Unit 5

Question 1

who initially designed chromatography?

how did he do this?

Answer 1

russian botanist, Mikhail Tswett

he separated pigments in plants using a column made of calcium carbonate

Question 2

define mobile phase

Answer 2

• phase which is moving over or through the stationary phase

- can be a liquid, gas or supercritical fluid

Question 3

define stationary phase

Answer 3

• phase which is fixed as a column or a planar surface

- can be solid, gel or liquid

Question 4

define chromatography

Answer 4

separation based on difference of rates at which the components of a mixture are migrating through a stationary phase in presence of the mobile phase

Question 5

define elution

Answer 5

process of separating components of a mixture using appropriate solvents

Question 6

what are the 3 main steps in chromatography?

Answer 6

1. sample introduced at head of column
2. elution of components
3. further elution and separation

Question 7

during elution of components, which chemicals are fastest to move?

Answer 7

unretained chemicals

retained chemicals move slowly

Question 8

define retention time

Answer 8

time between sample injection and peak maximum of sample component of interest

Question 9

which chemicals have the shortest retention time? which have longest?

Answer 9

shortest: unretained (will be furthest on left of retention time graph)
longest: retained

Question 10

how is u (linear velocity of mobile phase) related to:

a) length of column
b) velocities of retained analytes

Answer 10

a) proportional

b) reversely proportional

Question 11

what is retention factor?

Answer 11

measure of time the sample component resides in stationary phase relative to the time it resides in the mobile phase
- expresses how much longer a sample component is retarded by the stationary phase than it would take to travel through the column with the mobile phase velocity

Question 12

what is the equation of retention factor?

Answer 12

k’(B) = amount of B in stationary phase / amount of B in mobile phase

Question 13

what are 4 interactions between components and stationary phase?

Answer 13

1. adsorption
2. partioning
3. ion exchange
4. sieving

Question 14

what is elution chromatographic separation (plate theory)?

what is the equation that represents this?

Answer 14

a series of consecutive two-phase extractions

but actual equilibria are never really achieved since mobile phase is moving

K distribution constant: K = Cs / Cm

where
Cs = conc of A in stationary phase
Cm = conc of A in mobile phase

Question 15

define selectivity factor

Answer 15

a measure of the separation of two components

Question 16

how is selectivity factor (alpha) related to the column efficiency?

Answer 16

they are independent

Question 17

what factors affect selectivity factor (alpha)?

Answer 17

1. chemistry of compounds to separate
2. eluent type
3. adsorbent chemistry

Question 18

what happens if selectivity factor is 1?

therefore what is the requirement of selectivity factor?

Answer 18

there is no change to improve column efficiency to better separate them

selectivity factor must be bigger than a

Question 19

in the plate method, how is the column performance assessed?

Answer 19

by looking at the number of theoretical plates (N) of the column)

Question 20

what is the rate theory used for?

Answer 20

used to describe chromatographic efficiency

Question 21

what are the three terms in the rate theory?

Answer 21

A term: multi path (Eddy diffusion) in packed columns)

B term: longitudinal (multidrectional) diffusion coefficient

C term: mass-transfer coefficient

Question 22

define resolution

Answer 22

characterizes separation of 2 adjacent peaks by a column

Question 23

do we want a high or low resolution? why?

Answer 23

higher resolution = keeps analysis time short = good

Question 24

what are 2 types of non-ideal peaks?

Answer 24

tailing

fronting

Question 25

what are examples of practical problems which result in non-ideal peaks?

Answer 25

overload

flow rate too high

slow injection