[Dolphinsiu]

I get stucked when I meet this question in my test.

(a) Which mode of chromatography (state clearly whether it is GSC, GLC, LSC, LLC and the sorption mechanism e.g. adsorption, partition, ion exchange, bonded-phase (modified partition), ion-pairing, size exclusion and affinity) would you use to separate the following mixtures?

i) inorganic gases e.g. CO2, CO,  O2 and SO2 (GSC?)
ii) polyaromatic hydrocarbons, e.g. anthracene, phenanthrene, pyrene and chrysene (GLC?)
iii) a mixture of Cl-, SO42- and CO32- (LSC?)
iv) oligomers of an epoxy resin (LLC?)
v) a mixture of 1,2-, 1,3-, 1,4-dihydroxybenzene (GLC?)

What mechanisms belongs to for the parts from (i) to (v)?

Give reason(s) for your choice.(how to explain?)

(b) What type of samples can be separated by HPLC but not by GLC?
(non-volatile compound?)

All two questions are very difficult. And after the test I cannot find books that can mention these things.

[chiralic]

Hi Dolphinsiu:

You can fine a lot information on google about yours questions!.

So, I'll give some tips about it:

Adsorption - for polar non-ionic compounds

Ion Exchange - for ionic compounds
 + Anion - analyte is anion; bonded phase has positive charge
 + Cation – analyte is cation; bonded phase has negative charge

Partition - based on the relative solubility of analyte in mobile and stationary phases
 + Normal – analyte is nonpolar organic; stationary phase MORE polar than the mobile phase
 + Reverse – analyte is polar organic; stationary phase LESS polar than the mobile phase

Size Exclusion - stationary phase is a porous matrix; sieving

Also you will can find more information on Chapter 25(Gas Chromatography) and 26 (High Performance Liquid Chromatography) of Instrumental Analysis of SKOOG-LEARY

Comments to your answers:
a-i)  Gas-Solid Chromatography =>Adsorption (not absorption!)
a-ii)  Liquid-Solid Chromatography=> Bonded-Phase
a-iii) Liquid-Solid Chromatography => Ion Exchange
a-iv) Liquid-Solid Chromatography => Size Exclusion
a-v)  Liquid-Solid Chromatography => Try it to find a correct mechanism of separation (boiling point, molecular weight, solubitily...etc) 

b) For use HPLC instead GC you must be consider boiling point, molecular weight, solubitily...etc

Those questions ARE NOT DIFFICULT...are VERY EASY!

If you need more information about this and others question on Chromatography please send me a private message...

My best regards,

Chiralic

[Dolphinsiu]

Thank you very much! I will read this chapter when I go to the university.

I have done experiment on HPLC and GLC, but I still don't know when we will use them for separation.

For a) v) 1,2 - , 1,3 - and 1,4- dihydroxybenzene has same MWt, but only differ in their structural positions (meta-, ortho-, para-) . Actually I will think they have different pKa.(I just apply my knowledge from organic chemistry). Is partition (reverse phase) the correct mechanism? (It seems  LSC does not have partition, only GLC and LLC have partition, right?) or Adsorption?

I feel very confused in adsorption and partition (reverse phase). Since both of them are suitable for polar non-ionic compounds.

[chiralic]

Hi Dolphinsiu:

On this link you can find different between ADsorption and Partition, also another mechanism of separation in chromatography:
http://www.rpi.edu/dept/chem-eng/Biotech-Environ/CHROMO/be_types.htm

ADsorption and Partition occur on the packing surface, the main diffence IS:

Adsorption depend of intermolecular forces. Competition between
Liquid mobile phase and solid adsorbent

Partition depend of solubility of solute between two phases. Competition
between liquid mobile phase and liquid stationary phase

I recommend you read this book:
HPLC and CE: Principles and Practice
Andrea Weston and Phyllis R Brown

Read chapter 2 /Separations in high-Performance Liquid Chromatography, where you can find explain it very well Normal and Reverse-Phase Mechanism, then you'll compare with Adsorption phenomena. I hope that this information clarify your confussion.

(I'll send you a private message with a link to download this book)

Chiralic

[chiralic]

Dear Dolphinsiu:

Your wrote: "...HPLC and GLC, but I still don't know when we will use them for separation."

I wrote you before:"For use HPLC instead GC you must be consider boiling point, molecular weight, solubitily...etc",

When you have High polarity, High Molecular Weight, Thermal Instability, Tendency to ionise in solution, High Boiling Point (respect to your Stationary Phase and the use of High Temperature conditions to separations) you must consider to use HPLC.

Regards,

Chiralic

[Dolphinsiu]

a-ii)  Liquid-Solid Chromatography=> Bonded-Phase

But why can't we use Liquid-Liquid Chromatography => Bonded-Phase?

Do Bonded-phase has LLC and LSC?
Also do ion exchange and size exclusion has LLC? (I think this seems not!)