When running isocratically, peaks will have a width equivalent to (for early eluting peaks) or greater than (for all other peaks) the width of the injection profile. Therefore the greater the injection volume, the wider peaks will be.
In gradient mode, if the injection solvent is weaker than the starting mobile phase conditions, analytes can be "stacked" at the head of the column, eluting as a result of increasing solvent strength as the gradient profile progresses. In this case peak width is controlled by the slope of the gradient rather than the injection volume.
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