News
GC, supersonic beam UV, and TOFMS combine for 3-D t r a c e analysis Although GC/MS is routinely used for trace analysis of chlorinated aromatic compounds in environmental samples, it can be time consuming and involve complex cleanup procedures as well as long chromatographic run times. U. Boesl and colleagues at the Technische Universitat Miinchen (Germany) and the GSF-Forschungszentrum fur Umwelt und Gesundheit (Germany) have developed a threedimensional method that uses GC retention time, UV absorption, and mass to determine trace levels of chlorinated aromatic hydrocarbons with little
preserves the resolution and selectivity of both the capillary gas chromatographic and the UV laser spectroscopic analysis. Preliminary results indicate that the sensitivity of the method is better than 500 fg for toluene, and the researchers believe that an improvement in sensitivity of 2 orders of magnitude is possible. Isomerselective multiphoton ionization of large molecules such as PAHs, chlorotoluenes, and chloroanthracenes provides sufficienl selectivity to eliminate cleanup procedures and allows medium-resolution fast GC to be used. They also believe that by using smaller mechanical, electrical, and optical components a mobile instrument for use in the field is a possibility (Eur Mass Spectrom 1199 1 341-51)
C\X" X\C\
cleanup The researchers developed a pulsed valve that acts as a GC/MS interface and as a source for supersonic beam UV spectroscopy. Used in conjunction with a resonant enhanced multiphoton ionization source and a reflectron T^O F TTl 3 . s s 3.T1 3~
lyzer the pulsedvalve interface
Experimental setup for combination of GC, Jet-UV, and TOFMS using the pulsed valve as an interface. (Adapted with permission of IM Publications.)
A giant
photoacoustic effect A photoacoustic effect with an amplitude so large that a crackling noise can be clearly heard has been discovered by Huxiong Chen and Gerald Diebold of Brown University. The photoacoustic effect occurs when optical radiation produces a thermal expansion of a body which, in turn, generates mechanical motion and launches a sound wave into the surrounding medium. The anomalously large photoacoustic effect found by the authors takes place when an aqueous suspension of 30-nm-diameter carbon particles are ir80 A
radiated with a high-power pulsed laser. They report that the resulting sound amplitude is approximately 2000-fold greater than that produced by a dye solution with an equivalent absorption coefficient. The acoustic signal grows steadily in amplitude with repeated firings of the laser, reaching a maximum at about 3000 shots. Each laser shot imparts 160 mj. Electron micrographs reveal that the repeated irradiations form shell-like carbon structures with diameters from 30 to 400 nm. The researchers believe that the large photoacoustic response is due to high-temperature chemical reactions between the surface carbon and the surrounding water. (Sciencee,995,270, 963-66)
Analytical Chemistry News & Features, February 1, 1996
AFM identifies electroless deposition mechanism Although electroless deposition, a wet process used to add a metal layer to surfaces such as ceramics, has been used in industry for more than 50 years, its mechanism has not been conclusively determined. Four separate mechanisms (three chemical and one electrochemical) have been proposed, each with its own supporters. Bing Joe Hwang and Sheng Horng Lin of the National Taiwan Institute of Technology used tapping-mode atomic force microscopy (AFM) to monitor the morphological changes for the electroless deposition of cobalt from hypophosphite solution onto palladium-activated, highly oriented pyrolytic graphite. Tapping-mode AFM was chosen over scanning electron and scanning tunneling microscopies because it sharply reduces deformation or removal of surface features such as a nucleus on a surface If the deposition mechanism is chemical in nature, electroless films deposited on conductive noncatalytic substrates do not form new nuclei; however, if it is electrochemical in nature, some new nuclei are probably generated. Hwang and Lin observed the nucleation and growth during electroless deposition and compared the results with the four proposed mechanisms to uncover the reaction mechanism. They observed that new nuclei are formed between the original nuclei during the deposition; thus, the mechanism of electroless deposition is electrochemical in nature. The palladium particles acted as the catalytically active anode where oxidation of the hypophosphite occurred; the pyrolytic graphite substrate acted as the cathode (J. Electrochem Soc o995 142 3749-54)
Electrochemical mechanism for electroless deposition on a conductive noncatalytic substrate. (Adapted with permission of The Electrochemical Society.)
