Usually in LC gradient elution is popular than isocratic elution. But, what are the reasons of these two type of elutions in LC? If gradient elution has more benefits, what is the necessity of isocratic one?
Gradient elution sharpens the peak, as the elution power increases with the increasing salt concentration in the buffer; therefore, isocratic elution peaks are usually broader. Gradient elution also allows the whole chromatography to be faster.
As the elution power of the buffer does not change, isocratic elution peaks are more sensitive to changes in the eluate behavior, offer a better resolution. For instance, if you have a mixture of very similar proteins eluting in a peak, in gradient elution the peak shape may not change or only very slightly; in isocratic elution this often becomes more easily visible, with separate peaks or at least bumps or shoulders.
Isocratic elution is the norm in large-scale manufacturing, at least for protein drugs.
Sinply, you can save a lot of mobile phase and increase the retention time.
There are other advantages as well. You better refer Analytical Chemistry text book.
It depends on the situation. if you need more resolution, isocratic usually provides the best results. Assuming C18 or silica normal phase for the moment, if there is a mixture of non-polar and very polar compounds, a gradient may be the best way to measure or purify them since an isocratic run may take too long, if it could elute all of the compounds at all. Often, step gradients are run, or a mixture of a linear gradient and isocratic in the same run.
Generally, there are no disadvantages over each other, it depends on sample characteristics as described by Achim and Jack. For example, extract or fraction rich in flavonoids or triterpenoids (close polarity), then isocratic may give better resolution (with elongation of time, a suitable mobile phase, and pH). But when the extract varies in groups of compounds (probably based on phytochemical screening or TLC using different reagents) with different polarities, then gradient may be a choice as time of analysis can be shortened.
gradient elution give you the ability to control M.P polarety which make you able of separation of mixture of materials in low time.
Gradient elution provides the fastest elution rates but it totally depends on the type and nature of sample.
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