Question

Analytical Chem Tutorial: Partition Chromatography

Answer 1

\({\bf{Overview:}}\)
- definition: a type of high-pressure liquid chromatography in which the stationary phase is also a liquid and is immiscible with the liquid mobile phase
- best for nonionic polar compounds with low/moderate MW
- liquid-bounded phase columns: liquid is held in place by chemical bonding rather than adsorption
- bonded-phase is coated w/ siloxide (hydrolyzed silica + organochlorosilane)

Answer 2

\({\bf{Normal~vs.~Reverse~Phase:}}\)
- normal phase: stationary phase polar, mobile phase nonpolar
> least polar component eluted first
> increasing the polarity of mobile phase will decrease the elution time
- reverse phase: stationary phase nonpolar, mobile phase polar (good b/c you can use water as mobile phase which is cheap, UV-transparent, nonreactive)
> most polar component eluted first
> increasing the polarity of the mobile phase will increase the elution time

bonded phase:
normal if the coating is polar and reversed when coating is nonpolar

Answer 3

\({\bf{Method~Development:}}\) (basically, how to change the parameters of your experiment to maximize the efficiency of your chromatographic separation)
- must consider proper polarity of solutes, mobile phase, and stationary phase
- in order of increasing polarity: hydrocarbons, ethers, esters, ketones, aldehydes, amides, amines, alcohols. water is generally more polar than all of these.
- good rule of thumb is to match polarity of stationary phase and analyte, otherwise retention time is too short and separation is poor

- choosing mobile phase: goal is optimize the retention factor to 2-10 (my professor just recommends using 10)
- strong solvent: solvent that interacts *strongly* with its solutes. often but not always polar.
- polarity index (P'): a quantification of solvent strength based on solubility in dioxane, nitromethane, and ethyl alcohol. increases with polarity and decreases with nonpolarity.
- polarity index of a solution is also a linear combination of the solvent polarity indexes and their concentrations

\[\frac{ k_{2} }{ k_{1} } = 10^{(P_{1}'-P_{2}')/2}\]
for the normal phase. switch the polarity indexes (not the partition coefficients) for the reverse phase.

Answer 4

\({\bf{Solvent~Selectivity~Triangle:}}\)
- for normal phase, the three solvents are ethyl ether, methylene chloride, and chloroform, with n-hexane as the dilution solvent
- for reverse phase, the three solvents are methanol, acetonitrile, and tetrahydrofuran

change the position along the triangle by changing the relative amount of water and each type of solvent

Answer 5

Source material is section 28D of Principals of Instrumental Analysis, 6th edition by Skoog, Douglas A., Holler, James F., Crouch, Stanley R.