Void time

Measurement of the column s void time, and the retention time, and baseline width, w, for a solute. 

Besides the solute peak, Figure 12.7 also shows a small peak eluted soon after the sample is injected into the mobile phase. This peak results from solutes that move through the column at the same rate as the mobile phase. Since these solutes do not interact with the stationary phase, they are considered nonretained. The time or volume of mobile phase required to elute nonretained components is called the column s void time, or void volume. 

A solute s capacity factor can be determined from a chromatogram by measuring the column s void time, f, and the solute s retention time, (see Figure 12.7). The mobile phase s average linear velocity, m, is equal to the length of the column, L, divided by the time required to elute a nonretained solute. 

The difference between a solute s retention time and column s void time (t/). 

In a chromatographic analysis of low-molecular-weight acids, butyric acid elutes with a retention time of 7.63 min. The column s void time is 0.31 min. Calculate the capacity factor for butyric acid. 

First we must calculate the capacity factor for isobutyric acid. Using the void time from Example 12.2, this is... 

For complex mixtures, the k values should be distributed in the range of 0.5 to 20,3 corresponding to retention times of about 3 to 45 minutes on a column with a void time of 2 minutes. If the components of a sample are neutral, i.e., nonionizable, the correct proportion of water and organic solvent gives the optimum k value by which symmetrical, well -resolved peaks are obtained. Figure 1 is a chromatogram of the separation of a neutral... 

The Poppe plot is a log-log plot of H/uq = t(JN versus the number of plates with different particle sizes and with lines drawn at constant void time, t(). H is the plate height, Vis the number of plates, and u() is the fluid velocity (assumed equal to the void velocity). The quantity H/u() is called the plate time, which is the time for a theoretical plate to develop and is indicative of the speed of the separation, with units of seconds. In the Poppe plot, a number of parameters including the maximum allowable pressure drop, particle diameter, viscosity, flow resistance, and diffusion coefficient are held constant. 

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. 

FIGURE I A chromatogram showing retention time (t ), void time (Tg), peak at base width (Wb), and peak height (h). 

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. ... 

The concept of column void volume (Vg) is important for several reasons. Void volume is the volume of the empty column minus the volume occupied by the solid packing materials. It is the liquid holdup volume of the column that each analyte must elute from. Note that the void volume is equal to the void time multiplied by the flow rate (T). 

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. 

A constituent is characterised by its retention time. tR, defined by the time taken between the moment of injection into the chromatograph and the peak maximum recorded on the chromatogram. In an ideal case, the retention time fR is independent of the quantity injected. A compound not retained will elute out of the column at time tM, called the void time or the dead time1 (sometimes designated by t0). The separation is complete when as many peaks are seen returning to the baseline as there are components in the mixture. In quantitative analysis, it suffices to separate only the components that need to be measured. 

If the performance of different columns has to be compared for a given compound, more realistic values are obtained by replacing the total retention times R, which appear in equations (1.7) to (1.9), by the adjusted retention times /R, which do not take into account the void time rM spent by the compound in the mobile phase. The mathematical relationships thus become ... 

These corrected parameters are only useful if the void time is large compared to the retention time of the compound. This is the case in gas chromatography particularly when the performance of capillary columns is compared to that of packed columns. 

Vibrational-rotational spectrum, 166 Void time, 7 Voltammogram, 359... 

For studies on the relationship between log k and the percentage of strong solvent, olive oil was dissolved in the appropriate strong solvent at a concentration of 50 mg/ml. For time-normalization studies, olive oil was dissolved in the mobile-phase mixture at this same concentration level whenever possible. In cases where olive oil was not soluble in the mobile phase, it was dissolved in the strong solvent. The column void time was determined by measuring the av-... 

In the early years of LC-MS/MS application in clinical laboratories, chromatographic separation was looked upon as rather unnecessary with tandem mass spectrometers being understood as extremely selective measuring devices. Thus, many LC-MS/MS methods with minimal degree of chromatographic resolution and analyte retention times close to the void time of the chromatographic systems ( dilute and shoot approaches) have been described. However, from the issues discussed so far, the requirements of proper sample preparation and sufficient chromatographic separation prior to MS/MS detection have become evident. 

Void time. The time equivalent to the void volume (vQid time = void volume flow rate). 

The void time t,j is by definition equal to the void or channel volume, V0=bLw, divided by the channel flow rate V, which leads to ... 

To calculate the retention ratio R, one needs to know the void time t0 (See Eq. (7)) which for an asymmetrical channel is given by [249] ... 

If a particular HPLC system provides constant and stable mobile-phase flow (F), one can convert retention volume (Vr) and void volume (Vo) into the retention time (Ir) and a void time (to). 

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