Retention factor stationary phase film thickness

Phase ratio, /3 is used in GC to describe the thickness of the stationary phase, measured as the ratio of stationary phase volume. Kg, to mobile phase volume. Km- Thicker stationary phase films give higher capacity factors and therefore longer retention times ... 

Capillary gas chromatography is conducted with a cross-linked phenyl methyl silicone stationary phase (5%, 25 m x 0.2 mm x 0.5 pm film thickness, injector temp. 225°C, detector temp. 250°C, column temp. 40-200°C, 15°C/min after 1 min initial time). (Z)-2-Bromo-5-(trimethylsilyl)-2-penten-4-ynoic acid ethyl ester is observed to have a retention time of 14.0 min. GC data for bis(trimethylsilyl)butadiyne is as follows retention time 10.0 min, response factor (GC area produced relative to an equimolar quantity of (Z)-2-bromo-5-(trimethylsilyl)-2-penten-4-ynoic acid ethyl ester) 1.29. For (E)-2-[(trimethylsilyl)ethynyl]-5-(trimethylsilyl)-2-penten-4-ynoic acid ethyl ester the retention time is 17.0 min, the response factor 1.19. 

The discussion of IL-based stationary phases up to this point has centered around ILs that are either coated as a thin film on a capillary wall or on a solid support. Although ILs exhibit a variety of properties that allow them to be unique stationary phases, their most significant drawback lies with their drop in viscosity with increasing temperature. This results in an increased propensity for flowing of the IL within the capillary, which often produces pooling of the stationary phase and nonuniform film thickness throughout the column. These factors often contribute to diminished analyte retention time reproducibility as well as detrimental effects on separation efficiency. 

At the constant linear velocity in Figure 24-6, increasing the thickness of the stationary phase increases retention time and sample capacity and increases the resolution of early-eluting peaks with a capacity factor of k < 5. (Capacity factor was defined in Equation 23-16). Thick films of stationary phase can shield analytes from the silica surface and reduce tailing (Figure 23-20) but can also increase bleed (decomposition and evaporation) of the stationary phase at elevated temperature. A thickness of 0.25 pm is standard, but thicker films are used for volatile analytes. 

Band broadening and temperature The five terms of Equation (24-14) can be examined in the context of the influence of temperature on flow rates, retention volumes, and diffusion coefficients to obtain an estimate of the overall influence of temperature on band broadening. Through thermal expansion, temperature also influences such factors as thickness of a liquid film and particle and column diameters, and it may also influence slightly the empirical constants in (24-14). With a liquid mobile phase, flow velocity (with the same inlet and outlet pressures) is strongly dependent on temperature. But with flow velocity u maintained constant the first term of (24-14) becomes smaller as diffusion coefficients increase in the mobile phase. For flow rates near the optimum the first term is approximately inversely proportional to The second and third terms increase in direct proportion to the diffusion coefficients in the mobile and stationary phases D and D, whereas the fourth and fifth... 

It can be concluded that the theoretical model describing the retention of injected samples on an amorphous polymeric stationary phase makes possible the prediction of the effect on the retention diagram of the following factors polymer film thickness, non-uniformity, crystallinity, support specific suriace area, carrier gas flow rate, glass transition and the difference between thermal expansion coefficients at T and Tg. 

For a fixed column radius (t. = constant), the retention factor of a solute at a given temperature (/f = constant), doubles when the film thickness is increased by a factor of 2. Guidelines for the. selection of film thickness for the stationary phase methylsilicone are listed below (values depend on column length and stationary phase) ... 

As the film thickness decreases, k or retention factor also decreases at constant temperature, column length, and inner diameter. Conversely, with an increase in film thickness in a series of columns having the same dimensions, retention increases under the same temperature conditions. This effect of film thickness on separation is demonstrated in the series of parallel chromatograms appearing in Figure 3.31. Column diameter limits the maximum amount of stationary phase that can be coated on its inner wall. Small-diameter columns usually contain thinner films of stationary phase, while thicker films can be coated on wider-bore columns. The concept of phase ratio allows two columns of equal length to be compared in terms of sample capacity and resolution. 

To use Equation 4.13, we will need to relate the retention factor A to the distribution coefficient. Chromatographers can measure retention factors directly from the chromatogram if the unretained peak time is known. The distribution coefficient K in Equation 4.13, however, is not directly evident, but it can be computed from the measured retention factor, if the column film thickness d and inner diameter are known. These two column measurements determine the phase ratio P, which is the ratio of the gas to stationary-phase volumes in the column ... 

Retention time is also strongly influenced by the choice of stationary phase. Several factors must be taken into account when selecting a column for separating androgens. Retention time and resolution will be affected by the choice of column length, stationary film thickness and polarity, and the temperature program rate. The programmed temperature optimization of a mixture of anabolic... 

Di is the diffusion coefficient of the solute in the stationary phase, k the retention factor of the solute, // the film thickness of stationary phase, and r the radius of the capillary column. With capillary columns, C/ is small and becomes significant only with capillary columns having a thick film of stationary phase. The Golay equation may then be rewritten as... 

The retention factor, k, is governed by the column temperature, the film thickness of the stationary phase, and the column diameter. The term k/ - -k) in equation (3.17) can not exceed unity and, in fact, can only asymptotically approach unity, as illustrated in Figure 3.46. Also note in Figure 3A6b that resolution increases rapidly until k reaches a value of approximately 10, after which there is no further contribution of this term to overall resolution, only an increase in the time of analysis, an important consideration in a laboratory of high sample throughput. This increase in time of analysis may or may not be necessary, depending on the number of components present in the sample. 

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