[iam_psyche]

i already have some idea about it...but i just want to broaden my knowledge...and know whether my assumptions are correct.

these are the questions:
1. Why is the chromatogram developed in an essentially closed system

2. What is the main advantage of 2-dimensional paper chromatography over a 1-dimensional one?

3. what are the considerations in choosing a chromatographic solvent?

4. Compare and contrast between normal phase and reverse phase chromatography

5. What would happen to the Rf values of the fractions collected in the column chromatography if you were to increase the relative concentration of the ethyl acetate in the developing solvent?

[macman104]

Give us your thoughts first then people can build on them, and help point you in the right direction.

[iam_psyche]

1. the closed system maintained the column chromatography efficiency by preventing the formation of column bubbles. The system was efficient in reducing solvent evaporation as well as preventing water condensation at the column outlet. Since free solvent vapors were eliminated, the system provided an additional safety factor when a flammable solvent, was employed.While the columns were used for solvent cleanup, a small modification would transform the system into a solute purification apparatus.

2. it gives relatively high peak capacity, so less sample is required. also, no evaporation or dilution is necessary.
and sometimes in chromatography two substances will have the same Rf value and travel the same distance in a particular solvent, but only one spot will be seen although two substances are present.

3. by using the polarity property of the mixture, we can choose a development solvent which will allow us to separate the substance based on their rate of travel up the chormatographic plate. Depending on the polarity of the substance present, they will travel at different rates in the presence of different polarity solvent systems.

4.Reversed phase  has a non-polar stationary phase and an aqueous, moderately polar mobile phase. In reversed phase retention time is longer for molecules which are more non-polar, while polar molecules elute more readily. An investigator can also increase retention time by adding a polar solvent to the mobile phase, or decrease retention time by adding a more hydrophobic solvent
Normal phase chromatography separates analytes based on polarity. The polar analyte associates with and is retained by the polar stationary phase. Adsorption strengths increase with increased analyte polarity, and the interaction between the polar analyte and the polar stationary phase (relative to the mobile phase) increases the elution time. The interaction strength depends not only on the functional groups in the analyte molecule, but also on steric factors.


5. I am not sure with the last one, but i think the higher concentration of the ethyl acetate will yield higher Rf values.

[macman104]

Most of those answers sound good to me.

For 5, what other solvent were you using?  Is ethyl acetate more or less polar than the other solvent?

[azmanam]

re #2: do you know what it means when something 'degrades on silica gel'?  do you know how 2-d chromatography might be used in this circumstance?

[iam_psyche]

For no. 5... i think ethyl acetate is more polar than the other solvent... im not sure... but our result yielded higher spot for chambers with higher concentration....

actually, in the experiment, we mixed different concentration of ethyl acetate and hexane as our developing solvent...then we observed the distance traveled by the spots.

for no. 2.... i dont really know... all i know is that silica gel consists of a three-dimensional network of thousands of alternating silicon and oxygen
bonds, with O-H groups on the outside surface. It is highly polar and is capable of hydrogen bonding.

[azmanam]

ok, never mind then.  that discussion is probably outside the scope of the course.

[sjb]

Another take on 2-d chromatography then, as well as the decomposition stuff that's been mentioned.

There are times when you, for instance, have 3 spots to consider. It could happen that A co-elutes with B in solvent system X, and with C in solvent system Y. How do you discover if there is any A around?

S

[iam_psyche]

is it possible to use another solvent system? to know if there's an A present?

i think the chromatogram must be turned through 90o and eluted with a different solvent, in which the two substances have different Rf values, then two separate spots will develop, enabling identifcation of the two substances.

is that right???