gel electrophoresis. The researchers note that although precision needs to be improved, this preliminary result indicates the potential use of ESIMS to analyze PCR products on a rapid time scale, with potential for medical and taxonomic applications. (Rapid Commun. Mass Spectrom. 1996,10,29-35)
ESI-FT-ICFt mass spectrum of the PCR product ofB. thuringiensis with tentative charge states indicated. (Adapted with permission from John Wiley & Sons.)
Sample loss in electrothermal vaporization
Nonradioactive labeling of RNA Although the necessity of nonradioactive labeling methods for natural RNA has been known for more than a decade, a convenient method of attaching such labels has not yet been described. Gabor L. Igloi of the University of Freiburg (Germany) has synthesized a modified ribodinucleotide analogue of the commonly used radioactive [32P]pCp. Called pSEEp, the new reagent has a 3'-terminal primary amino group that can serve as a point of attachment for a variety of nonradioactive labels such as fluorescein. Igloi used a T4 RNA ligase catalyzed reaction to attach the tagged dinucleotide to natural RNA and analyzed the resulting RNA using a fluorescence-based DNA sequencer. The effects of ribose oxidation and aminoacylation on the ligation reaction were
plasma. Despite these advantages, analyte loss can still occur in the transport tube. James A. Holcombe and Christopher M. Sparks of the University of Texas and Thomas L Pinkston of SEMATECH have measured the amount of material reElectrothermal vaporization has several tained in the transport tube as a function of advantages over nebulization as a sample introduction technique for ICPMS, includ- the mass of sample vaporized. ing increased sampling efficiencies, Because physical carriers such as NaCl lower detection limits, reduced sampleare often added to the system to enhance volume requirement, and reduced interfer- condensation by providing additional nuences from matrix constituents in the cleation sites, the researchers first determined the percent of NaCl retained and then the percent of Ag retained, with and without NaCl present. Approximately 10% of the NaCl carrier remained in the transport tube. When the Ag was vaporized with NaCl, approximately 610 % of theAg was retained depending on the type of tubing. These numbers closely paralleled the amount of c cirrier rc_ tained which indicated that the Ag was probably transported on the carrier When Ag was vaporized alone only 1-3 % remained Effect of various masses of NaCl carrier on ionization of in the tube. (Appl. Specsilver. (Adapted with permission of the Society for Applied trosc. .996,50,86-90) Spectroscopy.)
Structure of the dinucleotide pSEEp and ts fluorescent derivative 3FpSEEp. (Adapted with permission of Academic Press.)
also studied. He suggests that any procedure depending on detection or targeting tagging of RNA can be adapted for use with pSEEp and its corresponding biotin and phenanthroline derivatives. (Anal. Biochem. 1996,233,124— 29)
Antarctic aerosol particles by LAMMS Because of its remoteness from human activity, the Antarctic is an ideal site for monitoring changes in the global environment. The Antarctic atmosphere is sensitive to changes in climate and air quality, including the effects of aerosol particles that scatter solar light and contribute to cloud formation. Earlier studies of particles have concentrated on size distribution and elemental composition using bulk analysis techniques, rather than on obtaining information on individual particles. Keiichiro Hara and colleagues at the Science University of Tokyo (Japan) Nagoya University (Japan) and the National Institute of Polar Research Oapan) used reflection laser microprobe MS (LAMMS) to clarify the seasonal behavior of Antarctic aerosol particles over the course of a year Because LAMMS suffers from poor reproducibility of spectrum intensity the researchers used the relative abundance of characteristic peaks to quantitate their results Matrix effects were eliminated bv analyzing a fixed number of individual nar tides and defining the relative abun dance of the species as the ratios of the frequency of detected characteristic peaks for a specific component (e.g., Pb) to the total number of analyzed particles.
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News
Seasonal variation of solar radiation, sulfates, MSA, and nitrates determined by LAMMS.
STM describes outer Helmholtz layer at Pt electrode In situ scanning tunneling microscopy (STM) has been used extensively to study the structure of electrode surfaces in electrolyte solution. Although STM has been used to elucidate the structure of the inner Helmholtz layer at Pt electrodes, chemical species such
Sampling took place from February to December 1991 at two sites at the Syowa station, and particles found on the third stage (1.6- 5.4 um) of a multistage high-volume air sampler were analyzed. The behaviors of sulfates, MSA, and nitrates in the Antarctic atmosphere correlated with the seasonal change in solar radiation (except in March, April, and August), which indicates that photochemical reactions are responsible for generating the aerosol particles The researchers postulate that the presence of metallic elements including V Mn Zr Cr and Pb may be caused by the transport of soil particles into the reeion (Fnviron Sci Terhnol 1996 on Qg5_gn
as solvated cations and anions might be expected to move too rapidly to be observed by in situ STM. Kingo Itaya and colleagues of Tohuku University and the Research Institute of Electric and Magnetic Materials (Japan) have used STM for what they believe is the first in situ description of the outer Helmholtz layer. They examined a well-defined Pt(lll) electrode in acidic and alkaline solutions containing NaCN or KCN and found a CN adlayer of 6-membered rings in the double-layer region. STM images revealed a well-ordered array of cations adsorbed to the CN adlayer when K+ or Na+ was introduced to the system. The cations appeared as bright spots in the STM images, and the corrugation height found to be potential dependent. On the basis of these results the researchers proposed a new model for the CN adlayer in which a hollow hexagon structure allowed the coordination of cations in the center of the hexaeon (7 Am Chem. Soc. 1996,118,
Stretching DNA How double-stranded DNA (dsDNA) packs into phage heads, bends on interaction with proteins, and loops to connect enhancer and promoter regions is a function of how it bends and twists. An easy way to explore DNA elasticity is to stretch a single macromoleculefromboth ends and measure the force as a function of its end-to-end distance extension. Carlos Bustamante and colleagues at the University of Oregon used force-measuring laser tweezers to observe the behavior of single molecules of dsDNA They used a dual-beam laser trap instrument with low numerical aperture lenses, to form an optical trap. Each end of a single A,,phage dsDNA molecule was attached to a separate microscopic latex bead. One bead was held by suction with a glass micropipette and the other was held by the optical trap, which also functioned as a force transducer. The DNA molecule was extended by moving the pipette relative to the laser trap, and the force acting on the DNA was inferred from the displacement of the laser beams on position-sensitive photodetectors. At a force of ~ 65 pN, ,he molecule assumed an extended form ~ 1.7x its B-form contour length. When the force fell below 65 pN, the DNA contracted to its normal contour length. The force rose rapidly again at extensions > 28 um when the molecule was entirely converted to the overstretched form. The narrow range of forces over which the overstretching transition occurs indicates high cooperativity, which implies that it is energetically easier to spread an existing region than to nucleate a new one. (Sciencce196,271, 795-98)
Possible conformations of stretched DNA. (A^x^oTfor^eMv^mmoTnCan Science.)
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