News
GC-rfGDMS for elemental speciation studies Glow discharge techniques, performed at low powers and low pressures, have achieved subpicogram detection levels, making these techniques attractive for studying elemental speciation. The reduced pressures and low-power plasmas provide fragmentation spectra that can identify and quantitate organometallic compounds at ultratrace levels. J. A Caruso and colleagues at Shepherd Color and the University of Cincinnati have performed a preliminary study using an rf-GD source as a detector for tetraethyltin (YET) and tetrabutyltin (YET) separated by GC. The rf incident power and GD cell were optimized for S/N based on the peak heights of the two compounds. The researchers found that powers 40 W caused significant heat generation on the direct insertion probe. Using operating conditions of 30 W and 0.64 mbar, they obtained detection limits of ~ 1 pg, near the lowest reported in the literature using other instrumentation. By coupling the GD cell with 3 nieiss spectrometer they also able to obtain structural information on each peak (J. Anal At Spectrom 1996 11 491-96)
The nose knows
Nature is still the best teacher. In this case, David R. Walt and colleagues at Tufts University and the Tufts School of Medicine have developed an "artificial nose" using a multianalyte fiberoptic sensor array that is modeled directly on the olfactory system. The array produces complex, time-dependent signals that provide a "signature" for each analyte following a model for vertebrate odor detection that involves response patterns widely distributed in space and time. The researchers constructed a bundle of 19 multimode optical fibers. Each fiber is coated with the solvatochromic dye Nile Red immobilized within a different polymer matrix having a different baseline polarity, Schematic of odor-sensing process showing 19 optical fibers and optical packed together and incorporated into an illumination and changes in the detection system. Fluorescence responses are plotted versus dyes on time. (Adapted with permission of Macmillian Magazines..
A close look at metal film formation
Mass spectrum for 5-ng injections of (a) TET and (b) TBT showing compound fragments produced in the GD. Both spectra show the elemental tin isotope peaks (hi/z = 120) and groups of peaks that correspond to methylated (mlz = 135) and ethylated (m/z = 150) fragments. .Adapted with hermission no the Royal Society of Chemistry.) 594 A
to organic vapors were monitored. Whereas other reported artificial noses respond with a single parameter, this system provided several response parameters, including intensity changes, fluorescence lifetime and wavelength. Video images of the sensor array's complex temporal responses to various analytes were used to train a neural network for vapor recognition. The network accurately identified different vapors at various concentrations. (Nature 1996 382 697-700)
Recent work has shown that elemental lithium and sodium anodes can be used in an LiCl- or NaCl-buffered A1C13/ l-ethyl-3-methylimidazolium chloride (EMIC) molten salt electrolyte. (In these experiments, a buffered melt forms when an excess of LiCl or NaCl is added to AICI3/EMIC melts with a molar ratio of greater than 1:1.) Moreover, the addition of 20- to 70-mM thionyl chloride to either LiCl or NaCl buffered melts promotes the reversible deposition and stripping behavior of the alkali metal with high efficiencies. J. Fuller and R. T. Carlin of Covalent Associates and R. A. Osteryoung of North Carolina State University have performed a series of electrochemical experiments and optical microscopy studies of the electrode surface at various thionyl chloride concentrations to better understand the alkali metal deposition and passivation. They found that the lithium deposit is dendritic in nature and fails to form a uni-
Analytical Chemistry News & Features, October 1, 1996
form film on the tungsten electrode. The sodium deposits, on the other hand, form as a uniform flat film with little or no dendritic growth and undergo complete stripping from the electrode without leaving dendritic or disconnected sodium metal on the surface. (J. Electrochem. SocS 1996,143, L145-47)
Lithium deposit on electrode at 200x in the presence of 106-mM thionyl chloride. (Adapted with permission of The Electrochemical Society.)
