Beat Krattiger

A Silicon Flow Cell for Optical Detection in Miniaturized Total Chemical Analysis Systems

Abstract The applicability of silicon micromachining to the fabrication of a small-volume flow cell for UV-visible absorption detection is demonstrated. With volumes ranging from 1 to 100 nl and lengths of 1 and 5 mm, this type of cell has a long path length relative to its volume. Light is transported through the cell by means of a series of reflections, so that the optical path length may be increased to values beyond the actual cell length, depending on the input angle of the light. Preliminary experiments using a 1 mm, 15 nl cell to measure dye-containing solutions demonstrate an application of multireflection to the measurement of absorbance.

Holographic refractive index detector for application in microchip-based separation systems

A novel detection scheme for capillary electrophoresis on planar glass microchips is presented. The application of a holographic-based refractive index detector to the electrophoretic separation of carbohydrates is described. The microchip device consists of a cyclic (square) separation channel having a circumference of 80 mm, a width of 40 µm and a depth of 10 µm. The volume of the injection scheme is approximately 16 pl. Separation and refractive index detection of a mixture of sucrose, N-acetylglucosamine and raffinose, each at a concentration of 33 mM, was achieved within 17 s of injection. Preliminary results demonstrate the feasibility of using hologram-based refractive index detectors in microchip separation systems. Although the initial detection limits are poor in comparison with alternative techniques, the potential of a universal detector of this kind is clear.

The pigtailing approach to optical detection in capillary electrophoresis

A novel approach, called pigtailing, is presented for the construction of optical detectors for capillary electrophoresis. Optical components and procedures from other fields, mainly telecommunications, are incorporated into these devices to give miniaturized detection systems. Suitable light sources for the construction of pigtail absorbance, fluorescence, refractive index and thermo-optical detectors are light-emitting diodes (LEDs) and laser diodes. The best optical components are gradient-index lenses, optical fibers or diffractive optical elements. These components are joined to the capillary with refractive-index-matching materials to avoid refraction and reflections at the optical interfaces and to reduce mechanical vibrations. These joints also facilitate fast thermal equilibrium. The performance of absorption detectors depends mainly on the brightness of the selected LEDs. Two types of refractive-index capillary detectors are described: one features a single-mode polarization-preserving fiber whereas the second uses a customized holographic plate as the main optical element.

Laser-based refractive-index detection for capillary electrophoresis: ray-tracing interference theory

The fringe pattern observed in a far field after a laser beam illuminates a fused silica capillary immersed in a refractive-index matching material and filled with an analyte fluid is exploited to develop a sensitive optical detector for capillary chemical analysis. The inner capillary interface splits the laser beam into a reflected beam fan and a refracted beam fan, which, on overlapping in the far field, lead to interferences. The intensity and the position of the fringes for capillaries with 250 microm >/= i.d. (inner diameter) >/= 25 microm are well reproduced by the presented model. The calculation predicts the fringe pattern for various beam/i.d. geometric configurations and is used to optimize the performance of the nanoliter-picoliter refractive-index on-column detection studied. It is found that the best contrast corresponds to a capillary that is illuminated with a beam waist of omega(0) ~ i.d./12, which is off-center focused with an offset of s ~ i.d./2. For a given interference pattern, the fringes that are found to be more sensitive to Deltan are those that appear near the optical axis but still retain high intensity and contrast. The sensitivity increases approximately linearly with the fringe number, and the maximal fringe number increases proportionally with the i.d.

Thermo-optical absorbance detection of native proteins separated by capillary electrophoresis in 10 μm I.D. tubes

Abstract Thermo-optical absorption (TOA) detection of native proteins separated by capillary electrophoresis is demonstrated in 10 μm I.D. tubes. The eluents were on-column pumped at 257 nm and probed by a hologram-based refractive index detector. The use of 10 μm capillaries allowed fast 2-min separations. Slower separations in wider tubes led to limits of detection (LODs) of 7·10−9 M for bovine serum albumin. These LODs are comparable to those obtained with laser induced fluorescence and two orders of magnitude lower than in absorbance detection. Since native fluorescence of proteins is rare, TOA detection appears as a more universal detection scheme and thus suitable for other proteins having smaller amounts of fluorescent amino acids.

Chapter 11 On-Column Refractive Index Detection of Carbohydrates Separated by HPLC and CE

Publisher Summary This chapter reviews the refractive index (RI) detection methods that have been used to analyze carbohydrates separated by capillary high-performance liquid chromatography (HPLC) and capillary electrophoresis (CE). The most popular detection scheme used in capillary based separations is ultraviolet/visible (UV/vis) absorption using conventional lamps as light source followed by laser induced fluorescence (LIF). RI detection is a universal method useful in the mM– μ M range, which can be used with a wide spectrum of mobile phases or buffers. On-column RI detectors can be classified, according to their principle of operation, into deflection or interferometry types. Within the interferometric type, a further distinction can be made depending on the illumination arrangement—namely, the off-axis and the (on-axis) hologram methods. In the hologram method, the RI detector features a laser diode (LD), and as the main optical element, a holographic optical element (HOE) that performs several optical functions resulting in a considerable miniaturization of the detector and an improvement in its sensitivity.