Anyone who has ever worked with antibodies or other proteins has found themself in a situation where they have the protein they need to work with in a buffer incompatible with their reaction, at too low of a concentration, or contaminated by a compound like sodium azide. One of the most common and straightforward ways to perform buffer exchange, concentrate samples or remove low molecular weight contaminates is through dialysis.
Dialysis relies on selective diffusion across a membrane to allow separation of components of a solution, or to perform buffer exchange. The pore size of the membrane determines the molecular-weight cutoff (MWCO), the molecular weight at which the solute is unable to pass through the membrane and is thus retained in the solution. With proper selection of dialysis buffer and MWCO, it is possible to retain your protein of interest and change the buffer in which it is dissolved, increase its concentration in solution or remove unwanted chemical compounds with, theoretically, minimal sample loss.
In practice, dialysis has historically been less straightforward for those inexperienced in the technique. Dialysis tubing is relatively expensive considering that it is normally ordered in lengths sufficient for hundreds of experiments and that it may be necessary to buy multiple types if more than one MWCO is needed. It in most cases, the tubing must be pre-wetted and sterilized before adding your sample. And maybe most importantly, all too many researchers have come into the lab in the morning to find the dreaded empty dialysis tube. The clip that you were positive was securely fastened came undone and your entire protein sample is now dissolved in the five liters of PBS you were dialyzing against!
Pierce’s solution to these problems is to offer single use (more on this later) dialysis cassettes. The dialysis membrane is encased in plastic, almost like a double-paned window, leaving space for your sample between the two sheets of membrane. The sample is added to the cassette via loading ports on the corners accessed using a standard syringe and hypodermic needle. The rigid framework makes the cassettes easy to handle and eliminates the dreaded orange clip. Cassettes are available in various sizes from 0.5 to 30 ml and with multiple MWCOs.
For first time users, dialysis can be a complicated frustrating procedure. With the Slide-A-Lyzer cassettes, even inexperienced users can be comfortable performing dialysis from the first use. Membrane preparation is as simple as pre-wetting with PBS; in our lab, this is done for a few seconds on each side using a squirt bottle kept especially for this purpose. Sample loss is much less likely and normally only occurs when the membrane is accidentally punctured with the hypodermic needle, a problem that is immediately obvious. We have also found that it is possible to save cassettes by draining them completely and allowing the membranes to dry. They can then be re-used the next time a new sample of the same protein needs to be dialyzed.
Overall, this is one of the few products that in practice lives up to its marketing. The cassettes are very useful tools that make dialysis extremely straightforward. If you find the need to perform dialysis, I would highly recommend using the Slid-A-Lyzer as a better alternative than conventional dialysis tubing in most cases.