A charge coupled device array detector for single-wavelength and multiwavelength ultraviolet absorbance in capillary electrophoresis

A fundamental limitation to the use of single-point absorbance detection fo r capillary electrophoresis is irradiance, since it is not possible to crea te an image at the detection point on capillary that is brighter than the l ight source. This limitation may be overcome by illuminating a length of th e capillary using a fiber-optic bundle and using a charge coupled device (C CD) camera that can image the full length of the illuminated zone. The pres ent paper describes design and development of a CCD detector for UV absorba nce that can be used in both multiwavelength and single-wavelength modes. T he CCD camera images analyte peaks in the capillary dimension, together wit h wavelength-resolved absorbance in the dimension perpendicular to the capi llary. Successive snapshots of the peaks are added together, after appropri ate correction for time-dependent peak displacement, without sacrificing sp atial resolution. Measured baseline rms noise values at 200 mu are 34 mu AU using a holographic grating in multiwavelength mode and 8 mu AU with the a ddition of a band-pass filter. Both values are in excellent agreement with calculations of limiting shot noise. Performance in multiwavelength mode is constrained by the 470-ms readout time of the CCD used, which sets a maxim um duty cycle of 2.3%, Noise contributions from source intensity fluctuatio ns are reduced by using a portion of the CCD image to provide a baseline re ference signal. With 4-hydroxybenzoate as test analyte, the linear dynamic range in multiwavelength mode is shown to be between 3 and 4 orders of magn itude. High-quality spectra of 2-, 3-, and 4-methylbenzoates are obtained o n capillary and used in deconvolution of closely migrating peaks of the 2- and 3-isomers.