Adjusted retention time volume

A chromatographic peak provides valuable information, namely, the elapsed time from the injection point or the difference in elution times of two peaks (qualitative information), the peak shape (qualitative or quantitative information), and the peak size(quantitative information). The simplest qualitative tool is simply the comparison of retention data from known and unknown samples. A chromatogram illustrating the commonly used retention nomenclature is given in Figure 4.1. The retention time (tp>) is the time elapsed from injection of the sample component to the recording of the peak maximum. The retention volume (VR) is the product of the retention time and the flow rate of the carrier gas. Generally, the adjusted retention time (t ) or adjusted retention volume (V >) and the relative retention (rA/B) are used for qualitative analysis. Adjusted retention time (volume) is the difference between the retention time (volume) of the sample and an inert component (usually air). The relative retention is the ratio of the adjusted retention time (or volume) of a standard to the adjusted retention time (or volume) of the unknown, (see Chapter 2). 

Adjusted retention time, 327 Adjusted retention volume, 327 Air, 139, 140... 

As mentioned in Section 11.8.4, the parameters that are most important for a qualitative analysis using most GC detectors are retention time, tR adjusted retention time, t R and selectivity, a. Their definitions were graphically presented in Figures 11.16 and 11.17. Under a given set of conditions (the nature of the stationary phase, the column temperature, the carrier flow rate, the column length and diameter, and the instrument dead volume), the retention time is a particular value for each component. It changes... 

Those characteristics of the chromatogram that have theoretical significance will be discussed later, but some common measurements that can be taken from the chromatogram are given now. The elapsed time between the dead point and the peak maximum is called the retention time, and the time between the dead point and the peak maximum the adjusted retention time. If the retention time is multiplied by the mobile phase flow-rate, the product is called the retention volume. Similarly, multiplying the adjusted retention time by the flow-rate gives the adjusted retention volume. 

The most commonly used retention parameter in gas chromatography is the Kovats index. When the adjusted retention times are used to calculate Kovats indices, retention parameters are obtained which depend only on the column temperature and the stationaiy phase used. Kovats indices are highly reproducible, and with a well designed experimental technique and an accurate timing mechanism, an inter-laboratory reproducibility of one unit for larger values of Kovats indices and two units for indices below 400 is possible [14]. Instead of Kovats indices, sometimes in QSRR studies the logarithms of retention volumes of solutes are used. 

Relative retention The relative retention, or the column selectivity, is defined as the ratio of the adjusted retention times or volumes of two components under identical conditions ... 

The retention time is the time that a solute spends in the stationary phase plus the time it spends in the mobile phase (r )- is the same for all solutes and includes time spent in the dead volume of the column, and so it is sometimes preferable to use the adjusted retention time (/ r), which consists only of the time a solute spends in the stationary phase. 

N calculated this way using is a measure of the efficiency of the whole system that includes the dead volume. In order to calculate and compare efficiencies of columns alone, it is necessary to use (adjusted retention time) in place of tR in the above expression, to obtain the effective plate number. 

It follows that the net retention volume and therefore the adjusted retention volume (Fr) and retention volume (Fr) vary as the logarithm of l/T. Similarly, adjusted retention time (/r) and retention time (Jr) will decrease logarithmically as temperature increases. 

SP is some free energy related solute property such as a distribution constant, retention factor, specific retention volume, relative adjusted retention time, or retention index value. Although when retention index values are used the system constants (lowercase letters in italics) will be different from models obtained with the other dependent variables. Retention index values, therefore, should not be used to determine system properties but can be used to estimate descriptor values. The remainder of the equations is made up of product terms called system constants (r, s, a, b, I, m) and solute descriptors (R2,7t2, Stt2, Sp2 log Vx). Each product term represents a contribution from a defined intermolecular interaction to the solute property. The contribution from cavity formation and dispersion interactions are strongly correlated with solute size and cannot be separated if a volume term, such as the characteristic volume [Vx in Eq. (1.6) or V in Eq. (1.6a)] is used as a descriptor. The transfer of a solute between two condensed phases will occur with little change in the contribution from dispersion interactions and the absence of a specific term in Eq. (1.6) to represent dispersion interactions is not a serious problem. For transfer of a solute from the gas phase to a condensed phase this... 

The concept of retention volume in chromatography was discussed in Chapter 21. One can distinguish between the retention volume Fr (or retention time and the adjusted retention volume F, (or adjusted retention time t l) ... 

