Beckman Flame Spectrophotometer - Analytical Chemistry (ACS

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ANALYTICAL CHEMISTRY

772 (7) Dobriner, K.,and Jonee, R. N., private communications. (8) Gore, R. C., McDonald, R. S., Williams, V. Z., and White, J. U:, J . Optical SOC.A m . . 37,23 (1947). (9) Hayworth, Richardson, and Sheldriok, J. Chem. Soc., 1935, 1576. ( I O ) Hayworth, Sheldrick, and Mavern, Ibid., 1935, 636. (11) Jones, R. N.,and Dobriner, K., Vitamins and Hormonca, 7, 293-363 (1949). (12) Jones, R. N.,Williams, V. Z., Wahlen, hl. J., and Dobriner, K., J . Am. Chem. Soc., 70,2024 (1948). (13) Lehrer, E., 2.tech. Phybik, 23, 169 (1942). (14) Liston, M. D., J . Optical Soc. Am., 37, 515A (1947). (15) Liaton, M. D., and White, J. U., Ihid., 40,36 (1950).

(16) MeAlister, E. D., hiatheson, G.. L.. and Sweeney, W. J., Rea. Sn'. Instrumenta, 12, 314 (1941). (17) National Technical Laboratories, Bull: 153 (May 1946). (18) Sutherland, G.B. B. M., and Thompson, H. W., Trans. Fareday SOC.,41, 171 (1945). (19) Trans. Faraday Soc., 41, 171 (1945). (20) White, J. U.,and Liston, M. D., J . Optical SOC.A m . , 40, 29 (1950). (21)Wild, R. F.,Rev. Sci. Instrumenfa, 18,436 (1947). (22)Wright, N.,and Herscher, L. W., J . Opfical SOC.Am., 37, 211 (1947). RECEIVED December 7, 1949.

BECKMAN FLAME SPECTROPHOTOMETER P. T. GILBERT, JR., R. C. HAWES, AND A. 0. BECKMAN National Technical Laboratories, South Pasadena, Calif. A new flame spectrophotometer, now commercially available, is characterized by a simple detachable atomizer handling ramplea of 1 ml. or less, a heated spray chamber for evaporating the spray, a versatile burner for oxygen-gas or other flames, and construction of the atomizer-burner unit ae an attachment for the Backman quartz spectrophotometer. Individual measurements can be made in very rapid sequence, and the precision of such measurements is a few tenths per cent of full scale. Interference effects and methods o f circumventing them are discussed. Flame spectra and detection limits are given for several dozen elementr, and excitation characteristics and methods of analysis are illustrated for a few cases.

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LAME spectrophotometry, as a quantitative analytical technique, has only recently aroused widespread interest in this country. Although fhne spectra have been used nearly 100 years for the qualitative identification of elements, the literature discloses little work on the use of flame spectra for quantitative determinations prior to 1929,when Lundeglrdh (7)published his first treatise on the method. A number of European papers on flame spectrophotometry have appeared since that time, but in this country it was not until 1939 (6) that any work was published, and only within the past 3 years has any apparatus been described which differs significantly from that of Lundeglrdh. Lundeglrdh and most of his followers employed a spectrographic technique. The solution to be analyzed waa sprayed into a flame placed in front of a spectrograph. After exposure, the photographic plates were developed and the optical densities of various spectral lines recorded thereon were measured. With the aid of suitable calibration data the optical densities could be correlated with chemical concentrations with relative accuracies of a few per cent. The photographic step introduced delay and inconvenience and also limited the accuracy of the method to the reproducibility of photographic emulsions. To overcome the disadvantages of the spectrographic method, direct-reading flame photometers were described as ea'rly as 1935. Instruments of this type were made by a t least three manufacturers in Germany prior to the war. In 1945 the first direct-reading flame photometer was described in this country ( 1 ) . Since that time many papers have appeared, describing applications and apparatus. Judging from the recent literature, it appears that the apparatus heretofore commercially available leaves much to be desired with respect to convenience and speed of operation, accuracy of measurement, and freedom from spectral interference in multicomponent solutions. To overcome these shortcomings the instrument described in this paper was designed. It has high optical resolving power with attendant freedom from the interference resulting from unresolved overlapping spectral bands. It provides high photometric accuracy. It is simple and fast in operation. Determinations

