Chromatographoc Theory Flashcards
column chromatography
stationary phase held in a tube with the mobile phase moving through the tube under the influence of gravity or pressure
planar chromatography
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
surface adsorption
relative polarities of analyte and solid stationary phase determine the rate of movement of the analyte through a column or across a surface
partition (sorption mechanisms)
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
ion exchange (sorption mechanism)
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
size exclusion (sorption mechanism)
separations occur because of variations in the rate at which anlyte molecules diffuse through the inert but porous stationary phase
t0
sample containing components A and B introduced onto column
t1
components begin to separate as they start to move throughh the column under influence of mobile phase
t2
components nearly resolved due to B interacting more strongly with stationary phase than A
t3
components fully resolved and A is detected
t4
B eluted from the column and is detected
distribution ratio
D = C(stationary phase)/C(mobile phase)
Increasing D = increasing associations with stationary phase
retention time (tr)
the time after injection for the peak maximum to be eluted
tm
time taken for an unretained compound to travel through the column, column void volume or dead volume
adjusted retention time
tr-tm=t’r
time the analyte spends on the column compared to an unretained compound
number of theoretical plates (N) (performance)
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
plate height (H) (performance)
H=L/N where L is the length of the chromatography column - the smaller H the more efficient the column
chromatographic resolution
resolution Rs of column is a quantitative measure of its ability to separate 2 analyses
Rs = 2[(tr)B - (tr)A]/(WA + WB)
Isotherms
express the concentration profile of an analyte as it moves through the chromatographic system
Chromatographic peak shapes
departures from the ideal Gaussian profiles reflect non-linear isotherms, i.e. poor chromatography
Tailing
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
Fronting
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)
Asymmetry factor
As = B/A (right of peak width over left side, taken at 10% of peak height)
Tailing factor
Tf = (A+B)/2A (A = left side of peak width, B = right)
Alternative plate height equation (van Deemter equation)
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
