Unretained time —

Partition Ratio. The partition ratio is the additional time a solute band takes to elute, as compared with an unretained solute (for which k = 0), divided by the elution time of an unretained band ... 

The time required for unretained solutes to move from the point of injection to the detector (tm). 

Using the data from Problem 1, calculate the resolution and selectivity factors for each pair of adjacent compounds. For resolution, use both equations 12.1 and 12.21, and compare your results. Discuss how you might improve the resolution between compounds B and C. The retention time for an unretained solute is 1.19 min. 

Gas holdup Vm is the volume of carrier gas that passes through the column to elute an unretained substance, such as argon or methane. The time required is tm. 

Adjusted retention time (r R) The retention time for a substance (rR) minus that of an unretained substance (fm) t R = tR - fm. 

The time elapsed between the injection and the elution of the unretained solute is called the dead time and has been given the symbol (to). The volume of mobile phase that passes through the column during the time (to) is called the dead volume (Vo) where... 

The time taken for an analyte to elute from a chromatographic column with a particular mobile phase is termed its retention time, fan- Since this will vary with column length and mobile phase flow rate, it is more useful to use the capacity factor, k. This relates the retention time of an analyte to the time taken by an unretained compound, i.e. one which passes through the column without interacting with the stationary phase, to elute from the column under identical conditions (to). This is represented mathematically by the following equation ... 

During their passage through the column, sample molecules spend part of the time in the mobile phase and part in the stationary phase. All molecules spend the same amount of time in the mobile phase. This time is called the column dead tine or holdup time (t.) and is equivalent to the tine required for an unretained solute to reach the detector frsolute retention time (t,) is the time between the instant of saiq>le introduction and when the detector senses the maximum of the retained peak. This value is greater than the column holdup time by the amount of time the solute spends in the stationary phase and is called the adjusted retention time (t, ). These values lead to the fundamental relationship, equation (1.1), describing retention in gas and liquid chromatography. 

The total retention of a given solute and the gas holdup time of an unretained solute eluting through two serially connected columns is the sum of the retentions in the individual column units, and thus we can write that... 

Vr = the retention volume (time or length) of our solute V0 = the retention volume (time or length) of an unretained solute. 

When the column is ready to be used, the chromatogram of a suitable test mixture should be obtained. The plate number and retention times of the test solutes should be noted, and the peaks should have a satisfactory shape (minimal tailing). For measurement of the plate number, the recorder should be used at a high chart speed. Fig. 5.1b(i) and (ii) show test chromatograms for a C-18 column prepared by the above method, and Fig. 5.1c and 5.Id show the data that you should report with the chromatogram. The retention for an unretained peak is taken as the small baseline disturbance just before the first peak. 

There has been an attempt to measure the peak capacity in 1DLC and 2DLC by assigning a range of useful retention time between the unretained marker that elutes at ti and some stated value of the retention factor k leading to a zone at tf and plugging in a value for the peak width W. This number is useful but will never be equal to the number... 

The zero retention time t0 for mobile phase or other unretained molecules to move from one end of the column to the other can be determined as follows ... 

Another parameter often measured is the adjusted retention time, Ur. This is the difference between the retention time of a given component and the retention time of an unretained substance, tM, which is often air for GC and the sample solvent for HPLC. Thus, the adjusted retention time is a measure of the exact time a mixture component spends in the stationary phase. Figure 11.17 shows how this measurement is made. The most important use of this retention time information is in peak identification, or qualitative analysis. This subject will be discussed in more detail in Chapter 12. 

Finally, a parameter known as the capacity factor may be determined. The capacity factor, symbolized k (k-prime) is the adjusted retention time divided by the retention time of an unretained substance, tM, such as air in GC or the sample solvent in HPLC. 

The capacity factor is a measure of the retention of a component per column volume, since the retention time is referred to the time for the unretained component. The greater the capacity factor, the longer that component is retained and the better the chances for good resolution. An optimum range for k values is between 2 and 6. 

Figure 1 shows a typical chromatogram, which includes a time axis, an injection point, and an analyte peak. The time between the sample injection point and the analyte reaching a detector is called the retention time (t ). The retention time of an unretained component (often marked by the first baseline disturbance cansed by the elution of the sample solvent) is termed void time (tg)- Void time is related to the column void volume (Vq), which is an important parameter that will be elaborated later. 

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. ... 

Possible difficulties in obtaining accurate retention data for physicochemical measurements should be recognized, however. The evaluation of to, the elution time of an "unretained peak (274) is often connected with systematic error and the measurement of the retention time of asymmetrical peaks may not be accurate. Moreover, no satisfactory methods are available for the precise evaluation of the phase ratio in the column. Consequently, the measurement of the equflibrium constant proper is beset with difficulties as discussed in Section VII.B. 

The uncertainty in the measurement of elution time / or elution volume of an unretained tracer is another potential source of error in the evaluation of thermodynamic quantities for the chromatographic process. It can be shown that a small relative error in the determination of r , will give rise to a commensurate relative error in both the retention factor and the related Gibbs free energy. Thus, a 5% error in leads to errors of nearly 5% in both k and AG. An analysis of error propagation showed that if the... 

The elution strength should be adjusted so that the sample components of interest are eluted within a reasonable time. Generally > 0.3 are necessary to separate the sample components from unretained substances including the solvent. On the other hand, values of A < 10 are desired to reduce analysis time and minimize sample dilution which increases with increasing k value. In high-speed analysis optimum conditions are obtained if the k values of the sample components vary between 0.3 and 3. 

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