Presented at the Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Cleveland, OH, March 3, 1975
The Other Face Richard W. Roberts National Bureau of Standards Washington, DC 20234
It is a pleasure for me to be here today, and somewhat of an historic occasion. The National Bureau of Standards has long contributed to the fields of spectroscopy and analytical chemistry. We have been closely allied with the Pittsburgh Conference since the first meeting back in 1950. Over the years many outstanding Bureau scientists have presented papers before this forum. But never before has an NBS Director addressed the Conference, and I consider it a privilege to be here today. The timing is most appropriate, as March 3 is the anniversary of the Bureau's founding in 1901. Analytical chemistry has a vital role to play in the years ahead. As we search for new materials, new energy sources, a cleaner environment, and better health, the central questions of composition and quantity will be raised almost daily. As you know, trace element analysis plays an important role in many fields and has at least two major health impacts. Better measurements are providing new information on the necessary role of trace elements in vital metabolic processes. And trace element analyses, both of biological specimens and of the environment, are revealing the level of and potential danger of many pollutants. Analysis has a major role to play in the materials area, since performance
is so intimately tied to composition. As shortages of traditional materials accelerate the search for substitutes, as we seek to recycle more materials, and as requirements for stronger, tougher, longer lasting materials arise, the role of the analyst will become even more important. Analysis affects the economy in several ways. Quality control based on valid analytical results can improve productivity, reduce warranty claims, and help American goods compete in world markets. Analysis has an especially important role to play in the energy field. As we edge our way closer to controlled fusion devices, spectral characterization of the plasma becomes an increasingly important diagnostic tool. As new sources of coal and oil are developed, analysis of the fuel itself or of the combustion products is necessary in meeting environmental standards. And the development of new converters of solar energy will require accurate data on composition. I could go on with other examples, but I think the point is clear: rapid, accurate, inexpensive analyses will play a major role in determining the type of world, and the quality of life, that we achieve in the years ahead. In some cases, using tried and proven methods of analysis, the answers will come easy. In others, totally new ap-
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proaches will have to be developed. But no matter what quantity is being measured, no matter how simple or sophisticated the technique, accuracy will remain the very foundation of chemical analysis. An accessible system of standards is necessary if accuracy is to be attained through the measurement chain. You chemists and spectroscopists provide the accurate measurement base that is of fundamental importance to our scientific and technological society. We at NBS provide what I call the other face of that measurement base: the standard reference materials, reference data, reference methods, calibration services, even the base units themselves, that help all of you achieve the accuracy you seek. NBS is no stranger to measurement standards, analytical chemistry, or spectroscopy. Congress created NBS in 1901 to meet a national need for a unified measurement system, a need that had many roots. The rapidly growing electrical industry, the growth of industrial research, the campaign for fairness in trade, all created demands for accurate measurement. The Act of Congress which established the Bureau assigned us responsibility for "custody, maintenance, and development of the national standards of measurement." Recognizing that having standards was only half
Metre bar, composed of 9 0 % platinum1 0 % iridium, served as the Nation's fundamental standard of length from 1893 to 1960
Report
of the
Measurement Base the job, we were also charged with providing the "means and methods for making measurements consistent with those standards." As might be expected, NBS started small, with a staff of 11 and with borrowed quarters in the Coast and Geodetic building. Work began immediately on fundamental measurements in electricity, photometry, and thermometry. While a chemistry division as such wasn't formed until 1905, the first annual report of the Bureau recognized the need of chemists for calibrated burets and graduates, and a program was started in this area. We began early in spectroscopy, and the fourth paper ever published by NBS was by Perley Nutting on the spectra of mixed gases. In 1903 William Noyes, one of the most distinguished chemists of his time, joined NBS. He started a line of contributions to the field of chemical analysis that has continued unbroken to the present time. As new staff and new programs were added, we soon outgrew our temporary quarters. By March of 1903 work had begun on a permanent NBS facility on what were then the outskirts of Washington; and within a few years the facilities, staff, and work of NBS were recognized on a par with the great national laboratories of Great Britain and Germany.
