Audiovisual Instrumental Analysis - Analytical Chemistry (ACS

Anal. Chem. , 1984, 56 (8), pp 918A–918A. DOI: 10.1021/ac00272a715. Publication Date: July 1984. ACS Legacy Archive. Cite this:Anal. Chem. 56, 8, 91...
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Audiovisual Instrumental Analysis Sir: One problem of a college with a small student population is the lack of upperclassmen who know how to use the school's analytical instruments. The reason for this is that upper-level courses are usually taught every two years. Therefore, a minuscule number of experienced students are still enrolled while a new group of students is taking the instrumental analysis course. Two students at LaGrange College, Alan Ford and John Sullivan, initiated the idea of creating sound-on-slide presentations to provide step-by-step operating instructions for analytical instruments in the laboratory. The presentations, including photographs of the instruments and written descriptions of instrument operation, were prepared on a 3M Model 625 Sound-on-Slide Projector/Recorder. Information for these presentations was gathered from textbooks used in the instrumental analysis course, instrument manuals, laboratory procedures, and library references. These presentations have been very useful in two ways: Students can view the material at their convenience, and the presentations free the professor to go into more detail on a technique during class than would have been possible otherwise. Alan Ford John Sullivan Kenneth Cooper Jr. Department of Chemistry LaGrange College LaGrange, Ga. 30240

More on Stray Light Sir: Nelson L. Alpert's letter on Stray Light (Anal. Chem. 1984,56, 532 A) further confuses a difficult subject. Suggesting that stray light is neither a user-oriented specification nor a useful concept merely sweeps an important issue under the rug. Three factors are largely responsible for the present situation. Foremost is insufficient appreciation of the influence of any sample, including test reagents, on the stray radiation. This situation is ag-



gravated in instruments using blocking filters to reduce stray radiation. With the potential stray radiation thereby confined to a relatively narrow wavelength interval the sample can have a profound influence on the stray component. The second important factor is the method of measuring the stray radiation. That most widely used is the ASTM method ("Standard Method of Estimating SRE," ASTM Designation E.387-32, American Society for Testing Materials, Philadelphia, Pa.). It underestimates stray radiation and does not allow a measurement in the presence of an arbitrary sample, therefore does not provide a measure of the resultant photometric nonlinearity. These factors defeat Dr. Alpert's inference that the stray light specifications could be replaced by a set of linearity deviations. The third factor resides in loose terminology. The ASTM definition does not provide a mathematically exact and experimentally verifiable distinction between stray and primary radiation. With properly defined terms there should be no question whether or not doubly diffracted light rays contribute to stray light. These factors are not insurmountable. Methods for the measurement of stray radiation in the presence of any sample do exist. Terms can be defined with mathematical precision. Furthermore, application of the proper concepts need not be so complex as to elude everyday use. A forum for resolution of these difficulties exists in ASTM subcommittee E-13.01. Those wishing to contribute ideas or participate in decisions should contact the chairman, Mr. R. J. Darnowski, Merck and Company, Inc., R71-22, P.O. Box 2000, Rahway, N.J. 07065. Wilber Kaye Beckman Instruments, Inc. P.O. Box C-19600 Irvine, Calif. 92713 Nelson Alpert replies: I did not mean to imply in my letter that stray light is not a useful concept. My key message was to propose a pragmatic, user-oriented approach to an admittedly complex problem. This new approach does not purport to solve the problem, but only to state the problem in terms that are more comprehensible to users.