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Pesticide sorption equilibria
Schematic of the solid-inseriion probe. (A) water-cooiing coil, (B) thermocoax heating element, (C) Nichrome plug, (D) ceramic tip, (E) thermocouple, and (F) sample cavity.
Thermal analysis meets MS Because it only takes a slight change in a material's composition to significantly affect how it responds to changes in temperature, thermal decomposition reactions are of great importance in the manufacturing and characterization of materials ranging from ceramics to heterogeneous catalysts. High-resolution thermal analysis techniques are needed to better understand and quantify the effects of various components on solid-state decomposition reactions. P. A. Barnes snd co-workers sa the University of Huddersfield (U.K.) report on the development of a new solid-insertion probe (SIP) mass spectrometer with enhanced resolution, which provides information on the energetics and mechanisms of thermal decomposition reactions. The SIP consists of a cylindrical micro-
DNA condensates Stretch DNA out and you have an amazingly long molecule—in some cases, 100,000 times longer than the cells they are found in. Like the contortionist at the circus, DNA can condense itself into a tight package of chromosomes to fii into its "confined" quarters. Eileen M. Spain and Michele Y. Ono of Occidental College demonstrate that atomic force microscopy (AFM) can provide realtime images of DNA's remarkable compaction process, which should help in understanding the mechanism. AFM offers several advantages, including better spatial resolution than real-time fluorescence techniques.
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furnace located at the end of a water-cooled stainless steel rod, which can be positioned directly in the source chamber of a doublefocusing magnetic-sector mass spectrometer. Temperature is measured via a thermocouple, which is positioned at the base of the sample holder. SIP-MS requires small sample masses (10 ug-10 mg) and operates under high vacuum, leading to reduced temperature and concentration gradients within the sample. The system is capable of conventional linear heating and various sample-controlled thermal analysis (SCTA) techniques, in which the heating rate is dependent on the measured reaction rate of the process being studied. The researchers measured activation energies for the decomposition of NaHC03 and CaC03 by SIP-MS using both a linear heating and a relatively new SCTA technique—constant rate thermal analysis (CRTA). In CRTA the reaction rate rather The researchers followed the interaction of two DNA condensates that form at the solid-liquid interface of their study system, which is comprised of chemically modified mica and an aqueous buffer. The AFM images show that the DNA condensate molecules and their strands interact and are dynamic in solution. Moreover, strands of one condensate organize commensurately with strands of a nearby condensate. This interaction leads to self-assembly of the component DNA strands and be an intermediate in the formation of a larger condensate. (J. Am. Chem. Soc. 1919 121 7330-34)
Analytical Chemistry News & Features, November 1, 1999
When remediating soils of pesticides and toxic chemicals, it is necessary to know how much of the contaminant is adsorbed to soil particles and how much is in the surrounding waters. These levels are dictated by sorption/desorption kinetics and equilibria. Conventional approaches for studying partition equilibria of pesticides in soils require the separation of the solids from the liquids and analysis of the separated phases by LC or GC. L Ramos and co-workers at Free University (The Netherlands) and ASL Laboratories (Canada) have developed a much faster approach for determining partition coefficients in soils using on-line filtration and LC microextraction followed by GC/MS. TTic method C3.ii determine the amount of pesticide adsorbed in the soil and in solution with only injection
than the heating rate is predetermined. When CRTA is combined with the "rate jump" method, the reaction rate is alternated between two predetermined values while the temperature is monitored. This combination minimizes the effects of heat and mass transfer, resulting in data that are less dependent on experimental conditions. However, many other factors, including grain size, crystallinity, purity, and thermal history of the material, were found to influence activation energies. (J. Phys. Chem. m 1999 103, 6944-49)
Multicondensate intermediate structure formed from several linear DNA structures.
Simultaneous filtration/LC microextraction setup.
Using this method, the researchers determined partition coefficients for pesticides ranging from polar triazines to nonpolar hexachlorobenzene in an organic soil. Whole slurries containing particles and dissolved phase were injected into the sample loop and pushed through a hollow filter. The particles were retained on the
Evanescent microwave microscopy The new microscopies, such as atomic force and near-field optical, are revolutionizing how scientists image micrometersized features. In two papers, Massood Tabib-Azar and colleagues at Case Western Reserve University, NASA Lewis Research Center, and Wright-Patterson Air Force Base discuss another new microscopic technique, evanescent microwave microscopy, and demonstrate its applications. The evanescent microwave method is nondestructive, features fast scan rates, and measures displacements in the range of 100 A-5 mm on materials as diverse as metals, insulators, and composites. To collect data, an object is placed near a microwave resonator probe, which is held at a fixed operation frequency. The sample or probe is vibrated at —100 Hz. The resonance frequency and the quality factor of the resonator probe are then affected by the presence of a sample; changes in these parameters depend primarily on the microwave properties of the sample, distance between the tip and the sample, and the tip's effective area. The authors report a vertical resolution of 0.01 vim at 1 GHz znd day that higher resolutions are possible. Moreover, the characteristics of the probe can be easily altered for various applications by changing the probe geometry and frequency of operation.
filter, and the pesticides remaining in solution were preconcentrated on a solid-phase extraction cartridge. Both the filter and the cartridge were extracted with methyl acetate, and the desorbed pesticides were analyzed by GC/MS. Linear responses were obtained over the concentration range 0.004-0.36 ug/mL,
and limits of detection in real soil slurries were in the 2- to 15-ng/mL range. Good correlation was obtained between the calculated soil-water partition coefficients and the previously publlshed octanol-water partition coefficients. (Environ. Sci. Technol. 1999, 33, 3254-59)
One unique application of the method is transient thermography—monitoring the continuously changing heat buildup and dissipation in materials such as microelectronics. In the second paper, the evanescent mi-
crowave method is shown to map temperature distributions with resolutions of ~1 um, detect 0.01 K temperature changes, and respond at rates faster than 1 us. .Rev. Sci. Instrum. 1999, 70, 3381-86; 3387-90)
The experimental setup. (Adapted with permission. Copyright 1999 American Institute of Physics.) Analytical Chemistry News & Features, November 1, 1999 7 2 5 A
