INSTRUMENTATION by Ralph H. Müller
N e w infrared sampling technique described. Commentary on the difficult art of interpolation and extrapolation Paul A. Wilks, Jr., vice-president of the Connecticut Instrument Corp. has brought to our attention a new principle—attenuated total reflection (ATR) which affords a new infrared sampling technique particularly useful for solids, films, and strongly absorbing liquids. Our brief outline is based upon a paper presented by Wilks at the 1961 Ohio State Symposium on Molecular Structure and Spectroscopy. This paper will be reprinted in the September CIC newsletter. According to Wilks, the principles of attenuated total reflection were developed by J. Fahrenfort in a paper presented in 1959 in Bologna. A very complete mathematical explanation of the phenomenon was given at these meetings. When a beam of radiation is passed into a prism so that it is totally reflected from the back face, it has been shown, both mathematically and experimentally, that some portion of the energy of the beam escapes from the totally reflecting face and then is returned into the prism. I t would appear that, if an absorbing material is placed in contact with the reflecting surface, the energy that escapes temporarily from the prism would be selectively absorbed, much as in a transmission
spectrum. Under the proper conditions this is indeed the case. The absorption-like spectra obtained by this method have two important and unique features: 1. The band intensities are the equivalent of an extremely shallow (5 microns or less) penetration into the sample. 2. They are completely independent of the sample thickness. All of this comes as a relief to manufacturers of absorption cells who are required to find a sampling procedure which eliminates precise and extremely short path lengths as requirements. Employing these principles, several attempts have been made to develop simple optical systems, which can be mounted in any conventional infrared spectrophotometer. The one finally adopted by CIC permits freedom of sampling procedure and a wide range of angle of incidence of the primary beam. The next, problem is to mount the sample in such a way that the ATR effect can be obtained. Here the requirement is that the sample be in close contact with the reflecting surface. This can be done easily if the sample can be deposited on the reflecting surface by evaporation, thermosetting, spraying, or some similar way. The re-
flecting plate may be a prism or half cylinder. However, these forms require considerable amounts of expensive crystal. Except for Irtran-2, most of the materials are soft and subject to damage by repeated usage. A third form, shaped like a corduroy or grooved plate, has been developed to provide an inexpensive but effective ATR plate. Silver chloride can be pressed into this form with a proper die. After pressing, the sample may be deposited as described above. Or, if the sample is solid, the AgCl plate may be formed and pressed on the sample in one operation in a KBr press. The ATR technique is a relatively low energy technique. However, enough energy is present for practical analyses, especially when an ATR system is placed in both beams. It has been shown also that liquids can be examined by ATR. In such applications the absorption cell is a half cylinder of silver chloride with a liquid compartment attached to the reflecting surface. " O n the Difficult Art of Interpolation and Extrapolation"
The mathematicians have given us many schemes for interpolation by successive difference—the Newton, La-
VOL. 33, NO. 10, SEPTEMBER 1961
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137 A
INSTRUMENTATION
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ANALYTICAL CHEMISTRY
grange, and other methods. Scientists have always been understandably cautious and suspicious about all but short range extrapolation. In the direction of research and in almost all human affairs decisions must be made. Many of these involve simple interpolation from known facts, others in connection with long range planning, involve extrapolation. In these days of elaborate computers and an advanced state of information theory it is perfectly astonishing how many grave decisions are made, with the facts available and exotic aids for their interpretation at hand, by little more than hunch, or guesswork. In our ignorance, it would be little short of impertinent for us to explain the recent Cuban fiasco, but we continue to be bewildered, as many others must be, by the fact that there is probably no single or even one hundred agencies having the informational resources of the Government of the United States. When we were very young, the mighty Manhattan bridge was being built between Brooklyn and Manhattan. For two decades after its completion, access to this bridge was limited by the bottleneck slums of Brooklyn and Manhattan's lower East Side. Eventually broad access plazas were constructed on each side. The system was inadequate at its inception and needed only interpolation, a process simple to begin with because there were three adjacent bridges for accurate intereomparison. Even plain ordinary people come up with more sensible approaches. The matron who explained to the shoe clerk, "My size is really three, but fours are so comfortable that I always wear fives," really had something. When Mark Twain explained that proficiency in playing poker involved only "simple Christian faith and four aces" he was being delightfully irresponsible as humorists, God bless them, are entitled to be. It is true that decisions are made by men and the decisions of a grave nature are often an intolerable demand upon a human being. Despite this, it is often too apparent that relatively important planning is relegated to ignorant, prejudiced, and willful executives and in such flagrant examples, readily available data are either ignored or misinterpreted. As an example—we have had occasion to visit two large establishments both working in sensitive areas, and with extensive security precautions. In one of these, the employees' cars must be parked as much as a quarter mile from their respective buildings, in which both vehicle and driver are exposed to snow
INSTRUMENTATION and sleet in the winter and a broiling sun in the summer. As much as onehalf hour a day is consumed in four trips to and from the parking lot. In the other example, an equally modern building provides roof overhang to protect each car and its alighting oc cupant who is never more than twenty feet from the building. Each parking slot bears an employee's name and is allocated in a manner to assure the shortest distance to his office or bench. To discourage careless or thoughtless parking each slot is flanked by fourinch curbs. Each establishment has approximately the same amount of barbed wire fencing and number of security offices. Small wonder that the .vt.l planned establishment is in South ern California. Although that state has its decent share of screwballs, it has a habit of thinking for the future and intelligent, efficient, and comfortable ar rangements of this sort may well ex plain why so many manufacturing and research organizations have followed Horace Greeley's advice. Λ minor official, bored to death de spite the exciting things which others are doing around him, can work all sorts of mischief by planning new rules and regulations with the help of a mis aligned slide rule. Human prejudice can warp the de cisions of even the highly competent. We know of one gifted and highly educated research director who has had a lifelong dislike for organic chemistry, an attitude strengthened largely by the fact that his wife flunked a course m that subject and, for some strange reason, is proud of it. An unkind twist of Fate has directed his talents to a company engaged primarily in organic chemistry ! For many years the throttle on a locomotive or subway train has been equipped with a "dead-man's button" so that if the engineer is incapacitated and releases his grip, the train will stop automatically. Being instrumentally inclined, and not always responsibly so, we envision a simple encephalograph attached to rules-making officials. Whenever they are inclined to ignore well established facts, reasonably ac curate interpolation from those facts, and to an inability to make short range extrapolation coupled with a little imagination and foresight, the device will sound an alarm to summon someone with competence. Whenever policy making decisions are in the hands of people only one tenth as competent as their technical associates, we are in trouble. In large establishments, such weaknesses are worth several division- to the enemy. So let's PLAN A1 i E.\d. Circle No. 109 on Readers' Service Card
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