Atomic Absorption Spectra of Vanadium, Titanium, Niobium, Scandium, Yttrium, and Rhenium Sir: I n several recent communications
Top View
(5,6 ) , we reported on the striking enhancements in the atomic emission spectra of some elements (vanadium, titanium, niobium, scandium, yttrium, rhenium, and the lanthanides) when solutions of their compounds viere aspirated in fuel-rich oxyacetylene flames. These observations, plus other considerations discussed in our papers, have led us to conclude t h a t fuel-rich, osyacetylene flames provide an environment more favorable to the existence of free atoms of those elements which have a strong predilection to form stable monoxide molecules in stoichiometric flames. The greater free-atom populations in these fuel-rich flames should produce a parallel increase in the atomic absorption spectra of these metals. Indeed, this effect has been observed for several e l m e n t s not included in the group under present consideration. Thus, Allan ( 1 ) reported a 30% increase in the atomic absorption sensitivity for manganese and a fourfold increase for chromium ( 2 ) by increasing the fuel to osidant ratio in air-acetylene flames, and David (3) found that the ground state molybdenum and strontium atom population in a similar flame was critically dependent on the oxidant to fuel ratio. Gatehouse and Willis (?’) noted that the optimal conditions for the atomic absorption detection of vanadium involved the use of a “very rich” air-acetylene flame, but they were unable to detect less than 1000 p.p.m. by
Table I. Experimental Conditions for Photographic Recording of Absorption Spectra
Spectrograph
Order Reciprocal linear dispersion Wavelength regions and pliotographic emulsions Exposure time Sector
~
252
Jarrell-Ash Co., 3.4meter Ebert mounting plane grating spectrograph, using 6-inch, 15,000 lines per inch grating blazed for 5000 A., first order (8) Second 2.6 A./mm. Spectrum analysis #1
8 seconds All exposures were made through a rotating 8-step sector ~
ANALYTICAL CHEMISTRY
U
Three Flames
Xenon Arc
Figure 1. Multipass optical system for photographic recording absorption spectra
weight of this element in solution in their flame. ilnalytically useful atomic absorption spectra of vanadium, titanium, niobium, scandium, yttrium, and rhenium can be observed in highly fuel-rich, oxyacetylene flames. Since these elements have evaded detection by flame atomic absorption techniques using stoichiometric or less fuel-rich flames, the observations reported in this paper should widen the potential scope of application of this analytical technique. Slavin and Manning (9, I O ) , using a commercial atomic absorption spectrophotometer, have already confirmed our observations on vanadium and titanium.
atomic
~~
~
Table 11. Wavelengths and Detection Sensitivities of the Strongest Absorption Lines
Element Vanadium
wavelength, A. 3183.41 3183.98 3185.40 3828.56 3840.75
3855.84 3902.25 4111.78 4379.24 4384,72
Titanium
EXPERIMENTAL
Apparatus. T h e atomic absorption spectra reported in this communication were observed photographically. A schematic diagram of the experimental arrangement is shown in Figure 1. T h e background continuum source was a IjO-watt, high pressure xenon arc (Hanovia Chemical and Mfg. Co., k2931). The radiation from this lamp was passed through the flames of three integral atomizer, oxyacetylene burner assemblies (Beckman Instrument, Inc., $4030) operated under the fuel-rich conditions previously described (5, 6 ) . The effective path length of these flames was increased by a triple pass arrangement. The latter consisted of a Jarrell-Ash #82330 multipass attachment modified so that three traversals of the xenon continuum passed through the portion of the flame exhibiting greateqt absorption. The modifications made in the JarrellBsh attachment were as follows: -4s is apparent in Figure 1, the mirrors and lenses in the modified attachment were mounted in the horizontal plane. The
of
Siobium Scandium
4359.97 3635.46 3642.68 3653.50 3729.82 3741.06 3752.86 3948.67 3966.34 3958.21 3981.76 3989.76 3998.64 4058.94 4079.73 4100.92 3255.69 3269.91 3273.63 3907.49 3911.81 3933.38 3996.61 4020.40 4023.69 4054.55 4082,40
Yttrium
Rhenium
4077.38 4102.38 4128.31 4142.85 3451.88 3460.46 3464. 73
Sensitivity (p.p.m. weight in py solution) 25 10 25
100 100 50
100 100 50
100 100 100 50
50 100 50 50
100 100
100 100 100 50
250
250 250 50
10 10 5 5
100 50
5 5 50 50 50 100 100 100 50 25 25
three flame< were situated centrally between the mirror-lens assemblies. Continuous radiation from the off-axis xenon arc nas focubed on the central flame by the source lens. The light traversed all three flames a t a distance of 25 mm. above the burner tip. The transmitted radiation was then reflected by mirror .-I t o mirror B. After reflection a t mirror B, the light again passed through all three flames 25 mm. above the burner tip; the transmitted light \\-as then reflected by mirror C to mirror D. The latter formed a n image of the source on the central flame 25 mm. above the burner tip. A conventionai crossed cylindrical lens sl-stem was employed to form a n image of the xenon Source on the collimating mirror of the spectrograph
(4).
