The Design and Construction of an Automatic Spectroscope a Thesis in Electrical Engineering

THE DESIGN AND CONSTRUCTION OF AN AUTOMATIC SPECTROSCOPE A THESIS IN ELECTRICAL ENGINEERING Zackie D, Reynolds Approved Texas Technological College May, 1951 Tp: DESIGN AND CQNSTEUCTION OP AN AUTOMATIC SPECTROSCOIE A THESIS IK ELECTRICAL ENGINEERING Submitted to the Faculty of the Division of Graduate Studlea of Texas Technological College in Partial Fulfillment of tê Requirennts for the Degree of MASTER OP SCIENCE Zaokie D» Reynolda, B. S« It Lubbock, Texaa May, 1951 ACKNOTIXEDGSMENT Ihe aulhor hereby expresaea hie appreciation to the m9sft)era of his Oliesls Committee, Professors C, £• Houston, chairman, H, A. Spuhler, and v . i:. Craig, ô have given helpful criticism and suggestions in the preparation of tîs paper* La particular, appreciation la expreaaod to Dr« Craig, Professor of Chemistry, for providing the opportunity of working on this project, for his much needed moral support, and for hia assistance in coordinating the spectrographic science with the electronic* in U TABLE OF CONTENTS Introduction •••••••••••••••• 1 1* Principls of Operation ••••• 9 2. The Spectroacope • • • • • • • • • 14 3, The Multiplier Phototube 21 4« The Power Supplies • 30 Theoretical Considerations Specific Construction Requiremonts Circuit resign The High Voltage Supply IhB / 250 Volt Supply 5* The Sequence and Timing Control •••••• 54 6. The Amplifiers • •••• 65 7* The Recordera • 76 8« Teated Operation and Suggested Improvementa 80 Bibliography • • 65 ill IHTRODUCTIOH 2 In 1666,^ Sir laaao Newton first separated white light into a band of colors with a prism* In 1814, v • H. V.oHas ten observed spectral lines, and In 1859, 0, l • Klrchhoff and R. Bunsen made the first practical spectroscope. Thus, it v/as that the visible electromagnetic radiation of ex* cited bodies was separated into regions of different wave lengths* It has since been determined that electromagnetic radiation exists not only in the visible region but extends also into the short wavelength region of the ultraviolet and the X-raya as well as into the relatively long wave length region of ttie Infrared, or heat waves and the longer radio waves* Radiation near and including the visible region is very useful for the Investigation of matter, v1thin the wavelength limits of approximately 1000 Angstrom units (lA: 10-^^ meter) to 80,000 A, lies the region of visible llpht, the transmission limits of both glass and quartz, and the spectral sensitivity regions of photocell cathodes. By en ergy shifts and vibrations in the outer electrons in atoms and molecules, excited matter produces radiation in this region* Thla excitation may easily be produced by electric arc, spark, or gas discharge. The specific frequency of radiation from a sample of inaterlal is determined by the ^0, R* Harrison, R. C« Lord, and J. R, Loofbourow, Practical Spectroscopy* (New York: Prentice-Hall, Inc*, 1943), pp. 2-4* state of excitation and by the elemental constituents of the Sample, each elemsnt of the periodic table radiating energy at specific frequencies in its characteristic spectra* The more restricted spectral region near to and including vis ible light, 1*0* from approxlnately SOOO A to 8000 A con tains spectral lines of most elements with sufficient avail able Intensity to observe and measure. A large portion of spectroscopic work is now done in this region. The spectroscope has been an important aid to the re search scientist in determining the kind and magnitude of the fundamental constituents of matter. The scope of its use in technological laboratories Is still increasing, and its usefulness is not restricted to any one field of science. As the information furnished by the spectrograph becomes more and more important in Industry and research, the im provement of the performance of the spectrograph likewise becomes necessary. Specific improvements would be greater accuracy, shorter time limits between sample injection and the determination of useful information, and Increased con venience* This paper presents a combination of electronic and optical arrangemsnts designed to facilitate the rapid and accurate determination of quantitative measuremsnts of known materials in a spectroscopic sample. Instruments performing these functions are commercially on the market; one is manu factured under the trade name of "ûantometer." The spectroscope hez^ described was designed and constructed to off set the very high cost of the commercial instruments while providing an adequate instrument for college laboratories, small industries, etc. This work does not represent an in tention to compete with the quality or to knowingly infringe upon any of the circuit details used in ccmsnerclal instru ments of this type. Previously, nearly all quantitative analyses of test samples of material were accomplished by photographing the pattern of spectrum lines produced when the material was burned in an electric arc or spark. Since the intensities of the spectral lines are functions of the amount of each element present in the sample, the amount of exposure, and