Major Urinary Proteins retain folded conformations as gas phase ions under conditions of electrospray ionisation at low or neutral pH values.

Individual MUPs vary in their gas phase stability, despite modest differences in primary sequence and three-dimensional structure.

The disulphide bond in one of these proteins, darcin, is critical to its pronounced stability.

Variation in gas phase behaviour means that quantification of individual MUPs by intact mass phenotyping is restricted to relative quantification.

A QconCAT approach can be used to address individual MUP absolute quantification, and to solve the ‘isoform problem’ for this group of proteins.