Chromatography

Question 1

Briefly explain the principles of HPLC and how the various components of a mixture are separated in this process. (4)

Answer 1

• The sample to be separated is loaded onto the stationary phase (column)
• Components of the mixture adsorb into the stationary phase with differing strengths.
• Components interact with the mobile to differing degrees
• The interactions between components and the stationary and mobile phases means they move through the column at differing speeds and therefore become seperated.

Question 2

If all components of the sample are non-polar, what characteristic allows them to be separated in HPLC. (2)

Answer 2

• Separate based on molecular weight.
• Low molecular weight elutes first (short retention time), high is opposite.

Question 3

Briefly describe the principles of gas chromatography and state one reason why GC may have been chosen over other forms of chromatography to perform the analysis of effectiveness of a catalytic converter. (4)

Answer 3

*GC involves passing a sample of gas through a thin tube which is packed with a inert substance (stationary phase - e.g. silica) coated in a viscous non-volatile liquid
*an inert carrier gas (e.g. helium) acts as the mobile phase and moves the gas through the chromatography column where the various gas components move at different speeds as they interact with the stationary phase
* the gases separate based on their size/molar mass as well as their polarity and can be identified via their retention times
* GC works best for separating gaseous mixtures or mixtures that can easily be vaporised, which is why it is suitable for analysing car fumes