Quantitation of functional groups on solid supports

A method for quantifying target functional groups immobilized on a solid support comprising one or more paramagnetic particles, the method comprising the steps of:

• (a) contacting the target functional groups immobilized on the one or more magnetic paramagnetic particles with an amount of a selective compound, wherein an amount of the selective compound binds directly to the target functional groups and forms bound selective compound, and an amount of the selective compound does not bind to the target functional groups and is present as unbound selective compound, wherein the target functional groups comprise aldehydic functional groups, and wherein the selective compound comprises propylamine;
• (b) obtaining a byproduct having an amount of the unbound selective compound;
• (c) adding an indicator to the byproduct which binds to unbound selective compound in the byproduct to provide a colorimetric result, wherein the indicator comprises a solution comprising 2,4,6-trinitrobenzene sulfonic acid (TNBSA);
• (d) measuring the colorimetric result;
• (e) determining from the measured result an amount of unbound selective compound in the byproduct;
• (f) determining an amount of bound selective compound immobilized on the one or more paramagnetic particles from the amount of unbound selective compound;
• and
• (g) determining from the amount of the immobilized selective compound an amount of the target functional groups immobilized on the one or more paramagnetic particles;
• and
• (h) reacting the paramagnetic particles with an effective amount of a first member of a binding pair to attach the first binding pair member to the paramagnetic particles, wherein the effective amount of the first binding pair member is determined based upon the amount of target functional groups immobilized on the paramagnetic particles determined in step (g), and wherein the second binding pair member is a target analyte in a sample.