To obtain accurate and consistent measurements from circular dichroism (CD) instruments over time and from different laboratories, it is important that they are properly calibrated. The characteristics of the available reference materials are not ideal to ensure proper calibration as they typically only give peaks in one or two spectral regions, and often have issues concerning purity and stability. Currently either camphor sulfonic acid or ammonium camphor sulfonate are used. The latter can be an unstable, slightly hygroscopic secondary standard compound with only one characterized CD band. The former is the very hygroscopic primary standard for which only one enantiomer is readily available. We have synthesized a new reference material for CD, Na[Co(EDDS)]· H2O (EDDS = N,N-ethylenediaminedisuccinic acid) which addresses these problems. It is extremely stable and available in both enantiomeric forms. The CD spectrum of Na[Co(EDDS)]·H2O has nine distinct peaks between 180 and 599 nm. It thus fulfils the principal requirements for CD calibration chemical standards and has the potential to be used to ensure good practice in the measurement of CD data, providing two spectra of equal magnitude and opposite sign for a given concentration and path length. We have carried out an interlaboratory comparison using this material and show how it can be used to improve CD comparability between laboratories. A fitting algorithm has been developed to assess CD spectropolarimeter performance between 750 and 178 nm. This could be the basis of a formal quality control process once criteria for performance have been decided.
- Circular dichroism