A centrifuge is a machine used in laboratories to separate liquids, gases, and occasionally solids into their separate components. Because they are useful in a wide variety of applications, centrifuges are commonplace in many different types of research and clinical labs. For example, a centrifuge may be used by a clinical lab technician to separate a patient’s blood sample into red blood cells and plasma. After processing the sample, the technician can then remove the red blood cells and keep the plasma for a diagnostic procedure.
How Do Centrifuges Work?
The foundational concept of centrifugal technology is sedimentation, which states that a solution comprised of different base substances will separate when gravitational force is exerted upon them. Centrifuges utilize this phenomenon by spinning samples at a very high RPM (revolutions per minute). The resulting centripetal force causes the substances to move outward, where they naturally stack from least dense to most dense. The densest portion moves to the bottom of the stack and the least dense gathers at the top.
One spins samples by dispensing the fluid into test tubes, then loading the tubes into a rotor. The rotor is placed on the centrifuge’s spindle. After closing the chamber, the technician sets parameters on the centrifuge’s control panel and initiates the run. The run's parameters, such as duration, temperature, and speed, are determined by a user’s application and the sample in question.
Should the application demand it, a technician can spin samples in microplates rather than test tubes. While this can increase sample throughout, it will require a microplate-compatible rotor.
Types of Centrifuges
On the market, centrifuges are typically classified according to two metrics: size and gravitational force. Every variance in these categories come with unique pros and cons that a buyer must prioritize in accordance with their budget and use case.
Size
All centrifuges are either benchtop units, or floor units. There are also microcentrifuges, which are, as the name suggests, more compact versions of benchtop systems.
With each size comes different processing capacities. Floor centrifuges have the largest chamber; thus, they can fit the largest rotors and process more samples per run than most other benchtop units. Additionally, virtually all floor models are refrigerated. A refrigeration system and expansive chamber are not easily compressed into a small form factor, so floor centrifuges come at the cost of space -- a precious resource in itself.
One of the main draws of a benchtop centrifuge is its compact form factor. Such models don’t take up any floor space and are relatively portable. Opting for a model without refrigeration (should one’s requirements permit it, of course) can save both space and a decent chunk of change. Floor centrifuges don’t offer that kind of flexibility -- as stated, they’re all refrigerated.
Last but not least are microcentrifuges: the low-volume, high-speed, ultra portable powerhouses commonly used in hematology. Microcentrifuges are ideal for applications in which sample volumes are limited and bench space is tight.
Gravitational Force
RCF, or relative centrifugal force, is one of the fundamental concepts of centrifugation. Simply put, RCF is how we measure the gravitational force being applied to samples as they spin. RCF is a value that can be calculated based on a rotor’s RPM (speed) and its diameter. (Learn more about how RCF is calculated.) Just as each size of centrifuge is suited for different usage scenarios, each speed range is suited for unique applications as well.
The finer a component’s particles are, the stronger the centripetal force must be to adequately separate them. Thus, as granularity increases, RCF must increase as well. Because centrifugal force is a product of speed, one boosts RCF by increasing the centrifuge’s current RPM.
Typical speeds found in labs can be less than 1,000 RPM to over 100,000 RPM. Because this is such a broad range, it’s divided into the following three categories:
- Low-speed: 0-9,000 RPM
- High-speed: 10,000 - 39,000 RPM
- Ultra: 40,000+ RPM
Rotors & Accessories
Just as the various types of centrifuges are suitable for different applications, there are also different rotors geared for their own use cases. The three main types are fixed-angle, swing-bucket, and vertical.
Fixed-Angle Rotors
Fixed-angle rotors are the most common type of rotor. They consist of a series of tube cavities lining the rotor's diameter, angled in toward its center. These rotors are frequently used to strain target particles, or "pellets", from samples while allowing you to discard the rest.
Swing-Bucket Rotors
Also known as "swing-out" rotors, these rotors are the most ubiquitous type besides fixed-angle. They're designed to hold buckets (typically four of them) on trunnion pins, which allow them to swing freely as they spin. The centripetal force holds the buckets at a 90° angle for the duration of the run, which allows the sample to maintain its orientation before, during, and after processing.
Swing-bucket rotors require adapters to hold tubes. Adapters come in a variety of configurations and can have layers added or removed to accommodate vials of varying height.
Vertical Rotors
As the name implies, vertical rotors hold sample tubes upright for the entire duration of the run. Considerably less common than fixed-angle and swing-bucket rotors, vertical rotors are used when the sample in question can pellet after moving only a very short distance.
In Conclusion
When you get down to the nitty-gritty details, there's more to centrifuges than what's been presented here. Nonetheless, the scientific foundations and conceptual prerequisites remain the same across them all.
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