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Best of both worlds Neither FT-Raman with a single detector nor dispersive Raman with a multichannel detector is perfect, so spectroscopists would like to combine them and get the advantages of both. Richard L. McCreery and Jun Zhao of The Ohio State University reported the wavelength precision, resolution, and practical advantages of the combination of a Sagnac, or common-path, interferometer and a charge-coupled device (CCD) for "multichannel FT spectroscopy" (MCFT), which differs from dispersive Raman in its use of interferometry and from ordinary FT-Raman in its use of multiple detector elements. The spectrometer consists of the interferometer (a cubic beamsplitter and two mirrors), four lenses, and a CdTe absorption filter (to eliminate scattered light with wavelengths shorter than 850 nm). MCFT combines the high throughput, wavelength accuracy, and stability of an interferometer with the low-noise advantage of a CCD. The entire Stokes Raman shift range is available to the interferome-
ter with die upper limit determined only by the detector response. Spectral coverage does not depend on sensitivity or resolution, and the fixed nature of the Sagnac interferometer provides excellent wavelength precision. Because the large entrance aperture is usually underfilled, the intensity is less sensitive to laser focus and position. The response Optical diagram of the common-path MCFT interferometer. is only weakly depen(Adapted with permission from the Society for Applied dent on beam focal size Spectroscopy.) and associated power density, so lower power densities can be limited detector. They believe that MCFT used when necessary. will be useful in applications that do not require high resolution but do require Although the authors have not yet precision, stability, or low laser power done a detailed S/N analysis, they expect that MCFT will have a Fellgett (multiplex) density, such as the observation of monolayers on carbon surfaces. (Appl. Specadvantage similar to that of a Michelson trosc. 1996,50,1209-14) interferometer with a single shot-noise-
Getting the salt out Matrix-assisted laser desorption/ ionization (MALDI) time-of-flight (TOF) MS can be used to analyze intact cells and tissues, but it has not been applied to the analysis of cells directly from marine organisms, possibly because their salt content is so high. Jonathan V. Sweedler and coworkers at the University of Illinois— Urbana-Champaign have used MALDI-TOFMS to analyze peptides from neurons from the mollusks Aplysia californica and Pleurobranchaea californica, which have physiological Na+ concentrations of ~ 450 mM and a total ionic strength greater than 650 mM. A rinsing procedure allowed the neurons to be placed directly on the MALDI target and assayed for bioactive peptides. Mass spectra of the neuronal peptides were obtained through two strategies. In thefirstmethod, a group of 400 cells was progressively
Mass spectra from direct peptide profiling. (A) A neuron from the R3-14 cluster of abdominal ganglion of Aplysia. (B) Connective tissue near the B1 and B2 neurons of the buccal ganglion of Aplysia. (C) Neurosecretory cells containing egg-laying hormone from Pleurobranchaea. (Adapted with permission from John Wiley & Sons.)
divided into four groups by rinsing the cells in a 2,5-dihydroxybenzoic acid (2,5-DHB) matrix and transferring all but 100 cells from each rinse. The rinsing procedure removed excess salts, and the mass spectra indicated that alkali metal adducts were reduced but the peaks of interest were not. In the second procedure, the researchers replaced the physiological saline solution with 2,5-DHB before dissection and isolated the neurons and removed excess salt simultaneously. The cells were placed directly on the sample probe without any spotto-spot transfers. The spectra were similar to die ones obtained using progressive spot transfers. The matrix solution stabilized the cellular membranes so that propane-l,2-diol, which is often added for that purpose, was not needed. In addition, the low pH (~ 2.5) of the 2,5-DHB solution denatured proteolytic enzymes, minimizing peptide degradation. (J. Mass Spectrom. 1996,31,1126-30)
Analytical Chemistry News & Features, January 1, 1997 11 A
