The term supernatant describes the liquid fraction of a mixture that remains after the solid components have been separated or allowed to settle. This liquid layer sits above the compacted solid material, representing the lighter, non-sedimented portion of the original sample. The formation of a supernatant is the result of applying a force to a heterogeneous mixture, which drives the denser particles downward. The ability to cleanly separate a liquid phase from a solid phase is a basic technique used across laboratory science for isolation and analysis.
Defining the Supernatant and the Sediment
The supernatant is the liquid phase of a mixture, typically appearing clear or translucent, that sits above the separated solid material. This liquid contains substances that are either dissolved in the solvent or are very small particles that remain suspended because their density is too low to settle easily. The supernatant’s necessary counterpart is the sediment, also frequently called the pellet, which forms the dense, solid layer at the bottom of the container.
The distinction between the two components is based entirely on density and particle size. The sediment consists of the heavier, denser particles from the original mixture that have been pulled out of suspension. Conversely, the supernatant holds the lighter components, such as soluble molecules like salts, proteins, or small nucleic acids, along with the solvent itself. Understanding this physical separation is important because the desired product might be contained in either the liquid supernatant or the solid pellet.
How Separation Occurs
The most efficient and widely used method for generating a supernatant in a laboratory is centrifugation, which dramatically accelerates the natural settling process. A centrifuge is an instrument that spins samples at extremely high speeds, applying a powerful outward force known as centrifugal force. This force effectively magnifies the natural pull of gravity, causing particles to sediment much faster than they would if simply left to settle.
The rapid spinning forces the denser particles to migrate away from the axis of rotation and collect at the bottom of the tube, forming the compact pellet. The less-dense liquid and its contents remain in the liquid layer above the pellet, which is the supernatant. The speed of the spin is measured by the relative centrifugal force (RCF), expressed as multiples of the force of gravity, or “g.” Centrifugation can accomplish the same separation in mere minutes, making it an indispensable tool for time-sensitive experiments.
Practical Uses in Science
The primary purpose of creating a supernatant is to isolate one component from another for subsequent analysis or purification. In molecular biology, for example, a common application is the extraction of soluble proteins from a cell lysate. After cells are broken open, the mixture is centrifuged to pellet the heavy, insoluble cell debris and organelles, leaving the desired soluble proteins dissolved in the supernatant.
In clinical diagnostics, centrifugation is used to separate blood components, resulting in a supernatant of plasma or serum above a pellet of red and white blood cells. This plasma supernatant is then analyzed for biomarkers, glucose levels, or other dissolved substances. Even when the goal is to study the solid phase, such as isolating whole cells or subcellular organelles, the supernatant is removed and discarded to wash or purify the pellet.