I
G. A. Rechnitz Universitv,
of
Pennsvlvania Philadelphia
I
Atomic Absoration Sim,.alified . ----
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Spectrophotometer
The rapid increase in theoretical and applied papers featuring the new spectrochemical technique of atomic absorption spectroscopy indicates that experiments illustrating the features of this method might be a fruitful addition to some undergraduate instrument,al analysis courses. Unfortunately, the instruments described in the literature to date have been largely of a research nature with elaborate electronic circuitry and accompanying expense (5, 7, 8). This paper endeavors to describe a simple instrument which can be constructed of inexpensive components readily available in the average college laboratory. Atomic absorption spectroscopy has only recently been employed for analytical purposes, but shows COJIsiderable potential for elemental analysis of solution samples (1-4). The main advantages of atomic absorpt,ionspectroscopy over flame photometry are related to decreased interelement effects and increased sensit,ivity for certain elements such as zinc and cadmium. A recent count indicates that some 38 elements can be det,ermined by this technique a t present (6). AJI exrrllent review of the field is given by Robmson (7).
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Atomizer-Burner. Any of the standard atomizerburner units used for flame photometry may be employed in this instrument. Since it is only necessary to produce an atomic vapor of the sample (and excitation of the sample components is, in fact., undesirable), a simple Meeker type burner fitted with a gravity-flow at,omizer is entirely adequate for alkali metal det,erminations. A small tank of compressed propane serves as an excellent source of fuel for the burner system and provides flame temperatures of up to 1900°C. A supply of regulated compressed air is desirable for quantitative work but lot essential to operate t,he instrument. Filters. The filters serve as crude monochromators to redure the intensity of interfering radiation outside the desired waveleugth range reaching the detectors. Karro~vband pass color filt,ers (such as the Klett-Summerson rolorimeter filters) giving a band pass of 50-70 mfi are ent,irely adequate for this purpose. Detection System. The detectiou system employs t x o CdS photoconductive cells (such as the International Rectifier CdS cell type (S-120 MG)); one is placrd directly into the light path and the other in a posit,ion perpendicular to the light path and facing the
Figure 1. Schematic diogram of the rpedrophotomekr. A Atomizer and burner Dl and D? Detectors FI ond F2 Filters H Helipot S Source of resonance G Galvanometer radiation
Figure 2. Wiring diagram of the rpectrophotometer. B Dry cell (3 v) Dl and Dp CdS detectors Rl mnd Rr Rerirtorr 15 Kl G Galvanometer Icenter-zero) H Helipot, ten turn (200 KI
1:igurc 1 is a schematic diagram of the instrument. The particular instrument described is intended for the detection of sodium in solution samples but could easily be adapted for the determination of other alkali metals by simple replacement of source unit a.nd filter element,s. Sorcrce Unit. A sodium vapor polarimeter lamp serves as a convenient source of resonance radiation in the .5889-95 A region. This utlit is line operated but m;ly be slightly underrun to give a narrower wavelength band. Variations in intensity of radiation produced are nrgligible after a 30-minute warm-up period.
flame. The wiring diagram involves basically a bridge circuit and is given in Figure 2. Atomic absorption spectroscopy depends on the ahsorption of resonance radiation by an atomic vapor of ground state atoms in the flame. With presently available equipment it is impossible, however, to generate an atomic vapor of an element in a flame without producing a substantial fraction of excited atoms which may emit resonance radiation. The complicating effect, of emission in the flame is eliminat~din this instrument by the use of two detectors. One detector receives only that radiation originating in the flyme, while the other Volume
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sodium-containing sample is then substituted for the pure solvent and the galvanometer is again zeroed by means of the helipot. Both detectors will receive equal contributions of emitted radiation from the flame, hut detector Dpwill show a decreased response due to the absorption of some source radiation by the sodium vapor in the flame. The difference in helipot readimgs for blank and sample can be related to sodium concentration in the sample by means of a working curve. For reproducible results it is essential that flow rates for fuel, air, and sample be controlled as closely as possible. Figure 3 shows some results obtained for aqueous samples of NaCl in the 5-180 ppm range.
N=+ lppm) Figure 3.
Some resultsfor aqueous sampler of NaCI.
detector is exposed to any source radiation which has passed through the flame as well as that radiation due to emission in the flame. The operation of the instrument is extremely simple. After a 30-minute source warm-up period the atomizerburner is put in operation and the helipot is adjusted until the galvanometer reads zero. This compensates for the difference in detector response due to unequal light intensity. Results will be improved if solvent is atomized as a "blank" sample during this operation. The
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Literature Cited (1) ALKEMADE, C. T. J., A N D MILATZ,J. M. W., Appl. Sci. Research, 4b, 289 (1955). (2) ALLAN,J. E., Analyst, 83,466 (1958). (3) DAVID,D. J., Analyst, 83, 655 (1958). R., AND HAMES,G. E., Analyst, 87,385 (1959). (4) LOCKYER, ( 5 ) MALMBTADT, H. V., AND CHAMBERS, W. E., Anal. Chem., 32, 225 (1960). D. C., paper resented at 9th Annual Anaehem (6) MANNING, Conference, Detroit, October, 1961. J. W., Anal. Chem., 32, 17A (1960). (7) ROBINSON, (8) RUSSELL,B. J., SHELTON, J. P., AND WALSH,A,, Spectraehim. Acta, 8, 317 (1957).
