Unit 5 Flashcards
who initially designed chromatography?
how did he do this?
russian botanist, Mikhail Tswett
he separated pigments in plants using a column made of calcium carbonate
define mobile phase
- phase which is moving over or through the stationary phase
- can be a liquid, gas or supercritical fluid
define stationary phase
- phase which is fixed as a column or a planar surface
- can be solid, gel or liquid
define chromatography
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
define elution
process of separating components of a mixture using appropriate solvents
what are the 3 main steps in chromatography?
- sample introduced at head of column
- elution of components
- further elution and separation
during elution of components, which chemicals are fastest to move?
unretained chemicals
retained chemicals move slowly
define retention time
time between sample injection and peak maximum of sample component of interest
which chemicals have the shortest retention time? which have longest?
shortest: unretained (will be furthest on left of retention time graph)
longest: retained
how is u (linear velocity of mobile phase) related to:
a) length of column
b) velocities of retained analytes
a) proportional
b) reversely proportional
what is retention factor?
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
what is the equation of retention factor?
k’(B) = amount of B in stationary phase / amount of B in mobile phase
what are 4 interactions between components and stationary phase?
- adsorption
- partioning
- ion exchange
- sieving
what is elution chromatographic separation (plate theory)?
what is the equation that represents this?
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
define selectivity factor
a measure of the separation of two components
how is selectivity factor (alpha) related to the column efficiency?
they are independent
what factors affect selectivity factor (alpha)?
- chemistry of compounds to separate
- eluent type
- adsorbent chemistry
what happens if selectivity factor is 1?
therefore what is the requirement of selectivity factor?
there is no change to improve column efficiency to better separate them
selectivity factor must be bigger than a
in the plate method, how is the column performance assessed?
by looking at the number of theoretical plates (N) of the column)
what is the rate theory used for?
used to describe chromatographic efficiency
what are the three terms in the rate theory?
A term: multi path (Eddy diffusion) in packed columns)
B term: longitudinal (multidrectional) diffusion coefficient
C term: mass-transfer coefficient
define resolution
characterizes separation of 2 adjacent peaks by a column
do we want a high or low resolution? why?
higher resolution = keeps analysis time short = good
what are 2 types of non-ideal peaks?
tailing
fronting
what are examples of practical problems which result in non-ideal peaks?
overload
flow rate too high
slow injection
