Focus
How to minimize pressure drop in preparative LC. The high flow rates used in modern preparative LC can cause excessive pressure drops. For example, at 300 mL/min a standard Rheodyne analytical valve can cause a pressure drop of 150 psi. By substituting a Rheodyne valve with larger flow passages, one designed for preparative work, this can be reduced to 17 psi. Rheodyne's Tech Note #7 contains formulas and graphs that enable you to calculate pressure drop in valves and tubes. One handy graph plots pressure drop vs. flow rate for Rheodyne valves. The data is supported by theoretical calculations (considering both laminar and turbulent flow) and verified by experimental results.
Send for Tech Note #7 Please address Rheodyne, Inc., PO. Box 996, Cotati, California 94928 U.S.A. Phone (707) 664-9050.
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Conference booth, the company was showing two AOTF-based devices for specific applications—a flue gas analyzer and a fiber-optic-coupled AOTF for process control in hazardous locations. But Westinghouse was making no secret about the fact that its interest lay not only in selling complete instruments, but also in selling AOTF devices to other instrument companies. The AOTF device is made of thallium arsenic selenide, a single-crystal biréfringent optical material patented by Westinghouse. In operation, a highfrequency (MHz-range) acoustic wave from a transducer is propagated across the optical path of the filter. This sound wave modulates the refractive index of the crystal and creates a sort of diffraction grating. A solid-state device with no moving parts, the AOTF can change wavelengths rapidly—in a matter of microseconds, according to Westinghouse— and can be easily controlled by a microprocessor to perform "smart" spectroscopy. The filter is available in four versions—high (
