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significantly in recent years, CCC remains a relatively slow and inefficient method. Tiing Yu and colleagues at National Chiao Tung University (Taiwan) and National Cheng Kung University (Taiwan) have reIn countercurrent chromatography (CCC), placed the liquid mobile phase of CCC with supercritical C02 in an effort to increase as in all types of chromatography, solutes are separated because of their different par- the rate of chromatographic separations. tition coefficients in the mobile and staTo demonstrate the feasibility of supertionary phases. In CCC, however, the llquid critical C0 2 as a mobile phase, the restationary phase is not bonded to a solid searchers separated mixtures of acetophesupport phase but is instead retained by the none and benzophenone using mixtures use of gravitational or centrifugal force. Alof water and methanol as the stationary mothough its separation speed has improved bile phase. Unfortunately, they say, ,he addition of a phase separator to protect the UV detector from aqueous droplets increased the dead volume of the system and reduced the efficiency of the separation. Further work is underway to enhance the separation efficiency (/. Chromatoer. A 1996 Experimental setup for supercritical CCC. (Adapted with 724 91-96) permission of Elsevier Science.)
Supercritical C O 2 used as mobile phase in CCC
Impedance spectroscopy monitors graphite/ polymer composites Graphite fiber-polymer matrix composites are used for applications requiring structural integrity in the aerospace, marine, and automotive industries. The critical role that these materials play in aqueous environments has raised questions about their electrochemical degradation. G. L. Cahen, Jr.. and co-workers at the University of Virginia have used electrochemical impedance spectroscopy to study the interfacial changes in a bismaleimide/graphite fiber composite subjected to cathodic and anodic polarization in NaCl solution; to to caustic solutions; and to galvanic coupling with Al Fe Cu and Ti in NaCl solution At cathodic polarization, the composite showed porous electrode response characterized by a decrease in impedance, which was attributed to an increasing electrochemically active surface area. The researchers postulate that the increased surface area could be a result of either roughening of the exposed fiber or breakdown of the fiber-matrix interface. Scan294 A
ning electron microscopy images indicate, however, that roughening of the fiber did not occur. When the composite was coupled with various metals, interfacial changes were observed via EIS, which corresponded to the potential established by each redox couple. The correlation between EIS data and the polarization conditions suggests that EIS is a viable method for monitoring changes in the surface morphology of graphite fiberpolymer matrix composites. (J. Electrochem. Soc. 11996143,449-58)
Nyquist plot for BMI composite sample that has been polarized to-..4 VSCE. (Adapted with permission of The Electrochemical Society.)
Analytical Chemistry News & Features, May 1, 1996
Quantitative PCR by dot blot Quantitative polymerase chain reaction (PCR) has been used for measuring gene expression and viral gene concentration, for following the progress of AIDS in patients, and for cervical cancer screening by determining the concentrations of human papilloma vims type 16. Nevertheless, the application of quantitative PCR has been limited by the lack of a rapid, reliable, and inexpensive method of analysis for large numbers of samples. Typically, quantitative PCR involves simultaneous amplification of target and control gene sequences from a DNA sample; PCR products are separated by gel electrophoresis and Southern blotting followed by hybridization of radioactive oligonucleotide probes to the PCR products. The concentration of hybridized probes is measured by densiometric analysis or phosphor autoradiography. James M. Hill and colleagues at the Louisiana State University Medical Center have described an alternative approach that dot blot procedures lyze PCR products directly from unfractionated samples According to the authors this new approach can process 192 PCR samples in 3 h compared with just 20 samples in 5 h using gel electrophoresis and Southern blot
In the dot blot procedure, the control is a unique endogenous DNA sequence, such that the chances of nonspecific hybridization is low. As a result, target and control PCR products do not need to be separated prior to hybridization and can be analyzed by the dot blot method. The authors demonstrate the procedure by quantifying amplified endogenous actin DNA sequences and exogenous herpes simplex virus-1 ribonucleotide reductase DNA isolated from the ganglia of rabbits Results obtained by Southern and dot blot cedures were comparable but less variance is observed in the dot blot results sav the authors (Anal Biochem.996 235 44-48)
