Define(a) isocratic elution;(b) gradient elution;(c) normal-phase packing;(d) reversed-phase packing

(a) Isocratic elution refers to a chromatographic separation technique where the mobile phase composition remains constant throughout the entire process. In other words, the solvent mixture used to carry the sample through the stationary phase does not change. This results in a constant elution strength and a single peak for each component in the sample.

(b) Gradient elution, on the other hand, is a chromatographic separation technique where the mobile phase composition changes over time. A gradient is created by varying the solvent composition, typically by increasing the concentration of a more polar solvent over time. This allows for the separation of components with different retention times, resulting in improved resolution and peak shape.

(c) Normal-phase packing refers to a type of chromatographic stationary phase where the stationary phase is polar and the mobile phase is nonpolar. This means that the sample components with higher polarity will have stronger interactions with the stationary phase, resulting in longer retention times. Normal-phase packing is commonly used for the separation of polar compounds.

(d) Reversed-phase packing, on the other hand, refers to a type of chromatographic stationary phase where the stationary phase is nonpolar and the mobile phase is polar. In this case, the sample components with higher polarity will have weaker interactions with the stationary phase, resulting in shorter retention times. Reversed-phase packing is commonly used for the separation of nonpolar or less polar compounds.