In this contribution, a capillary electrophoresis microdevice with an integrated on-chip contactless four-electrode conductivity detector is presented. A 6-cm-long, 70-microm-wide, and 20-microm-deep channel was etched in a glass substrate that was bonded to a second glass substrate in order to form a sealed channel. Four contactless electrodes (metal electrodes covered by 30-nm silicon carbide) were deposited and patterned on the second glass substrate for on-chip conductivity detection. Contactless conductivity detection was performed in either a two- or a four-electrode configuration. Experimental results confirmed the improved characteristics of the four-electrode configuration over the classical two-electrode detection setup. The four-electrode configuration allows for sensitive detection for varying carrier-electrolyte background conductivity without the need for adjustment of the measurement frequency. Reproducible electrophoretic separations of three inorganic cations (K+, Na+, Li+) and six organic acids are presented. Detection as low as 5 microM for potassium was demonstrated.
Microchip capillary electrophoresis (CE) with integrated four-electrode capacitively coupled conductivity detection is presented. Conductivity detection is a universal detection technique that is relatively independent on the detection pathlength and, especially important for chip-based analysis, is compatible with miniaturization and on-chip integration. The glass microchip structure consists of a 6 cm etched channel (20 microm x 70 microm cross section) with silicon nitride covered walls. In the channel, a 30 nm thick silicon carbide layer covers the electrodes to enable capacitive coupling with the liquid inside the channel as well as to prevent interference of the applied separation field. The detector response was found to be linear over the concentration range from 20 microM up to 2 mM. Detection limits were at the low microM level. Separation of two short peptides with a pI of respectively 5.38 and 4.87 at the 1 mM level demonstrates the applicability for biochemical analysis. At a relatively low separation field strength (50 V/cm) plate numbers in the order of 3500 were achieved. Results obtained with the microdevice compared well with those obtained in a bench scale CE instrument using UV detection under similar conditions.
The dynamic structure factor S(q, col of electrons in highly oriented pyrolytic graphite for q~~c and qlc with 0.37 & q (2.06 a.u. was measured at 0.8 eV resolution by inelastic scattering of synchrotron x radiation. The energy-transfer regions both of interband transitions and of core excitations were investigated. By means of Kramers-Kronig transformation Re[e(q, cv)] and Im[e(q, tv}] were obtained, where peaks of the latter were attributed directly to maxima of projections off the joint density of states achieved by both dipole selection rules and q-dependent matrix elements. Making use of both x-ray emission and photoemission data, limits for the energy positions of some conduction bands could be established by means of the Im[e(q, to)] data. As far as weakly-k,dispersing bands are concerned, their energy positions were found to be in good agreement with recent band-structure calculations. On the other hand, the measured energy ranges of the bands that could be identified as belonging to the recently proposed interlayer states disagree with most of the relevant band-structure calculations, though they are not far from the results of other spectroscopies. The strict bulk origin of the inelastic-scattering-based band-structure information obtained is stressed. The experimental data are also discussed in view of the interpretation of previous lowresolution inelastic-scattering data in terms of correlation-induced fine structure for qlc.
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