In the 1960's NBS moved again, this time to a magnificent new site in Gaithersburg, MD. Here, on 576 acres, are located 23 major buildings, special facilities such as a nuclear reactor, anechoic chamber, a testing machine that can apply 12 million pounds in compression, and accommodations for conferences that bring over 20,000 scientists to our site each year. At Boulder, CO, is located a somewhat smaller lab that concentrates on cryogenics, time and frequency, and electromagnetics. Our total staff is about 3,600, and our budget this year is a little over $100,000,000. Measurement Language
Today, environmental concerns, the oil crisis, the unique perspective provided by recent space flight, all have created a growing awareness of the need for international cooperation. While we are many nations, we are but one people. For the mutual benefit of mankind, we must recognize and foster the sharing of knowledge, materials, and products. No nation can close its eyes—or its borders—to the needs and contributions of its neighbors. But there are barriers to the cooperation that is so vital to progress. One major hurdle is that of different languages, a distinct impediment to the interchange of knowledge and the creation of new understanding. Fortu-
nately, we in science have a measurement "language" that provides a uniform, universal basis for the exchange of information. This "language," the International System of Units (SI), was formalized by the 11th General Conference on Weights and Measures in 1960, and provides a set of definitions that is accepted and used world wide. Basically, the SI language is the essence of simplicity: seven base quantities provide the foundation from which are derived all other quantities. The seven are mass, length, time, temperature, luminous intensity, electric current, and amount of substance. In principle, these seven units are conceptually quite simple, but their realization requires continuing research at the frontiers of science and technology. Before I touch on some of this research, let me provide just a brief framework of history and procedure. While the metric system had its origins in the reforms precipitated by the French revolution, it made little headway except in scientific circles until 1875. At that time a Metric Convention was signed by the United States and 16 other nations. Under the agreement, prototype standards for length and mass were to be constructed and delivered to each member nation, an International Bureau of Weights and Measures was established near Paris
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In 1960 the metre was redefined by in ternational agreement in terms of wave length of light emitted by 86 Kr. To re duce thermal effects, lamp is operated inside Dewar flask at triple point of ni trogen
to coordinate the measurement sys tem, and a General Conference on Weights and Measures was to be held every six years to settle important questions. In 1890 after a drawing by lots, the United States received two metre bars (numbers 21 and 27), and two kilo grams (numbers 4 and 20), which in 1893 were declared to be the Nation's fundamental standards of length and mass. Since that time the yard and the pound have been defined in terms of metric units, a situation that makes us in many ways a metric nation whether or not we realize it. Length
The metre bars that came to this country provided the fundamental standard of length for over 70 years. These bars are composed of an alloy of 90% platinum, 10% iridium, and have two parallel, microscopic scratch marks that define the metre. There are, of course, fundamental difficulties associated with such a standard. Un known changes in length, owing to crystal structure changes, could take place; a bar could be dropped or dam aged in some other way; and most im portant, only NBS could have the na tion's primary length standard. As early as 1893 Michelson had pro posed a wavelength definition of the metre, based on an emission line of cadmium. As time went by, other sci
entists championed other wavelength standards, including Meggers of NBS, who developed a mercury-198 lamp that was both convenient and accu rate. Finally, in 1960 the metre bar was officially supplanted by a wave length definition. The General Con ference agreed that a metre was 1,650,763.73 wavelengths, in vacuum, of the orange-red spectral line of 86 Kr. While this was a giant step forward, in that any well-equipped laboratory could operate its own primary length standard, the definition had some in herent problems. First, to reduce ther mal effects, the krypton discharge lamp is operated at the triple point of nitrogen, an inconvenient arrange ment. Next, 86 Kr can only produce in terference fringes over a length of 50 centimetres or so, meaning that longer lengths must be "stepped off," with a resulting loss of accuracy. Finally, the emission line itself was discovered to be asymmetric, limiting the accuracy with which measurements could be made. In 1960, the year in which 86 Kr was adopted as the definition of the metre, operation of the first laser was an nounced. An immediate interest was shown at NBS in using the laser as a potential length standard. NBS mea surements, published in 1964, demon strated the production of bright inter ference fringes over a path length of 100 metres, a distance that was dictat ed by the length of an available mea suring room and not by the coherence of the He-Ne laser light. But progress rarely comes easy. Studies at NBS and elsewhere re vealed instabilities in the laser wave length large enough to limit its useful ness as a length standard. Various sta bilization schemes were tried, but the one found most effective was devel oped by Barger and Hall of our Boul der laboratories. Their approach was to lock the laser wavelength to a mo lecular absorption line in a cell of methane gas. This approach provided a basic line width of a part in a billion, and a reproducibility of a part in 100 billion for two independent lasers. Again, there was a minor problem, for the laser line stabilized by meth ane is at 3.39 μπι, in the infrared por tion of the spectrum. A search was made for a suitable molecule with which to stabilize the 633-nm visible line of the same laser, and iodine was found to be the answer by Schweitzer, Kessler, Deslattes, Layer, and Whet stone at the Gaithersburg labs. But stabilizing the laser is only half of the job: its wavelengths also had to be carefully determined in terms of the primary 8 e Kr standard. This was done both at NBS and at other na tional laboratories which followed our lead, and in 1973 two laser wave-
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Speed of light was determined by mea suring both frequency and wavelength of same He-Ne laser line. (Shown is Kenneth Evenson of NBS-Boulder, who was responsible for frequency determi nation)
lengths were adopted by the General Conference as secondary wavelength standards. This may have been the first step toward adoption of a laser wavelength as a replacement for 86 Kr. Recently, another elegant experi ment tied the X-ray scale to SI units for the first time. In this work, the lat tice repeat distance of a nearly perfect silicon crystal was measured by means of simultaneous X-ray and optical interferometry. X-rays were passed through three parallel silicon crystals, one of which was moved relative t
