Investigation of Spectral Overlap of the Neon 359.352-nm and Chromium 359.349-nm Spectral Lines in Atomic Absorption and Atomic Fluorescence Spectrometry of Chromium J. D. Norris and T. S. West Chemistry Department, lmperial Coiiege
ofScience and
Technology, London S W 7 2A Y . U . K
The overlap of the spectral lines of several pairs of elements has been observed in both atomic fluorescence (1-5) and atomic absorption spectrometry (6-11). In atomic absorption, spectral interference occurs when a n emission line of an element in the spectral source overlaps sufficiently with a n absorption line of a second element present in the atom reservoir, so that the atomic population of the second element absorbs some of the incident radiation. In atomic fluorescence, the spectral overlap absorption of radiation induces the atomic fluorescence of the second element in the atom reservoir. While this phenomenon invariably occurs as a spectral interference in atomic absorption, it may not do so in atomic fluorescence and may even be used to advantage since the fluorescence induced is not necessarily resonance fluorescence. The application of spectral overlap excitation in atomic fluorescence was first demonstrated by the determination of bismuth using a n iodine electrodeless discharge lamp (EDL)( I , 2 ) . The iodine 206.163-nm nonresonance line overlaps with the bismuth 206.170-nm absorption line to stimulate the direct-line atomic fluorescence of bismuth a t 302.464 n m . An arsenic EDL has been similarly employed to excite the atomic fluorescence of cadmium (3, 4 ) . The arsenic 228.812-nm resonance line overlaps with the cadmium 228.802-nm resonance line, thus stimulating the resonance atomic fluorescence of cadmium at 228.802 nm. In a carbon filament atom reservoir, arsenic radiation a t 228.812 nm was also shown strongly to stimulate the atomic luminescence of cadmium a t 326.103 n m in a nitrogen atmosphere ( 4 ) . Mercury and cadmium vapor discharge lamps have been reported to stimulate the atomic fluorescence of other elements (uiz.,chromium, iron, magnesium, palladium, and thallium) (5). Spectral overlap has been reported as a n interference in atomic absorption for several pairs of elements ( e g , europium and copper, platinum and iron, vanadium and aluminum, vanadium and silicon ( 6 ) , cobalt and mercury (7), gallium and manganese (8). antimony and lead ( 9 ) , and germanium and calcium ( I O ) . The overlap of the neon 359.352-nm line and the chromium 359.349-nm resonance (1) R. M . Dagnali. K C. Thompson, and T . S. West. Taianta 14, 1467 (1967). (2) R . S. Hobbs, G . F . K i r k b r i g h t . and T . S West. Taianta. 18, 859 (1971). (3) H . A . Fulton. K . C. Thompson and T. S. West. Anal Chim. Acta. 5 1 . 373 (1970). (4) J. F. Alder and T S. West. Ana/. Chim. Acta. 51. 365 (1970) (5) N. Omenetto and G . Rossi. Anal Chim Acta 40, 195 (1968). ( 6 ) V A Fassel. J. A Rasmuson a n d T G Cowley SpectrochJm 4cta Part B. 23. 579 (1968) (7) J. E. Allan Spectrochim Acta. PartB. 24, 13 (19691. ( 8 ) D. C. Manning and F . Fernandez. A t . Absorption Newsleft.. 7, 24 (1968) (9) S. Slavin and T . W . Sattur, A t Absorption Newsiett., 7,99 (1968). (10) K . C Thompson, Analyst f i o n d o n i 95, 1043 (1970). (11) D. C. Manning. A t . Absorption Newsiett.. 10. 97 (1971).
2148
line has also been reported ( 1 2 ) . Neon is frequently used as the filler gas in hollow cathode lamps, and a n analytical curve was obtained between 10 and 300 ppm for the atomic absorption of chromium with a neon-filled aluminum hollow cathode lamp (11). Omenetto and Rossi (12) attempted to excite the atomic fluorescence of chromium using a neon-filled hollow cathode lamp, but were unsuccessful because of the low emission intensity. This paper describes the preparation and operation of a neon EDL and its application as a spectral source for the atomic absorption and atomic fluorescence determination of chromium.
