Retention volume measurements pressure

If the mobile phase is a liquid, and can be considered incompressible, then the volume of the mobile phase eluted from the column, between the injection and the peak maximum, can be easily obtained from the product of the flow rate and the retention time. For more precise measurements, the volume of eluent can be directly measured volumetrically by means of a burette or other suitable volume measuring vessel that is placed at the end of the column. If the mobile phase is compressible, however, the volume of mobile phase that passes through the column, measured at the exit, will no longer represent the true retention volume, as the volume flow will increase continuously along the column as the pressure falls. This problem was solved by James and Martin [3], who derived a correction factor that allowed the actual retention volume to be calculated from the retention volume measured at the column outlet at atmospheric pressure, and a function of the inlet/outlet pressure ratio. This correction factor can be derived as follows. 

The compressibilities of solvents vary significantly from one solvent to another. The compressibility of cyclohexane is about 0.67% per thousand p.s.i. change in pressure [11] and, thus, for a column operated at 6,000 p.s.i. (mean pressure 3,000 p.s.i.), there will be an error in retention volume measurement of about 2%. In a similar manner, n-heptane has a compressibility of about 1.0% per 1,000 p.s.i. change in pressure [11] and, under similar circumstances, would give an error of about 3% in retention volume measurement. Fortunately, as already discussed in Part 1 of this book, there are other retention parameters that can be used for solute... 

Figure 7. Experimental high pressure gas chromatographic apparatus for retention volume measurements.

Unfortunately values cannot be directly calculated from retention volume measurements by Equation 3 since the interfacial surface area is changing as a function of pressure due to the carbon dioxide sorption. However, the relative magnitude of the equilibrium shift of the sorbate from the solid adsorbent into the gaseous phase can be estimated by calculating the capacity factor, k, according to Equation A as given below ... 

Not only variations in the pressure at constant temperature influence column-to-column retention data the role of the column hold-up volume as well as the mass of stationary phase present in the column is also important. The net retention volume caleulated from the adjusted retention volume corrects for the column hold-up volume (see Table 1.2). The specific retention volume corrects for the different amount of stationary phase present in individual colunms by referencing the net retention volume to unit mass of stationary phase. Further correction to a standard temperature of 0°C is discouraged [16-19]. Such calculations to a standard temperature significantly distort the actual relationship between the retention volumes measured at different temperatures. Specific retention volumes exhibit less variability between laboratories than other absolute measures of retention. They are not sufficiently accurate for solute identification purposes, however, owing to the accumulation of multiple experimental errors in their determination. Relative retention measurements, such as the retention index scale (section 2.4.4) are generally used for this purpose. The specific retention volume is commonly used in the determination of physicochemical properties by gas chromatography (see section 1.4.2). 

The accurate determination of relative retention volumes and Kovats indices is of great utility to the analyst, for besides being tools of identification, they can also be related to thermodynamic properties of solutions (measurements of vapor pressure and heats of vaporization on nonpolar columns) and activity coefficients on polar columns by simple relationships (179). 

Liquids have relatively low compressibility compared with gases and, thus, the mobile phase velocity is sensibly constant throughout the column. As a consequence, elution volumes measured at the column exit can be used to obtain retention volume data and, unless extreme accuracy is required for special applications, there is no need for the retention volume to be corrected for pressure effects. 

Having chosen the test mixture and mobile diase composition, the chromatogram is run, usually at a fairly fast chart speed to reduce errors associated with the measurement of peak widths, etc.. Figure 4.10. The parameters calculated from the chromatogram are the retention volume and capacity factor of each component, the plate count for the unretained peak and at least one of the retained peaks, the peak asymmetry factor for each component, and the separation factor for at least one pair of solutes. The pressure drop for the column at the optimum test flow rate should also be noted. This data is then used to determine two types of performance criteria. These are kinetic parameters, which indicate how well the column is physically packed, and thermodynamic parameters, which indicate whether the column packing material meets the manufacturer s specifications. Examples of such thermodynamic parameters are whether the percentage oi bonded... 

The effect of pressure (measured at the pump) on this separation can be seen in Figure 3 and Table I. As the pressure is increased, the retention volume of benzene and naphthalene continually Increase, while the retention volume of anthracene, chrysene, and benz(a)pyrene first increase, go through a maximum, and then decrease. The maximum separation occurs at 38.5 bar while the minimum satisfactory separation volume (shortest analysis... 

The instruments for polymer HPLC except for the columns (Section 16.8.1) and for some detectors are in principle the same as for the HPLC of small molecules. Due to sensitivity of particular detectors to the pressure variations (Section 16.9.1) the pumping systems should be equipped with the efficient dampeners to suppress the rest pulsation of pressure and flow rate of mobile phase. In most methods of polymer HPLC, and especially in SEC, the retention volume of sample (fraction) is the parameter of the same importance as the sample concentration. The conventional volumeters— siphons, drop counters, heat pulse counters—do not exhibit necessary robustness and precision [270]. Therefore the timescale is utilized and the eluent flow rate has to be very constant even when rather viscous samples are introduced into column. The problems with the constant eluent flow rate may be caused by the poor resettability of some pumping systems. Therefore, it is advisable to carefully check the actual flow rate after each restarting of instrument and in the course of the long-time experiments. A continuous operation— 24h a day and 7 days a week—is advisable for the high-precision SEC measurements. THE or other eluent is continuously distilled and recycled. 

