Abstract: A mixture of isolated or synthetic affinity molecules in a liquid carrier is disclosed. The mixture comprises at least two different affinity molecules, each with affinity for a predetermined analyte, for use in a single or multi flow cell piezoelectric crystal micro balance apparatus. Each isolated or synthetic affinity molecule forms together with the predetermined analyte an interaction pair selected from the group consisting of anion-cation, antibody-antigen, receptor-ligand, enzyme-substrate, oligonucleotide-oligonucleotide with complementary sequence, oligonucleotide-protein, oligonucleotide-cell, and peptide nucleic acid (PNA) oligomer-polynucleotide, wherein the polynucleotide may be selected from the group consisting of RNA, DNA and PNA polymers complementary to PNA oligomer. Use of the mixture for introduction into the liquid flow of a single or multi flow cell piezoelectric crystal micro balance apparatus is also described, as well as a kit containing the mixture.