Isocratic
elution and the gradient elution is decided base on the ratio (Composition) of the mobile phase in Mixture.
Generally, a mixture of two or more solvents is used as a mobile phase in chromatography
technique. The ratio of the individual mobile phase changes during the
chromatography or it may remain as per the requirement.
Isocratic Elution
Isocratic
elution means the composition of the mobile phase remains constant throughout the
chromatographic separation.
In
chromatography separation single mobile phase or mixture or mobile phase is
used. Nowadays generally a mixture of the mobile phase is used it may be a mixture
of two solvent or more than the two. The composition of the mobile phase
neither increases one of them or decreases and remains the same. That type of elution
is known as the isocratic elution.
Example: In
Isocratic Elution If you are using a mixture of A and B as the mobile phase. The
ratio of the mobile phase component A: B in
the mixture is 9:1. During the whole analysis the ratio of component A and
B remains the same. The Ratio of the mobile phase is 9:1.
Gradient Elution:
Gradient
elution means the composition of the mobile phase used in chromatography is changed
during chromatography. When composition (ratio) of one solvent from the mixture
of mobile phase increases or during elution. Gradient elution is one of the
most effective methods for liquid. Gradient elution also depends on the
instrumentation.
Example: In gradient
Elution If you are using a mixture of A and B as the mobile phase. The ratio of the
mobile phase component A: B in the
mixture is 8:2 During the whole analysis the ratio of the component A and B does not remain the same it may change as per the requirement. The composition of solvent A
or B may increase or decrease with time. After % min run the composition of
mobile Phase may 9:1 or 7:3.
In Gradient elution, the retained components move
faster. When the composition of the organic solvent in the mobile phase is increased, the
retained analyte moves faster.
Advantage of the Gradient
elution over the Isocratic elution:
In isocratic elution the component which does
not move faster it retained longer into the stationary phase, the peak is
wider. Peak width increases
with retention time and the last eluted peaks may be broad.
While
in gradient chromatography, the sharp peak can be obtained by changing in the
composition of the Mobile phase.
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