Windowless Spectrophotometric Cell Devised Optical response improved by captive liquid cell that has no windows, depends on surface tension Scientists at Oak Ridge National Laboratory have studied molten fluoride salts with a windowless spectrophotometric cell. With this cell, which they call a captive liquid cell, light is transmitted through the side of a vertical column of liquid held in a specially designed metal container. The cell has no windows or mirrors. Path length of liquid samples held in this cylindrical container is independent of the sample's weight or volume. But Dr. J. P. Young of ORNL postulates that it depends on the liquid's surface tension and density. Although ORNL has used the cell for spectrophotometric studies of molten fluoride salts, Dr. Young sees no reason why it can't be used for other corrosive solutions and molten salts at well over 1000° C. And, he adds, now that high-intensity monochro-
Dimensions such as these are typical of a windowless or "captive liquid" cell for spectrophotometric studies
Aperture for light transmission
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matic light sources such as lasers are available, the cell seems ideally suitable for Raman spectroscopy. As presently designed, the cell will contain liquids which have a surface tension greater than about 60 dynes per cm., and which wet the cell's walls. In other methods of confining corrosive liquids for spectrophotometric study, a large amount of the liquid's surface area is used for observation, Dr. Young told the Division of Analytical Chemistry during the 145th ACS National Meeting. Thus any solid material in the liquid will be located at these surfaces and affect optical response. With the captive cell, however, only a small percentage of total surface area is used for spectrophotometric observation, and solids generally will be located above or below the observation region. Spectrophotometric ob-
servations also show that the liquid's path length is held relatively constant ( ± 3 % ) and is independent of sample weight in excess of a certain critical minimum weight. What It Is. The captive liquid cell is simply a metallic vertical cylinder with holes in it. For spectral studies, light passes through the large, lower aperture. Above this, a series of smaller holes ("keeper" holes, Dr. Young calls them) maintain liquid level within the cell in the plane of these holes. Accordingly, the minimum critical amount of sample is that which fills the cell to this level. Excess liquid drains out of the lower hole until this condition is fulfilled. Effects of surface tension keep liquid in the cell. Dr. Young assumes that'the six keeper holes provide the support required to maintain liquid level above the observation aperture. Actually, he says, liquid will be held in the larger (observation) aperture even without keeper holes above it. But the meniscus then can be seen through the aperture and the cell becomes what he calls "a rather poor pendent drop container." The cell can be made with a wide range of dimensions for container diameter, spectral aperture, and "keeper" holes, as well as the distance from aperature to keeper hole. These dimensions aren't critical, although perhaps the most critical of them is the distance between the top of the spectral aperture and the bottom of the keeper holes. If this distance is too small, Dr. Young says, the meniscus will appear in the aperture. If it is too great, the cell won't function properly. Optical response with the captive liquid cell offers advantages over that of a pendent drop, according to Dr. Young. The big difference is that the liquid in the captive cell is confined in a shape which approaches a double convex lens. In a pendent drop, suspended between two surfaces, it forms a double concave lens. And convex lens shapes improve light transmission. Dr. Young admits, though, that determining path length and its reproducibility with windowless spectrophotometric cells is a problem. He postulates that path length is a function of both surface tension and density. Of course, he adds, it would be better if it were independent of these variables, but suspects that this may be impossible in windowless cells.
Sacrebleu . · . they're going to test another new dog food on me!
Ooops. On camera. I'd better stick to the sponsor's script.
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I don't give a woof about propylene glycol, myself. But the Jefferson people are leading producers and they can give you all kinds of technical information . . . Jefferson Chem ical Company, Inc., 1121 Walker Avenue, P. O. Box 53300, Houston 52, Texas. C'est bon!
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JEFFERSON CHEMICALS C & Ε Ν 37
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