With the ease of use of modern centrifuges, it may seem that putting tubes in and pressing start are all that’s needed for a successful run. While loading a centrifuge properly is not difficult, there are some important nuances to know ahead of starting. This article highlights tips that may seem obvious for proper centrifuge loading, but the details will save the centrifuge user from avoidable mistakes often made by even seasoned scientists.
Tip 1—Carefully inspect and choose the right tubes—Not all tubes are created equal. If centrifuged at speeds higher than what the tubes are rated for, tubes can warp, crack, and lose their caps. Check the manufacturer’s ratings and your centrifugation protocol to make sure that the tubes will hold up to the forces they face during the spin. It is also important to check that the tube chambers are accommodating to the bottom of the tube. For example, if you are using round-bottom tubes but the tube chambers are meant for conical, you should use an adapter. The bottom of the tube will bear the most force and if not supported properly, it’s much more likely that the tube will rupture.
When ready to load, check for pre-existing cracks in the tubes and double-check the labels. Beware of using stickers as they are prone to fall off during centrifugation. If using microcentrifuge tubes, be wary of only labeling the lid, as they can also come off during centrifugation. Instead, make sure to use a chemical resistant lab marker to label the sides. Finally, wipe down the outside of the tubes to avoid contaminating the centrifuge before loading.
Tip 2—Select the correct rotor and adaptors, then secure and select—Some newer centrifuges have rotors that are very simple to exchange out with no tools required and automatic rotor detection. However, if your centrifuge does not have these features, there are several precautions to be aware of. It is recommended that if this is your first time exchanging a rotor that you ask for help from an experienced user. There are often precise tools required for unlocking the rotor and resecuring. Rotors also tend to be heavy and awkward to handle. Once secured, select the correct rotor on the centrifuge display. Add in the required adapters, and the tubes are ready to be loaded in.
Tip 3—Balance the weight within the centrifuge—Balancing is a critical detail before starting a centrifuge run. It is best to have perfect balance within the centrifuge. If the balance is off, the centrifuge can experience violent shaking, which can harm the sensitive mechanical parts within. Not only do the weights of the tubes and tube contents need to be balanced, but if the rotor has removable buckets or adaptors, those will also need to be double-checked for balance. Just because two adaptors look alike, does not mean they weight the same. Balance can be checked on a standard scale, or double pan balance.
Scale Balancing—When balancing several tubes at a time, use a tube rack or beaker that sits on top of the scale surface. Tare the weight of the rack or beaker and then place in tubes from one side of the centrifuge, record weight, and then repeat with tubes from the opposite side. If the balance is off, tubes should be added to one or both sides, adding water until a balance is reached. When adding water into the tube while it is on the scale, make sure the cap is still included in your weight measurement. If the buckets or adapters are easily removable, it’s best to weigh them with your sample tubes inserted, instead of the tube rack. This method will give you the most accurate balance.
Double Pan Balancing—A double pan balance has two platforms, called pans, connected by a beam and acts similar to a see-saw. There is typically a sliding weight along the beam and a pointer to show when the two pans are balanced. When balancing the weight of multiple tubes, beakers can be used on each pan. These should be balanced first using the sliding weight on the beam and then tubes added and balanced using the same principles as in scale balancing.
Tip 4—Determine refrigeration conditions of the run—Many protocols require refrigerated centrifugation at precise temperatures, although the act of centrifugation itself can cause your samples to be heated. To minimize heating during centrifugation, the centrifuge should be pre-cooled to the desired temperature. Removable rotors can be stored in a refrigerated space as well. This is especially important during ultracentrifugation where a frictionless environment is also required to reach top speeds and prevent heating. Planning refrigerations conditions ahead of time will ensure that your samples will be kept at an ideal temperature.