QIAquick Kits contain a silica membrane assembly in a spin column for binding of DNA in high-salt buffer and elution with low-salt buffer or water. The purification procedure removes primers, nucleotides, enzymes, mineral oil, salts, agarose, ethidium bromide, and other impurities from DNA samples. Silica-membrane technology eliminates the problems and inconvenience associated with loose resins and slurries. Specialized binding buffers are optimized for specific applications and promote selective adsorption of DNA molecules within particular size ranges.
DNA bands resolved on agarose gels (low melting point agarose is not needed, normal agarose will do) can be cut out and dissolved in QG buffer. QG buffer contains a pH indicator which allows monitoring any change in pH that would ultimately affect the binding of DNA to the silica membrane. Optimal pH shows yellow color, but if the pH is higher, the solution may turn violet in color. This problem can be solved by adding 10 ul of 3 M sodium acetate. Agarose should be kept at 50ºC with intermittent vortexing until completely dissolved.
Care should be taken while cutting the DNA band from the agarose gel so as to minimize the amount of agarose. Use of clean and alcohol-wiped instruments is suggested to avoid any contamination from external sources. PCR amplified fragments can be applied directly onto the column without going through agarose gel electrophoresis. The whole PCR reaction mixture can be applied directly on the column and rest of the protocol remains same.
I have used this kit for the past 7 years with no problem ever. I have used it primarily for purifying vectors and inserts to be ligated and cloned. I have worked with plasmids in the range of 2 kb to 13 kb. Though the yields were never to my level of satisfaction, the kits ease-of-operation and the purity of DNA obtained surpasses those shortcomings. I have used kits from Sigma, Bio-Rad and GE Healthcare (formerly Amersham) for the same purpose, but none of them matches Qiagen. Though I have never needed to perform multiple purifications, columns can be used on a vacuum manifold for purifying multiple DNA fragments in one go.
Another concern is about the maximum size of the plasmid that can be purified. Plasmids only up to 10 kb can be purified. But I have successfully used this kit to purify 13 kb fragments also.
It would be nice if the columns were larger. The 1 minute centrifugation steps are very convenient, but the inability of the columns to handle volumes greater than 800 ul is a restriction. Though volumes more than 800 ul can be loaded onto the columns by repeated centrifugations, the job would have been much easier if columns were designed to handle larger volumes.
The DNA purified from this kit is suitable for all practical molecular biology applications like sequencing, Southern blotting, microarray analysis, cloning, labeling and PCR.
I would recommend this kit for all molecular biology labs.