Other variables that affect PCR are:
- The pH of the buffer which is usually pH 8.3-8.8. This can be increased to pH 9 for long PCR i.e. over 5 Kb;
- The concentration of magnesium ions (needed for the enzyme to work). Usual concentrations of magnesium are 0.5-5 mM.
The best range for a particular set of primers for both these factors may be determined experimentally. In some cases the reaction may not work at all if the conditions are not in the correct range.
In some cases 5% DMSO (dimethyl sulphoxide) or 5% glycerol can be added to improve the efficiency of the PCR and increase the specificity of the primer binding. This does not work in all reactions and may sometimes inhibit the reaction altogether.