the chart follows the Gray code input. “FORWARDREVERSE” is accomplished as previously described by locking out the inhibit signal from reaching SI. The driver amplifier is comprised of two identical push-pull sections which translate TTL logic levels to the levels necessary for driving the motor. This design allows the components shown to be used for both three-wire ac motors which require a center tap power supply and, with slight modification, for five-wire bifilar wound dc motors. CONSTRUCTION
Of prime importance in the implementation of this system is the availability of a recorder in which the scanning motor is easily accessible and in which the driver mechanism can be reversed without objectionable backlash. The recorder chosen for this project, a potentiometric strip chart recorder manufactured by the Bristol Division, American Chain and Cable Company, Inc., Waterbury, Conn., scored well on both points. It was possible to directly replace the existing motor with a slo-syn SS 25 manufactured by Superior Electric Company, Bristol, Conn. A single gear was utilized to directly couple this motor to the chart gear. The original gears were thereby discarded. Other recorders commonly found in the laboratory such as the Brown and Leeds & Northrup were also examined. In these units, motor replacement can also be accomplished with relative ease; however, backlash might be a problem if the original gears were employed. Although this is not a serious
consideration in many applications, it should not be entirely neglected. The circuit was constructed on three DEC flip-chip blank printed circuit boards. Separate cards were used for the motor power supply, control logic, and the drive amplifiers. Care was taken to separate input and output lines on the drive board and the control board. Use of 0.1-microfarad capacitors on the power supply pins of each device minimized coupling through the power leads. Trim pots are provided on the input buffer amplifiers for adjusting the filtered Gray code for a 5Oz duty cycle at the zero crossing detector input. Other than this, no circuit adjustments are required. CONCLUSION
This system has been in operation for over a year and has proved to be a virtual work horse in the laboratory. The analysis of data automatically acquired proceeds with much greater confidence now that each data point can be related to a real time event. Instrumental errors show up immediately because each scan is automatically overlayed. As the computer plotted spectrum is also related in real time, it can be easily compared with the original data as an additional check on performance and procedure. This would prove to be invaluable in analyzing the first run of a new data reduction program. RECEIVED for review February 25, 1971. Accepted July 19, 1971.
New Bubbler Design for Atmospheric Sampling Herman D. Axelrod, Arthur F. Wartburg, Ronald J. Teck, and James P. Lodge, Jr. National Center for Atmospheric Research, Boulder, Colo. 80302
THE USE OF BUBBLERS is a widely accepted technique for gas sampling. The typical apparatus is made entirely of glass, and various parts are sealed with standard tapered ground glass joints. The dispersing device is a straight glass tube with an open or fritted glass end. These bubblers are easily damaged and costly to repair, especially if a glass joint becomes damaged. Some of the many styles of bubblers have been described by Wartburg et al. ( I ) . Wartburg et al. ( I ) also improved upon the previous designs by incorporating a Teflon (Du Pont) top mated to a glass bottom, thereby making the interchangeable parts relatively inexpensive to replace. For airtight seals, this improved design, however, required precise glass tolerances in the diameter and roundness of the bubbler neck. If the glass bottom neck is slightly oversize, the top is difficult to remove and in the process, the frit stem can be damaged; if the neck is undersize, the top will not seal to the bottom. Furthermore, components of the top are sealed with epoxy glue, thus preventing the cleaning of the top in corrosive solutions. The new bubbler design described here has eliminated the above problems through several significant changes in design. Figure 1 shows the details of the top. The entire unit is
(1) A. F. Wartburg, J. B. Pate, and J. P. Lodge, Jr., Eizcirort. Sci. Techtzol., 3, 767 (1969). 1916
KNURLED BRASS C A ?
O - R I N G SEAL PLASllC HOSE CONNEClOR
1EFLON
Figure 1. The bubbler top
machined from l l / ~ - i n .diameter stock Teflon. The top screws onto the bottom of the glass bubbler in a fashion similar to that of a lid onto a jar. The silicone rubber 0ring seals the units along the outer edge of the glass bottom, providing an excellent seal regardless of slight differences in bubbler neck sizes.
