Column dead time

The chromatogram in Problem 4 was obtained on a 2-m column with a column dead time of 50 s. How long a column is needed to achieve a resolution of 1.5 What height of a theoretical plate is needed to achieve a resolution of 1.5 without increasing the length of the column ... 

Where a, b, and c = van Deemter coefficients, dp = particle size of column, L = column length, Dm = diffusion coefficients of analytes, t = column dead time (depends on flow rate F), tg= gradient time (determines analysis time via tA = tg + t0), Ac = difference in concentrations of the organic modifier at the end and the beginning of the gradient (a continuous linear gradient is assumed), and B = slope of the linear relationship between the logarithm of the retention factor and the solvent composition. 

Snyder et al. (146) derived a rather simple relationship between the elution time of an eluite and the rate of change of solvent composition in gradient elution. They found the elution time, is related to the column dead-time, and an experimental parameter b by... 

The capacity value (retention time - column dead time) is used for the calculation of the log D. For this purpose a standard set of compounds with known log D7.4 is used for calibration of the system. 

The dimensionless independent variables are given by Eq. (7.37). The length parameter is ratiod to the column length and the time parameter is ratiod to the column dead time, to-... 

Capacity factor, k This is a more useful measure of peak retention that retention time, as it is independent of column length and flow rate. To calculate k you need to measure column dead time, to- This is the time it takes an unretained component to pass through the column without any interaction with the stationary phase. It is the time taken from the point of sample injection until the first disturbance in the base line caused by the... 

The column dead volume can be defined as the space in the column which is not working for the chromatographic separations (i.e., not occupied by the stationary phase and its support). Its value is generally determined by an elution time or elution volume of a nonretained solute in the chromatographic system. If the column dead volume is measured by a retention time of a nonretained solute, one can refer to this as a column dead time t. ... 

Controversial opinions exist among scientists regarding the meaning of column dead time in LC [4]. In its broad sense, the term column dead time refers to the elution volume of an unretained and unexcluded... 

It is useful to measure the exact volume delivered by the pump during the determination of the column dead time. The total porosity is then calculated according to Eq. 

Nevertheless this region is also of importance for preparative chromatography as the Henry coefficient is obtained from it. For its determination the retention time of a substance, the column dead time and the total porosity are necessary (Eq. 2.16b) ... 

Two different components are separated in a chromatographic column only if they spend different times in or at the stationary phase, respectively. The time in which the components do not travel along the column, is called the solute retention time, f5. The column dead time, tm, is defined as the time necessary for a non-retained component to pass through the column. The gross retention time, solute retention time and the column dead time ... 

As mentioned, a number of important conclusions can be drawn from the occurrence of system peaks. In this connection, the investigations of Levin and Grushka [77,78] are worth mentioning. From the system peak area the authors derived the amount of eluent component adsorbed at the stationary phase and calculated the capacity ratio, k, from it. Remarkably, this approach allows one to calculate the capacity ratio without prior knowledge of the column dead time, fd, of the separator column being used. 

Since the capacity ratio is typically determined according to Eq. (80) from the chromatographic data, inversely, the column dead time may be calculated when k is known. 

Finally, Kvaalen et al have shown that the system of equations of the ideal model for a multicomponent system (see later, Eqs. 8.1a and 8.1b) is strictly h5q3er-bolic [13]. As a consequence, the solution includes two individual band profiles which are both eluted in a finite time, beyond the column dead time, to = L/u. The finite time that is required for complete elution of the sample in the ideal model is a consequence of the assumption that there is no axial dispersion. It contrasts with the infinitely long time required for complete elution in the linear model. This difference illustrates the disparity between the hyperbolic properties of the system of equations of the ideal model of chromatography and the parabolic properties of the diffusion equation. 

Capacity factor or retention factor k, capacity of a stationary phase to attract a component, measured as the ratio of corrected retention time (tR) and column dead time (tM)... 

Effective theoretical plates N ff, the effective or true number of separation steps in a column, takes into account the column dead time in the calculation of column efficiency (see column efficiency). 

Column dead time, time for unretained peak to pass through column... 

The retention times are in minutes for preceding ( ) and following (AO n-alkane peaks, and for each of the three D1-D3 unknowns (U). As mentioned in the text the column dead time was 1.08 min. 

As a solute is eluted, the distribution of its molecules along the longitudinal axes of the column changes, generating a Gaussian-like profile a band broadening effect whose maximum is known as time of retention as depicted in Fig. 1. This parameter, corrected for the time of retention of a solute, that is, not retained by the column (dead time, is related to the solute capacity factor k) by the relation ... 

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