News modulator vpi^ivij, trie researchers produce a heterodyne signal between a weak
CD-induced polarization grating and a stronger intensity grating, thus enhancing the CD signal. The pump laser is a pulsed Nd:YAG, so the method can be used for timeresolved CD measurements that are limited only by the pulse duration of the pump laser. The authors did not thoroughly study the method's detection limit, but the experimental results were background limited. They say that artifacts caused by polarization optics and sample cells must be Experimental setup. (P1 and P2, polarizers; IF, overcome. (J. Am. Chem. Soc. interference filter; SF, spatial filter; CF, color filter to 1997 119 8293-300) block excitation at 266 nm.)
In search of a combinatorial screening method Combinatorial chemistry techniques have led to an explosion of compounds that must be screened for biological activity. The analytical challenge lies in devising fast methods for screening these libraries. Solid-phase screening methods have been the mainstay of combinatorial chemistry, but solution phase assays could improve the specificity of screens. Paul Vouros and co-workers at Northeastern University and Cubist Pharmaceuticals combine MS with CE or LC to screen ligands in solution, but they add an extra step. The large protein complexes are isolated from the unbound ligands with gel-filtration spin columns. The protein-ligand complexes are then disso-
SCIENCE
Going mobile The aerosol time-of-flight mass spectrometers (ATOFMS) designed by Kimberly Prather's group at the University of California-Riverside frequently take field trips. When Analyttcal Chemistry spoke with Prather, the instruments were in central Los Angeles and Azusa (a small city located between Los Angeles and Riverside) participating in the Southern California Ozone Study (SCOS97-NARSTO, North American Research Strategy for Tropospheric Ozone), an effort to study real-time ozone formation. Prather's group uses MS to characterize the composition and source of atmospheric particles. 652 A
ciated, and the ligands are identified by LC/MS or CE/MS. They discovered that a substantial amount of the protein was lost in the spin columns; however, the low cost, speed, small sample volumes, and disposability of the columns still make them preferable to standard size exclusion chromatography columns. Human serum albumin was used as a test receptor for six ligands of varying affinity for the protein. The authors believe that the method is well suited to ligands with high binding affinity. The analysis could be performed with as little as 900 pmol of the receptor protein an amount that the authors could easily be reduced. LC sPPTT1s
better suited to the analysis than CE because it is more tolerant of matrix components and high salt concentrations (Rabid Commun Mass Sbectrom 1997 11 1178-84) The ATOFMS instruments, which use a dual reflectron design, are described in the Oct. 15 issue of Analytical Chemistry y(. 4083). The eositive end negative ions generated by the desorption and ionization of a single particle are analyzed simultaneously, which is particularly useful in identifying the source of atmospheric particles. "The dual reflectron with online detection gives you more than double the information," says Prather. "By knowing twice as much about the composition of the particle, it gives you that much more information about the origin of the particle and subsequent chemistry it may have undergone in the atmosphere." The biggest challenge that Prather's group faced in building the instrument
Analytical Chemistry News & Features, November 1, 1997
Binding constants by CZE Currently,fivemethods are available to determine binding constants by capillary zone electrophoresis (CZE): frontal analysis (FA), Hummel and Dreyer (HD), affinity CE (ACE), vacancy peak (VP), and vacancy affinity CE (VACE). M.A.H. Busch and co-workers at the University of Amsterdam (The Netherlands) review these methods and discuss their limitations. When the mobilities of a protein and its complex are equal, accurate binding parameters can be obtained by the FA, HD, and VP methods. However, when the mobilities differ, systematic deviation will occur in the constants if determined by those three methods; the ACE or VACE methods should be used instead. The ACE method can be performed only when the mobilities of the complex and protein differ. For a monovalent complex, all five methods work equally well; although FA is often favored because it is the simplest and most robust of the methods and requires the least amount of sample. When multiple equilibria are involved, the ACE method cannot be used. In addition, the VACE method is complicated by multiple equilibria and, unless the relationship between interacting species is well known, should not be performed. (J. Chromatosr. A 1197, ,77, 329-53)
was the arrangement of the detectors. The detectors are coaxial with holes in the center for ions to enter the flight tubes and need to be floated at high voltage to maintain a field-free region, while providing appropriate accelerating voltages for both ion polarities. The ion signal needed to be capacitively decoupled from the accelerating voltages, but that turned out to be a relatively minor problem, according to Prather. She says that through a major effort, everything just "came together", and the design was up and running in less than a year. It could have taken even less time if they hadn't been so "picky" about the signal she The instruments combine a conventional method of atmospheric particle analysis with mass spectrometric analysis.
