What is the purpose of the Q spin filters used in your HRP kit?

These are ion exchange columns with a positive charge on the resin surface. The pH of one of the solutions in the kit is used to alter the charge on the conjugated antibody – above the pI of the antibody, the antibody-HRP complex will have more negative charges, and will bind to the resin. At this pH, the HRP is still positively charged (a higher pI) and will pass through the column, thereby separating the free HRP from the Ab-HRP conjugate. Then, the pH is changed again, below the pI of the antibody, thereby increasing the positive charge on the Ab-HRP conjugate, such that it passes through the column. The same strategy is used in the PE kit.