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Double-beam detection for CE Of the various detection schemes used for CE, including absorption, fluorescence, electrochemistry, and MS, absorption detectors are still the most popular because of their versatility and simplicity. High background noise caused by low light intensities, short optical pathlength, and poor optical coupling from small capillary size, however, can lead to a poor limit of detection. Edward S. Yeung and Wei Tong of the Ames Laboratory and Iowa State University have developed two simple double-beam absorption detection systems for CE: one based on a diode laser and the other based on LEDs that can achieve detection limits well below those of commercial systems. In the diode laser-based system, two well-correlated laser beams reflected from a glass window are used as signal and reference beams, respectively. The researchers found that the gain in detectability results from a reduction in intensity fluctuations and a better optical coupling of the laser beam with the small capillary tube, which maximizes the effective optical
Characterizing polymers by ESI FTMS Most methods for determining the relative molecular weight of complex polymeric materials rely on an indirect property such as light scattering or binding affinity. As such, these methods require calibration with known, preferably monodisperse standards. Fred W. McLafferty and Peter B. O'Connor of Cornell University have used electrospray ionization FTMS to directly measure isotopically resolved polyethylene glycol) (PEG) oligomers and determine the weight- and number-averaged molecular weight and polydispersity of the polymer. Such ESI MS spectra of polymers can be obtained in minutes and can provide far more detailed irictss information than those obtained usifii? coilventional techniques
Experimental setups of (a) the diode-laser based system and (b) the LED-based system. (Adapted with permission of Elsevier Science.)
pathlength. Despite the difficulty of using the LED-based system for optical focusing and the poor coupling of the beam with a small-diameter capillary, the excellent stability of the LED augmented by doublebeam cancellation of the baseline fluctuations allowed the researchers to obtain de-
tection limits close to those obtained with the diode laser-based system. They postulate that the availability of shorter wavelength diode lasers in the future could lead to even broader application for these systems. (J. Chromatogr. A 1995, 71,7 177-85)
The researchers found that PEGs of fied to minimize artifact peaks while retain4.3,13, and 23 kDa give spectra containing accurate abundance information. (J. .m. ing resolved isotopic peaks that represent Chem. Soc. 1995,117,12826-31) the individual oligomers in the sample. For example, in the 23-kDa spectra, they identified approximately 5000 isotopic peaks of 47 oligomers in 10 charge states. The m/z discrimination appears to be less than in MS measurements from MALDI or other instruments and averaging the distributions from multiple charge states further minimizes errors in mass abundances. The researchers believe that such errors should be further reduced by calibration with mixtures of monodisperse samples Interpretation of the complex ESI FTMS spectra is simplified with a combination of ESI FT mass spectrum of PEG 14000 (a) and corresponding relative molecular weight distribution deconvolution algorithms modi- from summed oligomer abundances (b). Analytical Chemistry News & Features, March 1, 1996 163 A
