Ion spray goes parallel conventional IX now rates or i I mL/ null are problematic in 1^1^/ IVLJ because of trie inability of the vacuum system to remove the evaporated eluent. Thermospray and atmospheric pressure chemical ionization methods can be useci to allow the introduction of the total ix> elfluent without using a postcolumn split or Experimental setup for LC/MS using a parallel ion spray. (Adapted with permission micro LC techniques. While performof John Wiley & Sons.) ing ion spray experiments, K. Hiraoka and colleagues at Yamanashi University (Japan) found another way to perform The authors found that when the capLC/MS with ion spray using conven- illary for the ion spray was positioned tional LC flow rates by changing the parallel to the sampling orifice, they position of the capillary. could conduct the experiment at sam-
Chemical enhancement in SERS Although surface-enhanced Raman spectroscopy (SERS) has been studied and used as an analytical technique, the microscopic origins of the electromagnetic and chemical enhancements are not quantitatively understood. Examination of the chemical enhancement mechanism has been difficult because most SERS studies are conducted with roughened surfaces in which both above mechanisms contribute simultaneously. Alan Campion and colleagues at the University of Texas at Austin reported the preliminary results of a study on the chemical
hancement mechanism using a smooth, atomically flat, single-crystal of pyromellitic dianhydride (PMDA) adsorbed on Cu(lll). This compound was chosen because the electromagnetic contribution is small and well-understood. Chemical enhancement is thought to occur because of resonance Raman scattering via new electronic excitations of adsorbed molecules, which serve as resonant intermediate states. Direct spectroscopic detection of new charge transfer resonances would help confirm this and also allow quantitative testing of the proposed mechanisms. Raman spectra of PMDA indicated chemical enhancement, which ruled out electromagnetic enhancement, and electronic absorption spectra obtained by high-resolution electron energy loss spectroscopy (HREELS) was used to obtain evidence of new charge transfer Although the searchers have not yet mapped the frequency dependence of the Raman scattered intensity their observations provide evidence that what is being. seen is resonance Raman scattering via the new intermediate state: The scattered intensity observed when exciting near the peak of the absorption is much greater than when exciting
to the red of the maximum, and the observed enhancement factor roughly agrees with the expected value if the scattering process is J
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HREELS spectra of PMDA on Cu(111).
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ple flow rates up to 4.4 mL/min without any difficulty. They also achieved better detection limits because of stronger ion intensities, and there was little contamination from the sample solution because the larger droplets were entrained in the nebulized gas and were carried outside the ion-sampling system, leaving only the fine, highly charged droplets to be sampled into the analyzing system. The capillary position was not critical to the ion intensities measured, and a corona discharge did not cause a sudden decrease in the ion intensities. The parallel configuration was also found to be superior in electrospray experiments. (Rapid Commun Mass Spectrom 1995 9,1349-55)
A carbon nanotube electron gun Standard electron guns, such as those used in cathode ray tubes, generate electrons thermoionically from hot tungsten and heated materials with low work functions. Other electron guns are based on field emission from sharp tips and are used for producing monochromatic electron beams. These electron sources typically require ultrahigh vacuums and high voltages; currents are limited to several microamperes. Carbon fibers also emit electrons and do so under less stringent
Schematic of the carbon nanotube electron source. The emitting surface (a) consists of ^-aligned carbon nanotubes anchored on a polytetrafluoroethylene substrate. The nanotubes are oriented perpendicular to the substrate surface. A perforated mica sheet (b) with a 1 -mm-diameter hole is bonded to the nanotube film; the hole is covered with a 3-mm-diameter, 50% transmitting, 200-mesh copper grid. (Adapted with permission of the American Association for the Advancement of Science.)
Analytical Chemistry News & Features, February 1, 1996 81 A
