Hologram-based refractive index detector for capillary electrophoresis: separation of metal ions

A novel refractive index detector suitable for capillary separations is presented and demonstrated in the capillary electrophoretic separation of metal cations in 10-micrometer-i.d. tubes. The principle of the detector is interferometric; it features a laser diode and, as the main optical element, a holographic optical element which performs several optical functions allowing the use of capillaries with inner diameters as small as 5 micrometers. In contrast to the 'off-axis' method, the use of holographic plates permits probing of the capillary through its center, where the optical path is larger and diffraction effects are smaller. With an i.d. = 10 micrometers capillary the detector resolves 2 microns RIU and covers a linear dynamic range of three to four decades. Joule heat effects preclude the detector to operate at its maximum sensitivity which is delivered in the absence of electrical fields. For this reason, the use of smaller inner diameter capillaries and buffers having low conductivities is recommended. Under field-amplified sample injection conditions, the detection limits for the studied cations are in the low nanomolar range. The performance of this detector is compared with indirect UV-absorption detection, which also helps to elucidate the mechanisms leading to refractive index changes in ionic solutions.