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

Abstract

A novel refractive index detector suitable for capillary separations is presented and demonstrated in the capillary electrophoretic separation of metal cations in 10-itm-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 /*m. 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 /tm capillary the detector resolves 2 juRIU 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.Considering that holographic optical elements (HOEs) could greatly simplify the construction of optical instruments without compromising performance, their use is gaining acceptance in analytical instrumentation such as spectrographs.1-2 The present investigation represents the first application of HOEs to on-column refractive index (RI) detection for the analysis of transparent substances using capillary electrophoresis (CE).Optically dense compounds are most commonly detected by absorbance methods, and when the substances of interest * Author to whom correspondence should be addressed.(1) Tedesco,

Detection in capillary electrophoresis

“…In general, very small phase changes caused by optical path length differences, in response to the analyte, allow for high sensitivity [86,87]. The use of interferometric techniques has produced some impressive results toward measuring RI changes in the small volumes [78,79] characteristic of CE; however, sensitivity dependency on pathlength has persisted. Bruno and co-workers [78] further developed the forward scatter, off-axis technique originally developed by Bornhop and Dovichi [77] for HPLC detection.…”

“…Another technique for detecting changes in n within capillary tubes uses a holographic grating to produce twobeam interference in a forward scatter configuration [79].…”

“…Krattiger and co-workers [79] were able to separate and detect metal ions by CE using a holographic grating, a capillary that is encapsulated in an index matching glue, and a photodiode array wired to produce position-sensitive detection. Manz et al [84] recently reported the application of the holographic forward scatter RI detector for CE on a chip.…”

Carbohydrate Chromatography: Towards Yoctomole Sensitivity

“…The advent of cheap laser optics has seen the development of laser based RI detectors for capillary separation systems (Bornhop et al, 1987;Bruno et af., 1991;Krattiger et al, 1994). which exhibit remarkable sensitivities considering the lack of sensitivity usually associated with RI detection.…”

Simultaneous LIF and retro‐reflected beam interference detection for CE

“…Unfortunately, most analytes need to be derived for their non-fluorescent property under conventional LIF detection [28,29]. For RI detection, being of universal and quantitative analysis, its comprehensive application is still far away because of its unsatisfactory sensitivity and complicated optical train [30][31][32][33][34][35].…”