EQUIPMENT
Lasers Boost Spectrograph Utility Jarrell-Ash grafts laser energy source to standard spectrograph, comes up with micro-emission analysis The optical maser (laser) has now been put to commercial use as an analytical tool. Jarrell-Ash Co., Newtonville, Mass., is using it as an energy source for a conventional spectrograph (C&EN, Aug. 27, page 35). Called micro-emission spectroscopy, the new laser technique bridges the gap between the conventional arc-spark spectrographic technique and electron microprobe analysis. The optical maser energy source is a self-contained system that fits on any spectrograph. It consists of a ruby laser head trained into the eyepieceend of a metallurgical microscope. Photons of light energy focus on a sample, producing temperatures of up to 20,000° K. According to Frederick Brech, Jarrell-Ash research director, micro-emission spectroscopy is a two-step process, the second of which embodies a crossexcitation. The intense heat caused by absorption of light energy first produces vapor from a spot on the sample. However, the excitation energy of the vapor is inadequate and must be increased. In the second step, the gas reaches the gap between two electrodes pressured to 1000 volts. The gas bridges the gap between these electrodes, producing very high excitation energies, Mr. Brech says. Electrically Neutral. An important feature of micro-emission spectroscopy is that analysis is independent of the electrical conductivity of the sample. Excitation produced in the conventional arc-spark spectrographs technique has a preference for more electrically conductive areas than for nonconductive areas. However, light energy is electrically neutral and will be absorbed by nonmetals as readily as metals. According to Mr. Brech, this phenomenon is especially important in analyzing metals containing refractories as inclusions. The light energy produced by the optical maser is absorbed by the refractory as easily as by the surrounding metal. However, in 52
C&EN
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3, 1 9 6 2
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Bridging the gap between emission and microprobe techniques, microemission spectroscopy . . .
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Is independent of electrical conductivity
• Can be used for localized sample areas
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Isn't limited to any elements
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conventional arc-spark spectroscopic techniques, the surrounding metal alone is excited because the refractory is the least conducting material. Therefore, to analyze inclusions a sample must be prepared by cutting out the inclusions and mixing them with graphite. This makes the inclusion electrically conductive. Mr. Brech also points out that micro-emission spectroscopy overcomes problems encountered with the electron microprobe. In the electron microprobe, a finely focused beam of electrons is directed at a specimen surface exactly on the spot to be analyzed. This electron bombardment causes excited atoms to emit characteristic x-rays. Crystals of appropriate materials are used to diffract the x-rays into their component wave lengths and the selected rays are focused into x-ray detectors. Laser excitation has an area of superiority over the electron microprobe in that the former is not limited to any element in the periodic table. The electron microprobe, however, is limited to elements higher than potassium (without special provisions). Main reason is the long x-ray wave 1
lengths these elements emit, which are readily absorbed by air. Therefore, costly and burdensome vacuum techniques are required, Mr. Brech says. Also, the long wave lengths produced by the elements below potassium are difficult to sort out, Mr. Brech claims. Organic crystals with large interplanar spacings must be used for this purpose. However, organic crystals have the disadvantage of being unstable under irradiation and their interplanar spacings change, Mr. Brech says. Small Area. The name micro-emission is given to the new analytical tool because of the very small area in which the light energy is focused. In the conventional arc-spark spectrographs technique, pits formed by the electrical spark cover an area from 3 mm. to 6 mm. The optical maser can focus on a spot as small as 50 microns in diameter. For analyzing the average composition of a sample, the arc-spark spectrographs technique is preferred. But if localized information is required, the laser excitation technique is better, Mr. Brech says. The optical maser energy source sells for $12,000 and is available on three-month delivery. The lasers used are manufactured by Trion Instruments Co., Ann Arbor, Mich.
Motorola Details New Line of Process Control Instruments Company's solid-state Teletrak line covers recording-controlling instruments and transducers Features of a complete line of solidstate process control instrumentation have been detailed by Motorola, Inc. (C&EN, Aug. 13, page 4 4 ) . The new line, named Teletrak, will be manufactured by Motorola Instrument and Control, Inc., at Phoenix, Ariz. Teletrak products include indicators, recorders, controllers, transmitters, special-function computers, and accessory equipment. Teletrak is a two-wire system that uses a current signal in the field and a voltage signal within the control room. Compatibility with other makes of transmitters is achieved with Transmack, a transmitter-matching unit. This unit provides proper supply voltage for transmitter operation and adapts signal range of other make
