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ANALYTICAL CURRENTS
Single-cell analysis by MS Single-cell analysis provides insights into the chemistry of cells and the cells' relationship to overall biological function. Recent advances in microcolumn separations methods such as CE and microscale LC, along with miniaturized and highly sensitive detection schemes, have been used to detect cellular components in single cells. Such analyses, however, remain formidable because of the small sample volume and complex cellular matrix. Liang Li and colleagues at the University of Alberta (Canada) presented a mass spectrometric approach for analyzing cellular proteins from a single erythrocyte with a total cellular volume of 87 fL They used MALDI to generate ions from the cellular components. To improve the sampling efficiency, a microspot sample deposition system is used to reduce the sample presentation surface with respect to the laser desorption site and ion
Diffusion studies using IR microspectroscopy Because of its ability to obtain chemical information from areas as small as 10 um, IR microspectroscopy has been used to tackle problems in polymer science, forensic science, earth science, pharmaceuticals, and electronics. Jack L Koenig and colleagues at Case Western Reserve University have used spatially resolved IR microspectroscopy to conduct in situ diffusion experiments of photocured polymer-dispersed liquid crystals. Studying the diffusion of liquid crystals into polymers is important because of the dependence of the phase-separation dynamics of these blends on diffusion parameters. They measured the concentration profiles by monitoring the hydroxyl band of the monomer as a function of time and spatial position and calculated diffusion coefficients from least-squares fitting of the data. The diffusion of a low molecular weight liquid crystal into a photopolymerized monomer followed
acceptance volume of the mass spectrometer. Through the use of fused silica capil laries with internal diameters of 10-200 um the delivered volume can be varied over a wide range from as little as 20 pL to as much as 100 nL. The laser desorption beam is a 50- x 180-um oval that is prealigned with the center of die probe surface through the use of a video camera. By rotating the sample probe, MALDI mass spectra are recorded from different in the sample spot. The desorbed ions then analyzed using resolution time-of-flight mass spectrometer with a 1-m flight tube To illustrate the application of the method to single-cell analysis, the researchers analyzed red blood cells and found that the spectra displayed two intense peaks from the apohemoglobins present in the cell. A strong signal was expected because ~ 450 amol of hemogk bin is present in one cell; however, a surprisingly high mass resolution of 1500 fwhm was obtained with no sample clean-
Fick's Second Law of Diffusion at a temperature above the critical temperature of the mixture. Optical micrographs of
Microspot MALDI sample delivery system showing a sample and MALDI probe-tip holder and a movable xy stage.
ing and despite the high salt content of the cellular sample. The authors believe that the method should be useful for handling small amounts of material for other biological applications as well, such as analyzing proteins isolated by polyacrylamide gel electrophoresis. (J. Am. Chem, Soc. 1996,118,11662-63)
the system after diffusion and polymerization showed three distinct regions: a nematic liquid crystal molecularly mixed in a cross-linked network of the polymer, a nematic liquid crystal droplet gradient in the polymer matrix, and polymer fibers scattered in the continuous nematic liquid crystal. Research is underway to understand the morphological behavior and to determine the limits of solubility of the liquid crystal in the cured matrix using an IR mi-
Optical micrographs showing the three distinct regions of differing crosnertrometer morphology after 45 min (top) and 70 min (bottom) of diffusion; (Aftttl ^bertrnw (A) nematic liquid crystal mixed with holymer, (B( droplet gradient, and 199fi W \"W&(C) strands of polymer scattered in the eematic liquid crystal. (Adapted with permission ffom the Society for Applied Spectroscopy.) Analytical Chemistry News & Features, February 1, 1997 75 A
