the chromatogram terms: retention time peak area peak width (at half height, at base) peak...
TRANSCRIPT
The Chromatogram Terms:
Retention time Peak area Peak width (at half height, at base) Peak height Void time/volume Adjusted retention time Leading edge, tailing edge
The chromatogram
TERMS - Retention time, Peak Area, Void Time (volume), Adjusted Retention TimePeak Width ( at half height, at baseline), Peak Height, Leading Edge, Tailing Edge
Chromatographic Terms Retention volume
Volume of mobile phase needed to carry component through column to detector
VR
Retention volume Not convenient to measure volume directly Measure time taken from injection to
appearance of signal peak Record as retention time tR
-maintain a constant flow rate
VR = tRFc where Fc = flow rate
Retention time
Void volume/time A compound cannot possibly exit the column
in “zero seconds” Takes time to travel through a column even at
“top speed” Fastest a component can travel is at the speed
of the eluting solvent – ie the same speed as the mobile phase
Void volume/time Have a flow rate of for example 1 mL/min Column and tubing occupy a volume of 2
mLs of liquid (excluding volume occupied by solid phase)
Then fastest a component could travel would be 2 minutes – as fast as the MP
Void Volume/time The fastest speed an unretained component could
travel through the column and “system” is represented by the void volume/time
No peak can be seen before the void time If a column has a void time of 2 minutes, then no
peak can occur at <2 minutes
Void volume/time To measure void time, inject a component
known not to stick or be retained on your column
The peak you see will represent the void
Why is this important? i.e.Why is it important to an analyst to know the void time of their system when performing an analysis?
The adjusted retention time Retention time = the time taken for a
component to travel through the column However, this cannot be less than the void
time Therefore the adjusted retention time is the
actual retention time minus the void time
Adjusted retention time V’R = VR – Vo
t’R = tR – t - equation for calculating adjusted RT
Retention times and volumes are dictated by the distribution coefficient K
K= Cs/Cm
The chromatogram
TERMS - Retention time, Peak Area, Void Time (volume), Adjusted Retention TimePeak Width ( at half height, at baseline), Peak Height, Leading Edge, Tailing Edge
Capacity Factor k – describes the ability of the stationary
phase to retain components
Ratio between number of molecules in SP compared to MP
k=Cs/Cm
Capacity Factor k1= (V1-Vo)/Vo
k1 = (tR –to)/to
= t’R/to
The longer a component is retained by the column, the larger the capacity factor
Capacity factor Large capacity factors favour good separation
but increased elution times Capacity factors > 1 and < 5 are favoured If over time the capacity factor changes,
usually indicates degradation of the column
Separation FactorSelectivity A purpose of chromatography is to separate
compounds from each other If want to separate A from B, they must have
different retention times (ie different capacity factors)
Retention time
Selectivity Separation factor (alpha) aka selectivity - for
two peaks:
Alpha (a) = kB/kA
= t’R(B)/t’R(A)
For separation to occur, alpha must be greater than 1
Which peak must have the longer retention time?
Selectivity Separation factor depends on type and
properties of stationary phase used, the composition and properties of mobile phase, the interactive forces of the analyte and the column temperature
Must be optimized for each separation If maximize alpha, get good separation BUT
analysis times are too long
Separating efficiency of a column Every component should show up as a nice
narrow peak Must then move in a narrow band through the
column Peak width is an indication of the efficiency
of a column
Efficiency Observation: the longer a component stays
on the column, the wider the peak The band is dispersed as it travels through the
column The longer it is on the column, the more time
it has to disperse Dispersion leads to band broadening and wide
peaks
Peak width Simple measurement: peak width Gives an estimation of the efficiency of the
column when combined with retention time The number of theoretical plates, or the plate
count, N is used as a quantitative measure of efficiency
Plate theory (again) Chromatography is a continuous process Separation occurs on all particle surfaces Think of in terms of imaginary segments of
the column called plates – remember the molecular “hurdles”?
Each plate represents a surface at which the separation occurs (“separation event” – ie the analogy of the hurdles in a race)
Plates The more plates (hurdles) the better the
separation and the efficiency of the column Abstract concept used to compare the
efficiencies of different columns To give it a value, need two easy
measurements, retention time and peak width
Plate count N Relationship between Rt and W As the retention time increases, so does the
peak width If look at two columns, compare peak width
of peaks having the same retention time Can use W or W1/2
Formula for N – Theoretical Plates N = 16(VR/W)2
= 16(tR/W)2
Where both tR and W are in the same units (eg seconds or minutes)
Remember, N is a measure of the separation power (efficiency) of the chromatographic system
Measurements for N