Redetermination of the extinction coefficient of camphor-10-sulfonic acid, a calibration standard for circular dichroism spectroscopy.

Camphor-10-sulfonic acid (CSA)1 is routinely used for calibrating circular dichroism (CD) instruments. However, CSA is hygroscopic, so an accurate concentration can be obtained only by measuring its absorbance peak at 285nm. Hence, calibrations are highly dependent on having a correct value for the extinction coeYcient for this compound. Following an extensive search of the literature, the origins of the commonly accepted value for the molar extinction coeYcient for CSA of 34.5 M¡1 cm¡1 at 285 nm can be ascribed to Chen and Yang [1], who cited Lowry and French [2], although Lowry and French reported the value at 284 nm to be 35 M¡1 cm¡1, following measurements at 286 and 292 nm [3]. (There was actually no data point at the peak maximum.) The value of 34.52 M¡1 cm¡1 at 285nm is reported in instrument manuals without reference, usually quoted in the form of a 1 mg/ml solution producing an absorbance of 0.743 in a 5-mm cell (also see [4]). Recently, more stringent methods of standardizing the calibration of conventional and synchrotron radiation CD instruments have been developed [5]. Considering that CSA is the most widely used standard for calibration and cross-correlation for both types of CD instruments, we decided to re-examine the measurement of the extinction coeYcient of CSA, concentrating on reproducibility (error levels) and measurements at the actual peak maximum.