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Low-Z elements in aerosol particles Atmospheric aerosol particles that contribute to environmental pollution, such as sulfate, nitrates, ammonium, and carbonaceous particles, contain elements with low atomic numbers, which cannot be characterized by conventional energy-dispersive electron probe X-ray microanalysis (ED-EPMA). Although techniques such as aerosol time-offlight MS have been used to characterize small aerosol particles at the single-particle level, such analyses are only qualitative. Now, Chul-Un Ro and co-workers at Hallym University (Korea), KFKI Atomic Energy Research Institute (Hungary), and the University of Antwerp (Belgium) demonstrate that the quantitative deter-
mination of such chemical species is possible by using ultrathin window EPMA and Monte Carlo simulation combined with successive approximation. To evaluate the new EPMA technique, the researchers characterized standard particles (0.5–5 µm in diameter) containing chemical compounds commonly found in atmospheric aerosol particles. In most cases, good agreement was obtained between nominal and calculated concentrations using the iterative simulation approach. The technique was also used to characterize real atmospheric particles collected on the University of Antwerp campus. Among the 900 particles analyzed, about 80 different types were identified based on their chemical com-
positions, including “pure” particles containing only one major chemical species and internally mixed particles containing two or more chemical species. However, there are limits to the technique. When several chemical species exist in a single particle, such that the number of equations is less than the number of chemical species, quantitative analysis using the new EPMA approach breaks down. However, because many atmospheric particles contain only one or two chemical species, the technique could provide more details about airborne particles of environmental interest. (Environ. Sci. Technol. 2000, 34, 3023–3030)
Gas-phase circular dichroism Measuring a chiral compound’s optical
dichroism and circular birefringence in
ports the specific rotation for several
rotation of light is one of those fundamen-
the gas phase. They find that the specific
compounds, including a- and b-pinenes,
tal techniques taught to nearly all under-
rotation values and even the sign can dif-
limonene, and fenchone. All of the vaues
graduate chemistry majors. But those
fer from gas phase to solution.
vary in complex ways from gas phase
measurements are always run in solu-
The cavity ring-down polarimeter
to solution. For example, (S)-propylene
tion, and the solvent affects the results.
uses 355-nm light for measurements
oxide is 10.2 in the gas phase but 226.4
Patrick H. Vaccaro and colleagues at
at room temperature, and it is sensitive
in solution. The authors point out that,
Yale University adapt a cavity ring-down
enough to collect data on compounds
with improvements in the system’s de-
system to the measurement of circular
at pressures of ~1.5 Torr. The paper re-
tection limit, the polarimeter could be used as a GC detector. (J. Phys. Chem.
Keplerian telescope for spatial filtering and mode matching
Circular polarizer
Ring-down cavity
Circular polarization Imaging lens analyzer
A 2000, 104, 5959–5968) Parallel channel detector
l/4
l/4
l/4
Region sensitive to optical activity
576 A
A N A LY T I C A L C H E M I S T R Y / S E P T E M B E R 1 , 2 0 0 0
l/4 Perpendicular channel detector
Schematic of a cavity ring-down polarimeter. Light from the output mirror is imaged on identical photodetectors at right angles to monitor orthogonal components of the linear polarization.
