1.4. Retention..... 8

The position of peaks in Chromatogram is characterized by the Retention time(tR) or Retention Volumn(VR). VR is more correct than time for measurement. The time solutes used in the stationary phase is called Adjusted retention time, t'R. Without interactions with the column, solutes will be detected after the time, tM, needed for solutes transferring from the injection port to the detector. So the retention time, tR = Adjusted retention time t'R + tM Column hold-up time.

A volume of the mobile phase filled in the column is higher than V hold up because there is a fraction of the mobile phase trapped in the pores of column packing.

tR(Retention time) = tM(k+1)

Retention factor, k =(tR-tM)/tM = t'R/tM

Separation factor(selectivity factor, selectivity, or relative retetion), α = t'R(B)/t'R(A) = k(B)/k(A)

Distribution constants, K = C(concentration of solutes in stationary phase)/C(concentration of solutes in Mobile phase) = β*k

β (phase ratio)= V(mobile phase)/V(stationary phase)

"A funny story, three people come in to a room, each person is interested in different things in this room, the time they spend with these things depends on how they like these things, so one may go out first after he had something to do with what he is interested in, that also is the separation mechanism of the column in Chromatography"

The mobile phase physical properties, The physical properties include density, compressibility, viscosity,...The influence of mobile phase physical properties on the separation system is defined mainly by the compressibility.

The compressibility varies with the most compressible for gas, then supercritical fluid, and the least compressible for liquid.

The measurements of solute retention volumes and average mobile phase velocity. The process you can find here

For non-ideal carrier gas that is insoluble in the stationary phase we can calculate the net retention volume following equation:

The Gas-Liquid distribution constants can be measured:

For liquid mobile phases, the influence of drop pressure on the retention factor is eliminated, because liquids are far less compressible than gases. The retention factor also varies with the full range of pressure inlet and pressure column drops, but this is not important for analysis.

For Supercritical Fluids, fluids are highly compressible. Its density gradient relating to column pressure drops make retention factors changes significantly. So the influence of mobile phase on retention factor is complex and built up by empirical relationships.

Property Estimations, Use retention factors to estimate the properties of solution and adsorption thermodynamic.

Advantages of using gas chromatography to determine the properties of solution adsorption thermodynamic: impure sample, small sample sized, and ease of variation temperature. Retention measurement can evaluate entanpy entropy, and free energy at infinite dilution of solutes. The activity coefficient can be measured at infinite dilution by:

Liquids are not usual to estimate the physicochemical properities because it is difficult to figure out the true composition of mobile phase and the absence of quantitative models. There are some exceptions that is the measurements of equilibrium constants and indirect property based on an empirical correlation between retention property and chromatographic system.

Supercritical fluids

Linear Free Energy Relationships

- phase A-B is liquid-liquid: a cavity model is used to describe solvent-solvent, solute-solvent interactions.

i) a cavity of suitable sizes are constructed in the solvent

ii) the solvent molecules are re-organized into a equilibrium position around solute molecules

iii) finally, solutes are inserted into cavity

Exothermodynamic Relationships

The relationships between retention and solutes or experimental variables are discribed by

i) Temperature

ii) ...

General Elution Problems

- Solutes are strongly retained in the column-long retention time

- Many solutes are eluted simultineously causing overloading peaks-poor separation of early eluting peaks-poor detectability of late eluting peaks.

Solutions to these problem are the use of programmed separation modes including:

GC: Temperature and Flow programming

LC: Mobile phase composition, flow, and temperature programming

SFC: Density, composition and temperature programming