Chromatographoc Theory

Question 1

column chromatography

Answer 1

stationary phase held in a tube with the mobile phase moving through the tube under the influence of gravity or pressure

Question 2

planar chromatography

Answer 2

stationary phase supported on a flat plate or in the pores of a paper. the mobile phase moves through the stationary phase by capillary action or under the influence of gravity

Question 3

surface adsorption

Answer 3

relative polarities of analyte and solid stationary phase determine the rate of movement of the analyte through a column or across a surface

Question 4

partition (sorption mechanisms)

Answer 4

a liquid phase is coated onto an inert solid support, movement of the analyte is determined solely by its relative solubility in the two phases or volatility if one phase is a gas

Question 5

ion exchange (sorption mechanism)

Answer 5

the stationary phase is a permeable polymeric solid containing fixed charged groups and the mobile phase contains counter ions which can exchange with ions of a solute as the mobile phase carries them through the premeable solid

Question 6

size exclusion (sorption mechanism)

Answer 6

separations occur because of variations in the rate at which anlyte molecules diffuse through the inert but porous stationary phase

Question 7

t0

Answer 7

sample containing components A and B introduced onto column

Question 8

t1

Answer 8

components begin to separate as they start to move throughh the column under influence of mobile phase

Question 9

t2

Answer 9

components nearly resolved due to B interacting more strongly with stationary phase than A

Question 10

t3

Answer 10

components fully resolved and A is detected

Question 11

t4

Answer 11

B eluted from the column and is detected

Question 12

distribution ratio

Answer 12

D = C(stationary phase)/C(mobile phase)

Increasing D = increasing associations with stationary phase

Question 13

retention time (tr)

Answer 13

the time after injection for the peak maximum to be eluted

Question 14

tm

Answer 14

time taken for an unretained compound to travel through the column, column void volume or dead volume

Question 15

adjusted retention time

Answer 15

tr-tm=t’r

time the analyte spends on the column compared to an unretained compound

Question 16

number of theoretical plates (N) (performance)

Answer 16

N = 16(tr/W)^2
where W is width of base of the max peak and tr is measured to the top of the peak. Higher value of N more efficient the column and more chance of achieving the separation you need

Question 17

plate height (H) (performance)

Answer 17

H=L/N where L is the length of the chromatography column - the smaller H the more efficient the column

Question 18

chromatographic resolution

Answer 18

resolution Rs of column is a quantitative measure of its ability to separate 2 analyses
Rs = 2[(tr)B - (tr)A]/(WA + WB)

Question 19

Isotherms

Answer 19

express the concentration profile of an analyte as it moves through the chromatographic system

Question 20

Chromatographic peak shapes

Answer 20

departures from the ideal Gaussian profiles reflect non-linear isotherms, i.e. poor chromatography

Question 21

Tailing

Answer 21

Shape : right side concave
Exceeding capacity of stationary phase
additional adsorption processes i.e. active sites
Leaks and blockages
going through system too fast to interact

Question 22

Fronting

Answer 22

Shape: left side concave, right side straight down
Reaction then dissolution in stationary phase (HPLC)
Common: mass overload of mobile phase
- exceeding solubility (HPLC)
- exceeding partial pressure of component (GC)

Question 23

Asymmetry factor

Answer 23

As = B/A (right of peak width over left side, taken at 10% of peak height)

Question 24

Tailing factor

Answer 24

Tf = (A+B)/2A (A = left side of peak width, B = right)

Question 25

Alternative plate height equation (van Deemter equation)

Answer 25

H = A + B/u + C.u
A - rate of mass transfer within the mobile phase
B - diffusion of analyte molecules within the mobile phase caused by concentration gradients
C - relates to the rate of mass transfer to and from the stationary phase