There are a number of common formats for performing gel filtration for smaller (less than 4mL) volumes:
* Chromatography columns
* Gravity-flow columns
* Chromatography cartridges
* Centrifuge columns
* Centrifuge plates
Gravity-flow, or drip, columns use head-pressure from a buffer-chase to push the sample through the gel filtration matrix. Sample is loaded into the top of an upright column and allowed to flow into the resin bed. The sample is then chased through the column by adding additional buffer or water to the top of the column. During this process, small fractions are typically collected and each is tested for the macromolecules of interest. In some cases, several fractions might contain the protein and may have to be pooled to improve yield. In order to eliminate the time and monitoring assorted with drip columns, fractions often equal to the full exclusion volume of the column are collected regardless of sample volume resulting in significant dilution of sample.
Sealed chromatography cartridges or columns work similarly except the sample and buffer is pumped into and through the resin by an external device such as a liquid chromatographic (LC) system, also requiring collection and monitoring of several fractions. Even though this method is often semi-automated, using chromatography cartridges is typically limited to processing one sample at a time and some sample dilution from the chase buffer is still likely to occur.
To eliminate sample dilution and the collecting and monitoring of fractions, centrifuge column or plate -based gel filtration, also referred to as spin desalting, methods are commonly used. Spin desalting is unique in that a centrifuge is used to first clear the void volume of liquid in the resin, followed by sample addition and centrifugation. After centrifugation, the macromolecules in the sample have moved through the column in approximately the same initial volume, but the small molecules have been forced into the pores of the resin and replaced by the buffer that was used to pre-equilibrate the gel-filtration matrix. Spin columns and plates eliminate the need to wait for samples to emerge by gravity flow and require no chromatography system, allowing for multiple-sample processing simultaneously.