumes...

Temperature programming was introduced in the early days of GC and is now a commonly practiced elution technique. It follows that the temperature programmer is an essential accessory to all contemporary gas chromatographs and also to many liquid chromatographs. The technique is used for the same reasons as flow programming, that is, to accelerate the elution rate of the late peaks that would otherwise take an inordinately long time to elute. The distribution coefficient of a solute is exponentially related to the reciprocal of the absolute temperature, and as the retention volume is directly related to the distribution coefficient, temperature will govern the elution rate of the solute. 

Furthermore, after time (tp) between (tp) and (tp+At) during a temperature program at a rate of a°C per unit time and an initial temperature of (To), the effective retention... 

It follows that if the retention time in seconds is (nj) and (At) is conveniently taken as one second, then the mean value of the retention volume throughout the program will be... 

Thus, rearranging equation (5), the retention time at a particular program rate (a) can be calculated as that time (tr) nota bena tr=nt) that allows the following equation to be satisfied. 

It is seen that the viscosity of the gas will change significantly during a temperature program and, thus, at a constant gas mass flow rate, the inlet pressure will rise proportionally. This increase in inlet pressure will result in an increase in the inlet/outlet pressure ratio and, as a consequence, will extend the retention time and oppose the effect of any increase in temperature. It also follows that the effect of... 

Figure 5. Graphs of Retention Time of (R) 4-Benzyl-2-oxazolidinone against Temperature Program Rate for Three Different Initial Temperatures...

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