Types of centrifuges
Micro- and tabletop centrifuges can be divided into two groups based on how they regulate their temperature: ventilated and refrigerated. Because the process of spinning rapidly results in heat production, this causes temperatures within the rotor to rise higher than the surrounding room temperature by several degrees. The amount of heat produced will vary by rotor, centrifuge, and speed. A ventilated centrifuge draws ambient air into the bowl, which is then released through an exhaust. This dissipates heat but does not provide any degree of temperature control. By comparison, a refrigerated centrifuge will maintain the rotor temperature to a specified temperature. Refrigerated centrifuges come in multiple configurations with various temperature ranges. Though precise temperature control may not be required for many applications, some analytes are temperature-sensitive and require stable temperature control. Refrigerated centrifuges counteract the temperature fluctuations that occur during centrifugation, making them suitable for temperature-sensitive analytes.
Can I just put my centrifuge in the cold room?
Refrigerated centrifuges are generally more expensive and have a larger footprint. Though you may be tempted to transport your ventilated centrifuge to a cold room or refrigerator, this is likely not going to be sufficient for temperature-sensitive samples. Precise temperature control is critical for some analytes such as enzymes, hormones, or blood factors (e.g., hemoglobin, creatinine).
Importantly, refrigerated centrifuges not only lower the rotor temperature but also maintain it, combating the heat produced during centrifugation. Because refrigerated centrifuges are sealed, they can better moderate the heat generated during operation. Thus, even though placing a ventilated centrifuge in a cold room or refrigerator will lower the air temperature within the bowl, it will take much longer to lower the temperature of the rotor and it will not mitigate the heat generated during operation to maintain the temperature of the rotor at 4°C.
Just as a refrigerator set to cool to 4°C achieves that temperature by blowing air that is cooled to less than 4°C, a refrigerated centrifuge is circulating cooled air that is below the setpoint in order to maintain a chamber and rotor temperature of 4°C. Circulating 4°C air in a cold room through a ventilated centrifuge will not effectively cool the rotor or chamber to 4°C. Realistically, you will end up with a temperature that is a few degrees higher. It also will not compensate for differences in speed or load, which may affect the amount of heat generated.
Additionally, your analyte may require a specific temperature other than the temperature of your cold room, generally 4°C, while a refrigerated centrifuge will be able to maintain a range of temperatures.
Operating a refrigerated centrifuge for temperature-sensitive samples
If you’re working with temperature-sensitive analytes and have acquired a refrigerated centrifuge, there are still some best practices to ensure optimal sample handling. First, it is important to pre-cool the centrifuge. While it is not necessary to leave it on constantly or pre-cool overnight, you will want to give yourself time to cool the centrifuge to the desired temperature before placing your samples in the rotor. Otherwise, it may never reach the appropriate temperature during operation.
Also, simply turning on the centrifuge and allowing it to cool will only cool the bowl of the centrifuge. To cool the rotor, you will need to either place the rotor in a cold room overnight or, ideally, centrifuge the empty rotor during the cooling process. Many refrigerated centrifuges have a pre-defined cooling program that can achieve this. It is also important to perform regular maintenance to ensure proper cooling. Regular service to ensure compressor function and cleaning of condensation and any debris is critical.
Are all refrigerated centrifuges the same?
Refrigerated centrifuges use compressors for temperature regulation. Some centrifuges turn on the compressor only when cooling is needed. This can cause larger temperature fluctuations as the compressor works to lower the temperature. Centrifuges that instead maintain a specified temperature by modulating the compressor - rather than turning it off and on - have smaller temperature fluctuations. Another consideration is whether the temperature sensor registers air temperature or sample temperature. The choice of rotor will also contribute to cooling rates. Fixed angle rotors made of aluminum cool down more quickly and will hold temperature for longer than rotors made of carbon or titanium. When using a non-aluminum rotor, it is especially important to pre-cool the rotor.
The HERMLE Z326K Universal Refrigerated Centrifuge has 17 different rotor choices.