The experimental conditions for the photographic recording of the spectra are summarized in Tsble I. Solutions. T h e solutions were prepared in accordance with t h e procerlurrs de-c ribed previously ( 5 , 6)
RESULTS
The wavelengths of the strongest absorption lines of these elements and their sensitivities of detection are summarized in Table 11. The latter are expressed as the concentrations required to produce a visually detected absorption line. It is evident that these lines exhibit adequate sensitivity to satisfy many analytical requirements. ACKNOWLEDGMENT
The authors are grateful to D. W.Golightly for his experimental assistance and to Richard B. Kniseley and Robert B. Myers for helpful discussions during the course of this investigation. LITERATURE CITED
(1) Allan, J. E., Analyst 83, 466 (1958). (2) Allan, J. E., Spectrochim. Acta 18,
259 (1962). (3) David, D. J., A\'uture 187,1109 (1960). (4) Feldman, C., Ellenburg, J. Y., Spectrochznz. dcta 7, 349 (1956).
( 5 ) Fassel, V. A., Curry, R . H., Kniseley, R. K.,Ihid., 18, 1127 (1962).
(6) Fassel, V. 4.,Myers, R. B., Kniseley, R. S . ,Ibid., in press. ( 7 ) Gatehouse. B. M.. Willis. J. B.. Ibid.. 17, 710 (1961). (8) Jarrell, R. F., J. Opt. Soc. Am. 45, 259 (1955). (9) Slavin, W., Atomic Absorption Newsletter, Perkin-Elmer Corp., October 1962. (10) Slavin, W., Manning, D. C., ANAL CHEV. 35, 253 (1963). VELVERA. FASSEL VICTORG. Mossom Institute for Atomic Research and Department of Chemistry Iowa State University Ames, Ion-a RECEIVED for review December 3, 1962. Accepted December 19, 1962. Contribution No. 1258. Work was performed in the h i e s Laboratory of the U. S. Atomic Energy Commission. Presented in part at the Xth Colloquium Spectroscopicum International, College Park, Md., June 21, 1962, and at the International Symposium on Molecular Structure and Spectroscopy, Tokyo, September 12, 1962.
Atomic Absorption Spectrophotometry in Strongly Reducing Oxyacetylene Flames SiR. In the pre.jent state of development of atomic absorption spectrophotometry, about 35 metals can be determined in solution a t concentrations of parts per million or less. An equally large grouli of metals resist analysis becauce, n hen their compounds dissociate in the flames that are conventionally used, oxides or hydroxides are immediately formrd, binding the metal in a refrartory compound that will not dissociate at the temperatures available. It n-ill be shown in this comniunication that, \\hen the flame conditions are properly chosen. the forniation of the oxide or hydroxide can be significantly inhibited. and an atomic vapor will be produced IT hich will absorb energy a t the reconance n avelengths. I n thic way, inany more metals can be determined d o n n to the parts per million le^ el in a +elution. This observation has heen rcy~oitcdby FasTel (6, 6). Much of the published literature on atomic ah-orption spectrophotometry describe; the use of a solution atomizer of thr p r c n i i ~type ( 3 ) . When an airncetdene flame is used in this burner and the acetylene flow is increased to the point where the flame is brightlv incandevrnt. it is 1)ossible to determine some metalc-e g., NO,Be, Ru, Cr, Snn hich absorb only slightly in a stoichiometric fuel mixture. We and others ( 2 , 8 ) have attempted to observe the atomic absorption of aluminum, vana-
dium, boron, and other metals in such a flame, but without success. Robinson (11), using a n oxycyanogen flame, detected weak absorption due to vanadium, but none a t all from the other metals mentioned. However, Dean ( 4 ) ) Fassel ( 7 ) , Gilbert (9),and others have observed emission a t the resonance lines from many of these metals in conventional total consumption burners utilizing very rich oxyacetylene flames. It seemed clear that if an atomic vapor is present that can emit radiation, it must d s o be available to absorb radiation. K e therefore modified a Perkin-Elmer Model 214 atomic absorption yiectro-
photometer (IO) to use a Zeiss total consumption burner-atomizer. Using a very rich oxyacetylene flame, we observed strong absorption from aluminum, vanadium, titanium, and beryllium solutions. (Hollow cathode lamps were used for all the analyses reported here.) Typical curves of concentration 1's. absorbance for these elements are shown in Figure 1. To observe the sensitivity indicated in the figure, use a salt of the metal which ill readily dissociate a t the temperature of the acetylene flame. For example, aluminum was present in our experiment as
"1 0.8
0 IO0 CONCENTRATION ( P P Y I
io0
Figure 1. Working curves for various metals determined in very reducing oxyacetylene flames VOL. 35, NO. 2, FEBRUARY 1963
253
