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SUBODH S SATHEESH
It is expressed by the number of theoretical plates
It is determined by the formula
The number of theoretical plates is a measure of the
“goodness” of the column
If the retention time is high and peak width is narrow then it
shows excellent chromatograms
where tr is the retention time measured from the instant of injection
w is the peak width
W is determined by SD= σ ie w=4σ
Resolution is the ability to seperate two signals
In chromatography its the ability to seperate two peaks.ie
seperation of constituents
where tr1 and tr2 and w1 and w2 are the times and widths,
respectively, of the two immediately adjacent peaks.
If the peaks are sufficiently close w is nearly the same for both
peaks and resolution may be expressed as
Greater the distance more resolution and vice versa.
The rates of migration of substances in chromatographic
procedures depend on the relative affinity of the substances for
the stationary and the mobile phases
Its the difference in time between the point of injection and the
time of emergence of separation of component from the
It is actually the time required for 50% of the component to
It is measure in minutes or seconds
It is the volume of carrier gas required to elute
components from the column to the time the peak
maximum is obtained.
Retention volume depends upon flowrate and retention
It is the ratio of partition coefficient of two components to be
S= Kb/Ka = K’a/k’b= (tb-to)/(ta-to)
If peaks are far apart ie there is more difference in partition coefficient
between compounds hence more seperation factor and viceversa
HETP is numerically equal to the column length divided by
the number of theoretical plates in the column
It varies from to one column to another as well as one solute to
The more efficient the column the better the resolution and the
smaller the HETP.
HETP=Length of column / no of theoretical plates
A chromatographic peak should be symmetrical about its
centre to follow gaussian distribution
Asymetric factor is the measure of peak tailing or fronting.
It is defined as the distance from the centre line of the peak to
the back slope divided by the distance from the centre line of
the peak to the front slope.
The chromatographic peak in (a) is an example of tailing, which
occurs when some sites on the stationary phase retain the solute
more strongly than other sites.
The peak in (b) is an example of fronting, which most often is the
result of overloading the column with sample. For both (a) and (b)
the green chromatogram is the asymmetric peak and the red dashed
chromatogram shows the ideal, Gaussian peak shape.
Its function is to separate sample components to
It should have reasonable chemical and thermal
There are a lot liquid stationary phases available for gas
chromatography. But there is no solvent that meet all
the requirements of a perfect stationary phase
Liquid phase should not permeate too deeply into the fine
pores of the support structure as slow diffusion in and out of
pores affects column efficiency
It should be chemically inert
It should be a good solvent for sample component
Liquid phase should have low volatility and high stability at
elevated temperatures otherwise they can contribute to
interference in analysis
Stationary Phase Trade Name Max Temp Common
Dimethyl Polysiloxane OV – 1, SE – 30 350oC Hydrocarbons,
OV – 17 250oC Steroids, Pesticides,
Poly (Trifluoro propyl
OV – 210 200oC Chlorinated
Polyethylene Glycol Carbowax 20 M 250oC Free acids, Alcohols,
Essential Oils, Glycols
5% Diphenyl – 95%
DB – 5 325oC Flavors,
samples and aromatic
Typical liquid stationary phases
Nonpolar solutes like pentane butane etc can be effectively
separated by nonpolar solvents like squalene. And polar
solutes can be easily separated by polar solvents eg; PEG.
Boiling point is also a factor of consideration. among solutes
with similar polarity if there is sufficient difference in BP
effective separation can take place.
Eg. Squalene min/max temp= 293/423 SE4 423/573
A solvent that could generate different partition ratios
among solvents can only be useful in GLC
H KAUR Instrumental methods of chemical analysis
ninth edition 2013 ;1091-1092
Skoog holler crouch Instrumental analysis 2012 841-
B K sharma Instrumental methods of chemical analysis
twenty fourth edition 2005 c188-191
En.wikipedia.org gas chromatography