BIOCHEMISTS HAVE ALWAYS amazed me. Using mostly straightforward, inexpensive methods, these gifted researchers somehow manage to unlock many of the mysteries of life. And although the past decade has seen powerful (and expensive) new techniques brought to bear, discoveries are still being made by means that are well within the reach of a dedicated amateur. Sadly, biochemistry is a field that has so far been little explored by amateur researchers, and I think I know why. Few of them have access to what is perhaps the central tool of biochemistry--the centrifuge.

A centrifuge rapidly spins several small test tubes filled with a liquid suspension that is to be separated into its component parts. Like passengers in a car making a high-speed turn, every particle suspended within the tube is thrown outward by its own inertia. Biochemists often take advantage of this effect by adding something to a solution that causes certain components to precipitate. For purifying proteins, for example, this change is often provoked by adding a weak acid or base (vinegar or baking soda, for instance). The high "g forces" generated by the centrifuge then induce the solid particles to settle out in no time flat.

There is no other method that can quickly isolate relatively large quantities of key biochemicals. So whether you want to extract cultured bacteria from their incubating broth, purify proteins or isolate antibodies, you are going to need a centrifuge. Unfortunately, professional models can cost thousands of dollars.

To overcome that financial obstacle, Charles Carter, an amateur biochemist and innovative entrepreneur in London, Ontario, designed a centrifuge that is inexpensive and easy to build. Thanks to his cleverness, any amateur can now construct a practical centrifuge in an afternoon for about $20, using an old kitchen blender, a small plastic pipe fitting and a plastic food storage container. Carter fashioned his prototype from an Osterizer brand of blender, but his technique can be adapted to work with just about any make and model.

Carter's centrifuge (or more correctly, "microcentrifuge," because it uses miniature plastic tubes to hold the samples) consists of three parts: a motorized base, a rotating inner cylinder with the sample tubes, and a fixed outer barrier, which protects you and your family from the high-speed motion inside. Before building this device, you will need to secure a set of miniature sample tubes. Suppliers are listed on the bottom of this page.

Most blenders have a ringlike plastic piece directly at the top of the base. Cut this piece away from the base with a saw and smooth any jagged edges with sandpaper. Then fashion the protective housing using a large plastic food storage container, one that is at least 20 centimeters (eight inches) in diameter and 10 centimeters (four inches) deep. Cut a large hole in the center of the lid and glue it upside down on the base using Krazy Glue or a similar product so that the rotating shaft pokes up through it. Make sure that none of the glue gets on the rotating parts or blocks any air vents, which help to cool the electric motor.

The centrifuge rotor itself is fashioned from a PVC pipe cap, which is basically a short plastic cylinder with one end closed off. Select one from your local hardware store that is about 10 centimeters in diameter. Make sure the plastic is at least 0.3 centimeter (1/8 inch) thick and that it is stiff and inflexible. Carefully drill a hole halfway up the side of the cap using sequentially larger drill bits until the hole is just wide enough to hold one of your miniature sample tubes. Repeat this process three more times to create a total of four holes equally spaced around the circumference.

The motor shaft must be aligned directly through the center of mass of the cap, which might not lie exactly at its geometric center. To find the right place to drill, suspend the cap from a short length of string so that its flat surface hangs at right angles to the floor. The center of mass lies somewhere along the line running straight down from the suspension point. To find out exactly where to drill, coat the string of a carpenter's plumb bob with colored chalk and align it with the other string from which the PVC pipe cap hangs. Carefully edge the string from the bob as close as possible without touching the cap's flat surface. If you hold the string in place just under the cap and then pluck it so that it snaps lightly onto the plastic surface, the string will leave a chalk line that crosses the point where you need to drill. Repeat this process at least twice more, suspending the cap from other points along the side. The multiple chalk lines will then intersect at the point where you should put the hole.

Next, you must drill precisely through the point you've found, making sure the hole is exactly vertical. A drill press works best here, but with a little care, you can do it with a handheld electric drill. The shafts of many old blenders have metal fittings with square cross sections, which will not fit your round hole very well. Carter simply drilled out a round hole that was just large enough to accommodate the square fitting. Then he doped the metal skirt at the base of the fitting with Krazy Glue, pressed the pipe cap down over it until it set and finally filled the gaps between the fitting and the cap with many applications of glue, letting the assembly harden overnight.

Depending on the design of your blender, you may have to cut a sizable hole in the end cap to accommodate a large plastic fitting on the shaft. In that case, use a hole saw to enlarge the first hole you drilled. Then glue the end cap to the plastic fitting using a carpenter's level to ensure that the cap is not canted. You might also find that an adhesive such as Plastix (Loctite Co., item no. 82565) works better than Krazy Glue.

When the glue sets, you will be ready to test the rotating cap for balance. Place an empty sample tube into each of the four holes and quickly pulse the motor at its lowest setting, being careful to keep your fingers out of harm's way. Should the device rattle loudly and quiver across the table, you will have to make some adjustments. Carter's clever method was to take out each tube in turn and repeat the test. If removing one tube reduces the shaking, you will know to shave some mass from the corresponding side of the cap. Gently file away some plastic from the rim of the cap, directly above the appropriate sample tube. Continually pulse the centrifuge to check on your progress until the cap spins without excessive vibration.

If you want, you can take your apparatus to a motor repair shop, where, for a small fee, a technician should be able to determine the rotation rate for each setting of the blender. (How much faster is "puree" than "mince" anyway?) With this information, you can calculate the precise accelerations your samples experience with the formula given at the left.

To test your centrifuge, fill two clean sample tubes with one milliliter of milk each. Top off one with water and the other with vinegar (5 percent acetic acid). Place the samples into opposing holes so that their lids are inside the rotating PVC cap and their hinges are downward. (Never run the centrifuge with just one tube of liquid, because it would be unbalanced.)

Put on the protective plastic cover and spin for three minutes at the lowest setting. When the rotor comes to a complete stop, remove the cover, extract the tubes and observe what's inside. You'll find that the white of the milk has settled to the bottom of the tube containing the vinegar. This is because the acid has lowered the pH, causing the casein protein molecules, which give milk its white color, to precipitate. Pouring off the remaining solution will leave you with a solid slug of pure protein—a visible token that you have entered the wild world of biochemistry.

Suppliers

Fisher Scientific (800) 766-7000 or (973) 467-6400 (800) 926-1166 (fax) Catalogue no. 05-406-22 600 tubes for $14.06

Scientific Supply Source 15201 E. Moncrieff Pl., Suite C Aurora, CO 80011 (800) 377-8775 or (303) 375-1664 Catalogue no. 265-1550 500 tubes for $13.95

For more information about this and other amateur science projects, visit the Society for Amateur Scientists's Web site. You may also write the society at 5600, Post Road, #114-341, East Greenwich, RI 02818, call 1-401-823-7800.

Correction: After the December 1997 Amateur Scientist went to press, the rules governing the FINDS prize, renamed the Cheap Access To Space (CATS) prize, were changed. The organizers offer $50,000 for the first group to achieve 120 kilometers altitude and $250,000 for the first to reach 200 kilometers.

The Society for Amateur Scientists (SAS) is a nonprofit research and educational organization dedicated to helping people enrich their lives by following their passion to take part in scientific adventures of all kinds.