News
Two-photon absorption by a single quantum system
sity, but at higher intensities, R increased more slowly than P, indicating saturation. Thermal heating alone could not explain the saturation, Optical experiments have been conand they determined ducted on single quantum systems in the past but each transition was the re- that the saturation sult of a single system absorbing a sin- was caused in part by gle photon. In other words, the interac- intersystem crosstion was linear. Urs P. Wild and co- ing to the triplet state. Although the crossworkers at the Swiss Federal Institute of Technology have observed the nonlin- section for twophoton absorption ear interaction of light and single molecules of diphenyloctatetraene (DPOT) was extremely small (6 x 10"45 cm4 s) ,hey that occurs through the absorption of Excitation scan over the inhomogeneously broadened were able to meatwo photons, each with half the energy two-photon absorption band. (Adapted with permission needed for the optical transition from the sure kinetic paramefrom the American Association for the Advancement of ters for the triplet deground state to the lowest excited sinScience.) glet state A Tisapphire laser at 888 nm which were in was used to excite the 444-nm transireasonable apreement tion of the DPOT with the triolet decay sion resulting from the excitation of shorter diphenylpolyenes. They believe that DPOT by two photons. {Science 1996, The count rate R should be propord. i• it, a i tional to the square of the laser inten271,17 03-05) they were observing thefluorescentemis-
PIXE CZE detection Metal-specific detection with capillary separation techniques such as GC, CE, or microbore HPLC could greatly increase the information content of biological separations. The detection volume is so smalll however, that atomic spectroscopy cannot generate an adequate signal. Protoninduced X-ray emission (PIXE) has been used previously as an off-line detection technique, but on-line detection is preferable. C. Vogt and co-workers at the University of Leipzig (Germany) have devel-
Configuration of coupled CZE separation and PIXE detection. 1, buffer reservoirs; 2, power supply; 3, needle valve and pressure regulation system; 4, Pt electrodes; 5, quartz capillary with 7, ion window; 6, Si(Li) detector; and 8, incident ion beam inside the beam tube. (Adapted with permission from Elsevier Science.)
346 A
oped an on-line PEE detector for capillary Monitoring methods and have successfully demoncorrosivity in the strated its capabilities for CZE detection. They compare the sensitivity of the PIXE atmosphere with a detector with traditional direct and indiQCM rect UV detection. To guarantee excitation of the analytes, The increasing use of electronic equipment the capillary thickness must be < 10 um, cre- in polluted atmospheres has increased the ating an "ion window." Two methods were need to monitor atmospheric corrosivity. used to produce the ion windows: laser abla- This is usually accomplished by exposing tion by using a Q-switched ruby laser or small metal plates or "coupons" to the atmoetching of the quartz capillary following the sphere and then taking them back to the removal of 5 mm of the polyimide coating. lab to measure weight gain or loss or caLaser ablation did not provide adequate re- thodic reduction. Problems with this stratproducibility or the ability to control the ion egy, including the amount of time (months window dimensions, so etching was used or years) needed to incur a detectable in all subsequent experiments. Because the change and having to send the samples etching weakens the capillary, it must be back to the lab necessitate concern for the fixed on a silica slice in n 180° arrangement reliability of the equipment C. Leygraf and or on cin scrylic crlsss holder in 3 90° M. Forslund of the Royal Institute of Techrangement The 180° confisfufcition wss nology (Sweden) described a quartz crystal found to be better for metals with X-ravs microbalance used to construct an atmo> 8 keV- ;he opppsite was true for the 90° spheric corrosivity monitor for in situ mass setup measurements and an accompanying moThe PIXE detector measured metals in bile field station the 10"5 M range, which is comparable to The probe consisted of a sensing eleindirect UV detection for some metals, ment inside a filtering device on top of a but is worse than direct UV detection for water-proof casing containing the oscillaall the metals studied. However, the re- tor and supporting electronics, connected sults indicate that an element-specific deto a frequency counter through a coaxial tection scheme for electrophoretic separa- cable. The sensing element was a 9-mm tions is possible. (J. Chromatogr. A 1996, circular AT-cut 35° 14' quartz crystal disk 727,301-10) with a resonance frequency of 6 or 8 MHz.
Analytical Chemistry News & Features, June 1, 1996
