Measurement of retention time

Figure 23-7 Schematic gas chromatogram showing measurement of retention times.

The capacity factor (Equation 23-16) is a measure of retention time, fp in units of the time tw required for mobile phase or an unretained solute to pass through the column. Reasonable separations demand that the capacity factors for all peaks be in the range 0.5-20. If the capacity factor is too small, the first peak is distorted by the solvent front. If the capacity factor is too great, the run takes too long. In the lowest trace in Figure 25-12, tm is the time when the first baseline disturbance is observed near 3 min. If you do not observe a baseline disturbance, you can estimate... 

The majority of chromatographic separations as well as the theory assume that each component elutes out of the column as a narrow band or a Gaussian peak. Using the position of the maximum of the peak as a measure of retention time, the peak shape conforms closely to the equation C = Cjjjg, exp[-(t -1] ) The modelling of this process, by traditional descriptive models, has been extensively reported in the literature. 

Since multi-wavelength detection has now been mentioned and will feature quite prominently in other apphcations, it is worthwhile discussing the reasons for this and why forensic laboratories have been keen to use and develop new multiwavelength detection techniques. Traditionally, solute characterisation in HPLC is based upon the measurement of retention time. However, for the purpose of sample identification or discrimination retention time is a very poor parameter because of the possibility of this being similar or identical to other compounds. 

Figure 5.2.3. Measurements of retention time tR and peak broadening Wb and Wh on a recorded chromatogram.

Retention factor is a measure of retention time and, therefore, resolution capacity. 

Pathways 8, 9 and 10 all involve two-parameter, linear regression equations using the log of each parameter. The utility of pathway 9 is enhanced by an available compilation of various solvent/water partition coefficients (K. ) for thousands of chemicals [28). The utility of pathway 09 is fairly well recognized the regression equations are included in Chapter 2, which covers estimation methods for solubility (S). Pathway 10 is more of a laboratory estimation method than a computational method it derives its main benefit from the fact that the measurement of retention time takes only about 25 minutes [44]. In a test of 18 compounds, the HPLC/RT method estimated values of log Kow with average absolute error of 23% [44]. 

The capacity factor (k ) is a measure of retention time of the peak of interest (tf) proportional to the column void time (tf )—that is, the retention time for unretained material. The capacity factor is given by the following equation ... 

The Measurement of Retention Time.— The accurate measurement of retention time /e [equation (4)] should not be determined from ruler measurements on the chromatogram but should be obtained from good stop-watch measurements. Paper stretching and mains-frequency fluctuations (which alter the rate of drive of synchronous motors) are two important factors that are responsible for the inaccuracies of the ruler method. 

Figure 21-3 Schematic gas chromatogram showing measurement of retention time (0 and width at half-height (wi/2). The width at the base (w) is found by drawing tangents to the steepest parts of the Gaussian curve and extrapolating down to the baseline. The standard deviation of the Gaussian curve is a. In gas chromatography, a small volume of CH injected with the 0.1- to 2-fxL sample is usually the first component to be eluted.

There are many methods which enable determination of activity coefficients in infinite dilution. They are mostly based on differential ebulliometry or on gas chromatographic measurement of retention time, subsequently retention volume. The headspace chromatographic analysis is another popular technique which enables measurement of equilibrium compositions at given temperature. Some similarity with static methods may be found, however, degassing is not required since the pressure is not measured. The data may be obtained rather quickly, nevertheless their accuracy is not very high. Methods for measurement of activity coefficients in infinitely diluted solutions are not described here in detail because such data are not included in this volume. 

Mass spectrometer equipped with desorption electrospray (DESI) ion source is capable of detection and identification of different substances from the surface. Moreover, the information about spatial distribution of those substances is retained. Connection of this technique with TLC not only allows for the measurement of retention times for separated chemicals (spatial distribution), but also for their unambiguous identification, based on the molecular weight of certain substances and their fragmentation spectra. Additionally, because DESI works under ambient conditions, there is no need to apply a high vacuum system for the sample introduction. Moreover, samples analyzed by DESI practically do not require any kind of preparation (e.g., covering with matrix prior to MALDI analysis), thus connection of those two techniques is relatively easy. Certainly, not all the substances, due to their chemical features, may be detected with this technique. Only compounds, which are able to ionize in this type of ion source, may be analyzed. 

Systematic studies in our laboratory have indicated the presence of a second lift force that acts on the particles. These studies involved the measurement of retention times for a set of PS particle standards under wide ranging conditions of flowrate and field strength. 

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