72 p2 7i pi

For solutions that approximate to ideal behaviour, y & 1 and the separation depends on differences in the vapour pressures; this is the case with nonpolar solutes in a nonpolar stationary phase. The existence of polar interactions between the solute and stationary phase molecules introduces nonideality into the system and y O 1. This can be made use of to provide a separation. A good example is the separation of benzene (C6H6) and cyclohexane (C6H12), which have boiling points of 353.2 and 353.8 K, respectively. In a nonpolar stationary phase the predominant molecular interactions, between benzene or cyclohexane and the stationary phase, will be dispersion forces, and the activity coefficients for both solutes are &1. Because the difference in the vapour pressures is small, little separation is observed and the solutes are eluted from the stationary phase in order of their boiling points, i.e. benzene is eluted before cyclohexane. If a polar stationary phase is used, cyclohexane, because of its saturated nature, still only exhibits dispersion interactions and y & 1. However the n electrons in benzene cause it to undergo dipole-induced dipole interactions, leading to a decrease in y and an increase in its distribution coefficient. Hence in a polar stationary phase benzene is eluted some time after cyclohexane.

The relationship between retention and molecular structure has long been used in GC as an aid to the assignment of chromatographic peaks. Many homologous series of compounds show a linear relationship between log (retention) and boiling point or carbon number. Since different solute types (e.g. n-alkanes, n-alkyl alcohols, n-alkyl esters) give different linear relationships with different slopes, these plots can be used to assign a chromatographic peak to a particular class of compound and to determine its carbon number and boiling point. These relationships have also been used to provide data in the form of a Retention Index as an aid to peak identification. However, with the widespread use of mass spectrometer detectors these techniques are used less than formerly.

In adsorption chromatography the stationary phase volume is replaced by the surface area (AS) of the stationary phase, and the distribution coefficient is replaced by the adsorption coefficient (KA). In GC both VR and VM have to be corrected for gas compressibility.

An alternative expression (the retention factor, k) for the distribution of a sample component is in terms of the relative number of moles (n) of a component in the stationary and mobile phases, such that:

The ratio VS/VM is the phase ratio. Early literature will refer to the retention factor as the capacity ratio (k').

Since a sample molecule only migrates through the column when it is in the mobile phase, the retardation factor (R) may be written:

amount of solute in the mobile phase amount of solute in mobile + stationary phases

R = nM/(nM + ns) = 1/(1+k) Substituting the retention factor into the equation: VR = Vm # KcVs gives:

VR= Vm(1 +k) or using retention times:

Solar Panel Basics

Solar Panel Basics

Global warming is a huge problem which will significantly affect every country in the world. Many people all over the world are trying to do whatever they can to help combat the effects of global warming. One of the ways that people can fight global warming is to reduce their dependence on non-renewable energy sources like oil and petroleum based products.

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