can be made in a few minutes, and no cleaning is required between samples. A single drop of sample is consumed in making a reading, and detectable concentrations may be as low as a few parts per 108 for the alkali metals and somewhat higher for other elements. The instrument has already been used for the determination of twenty elements and undoubtedly can be used for many more. Briefly, the features which distinguish the present instrument, described in greater detail below, from other direct-reading flame photometers, include : ( 1) a one-piece, high-suction, concentric atomizer requiring no rinsing and providing exceptionally constant and low rate of consumption of sample; (2) a heated spray chamber, which completely evaporates the spray, enhances luminous intensity, and improves stability of performance; (3) use of an oxygen-natural gas flame; and (4) construction of the atomizer-burner unit as a separate attachment to be used in conjunction with the Beckman quartz spectrophotometer whose utility for other purposes is not interfered with by its use as a flame sDectrouhotometer. DESCRIWION OF BECKMAN FLAME SPECTROPHOTOMETER

A satisfactory flame spectrophotometer must have adequate resolving power to differentiate between the spectral emission of the element being determined and the emissions of any interfering substances that may be present. For general utility and precise measurements an essential part of a flame spectrophotometer is necessarily a monochromator which provides a continuous selection of wave lengths with resolving power sufficient to separate completely most of the easily excited emission lines, and freedom from scattered radiation sufficient to minimize interferences. These requirements are provided by the monochromator of the Beckman Model DU spectrophotometer (4). As there are many hundreds of these spectrophotometers already in use throughout the world, it was decided to design a flame spectrophotometer which could take advantage of this monochromator and the aensitive and accurate photometric equipment of these instruments. In a paper presented in November 1947 before the Soil Science

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V O L U M E 22, NO. 6, J U N E 1 9 5 0 FLAME CHARACTERISTICS

Figure 1. Flame Spectrophotometer Accessory

The oxygen-gss flame wa8 selected beesuse of its high temperature and convenience. An air-gas flame, because of its lower temperature, is suitable only for the determination of elements which are easily excited, such as the alkali metals. Oxygen-acetylene and oxygen-gas produce much hotter flames and correspondingly greater excitation. The former is somewhat superior in this respect to the latter, hut this advantage is offset by the higher level of background illumination and the more stringent requirements imposed upon the design of B burner which will produce B steady, quiet flame with an oxygen-acetylene mixture without danger of flashing baok. Mixtures of oxygen and artificial gas containing much hydrogen m d carbon monoxide also &ah back rather readily because of the high flame propagation velocity. Satisfactory fuels having relatively low flame velocity include naturalgas and bottled propane and butane (of. Table I, B). Dilution of the flame by the air used to spray the sample reduces both the background illumination and the intensity of metallic excitation, m d the ratio of spray to flame must be taken into account in the design of B flame photometer. Whereas tank nitrogen can be used instead of sir lor the atomizer, its use entails a not inconeiderable loss of sensitivity and increases oxygen requirements of the flme. On the other hand, oxygen can be used advantageously to atomize the sample. The flame is thereby altered in the direction of greater background intensity, but at the Bame time the intensity of metallic emission is roughly doubled. Whereas the present instrument is designed for an

Society of America, Rogers ( 9 ) told of his use of the Model DU instrument with rn air-acetylene flsme for the routine determination of patsssium and calcium in sails. Others also have used the instrument for emission photometry. General construction of the Beekman flame spectrophotometer accessory is shown in Figure 1. It consists essentially of B burner and s spray atomizer, with sssociated gages and regulators for air, gas, and oxygen. The components are assembled in a unit which pmvides a mounting for the Model DU spectrophotometer. The eontml box on the left contains the pressure gages, control for air, gas, snd oxygen, and a. pressun lstor and filter for the air. The w a t e ~ burner (Figure 5 ) is designed for natural oxygen and pmduees a broad steady J3a.m does not require precise optical aligrtmer the monochromator. The fuel is burned B wall porta, which are located amund Iarg< through which the air stream containi atomized sample is introduced. The ~ m t combustion 8es upward through a Gate, chimney. %e sample is contained in th beaker a t the extreme right (Figures 1 I The capillary inlet tube of the atomizer 6) which dips into the sample has its inle sharply tapered to about 70 microns to pre, trance of solid particles that might clog th lary With air at 10 to 30 pounds per inch pressure flowing through the atomizer tion of about 1to 3 meters of water is ore& &ect of change in level of the sample is, th entirely negligible. The atomizer and spm! ber are made of borosilicate glass and the is constructed of nonreactive materids contaminrttion. The spray tip which is trio with the air nozzle, has an' internal d of about 100 microns and Droduees a vem f runrtant spray which ew