Retention times and capacity factors

The above discussion can be quantified as follows. A solute i distributes itself over the two phases, resulting in a total quantity qim to be present in the mobile phase (m), and a quantity q, s in the stationary phase (s). The solute molecules which find themselves in the mobile phase will be transported through the column at the same speed (u) as the molecules of the mobile phase. However, this is only a fraction of all the solute molecules, so the average speed for all solute molecules will be only a fraction of u given by 

6) is the fundamental equation for retention in chromatography. Throughout this book, extensive use will be made of the capacity factor as a convenient means to describe retention. A major advantage of the use of k for this purpose is the fact that it is a dimensionless quantity. It follows from eqn.(1.6) that 

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RETENTION TIMES AND CAPACITY FACTORS OF THE DYES, BLUE F-R AND YELLOW F4-G, SEPARATE AND IN MIXTURE (1 1, WAV)... 

Trithiolane is a stable compound at room temperature, although it will polymerize eventually and is best kept cool and sealed from the atmosphere. Separation of cisjtrans mixtures of 3,5-dialkyl-1,2,4-trithiolanes is possible on alumina. Reverse-phase HPLC has been used to separate cyclic methylene sulfides, and tellurides with retention times and capacity factors dependant in a systematic way on ring size, number and type of chalcogens and the number of heteronuclear bonds within the ring. 

A mixture of benzene, toluene, and methane was injected into a gas chromatograph. Methane gave a sharp spike in 42 s, whereas benzene required 251 s and toluene was eluted in 333 s. Find the adjusted retention time and capacity factor for each solute and the relative retention. 

The retention times and capacity factors depend, of course, on the chromatographic system and its operating... 

Table 2 Comparison of retention times and capacity factors of flavones analyzed using capillary columns DBS and BPl with supercritical CO2 density program P2...

Establish control charts of instrumental performance. Day-to-day variations in pump flow rate, relative response factors, absolute response to a standard, column plate counts, and standard retention times or capacity factors are all useful monitors of the performance of a system. By requiring that operators maintain control charts, troubleshooting is made much easier. The maintenance of control charts should be limited to a few minutes per day. 

The relative retention of two components is the quotient of their adjusted retention times. The capacity factor for a single component is the adjusted retention time divided by the elution time for solvent. Capacity factor gives the ratio of time spent by solute in the stationary phase to time spent in the mobile phase. When a separation is scaled up from a small load to a large load, the cross-sectional area of the column should be increased in proportion to the loading. Column length and linear flow rate are held constant. 

Experimental retention times or capacity factors generated by reverse phase high performance liquid chromatography (RP-HPLC) (Vowles and Mantoura, 1987 Hodson and Williams, 1988 Pussemier et al., 1990 Szabo et al., 1990a,b Hong et al., 1996) also have been correlated with Koc. 

Experimentally determined retention times or capacity factors (k) generated by reverse phase, usually octadecylsilane (ODS), high performance liquid chromatography (RP-HPLC) have been used widely to estimate Kow values (McDuffie, 1981 Haky and Young, 1984 Sarna, 1984 Doucette and Andren, 1988). More recently, this approach has been used to directly estimate Koc (Vowles and Mantoura, 1987 Hodson and Williams, 1988 Szabo et al., 1990 Kordel et al., 1993 Kordel et al., 1995 Hong et al., 1996). This is not strictly an estimation method because it relies on the acquisition of experimental retention times. 

In order to understand the thermodynamic considerations of the separation process, we need to look at the basic equations which describe the retention of a solute and relate the parameters of retention time, retention volume, and capacity factor to the thermodynamic solute distribution coefficient. 

The modern analytical laboratory employing instrumental chromatography uses a computer data collection system and associated software to acquire the data and display the chromatogram on the monitor. Parameters important for qualitative and quantitative analysis, including retention times and peak areas, are also measured and displayed. The software can also analyze the data to determine resolution, capacity factor, theoretical plates, and selectivity. 

While retention time is used for peak identification, it is dependent on the flow rate, the column dimension, and other parameters. A more fundamental term that measures the degree of retention of the analyte is the capacity factor or retention factor (k ), calculated by normalizing the net retention time (% > retention time minus the void time) by the void time. The capacity factor measures how many times the analyte is retained relative to an unretained component. ... 

Efficiency or plate count (N)—an assessment of column performance. N should be fairly constant for a particular column and can be calculated from the retention time and the peak widths. Selectivity (a)—the ratio of retention k ) of two adjacent peaks. Sample capacity— the maximum mass of sample that can be loaded on the column without destroying peak resolution. Capacity factor k )—a measure of solute retention obtained by dividing the net retention time by the void time. 

The efficiency of a HPLC column varies with column temperature. In general, capacity factor k decreases with temperature, and hence the retention of the analysis decreases with temperature [16]. Retention drops by 1 to 3% for each increase of 1°C [17]. The ability to maintain a steady and accurate column temperature is critical to achieving the desired retention time and resolution... 

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. 

Chromatographic Parameter-Relationships Correlations between Kov/ and various chromatographic parameters (CGP), such as HPLC retention time and thin-layer chromatography (TLC) capacity factors, allow the experimental estimation of Kow [19]. Usually, the CGP-A ow correlation is evaluated for a calibration set of compounds with accurately known K0w values. The Kow of a new compound can then be estimated by determining its CGP under the same experimental conditions as those used for the calibration set. 

Calculate the apparent capacity factor as kg = (tg - t0)/t0, where t is the retention time and t0 is the mobile phase hold-up time. 

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