Let us assume that we found a column that retains the more polar EG with respect to 1,2-DCA. Equation (4.13) would suggest that the 0 2-dca is much larger than The unretained solute peak in the chromatogram, often called the chromatographic dead time or dead volume, might be due to the presence of air in GC or a solvent in HPLC. The adjusted retention volume, Vj(, and adjusted retention time, tjj, are defined mathematically as... 

Adjusted retention time (t/). The adjusted retention time (t/) and the adjusted retention volume (V/) are sometimes quoted. They are calculated by subtracting the retention time or retention volume of an unretained solute (t or V ... 

Liquid Chromatography, Fig. 1 Measurement of the retention times (/,), void time (t ), and adjusted retention times (t/) is shown. The void time is equal to the elution time of the un-retained substances, and it can be related by the flow rate to the void volume, which is the volume of solution between the injector and detector. Two analyte peaks are shown with their corresponding retentirai times. The retention time of the first eluting analyte is and the... 

Another important parameter for characterizing the retentitm of a compound is the capacity factor, k, which is defined as the mass of solute in the stationary phase divided by the mass of solute in the mobile phase as shown in Eq. 4. In contrast with partition coefficient, k is not a state function and is dependent on the cohunn s characteristic phase ratio, which is equal to the volume of the stationary phase (yj divided by the volume of the mobile phase (ym). A greater stationary phase volume increases interactiOTis with the stationary phase, thereby increasing the retention time and A . It is important to note that the capacity factor is also equivalent to the adjusted retention time divided by the void time, which are obtained directly from the chromatogram. This important parameter can be intuitively described as a measure of the strength of an analyte s interaction with the stationary phase in a particular chromatographic system. 

To find the Kovats index for a given solute on a given stationary phase, a few members of the paraffin homologous series are chromatographed and plotted. Then the solute is run under the same conditions and its Index value is determined from the graph. It is best if the paraffins chosen bracket the retention volume of the analyte. If the flow rate is kept constant during the gathering of these data, then adjusted retention times can be plotted. Alternatively, the index can be calculated from equation 4,... 

Sixteen alkylaromatic compounds (e.g., o-xylene, ethylbenzene, n-amylbenzene, 1-ethylnaphthalene and 2-n-hexylnaphthalene) were chromatographed on a silica column (A = 254nm) using hexane and 0.5-2.0% dichloromethane, chloroform, carbon tetrachloride, or ethyl bromide as the mobile phase modifier [599]. For the stronger solvent modifiers (chloroform and dichloromethane) linear decreases in the reciprocal of the adjusted retention time (i.e., 1 /[(retention time) — (void volume)]) versus mole fraction of modifier in the mobile phases resulted. However, for the weaker modifiers (carbon tetrachloride and ethyl bromide) the plot was distinctly nonlinear at low mole fractions of modifier (0.01-0.02) and linear at higher mole fiactions of the modifier (>0.05). 

GLC determines directly the adjusted retention time, defined by eqn (2.30). It depends, obviously, on K of the solute-solvent system in question. Eqns (2.33), (2.35) and (2.37) transform t n into the specific retention volume, F, [16] which is also a fimction of JT. Taking account of eqns (2.37) and (2.42) the following equation relating K and F, results ... 

A certain volume of gas is required to carry a component which does not dissolve in the liquid phase through the column, and this is known as the hold-up volume, although in practice, it is more convenient to measure hold-up time (t p). This is usually measured by injecting methane onto the column and determining the time for the leading edge of the methane peak to emerge. Every solute molecule must spend the same time (tp,) in the gas phase, the true time spent in the liquid phase is therefore an adjusted retention time (f) ... 

While it is possible to calculate the corresponding adjusted retention time, t, by subtracting the retention time of the unretained peak, t, it is better to work with volumes since average flow-rate variations between individual analyses are then accounted for. Note that with each level of refinement beyond the raw retention time, a facet of instrument dependence from the resulting parameter is removed. is independent of the flow rate is, further, independent of column geometry. Continuing with this approach, the net retention volume, is defined, by applying a factor, j, to account for the pressure drop across the column ... 

Here, I, is the dimensionless Kovats retention index that is a function of both temperature and the stationary phase employed. The terms represented by X are retention parameters of the sample and standards. Following this convention, is the retention parameter of the sample under consideration. Any retention parameter, such as the adjusted retention time, t, the net retention volume, the adjusted net retention volume, and the relative retentions, can be used. X is the retention parameter of the sample under consideration, X is the retention parameter of a normal alkane (that is, straight chain or unbranched) of carbon number that elutes earlier than the sample, and X is the retention parameter of a normal Jkane having a carbon number greater than +1 that elutes after the sample. The retention index of a sample is, therefore, 100 multiplied by the carbon number of a hy-... 

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