Facile hyphenation of gas chromatography and a microcantilever array sensor for enhanced selectivity

The very simple coupling of a standard, packed-column gas chromatograph with a microcantilever array (MCA) is demonstrated for enhanced selectivity and potential analyte identification in the analysis of volatile organic compounds (VOCs). The cantilevers in MCAs are differentially coated on one side with responsive phases (RPs) and produce bending responses of the cantilevers due to analyte-induced surface stresses. Generally, individual components are difficult to elucidate when introduced to MCA systems as mixtures, although pattern recognition techniques are helpful in identifying single components, binary mixtures, or composite responses of distinct mixtures (e.g., fragrances). In the present work, simple test VOC mixtures composed of acetone, ethanol, and trichloroethylene (TCE) in pentane and methanol and acetonitrile in pentane are first separated using a standard gas chromatograph and then introduced into a MCA flow cell. Significant amounts of response diversity to the analytes in the mixtures are demonstrated across the RP-coated cantilevers of the array. Principal component analysis is used to demonstrate that only three components of a four-component VOC mixture could be identified without mixture separation. Calibration studies are performed, demonstrating a good linear response over 2 orders of magnitude for each component in the primary study mixture. Studies of operational parameters including column temperature, column flow rate, and array cell temperature are conducted. Reproducibility studies of VOC peak areas and peak heights are also carried out showing RSDs of less than 4 and 3%, respectively, for intra-assay studies. Of practical significance is the facile manner by which the hyphenation of a mature separation technique and the burgeoning sensing approach is accomplished, and the potential to use pattern recognition techniques with MCAs as a new type of detector for chromatography with analyte-identifying capabilities.     

The `Prelude' ATD experiment: assessments and future prospects

The Prelude experimental model represents a complete network which can integrate any bit rate from data to moving pictures by an asynchronous time-division (ATD) technique. To get reliable and full assessment of ATD, the model is made up of a switch, a concentrator, two home networks, and terminals. The project started in 1982 and was completed in the middle of 1987. The success of the experiment proves the feasibility of ATD, which is now agreed on as a possible solution for future ISDN (integrated-services digital network) use. The authors address the results derived from the experiment and analyze the trends that can be expected for the future. First, an outline of Prelude is given to describe the state of the arc reached within the experiment. Each element of the system is described with special emphasis on the switching matrix, as its optimized and efficient implementation could lead to short-term industrial developments. Then a thorough assessment of the whole system is given from which future evolutions are most likely to be derived. Possible ATD fields of application are given     

Herbicide accumulation and evolution in reservoir sediments

The aim of the present study was to understand the effect of reservoir configurations on sediment pesticide fate. Two dams were selected on the River Garonne, in southwest France: Carbonne and Golfech, both with reservoirs subject to accumulation of herbicide-contaminated sediment. They are situated upstream and downstream respectively of an agricultural and urban area: the Mid-Garonne. The results presented include pesticide concentrations and C/N ratios in the smaller sediment particles (< 2 mm) and values of oxygenation and herbicide concentrations in the water.The dynamic behaviour of sediment in the reservoirs is discussed. The present study shows that the theoretical lifespan (weak remanence in vitro) and the results actually observed in the sediment are conflicting. Pesticide contamination in Carbonne indicates conservation, even accumulation, of herbicide molecules while in Golfech transformation processes clearly dominate. The hydromorphological position of Golfech reservoir, i.e. located at the junction of two rivers with contrasting hydrological regimes and very different oxygenation conditions, leads to accelerated pesticide desorption or degradation. Unfortunately, this configuration is rare.     

On-column surface-enhanced Raman spectroscopy detection in capillary electrophoresis using running buffers containing silver colloidal solutions

Direct on-column surface-enhanced Raman spectroscopy (SERS) detection is demonstrated in capillary electrophoresis (CE). Distinctive SERS spectra of two test compounds, riboflavin and Rhodamine 6G, are obtained in 100 microm i.d. fused-silica capillaries under CE conditions using running buffers that contain silver colloidal solutions. Detection is performed using an unmodified commercial Raman spectrometer in a confocal microscope mode of operation. The effects of laser power, wavelength, spectra acquisition time, silver colloidal concentration, and applied voltage (i.e., flow rate) on the quality of SERS spectra are evaluated. Using laser powers of 17 mW (at the sample) at 515 nm and employing 1 s spectral acquisition times, spectra with bands exhibiting signal-to-noise ratios greater than 10 could be obtained for 1.0 x 10(-6) M riboflavin and very low nanomolar concentrations of Rhodamine 6G. This was accomplished without optimization of silver colloidal solution compositions and by using a low-throughput spectrometer. Incorporation of the colloidal solutions into running buffers is shown to have little effect on the separation of the test compounds as monitored using a laser-induced fluorescence instrumental scheme. However, SERS spectra degrade if the capillary is not rinsed between experiments. Riboflavin and Rhodamine 6G spectra are obtained on-the-fly for actual CE separations. In the case of the latter solute, the injected quantity was approximately 90 amol.