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1. Introduction CE has become a mature technique for analytical separation and has been increasingly important in separation science. It has been widely utilized in biological, environmental, pharmaceutical, clinical, and food analysis. Furthermore, CE is superior over other separation techniques in terms of fast separation, high solution and minimal requirements of sample and reagent amounts. However, since the volume or length of the injected sample zone in capillary is very small or short, CE suffers from poor concentration sensitivity especially when on-line Ultraviolet (UV) detection is used. Several strategies have been developed to solve the problem. Besides the use of high sensitivity detectors such as laser-induced fluorescence (LIF), chemiluminescence or electrochemical detectors, electrophoresis-based and extraction preconcentration techniques are used widely [1-3]. Electrophoresis-based techniques include transient isotachophoresis preconcentration [4], field- enhanced sample stacking [5], dynamic pH junction [6], pH mediated stacking and sweeping [7]. Electrophoresis- based techniques rely on differences in the velocity change of the electrophoretic mobilites of the analytes in different zones.The differences in migration velocity are caused by the change in electric field strength between the sample solution and separation zone or the change in effective charge on the analyte. This is very useful in on-line sample preconcentration techniques in CE, where a larger volume of the sample solution is injected into the capillary and the analytes in a long sample zone can be focused into a narrow zone before separation and detection. Sweeping is based on the chromatographic principle, that is, the migration velocity is different either due to the analyte is complexed by the pseudostationary phase or free from the pseudostationary phase. Sweeping is based on the chromatographic principle that the migration velocity is variable either due to the analyte being complexed by the pseudostationary phase or free from the pseudostationary phase. A complexing reagent that forms a complex with the analyte can be also used for sweeping [3]. Extraction sample preconcentration techniques, such as solid-phase extraction, solid- Central European Journal of Chemistry * E-mail: [email protected] College of Chemistry and Chemical Engineering, Lanzhou University, Lanzhou 730000, China Yonglei Chen, Wenjuan Lü, Xingguo Chen * , Min Teng Review of recent developments of on-line sample stacking techniques and their application in capillary electrophoresis Review Article Abstract: © Versita Sp. z o.o. Received 4 September 2011; Accepted 17 December 2011 Keywords: Capillary electrophoresis • Sample stacking • Transient isotachophoresis • Dynamic pH-junction • Sweeping • Extraction Capillary electrophoresis (CE) has become one of the most useful tools in separation science because of its high separation efficiency, low cost, versatility, ease of sample preparation and automation. However, some limitations of CE, such as poor concentration sensitivity due to its lower sample loading and shorter optical path length, limits its further applications in separation science. In order to solve this problem, various on-line sample preconcentration techniques such as transient isotachophoresis preconcentration, field-enhanced sample stacking, micelle to solvent stacking, micelle collapse, dynamic pH junction, sweeping, solid phase extraction, single drop microextraction and liquid phase microextraction have been combined with CE. Recent developments, applications and some variants together with different combinations of these techniques integrating in CE are reviewed here and our discussions will be confined to the past three years (2008-2011). Cent. Eur. J. Chem. • 10(3) • 2012 • 611-638 DOI: 10.2478/s11532-012-0007-4 611

Spotting a sparse crystal of trapped ions

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Sign of spring: Science finalists picked Ten high school girls and 30 boys last Park-Tudor School, Indianapolis.

week got the phone calls they’ve been dreaming about: news that they’d beaten 1,541 other top science students to become finalists in the 57th Annual West- inghouse Science Talent Search.

The next stage of the competition starts March 4, when the finalists face a panel of scientists in Washington, D.C. These judges will interview the students and on March 8 announce the winner of a $40,000 scholarship. Nine other students will receive between $30,000 and $10,000, and the remaining 30 finalists will each be awarded a $1,000 scholarship. Sci- ence Service, the publisher of SCIENCE NEWS, administers the competition.

This year’s projects ranged widely across disciplines, including a hurricane prediction model, a virtual cricket, and a biochemical analysis of why Korean rnan- du dumplings turn bright red during cooking. One student examined current efforts to control loosestrife, an invasive weed, by introducing a predator and dis- covered a ladybug that preys on the new- comer. Another finalist, who reads Russ- ian and Ukrainian, as well as French, developed a computer model for study- ing how children learn languages. Five math projects made the finals, as did an astronomy study of time dilation and redshift in cosmic gamma-ray bursts.

