A supernatant is the liquid portion that remains above a solid material after a separation process, free from the denser particles that have settled out of the solution. A common analogy is a bottle of oil and vinegar salad dressing after it has been sitting for a while. The oil that rises to the top layer is similar to a supernatant, while the vinegar and seasonings that settle at the bottom represent the solid phase.
The Process of Separation
The most common method for generating a supernatant is centrifugation. This technique involves placing a sample into a machine called a centrifuge. The centrifuge spins the samples at extremely high speeds, creating a strong centrifugal force that can be thousands of times greater than gravity. This force causes the denser components of the mixture to move to the bottom of the tube.
These compacted solids at the bottom form what is known as a pellet, and the remaining liquid above this pellet is the supernatant. The characteristics of the supernatant, such as its clarity and composition, depend on the nature of the original sample and the specifics of the centrifugation process, including the speed and duration of the spin.
Another, much slower, method of separation is sedimentation. This process relies solely on gravity to pull denser particles to the bottom of a container over time. It is less efficient and used for mixtures where the solid particles are large and settle out relatively easily without the need for high-speed spinning.
Applications in Scientific Research
In clinical medicine, supernatant analysis is a routine part of blood testing. When a blood sample is centrifuged, the red and white blood cells form a pellet at the bottom, leaving the yellowish fluid called plasma as the supernatant. This plasma is then analyzed for a wide range of substances, including glucose levels, hormones, and other biomarkers that indicate health or disease.
In cell biology, scientists often grow cells in a liquid known as a growth medium. These cells can secrete proteins and other molecules into the medium. To study these secreted substances, researchers will separate the cells from the medium. The collected supernatant, which is the cell-free growth medium, can then be analyzed to understand cellular communication or to identify potential drug targets.
Environmental science uses the separation of supernatants to assess water quality. When a water sample from a river or lake is collected, it contains suspended sediment and microorganisms. By centrifuging the sample, these solid particles are pelleted, allowing the clear water supernatant to be removed. This water is then tested for dissolved pollutants, such as heavy metals or pesticides, to assess the chemical contamination in the water body.
Handling and Collection Techniques
Properly collecting the supernatant is an important step for accurate analysis. The primary goal is to remove the liquid without disturbing the underlying pellet, which could contaminate the sample. Two main techniques are used for this purpose: pipetting and decanting.
Pipetting involves using a pipette to carefully aspirate the liquid. The tip of the pipette is slowly lowered into the tube, just above the surface of the pellet, and the liquid is drawn off. This method offers a high degree of control and is preferred when the volume is small or when it is important to avoid any transfer of the pelleted material.
Decanting is a simpler but less precise method that involves carefully pouring the liquid out of the tube into a new container. This must be done in a smooth, steady motion to prevent the pellet from being dislodged. While faster than pipetting, decanting is used when the pellet is very compact and stable, and a small amount of contamination is not a major concern.