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
series these bubblers or to perform some other operation, the plastic tube fitting can be replaced by a more elaborate one, such as a Teflon fitting, but regardless of the fitting selected, epoxy glue is not required. When chemicals are to be stored in the bubbler, the top can be replaced by a separate screw top to seal the bottom section, thereby lessening the chance of leakage or contamination. The frit is made from ceramic porcelain (100-pm pore size) which offers much greater uniformity of hole size than the often used glass type. Because the previously used glass frit tended to have large variations in pore size, only a few holes were used for dispersion. The uniformity of the ceramic type allows air to be dispersed through many more holes, which should aid in trapping efficiency. A typical pressure drop for this frit is 22 mm Hg. The bubbler bottom is obviously threaded to match the top. The long bottom stem has 0-25 ml graduations which allows easy addition of reagents from plastic squeeze bottles rather than pipets. The lengthy barrel should also increase trapping efficiency. The entire system shown in Figure 2 is leak-proof and all parts are interchangeable; therefore, any top will attach to any bottom. The bubbler also has extreme flexibility in that the lower section can be designed for any application as long as the bottom has threads to match the top. In summary, this design offers excellent reliability, ease of operation in the field, and low replacement costs for broken parts.
Figure 2. The assembled bubbler unit with ceramic frit
Similar to the Wartburg design, incoming air passes through a glass ball joint into the glass stem of the frit, but in our design the exit gases pass through an off-center hole and exit through a plastic (polyethylene) fitting. If it is desirable to
RECEIVED for review June 28, 1971. Accepted August 10, 1971. The National Center for Atmospheric Research is sponsored by the National Science Foundation.
Simple Optical Geometry for Obtaining Raman Back Scattering James R . Scherer, G . F. Bailey, and Saima Kint Western Regional Research Laboratory, Agricultural Research Seroice, US.Department of Agriculture, Berkeley, Gal$ 94710 RAMANSCATTERING from solid materials that exhibit high surface scattering or absorption of the incident radiation can be obtained by using a 180" back scattering geometry in which the direction of observation is opposite the direction of beam propagation. This general optical arrangement has been described earlier by Hawes et al. (1). The incident beam is focused by means of a long focal length lens onto a small diagonal prism placed against a hemispherical lens in optical contact with a solid pellet or capillary cell (for liquids). This design was not intended for excitation of small samples that might be difficult to find with a focused laser beam. Furthermore, it is difficult to irradiate selectively different portions of the sample without moving the sample relative to the hemispherical lens/diagonal prism assembly. A simpler arrangement that uses the light collection optics for focusing the laser beam is schematically shown in Figure la. D is the diagonal that deflects the unfocused laser beam through the ( 1 ) R. C. Hawes, K . P. George, V. C. Nelson, and R. Beckwith, ANAL.CHEM., 38, 1842 (1966).
light collecting lens, L, and onto the sample, S. The lens, L, collects the scattered light and images it on the entrance slit of the monochromator, M. For practical image magnification, the focus of the laser beam falls short of the point at which the samples should be placed for good image formation at the entrance slit. The focus can be extended by curving the surface of the mirror, D. The beam shape at the sample can be made circular or elliptical (slit shaped) by fabricating the diagonal from a convex astigmatic lens. If the sample is a capillary whose axis lies along Y and if it is aluminized on its back surface to increase light collection efficiency, the beam returning from the capillary is spread out in a horizontal (XZ) plane. If this light enters the spectrometer, we observe intense grating ghosts which can interfere with Raman scattering. This return beam can be interrupted by a horizontal bar (supporting the diagonal D) which is attached to a ring that adapts to the lens assembly, L (Figure 16). When the incident laser beam is not returned by an aluminized surface, the horizontal mask bar is not required and a simpler arrangement is to use a prism diagonal mounted at the center
ANALYTICAL CHEMISTRY, VOL. 43, NO. 13, NOVEMBER 1971
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