Two-Point Calibration of Circular Dichrometer with d-10-Camphorsulfonic Acid

Abstract The circular dichroism (CD) of d-10-camphorsulfonic acid (CSA) in terms of ∊L - ∊R is 2.36 and -4.72 cm2 mmol−1 at 290.5 and 192.5 nm, respectively. The corresponding molar ellipticities, ⊝, are 7,800 and -15,600 deg cm2 dmol−1. A ratio of -2.00 for the negative and positive bands provides a simple two-point calibration. If a standardized spectropolarimeter is available, the ratio of ⊝290.5/M306 = 1.75 to 1.76 or ⊝290.5/(M306−M270) = 0.77 at the extrema is recommended for CD calibration at the 290 nm region, which is unaffected by the presence of optically inactive impurities such as water. Alternately, CSA has a molar absorption coefficient of 34.5 cm2 mmol−1 at 285 nm, which can be used for concentration determination. This leads to (∊L−∊R)290.5/∊285 = 0.068 or ⊝290.5/∊285 = 226. However, CSA must be purified to remove any impurities that are optically active or absorb light or both in the UV region.