subsequent darkening of the film, may be used as indications of the percentage of each material present in the sample. In order to Interpret correctly the correlation between film exposure and percentage composition it becomes necessary to plot ''working grains'* of percentage composition versus den sity of the blackened film for each element. Ihls is ac complished b^ putting known samples of material in the spect roscope and using the film densities to plot the working cuz*vea* Since exposure time^ arc intensity, and densito meter measurements may vary between different runs, it be comes advantageous to refer all measurements to a standard, or known amount of a given material, in each sample* Thus, any variations in exposure from one run to another will af- foot the standard lines aa well as the "teat" lines* The relative denaitioa of the photo|praiphed lines may then bo uaod in calculating par cent age compoaition. Usually, the internal standard la a major conatituant of the sample, such aa the baao metal in an aHoy* In order to prevent large orrora, more than one teat is usually run on each sample, and the results are then averaged* 'Although the photographie method of quantitative analyaia la sexialtlve and pz<oaenta reaults which are generally bettor than those of chemieal analyaia, the process is re latively "round about*** Film sensitivity, developing pro- ceaa^ and density meaauroinenta nay all vary anS will affect tha reaulta* Also, by the time aeveral runa are made, the time consumed la conaiderable* For aeveral yeara the idea of applying phototubea in the direet meaaurement of apootral line intensity has been tried or suggested* However, until recently, the sensitiv ity of available tubes haa been too aaall and too variable to allow accurate meaauromenta of any but the atrongor linea * Alao, the range of relative intenaitiea, froei approxlmatoly 2 to 10,000 arbitrary unite, preaented a aerioua working limitation on the electrical circuita* v;ith the development of the electron multiplier phototube^ (in 1959-1940), hcm^ Â* M* Olevor, ""A Roviev of the Developmnt of Senai* tivo Phototubea,** Froc* j2£ !•£«£•# Vol* 29 (Auguat, 1941), pp* 4X5«4^* 6 ever, the aensitivity of the light receiver could be in- creaaed to one million times that of the conventional photo* tube, and this sensitivity could be controlled over a con siderably large range, to "handle" lines of different in tensity octaves, applied to logarithmic intensity scales* A few instruments have been developed which utilize multiplier phototubes for qualitative and quantitative an alyaia* Generally speaking, these units have been larê and expensive, notwithstanding the fact that the output of a multiplier phototube la generally sufficient to operate sensitive recorders with no intermediate stages of amplifi cation* The jnajor considerations are then the stability or constancy of amplification, short term reproductibility, long term stability, and general linearity*^ These are ea- aential for accuz*ate quantitative analysis, although not so important in qualitative measurements* An automatic apectroaonpe is herein described which uses multiplier phototuben aa detectors* Comblnationa of accepted electronic circuita and spectrographic procedure are applied to produce an inexpensive Instrummit designed to provide dependable and reproducible operation* 1E, A. Doettner and 0. F^ Brewing.ton, "The Application of Multiplier Photo-Tubea to Quantitative Spectroohenioal Analyaia," Jour* Opt* Soc* of Awer** Vol* 34 (January, 1944), "Âpplied Haaearch Laboratoriea, SpectroRrapher*a Newa Letter, Vol* 11* No* 11 (San Pranciacoi April, 1949), pp, 1-5* Thla unit was cona true ted for Speotrographio Roaearoh in the Department of Chemiatry, where ita primary uae will be for the quantitative analyaia of aaxsplea of material with known oonatltuenta, but with unknown amounta of these mater ials* 1h9 time of analyaia for a aample is, after insertion in the arc, approximately one and one«half minutes for in- atrianent operation and fifteen seconds per element for in- torprotation and recording of percentage composition* Sineo oertain spectral linea of an instrument have move definite line-intensity funetlcna of concentration than others, the choice of the indioatlng line for any specific element will affect the accuracy of obaervation for that element* Since analyaia by thla method la quite rapid, several runs may bo made and the results averaged to give very good results in a short time* Conmeroial Instruments give results with a deviation of approximately 0«1I^ to a naxlBRin of 4^ over a large aver age of runa*^ There la no reaaon to expect greater error in the inatrunent dlacuaaed if operated under identleal con- ditiona, i*e« whan the factor a of traiperature and atray light are controlled to the aame extent* <^mm It2iUl< ^tttr. PRINCIPLE OF OPERATION 9 General circuit analyaia is best aided by reference to Fig. 1, the block diagram of liie apparatus. The sample to be analysed is first prepared and in serted in a small recess cut in the tip of the lower arc electrode.

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