Separation Science and Mass Spec Flashcards
What is the minimum value of Rs, for baseline resolution?
Rs > 1.5
Describe what the capacity factor kβ is?
Tells us the amount of time solute spends in the stationary phase relative to the amount of time spent in mobile phase. Longer component is retained on column larger kβ.
Describe what the selectivity factor, πΌ, is?
Selectivity factor is the ratio of capacity factors of 2 different analytes.
What does migration velocity through the column depend on?
The distribution of the analyte between the mobile and the stationary phases
What is the rate theory of chromatography?
symmetrical spread of velocities around a mean value. band broadening is a statistically random process
How does the area of a peak change overtime
It doesnβt, total area under a peak remains constant
What is π?
standard deviation in length units
What is π?
standard deviation in time units,
is the half width at inflection point
π = W/4
What is diffusion?
The diffusion coefficient ,D, is the rate at which a substance randomly moves from an area of high conc to low. Diffusion is the main cause of band spreading
When do asymmetric band shapes/ Skewing occur?
Skewing occurs when the partition function, K, is dependent on the concentration
What causes fronting and how to correct it?
Occurs from overloading with too much solute.
Correct by smaller/more dilute sample
What causes tailing and how to correct it?
Occurs when small quantities of solute are retained more strongly than large quantities.
Correct by masking strong adsorption sites on SP
How can plate height be described?
Plate height can be thought of as approximately the length of column required for 1 eqm of solute between the mobile and stationary phase.
How does plate height related to peak width
The smaller the plate height the smaller the varience therefore π is smaller and peak width is reduced = good
In the Van Deemter equation what is A?
A = multiple flow paths, in a packed column mobile phase can flow through different pathways that can vary tR
In the Van Deemter equation what is B/u?
B/u = longitudinal diffusion - the longer the analyte spends on the column the more it spreads
In the Van Deemter equation what is Cu?
Cu = mass transfer/ equilibration time
This is the finite time for analyte to equilibrate between mobile and stationary phases
How can you reduce value of A
Using smaller particles
How can you reduce value of Cu
Can be reduced by increasing temp, reducing sp thickness, column radius - basically anything to increase rate of diffusion
How does H vary with flow rate?
Extent of band broadening depend on length of time mobile phase is in contact with stationary phase - i.e. depends on flow.
Ideally we want to run at flow rate giving lowest plate height
In GC within a compound class what order do analytes elute?
In order of boiling points
In GC comparing compound classes what order do analytes elute?
Molecules with greatest similarity to the SP will elute last.
