Wavelength calibration method for a CCD detector and multichannel fiber-optic probes

A wavelength calibration method for a charge-coupled device (CCD) array detector and seven-channel fiber-optic spectrograph is described. The method was developed for the extraction of wavelength-calibrated spectra for in situ dissolution testing of tablets. The method includes automatic recognition of the positions of the seven fiber channels and recognition of mercury line positions in pixel numbers. A wavelength calibration model with two trigonometric terms was used for least-squares fitting of horizontal pixel numbers to the known wavelengths of five lines from a low-pressure mercury discharge lamp. Four other models were used for comparison. The standard error of estimate (SEE) was minimum for the model with two trigonometric terms. Each fiber channel was calibrated separately. After least-squares fitting, linear interpolation was used to obtain the wavelength-calibrated spectra in the range of 190-450 nm at 1-nm intervals. With this method, spectra from seven fiber-optic probes can be acquired simultaneously and rapidly for in situ dissolution testing. The wavelength calibration procedure is good enough to permit solution spectra acquired on one channel to be used for multivariate calibration of the other channels under appropriate circumstances.