EXPERIMENTAL Apparatus. T h e EDL's were operated at 2450 f 25 MHz in a three-quarter wave resonant cavity, powered from a Microtron 200 microwave generator. Discharge was initiated with a Tesla high-frequency vacuum tester. The instrumentation employed for the atomic fluorescence measurements and the optimal operating parameters for chromium have been described previously (131. T h e atomic absorption measurements were made with a Techtron AA4 atomic absorption spectrometer. An air-acetylene flame. with an acetylene flow rate of 1.0 d m 3 min-l and a n air flow rate of 8 d m 3 min-', was supported on a 100-mm long-path burner. The part of the flame from 4 to 8 m m above the burner head was viewed by the monochromator. The EDL's were modulated 285 Hz in-phase with t h e Techtron amplifier, using a Microtron modulator unit. The monochromator slit-width employed corresponded to a spectral band pass of 0.3 nm. Preparation of Neon EDL's. T h e equipment and general procedure for the preparation of EDL's has been described elsewhere (14). The neon lamps were prepared in the following way. A blank tube ca. 40 m m long was prepared in the usual manner from 8 - m m internal bore quartz tubing. The blank tube was connected to the vacuum line and evacuated. It was flushed with neon (Ordinary grade: British Oxygen Ltd., Special Gases Division, London. SW19) and reevacuated several times. The tube was heated under vacuum to red heat for 3-4 min, and flushed several times with neon t o remove any occluded gases. T h e tube was cooled to room temperature. pressurized with neon a t the required value, and sealed. Freshly prepared lamps were conditioned in the resonant cavity for ca. 20 min before use.
RESULTS AND DISCUSSION Spectrum of Neon EDL's. The EDL's were a n intense
red in color. The spectrum of a neon electrodeless discharge lamp, between 200 and 500 nm, is shown in Figure 1. The most intense emission lines are identifiable as those of the neon (I) spectrum (15). Other, more intense, emission lines appeared a t longer wavelengths, but are unlikely to he of use in the excitation of atomic fluorescence. The 359.352-nm line is attributable to transitions (12) N Omenetto and G . Rossi, Spectrochim. Acta. Part B. 25, 297 (1970). (13) J. D. Norris and T. S. West, A n a / . Chim. Acta. 59, 355 (1972). (14) R . M . Dagnall and T. S. West. Appi. Opt.. 7, 1287 (1968) (15) ' M.1 T Wavelength Tables," 1969 ed, Massachusetts Institute of Technology, 1969.
A N A L Y T I C A L C H E M I S T R Y . VOL. 4 5 , N O . 12, O C T O B E R 1973
(r
t T
09
f
?
9
K
500 Figure 1. Spectrum
of neon
EDL
Wavelength nm.
between 200 and 500 n m
between the 4p'(3/2)2 and 3s'(1/2)lo excited states, while the ground state of neon is 2p6 IS0 (16). Operation of Neon EDL's. Incident power and emission intensity plots for neon EDL's pressurized a t 1, 5 , and 10 Torr, at 359.352 nm, are given in Figure 2. The lamps containing 5 Torr of neon were slightly more intense than those with other pressures. At operating powers above ca. 30 W, the intensity ceased to increase significantly with the incident power. Therefore, the lamps were normally operated a t 25-30 W of incident power. When operated a t this power the EDL's required no warming-up time and exhibited the maximum emission intensity immediately upon initiation of the discharge. The stability of the lamps a t 359.352 nm over a period of 1 hr was within k1.570.The line-to-background ratio a t this wavelength was greater than 1OO:l. We are unable to comment further on the lifetimes of these lamps, since after well over 100 hr of operation they are still as intense as they were initially. Atomic Fluorescence Behavior of Chromium. The atomic fluorescence determinations were carried out using an argon-separated air-acetylene flame, under the optimal operating conditions previously established for chromium ( 1 3 ) . An incident power and atomic fluorescence plot obtained with a neon EDL pressurized a t 5 Torr, while nebulizing a 20-ppm solution of chromium, is included in Figure 2. At powers above ca. 30 W, the atomic fluorescence decreased. A detection limit of 2.5 ppm was obtained for chromium atomic fluorescence a t 359 nm, using a monochromator slit-width which corresponded to a spectral band-pass of 6 nm. Due to the use of integration, the detection limit was defined as the concentration of chromium in aqueous solution which produced a signal equivalent to twice the (16) C E Moore Nat Bur Stand fU S ) Circ
467. Vol 2, 1952
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