Adsorption/Chromatography Flashcards
What are the two phases in chromatography?
Mobile and Stationary
What is HETP?
HETP is the height equivilnet to a theoretical plate
HETP = (column length (L)/N)
What is the HEPT equation?
HEPT = A + B/μ +Cμ
What does the A term represent?
The multipath term.
Eddy currents exist within the column. Solutes move differently depending on which current they follow.
i.e. the distance travelled by molecule A is longer than molecule B, thus Molecule A will take longer to elute.
- Not affected by mobile phase velocity
- Proportional to the particle size of the packing
What does the B term represent?
Longitudinal Diffusion:
- Molecules spread out – diffuse
- Very high diffusion in gases – important in gas chromatography
- In liquid chromatography this term is close to zero
- Faster movement, less time to spread out – faster is better
Molecules diffusion from areas of high concentration to areas of low concentration.
What does the C term represent?
Mass Transfer:
- Molecules move into and out of the stationary phase at different rates
- Molecules spending too much time in the stationary phase are left behind
- The faster the mobile phase velocity the farther these molecules are left behind.
- Fast mobile phase is not good from the ‘c’ terms perspective
What is the effect of the mobile phase velocity on separation? (GAS)
There is a mobile phase velocity which provides the minimum plate height and therefore the best operating velocity. (Minimum of the sum of three A, B and C terms)
Draw the A, B and C term on a graph of HEPT vs Linear Velocity: (GAS)
A = horizontal
B/viscosity = Negative exponential graph
C*viscosity = positive linear
To get optimal flow rate minimise… (GAS)
longitudinal diffusion B/viscosity
What is the effect of the mobile phase velocity on separation? (Liquid)
Since diffusion term is small -> plate height is smallest at its lowest mobile phase velocity.
Plate height = smallest as lowest linear velocity
Draw the A, B and C term on a graph of HEPT vs Linear Velocity: (Liquid)
A is horizontal
B/viscosity horizontal at 0 HEPT
C is positive linear graph
5 important points to remember:
- Based on the principle of adsorption
- Affinity between the mobile phase and stationary phase
- 1-D chromatography separates molecules based on their MW
- Small HEPT => better separation
- Velocity of mobile phase has crucial effect on separation performance (The A, B and C terms)
What is adsorption?
Adsorption is a surface phenomenon.
Matter is extracted from one phase and concentrated at the surface of a second phase (solid).
Why is adsorption an exothermic process?
Adsorption decreases the entropy of the system (as you immobilize bulk onto a surface) therefore this is not entropically favourable (negative change in entropy).
What are the two important factors of adsorption?
and two major factors controlling the RATE of adsorption?
Capacity and Kinetics
Film and diffusional transport rate