“I think we should be very encouraged because at this very high level we are producing in America today students of extraordinary talent,” says Princeton University astrophysicist J. Richard Gott, who heads the judging panel.

The 40 finalists are:

Arizona: Ravi Vikram Shah, Corona del Sol H.S., Tempe.

California: Ann Kromsky, Corona H.S., Corona; James Greg Marsden, North Hollywood H.S., North Hollywood; Sohini Ramachandran, Rio American0 H.S., Sacramento; Jeremy Walter Shaw, Villa Park H.S., Villa Park.

Colorado: Heather Margaret Mat- thews, William J. Palmer H.S., Col- orado Springs.

Delaware: Calvin K. Huang, Sale- sianum School, Wilmington.

Florida: Carey Marie Tanner, Canter- bury School, Fort Myers; Kimberly Anne Fitzgerald, Gulliver Preparatory School, Miami; Aaron M. Seider, North Miami Beach Senior H.S.. North Miami Beach.

Maryland: Kirk Doran, Walt Whitman H.S., Bethesda; Josh Evan Greene, Oak- land Mills H.S., Columbia; Sabyasachi Guharay, Wilde Lake H.S., Columbia; David Jacob Wildstrom, Montgomery Blair H.S., Silver Spring.

Michigan: Cheryl Kyung Kim, Detroit Country Day School, Beverly Hills.

Minnesota: William J. Greenleaf, Mayo H.S., Rochester.

New Mexico: Stephanie Lynn Levine, Eldorado H.S., Albuquerque.

New York Grace Yew-Mih Lee and Keith Jarred Rotbard, Lawrence H.S., Cedarhurst; Mark Evan Abraham, Jamesville-Dewitt H.S., Dewitt; Ameet Shrirang Talwalkar, Half Hollow Hills H.S., Dix Hills; Jeremy C. Hwang, Great Neck

South H.S., Great Neck Michael Joseph Smolow, John L. Miller Great Neck North H.S., Great Neck Parker Rouse Conrad, Collegiate H.S., New York; Jesse Keith Anttila-Hughes, Stuyvesant H.S., New York; Jonathan Adam Kelner, The Wheat- ley School, Old Westbury; Christopher Michael Gerson, Grace Ann Lau, Clyde Law, and Thomas Petersen, Ward Melville H.S., Setauket; Mark Anthony Grishaj, Ramapo Senior H.S., Spring Val- ley; Steven Mark Tobias, Syosset H.S., Syosset.

Oklahoma: Michael Yanche Lee, Nor- man H.S.. Norman.

Texas: Patrick William Goodwill, Texas Academy of Math & Science, Den- ton; Stephen Alexander Tinnin, McKin- ney H.S., McKinney.

Virginia: Paul Julius Bracher, David Hayes Marcus, and Mary Ellen Matyskiela, Thomas Jefferson H.S. for Science and Technology, Alexan- dria. -S. Milius

still have the orderly arrangement shells (SN: 7/30/88, p. 69). The new MST expected of a crystal. results show that with a sufficiently large

Wayne M. ltano and his coworkers number of ions, at least those in the inte- at the National Institute of Standards rior fall into a cubic pattern. and Technology in Boulder, Colo., “We’re studying various properties of obtained evidence of such a pattern in these crystallized ion plasmas, now a rapidly rotating cloud of nearly 1 that we can image them and control the million ions confined in a special trap. rotational frequency,” ltano says. Plas- They report their findings in the Jan. ma crystals may prove useful as a mod- 30 SCIENCE. el of a neutron star’s crust, for example,

The NlST team is the first to observe aiding efforts to determine the ages of this simple crystalline state in the lab+ these stars. -I. Peterson

Illinois: Travis Jeremy Schedler, Illi- nois Math &Science Academy, Aurora.

single crystal.

method for precisely controlling the cloud’s sDin so as to svnchronize it with

bright spots that are smeared into rings The NlST scientists developed a by the rotation of the ion cloud within

the hap. The shadow of a laser beam deflector musks Dart of the imape.

Indiana: Christopher Colin Mihelich,

JANUARY 31,1998 SCIENCE NEWS, VOL. 153 69