Ammonium d-10-camphorsulfonate (ACS) is a NIST traceable standard that has been the CD scale calibration standard for over 30 years. ACS has two well-defined peaks in both the near- and far-UV, which makes it an excellent calibration standard since protein structure studies are measured in the same spectral region.

Solution preparation is quick and easy since ACS is a non-hygroscopic material and is stable for an extended period of time when refrigerated.