The injector and the detector have dead volumes that affect the total retention volume. In gas phase chromatography, because the mobile phase is compressible, the flow rate measured at the end of the column has to be corrected by the compressibility factor J, which accounts for increased pressure at the head of the column (c.f. 2.2). 

For most accurate flow measurement (required when retention volumes are to be calculated) a correction for water vapor pressure must be applied. The flow should also be corrected to the column temperature. Dal Nogare (8) gave the equation for correcting the desired flowrate to column temperature T and outlet pressure, p0, as ... 

The method of determination of virial coefficients by GC consists of measuring the retention volumes at various carrier gas pressures and extrapolating to zero pressure. Three procedures of extrapolation have been suggested, although they do not give the same results. The method of Cruickshank, Windsor, and Young (30), which takes into account carrier gas flowrate and local pressure, seems to be most promising. 

Experimental assessment of the column void volume proved to be critical since the solute retention volume approaches the void volume as pressure is increased. Following the recommendations of Kobayashi (24), we used an unretained solute, methane, for this measurement. Values for the void volume determined over an extended pressure range were 1.8 and 0.5 ml. for the crosslinked resin and alumina columns, respectively. These figures were in excellent agreement with void volume approximations of 1.4 and 0.45 ml. based upon the geometric volume of the column assuming a porosity of 0.6 for the packed beds. 

The adsorbates and adsorbents in this study were chosen to reflect a range of different types of molecular interactions as well as to observe whether the retention volume data trends could be generalized. In addition, solutes were picked which would rapidly equilibrate with the chosen adsorbents (no hysteresis) and whose distribution coefficients could be measured conveniently over as wide a pressure range as possible. As shown in Table I, the adsorbents corresponded to two distinctly different chemical types as classified by the criterion of Kiselev (40). The alumina represented an adsorbent capable of specific interactions with sorbates having localized peripheral... 

There is a fundamental relationship described in chromatographic theory between the retention volume of a elution peak and the mid-point of a breakthrough curve achieved by operating the column under frontal analysis conditions (41 ). In the Henry s Law region of the adsorption isotherm, the net retention volume and its measurement can be used to describe the variation of sorbate breakthrough volume as illustrated in Figure 8. Utilizing the experimental apparatus described in the last section, retention volumes were measured as a function of pressure at 40°C (T =... 

This is largely due to the fact that retention data depend on certain factors the effects of which are difficult to eliminate completely or control and which are normally neglected. These factors are the imperfections in the gas phase and the compressibility of the stationary phase (cf., the quantities vh v , zq and 0 in eqn. 1), the finite rate of equilibration of the solute, variations in the composition of the sorbent, spurious sorption of the solute, solubility of the carrier gas in the stationary phase, etc. Hence, even relative retention volumes and/or retention indices must depend to some extent on the kind, flow-rate and absolute pressure of the carrier gas, the load of the liquid stationary phase on the support, which production batch of the stationary phase has been used and the kind of support. The absolute column pressure will obviously vary with the column length and particle size of the support. Moreover, adjusted retention data are required in all instances, which renders it necessary to measure the dead retention time. This is a crucial step in obtaining accurate retention data and presents a problem per se. 

The opportunity to measure the dilute polymer solution viscosity in GPC came with the continuous capillary-type viscometers (single capillary or differential multicapillary detectors) coupled to the traditional chromatographic system before or after a concentration detector in series (see the entry Viscometric Detection in GPC-SEC). Because liquid continuously flows through the capillary tube, the detected pressure drop across the capillary provides the measure for the fluid viscosity according to the Poiseuille s equation for laminar flow of incompressible liquids [1], Most commercial on-line viscometers provide either relative or specific viscosities measured continuously across the entire polymer peak. These measurements produce a viscometry elution profile (chromatogram). Combined with a concentration-detector chromatogram (the concentration versus retention volume elution curve), this profile allows one to calculate the instantaneous intrinsic viscosity [17] of a polymer solution at each data point i (time slice) of a polymer distribution. Thus, if the differential refractometer is used as a concentration detector, then for each sample slice i. 

Figure 1 illustrates differential distributions of the adsorption energy of n-hexane on partially dehydroxylated silica gel (1) and on carbon-silica adsorbents prepared through the pyrolysis of n-heptyl (2) and benzyl (3) alcohols and their different mixtures (4-6) on the solid surface. The energy distribution function X(E) of n-hexane was measured chromatographically from the pressure dependence of the retention volume [24,25]. 

A simplified version of equation (18), which does not contain the pressure-dependent exponential term, can be used to describe the temperature dependence of retention measurements. In this case retention volumes are measured at different temperatures and an infinite dilution of the sample. This simplified equation expressed in terms of the adsorption energy has the following form [144] ... 

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