W10 Efficiency and Band Spreading Flashcards
what are the two factors that contribute to how well compounds are separated by chromatography
difference in elution times between peaks: farther apart > better separation
how broad the peaks are > thinner peaks > better separation
how is standard deviation used in chromatography
used to represent how much the data (or peaks) spread out from average retention time
smaller sd: peak is narrow and well-defined
larger sd: peak is wider and more spread out
what is theoretical plate
imaginary layers inside a chromatography column > greater number of plates > greater chromatography efficiency
how is column efficiency measured
by plate height: lower height > greater efficiency
number of theoretical plates: greater number > greater efficiency
what is band spreading
the widening or dispersion of a sample’s analyte as it passes through the chromatographic column
ideally, analyte should move through column as a narrow, sharp peak, but due to several factors, the analyte often spreads out > broader peaks
how does multiple flow paths affect band spreading
solute molecules passing through column travel separate paths that may differ in length > solutes elute at different times
non-homogenous packing and differences in particle size
improve by using smaller range of particle size and a more consistent packing
how does longitudinal diffusion affect band spreading
since concentration of solute is greatest at center of chromatographic band > more solute diffuses towards the band’s forward and rear edges than towards band’s center
faster the linear flow > less time spent in the column > less diffusional broadening occurs
solute’s diffusion coefficient is larger in gaseous mobile phase than in liquid phase > longitudinal diffusion more serious in gas chromatography
how does finite equilibration time between phases affect resolution
delay between the switching of solute from mobile to stationary phase > band broadening > lower resolution
how can finite equilibrium time between phases be improved
use slow mobile phase velocities: mobile phase move slower > more time for solute to equilibrate between 2 phases
use smaller diameter packing materials: give solute more surface area to interact with stationary phase
use thinner films of stationary phase: allow solute to move more easily between the 2 phases
definition of finite equilibrium time and mass transfer resistance
finite equilibrium time: the delay as molecules move back and forth between the moving and stationary parts of the column
mass transfer resistance: the difficulty or slowness of this movement
properties of van deemter equation
below optimum velocity > longitudinal diffusion (B/Ux) most significant
above optimum velocity > equilibrium time broadening (Cux) most significant
how to improve efficiency
use open tubular columns
use different separation mechanisms like electrophoresis
cause of fronting peaks
when [solute] exceeds binding capacity of stationary phase > rapid elution of solute molecules that cannot bind
often due to overloading of stationary phase in non-linear adsorption systems
cause of tailing peaks
due to heterogenous interactions of stationary phase > non-ideal binding sites can bind some solute molecules too strongly > delayed elution
