Direct Determination of Sulfur by Atomic Absorption Spectrometry in a Nitrogen Separated Nitrous Oxide-Acetylene Flame G. F. Kirkbright and Maurice Marshall Chemistry Department, imperial College, London S . W .7., U.K.
MUCHWORK has been reported on the determination of sulfur by methods which involve measurement of the emission intensity from SP molecules in cool flames; this work has recently been reviewed by Gilbert (1). Sulfur has long been determined routinely by arc or spark emission spectrometry at its principal resonance line at 180.7 nm, and the determination of traces of sulfur by atomic absorption spectrometry (AAS) by pulse vaporization of small samples in a graphite tube has been reported by L‘Vov (2). No analytical data are available, however, for the determination of sulfur by AAS in flames at 180.7 nm. The energy available in a flame cell of moderate or high temperature should be sufficient to enable the formation of sulfur atoms from sulfur-containing species introduced into it, even if insufficient energy is available to excite the 180.7-nm resonance line. While it is not surprising, therefore, that analytical flame atomic emission spectrometry of sulfur at 180.7 nm has not been successful, the determination of sulfur at this wavelength by AAS should be possible. Several difficulties must be resolved in order to exploit this possibility. While no difficulty exists in the detection and measurement of radiation at 180.7 nm when a vacuum or inert-gas purged monochromator is used, the flame employed must not only be hot enough to atomize sulfur efficiently, but it must transmit radiation from the primary source at this wavelength. Additionally, a suitably intense sulfur atomic line source is required which does not exhibit excessive collisional or selfabsorption broadening. In recent work in this laboratory, the remarkable transparency below 200 nm of the fuel-rich separated nitrous oxideacetylene flame has been demonstrated and utilized for the determination of arsenic and selenium (3). The transparency of this flame at short wavelengths is attributed to the lack of absorbing oxygen species in the interconal zone of the flame. It was reasonable to expect that this flame would also transmit an appreciable fraction of radiation incident upon it at 180.7 nm. The successful production and operation of a microwave-excited electrodeless discharge lamp (EDL) source for sulfur has also been reported (2), although little data are available on the general suitability of EDL’s as atomic line sources for AAS of this element. This communication describes the results of our preliminary work on the use of the nitrogen separated nitrous oxide-acetylene flame in conjunction with a sulfur EDL source and vacuum monochromator for the determination of sulfur by AAS at 180.7 nm. EXPERIMENTAL Apparatus. The instrumental assembly employed is shown in Figure 1. The sulfur EDL was made from silica tubing (8-mm internal diameter, 1-mm wall thickness) to form a bulb
(1) P. T. Gilbert, in “Analytical Flame Spectroscopy,” R. Mavrodineanu, Ed., Macmillan, London, 1970. (2) B. V. L‘Vov, “Atomic Absorption Spectrochemical Analysis,” American Elsevier, New York, N.Y., 1970, p 255. (3) G. F. Kirkbright and Leslie Ranson, ANAL.CHEM.,43, 1238 (1971). 1288
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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Figure 1. Schematic diagram of instrumental assembly EDL source 7-cm slot burner Vacuum monochromator Indirect nebulizer and expansion chamber 5. Nitrogen-filled glass tubing with silica end-windows 1. 2. 3. 4.
of length 60 mm containing ca. 1 mg of sublimed sulfur and an argon filler gas pressure of 3 Torr. The lamp was operated in a a/,-wave resonant cavity (Model 210L, Electromedical Supplies, Wantage, U.K.) powered by a 2450-MHz Microtron microwave generator (EMS, Wantage, U.K.) at 20 Watts using a - 3 dB attenuator, so that the effective power to the lamp was 10 Watts. The burner employed was a 7-cm nitrous oxide-acetylene slot burner (Beckman-RIIC, Glenrothes, Scotland) with facility for separation of the flame by inert gas shielding (4). This burner was mounted on the indirect nebulizer from a commercial AAS instrument (PerkinElmer Corp., Norwalk, Conn., Model 290B). Sample solutions were nebulized on nitrous oxide (8.2 liters/min) and the sample uptake rate was 6.5 ml/mit. The optimum acetylene flow rate was 4.2 liters/min, and the flame was separated by nitrogen. The monochromator used was a 4-channel fluorite prism polychromator (Hilger and Watts, London, Model E 796) evacuated to 0.15 to 0.20 Torr. The four channels of this monochromator were preset to the S 180.7 nm, P 178 nm, C 165.7 nm, and Fe 171.3 nm lines, and each channel was fitted with a 2-inch end-window multiplier phototube (EM1 6256B). Only the sulfur channel was employed in the present work. The photomultiplier output was amplified by a sensitive microammeter (RCA Model WV-84C) and then displayed either at its meter or on a potentiometric chart recorder (Servoscribe Model 211 R, Smiths Industries, U.K.). Glass tubing of 25mm diameter, fitted with optically flat fused silica windows of the same diameter, was purged with nitrogen and placed between the source and silica lens (25-mm diameter, 63-mm focal length) used to focus the radiation into the flame, and also between this lens and the flame and the flame and collimating lens of the monochromator. These tubes were used to provide a light path appreciably more transparent than the atmosphere and assisted the collection of as much radiant energy as possible at the monochromator. Reagents. Standard Sulfur Solution. A 5000 ppm stock solution of sulfur, as sulfate, was prepared by dissolving 27.17 grams of potassium sulfate, K2S04,AR grade, in 1 liter of distilled water. This solution was then diluted as required. Diverse Ions. Stock 50000-ppm solutions of the diverse ions used in the interference studies were prepared from (4) G. F. Kirkbright, M. Sargent, and T. S. West, Talanta, 16,
1467 (1969).
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Figure 2. Transmission of source radiation by flame system
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4.0 4.3 4.6 ACETYLENE , LITER / MIN.
Figure 3. Effect of variation in acetylene flow rate on flame background absorbance at 180.7 nm
1. No flame, nitrogen separation gas only. Transmission set to 100 2. No flame, separation gas off. Air only in light path across burner 3. Air-acetylene flame 4. Unseparated nitrous oxideacetylene flame, fuel-rich 5. Nitrogen separated nitrous oxide-acetylene flame, optimum acetylene flow-rate for maximum sensitivity for sulfur
analytical reagent grade salts of the elements. The potassium thiocyanate, sodium thiosulfate, and thiourea solutions in distilled water were also prepared from analytical reagent grade chemicals. RESULTS
The particular instrumental system employed in this preliminary work was operated in the dc mode, Le., the EDL source emission is unmodulated and a dc amplifier is employed. In addition, the monochromator is preset at the wavelength of the sulfur resonance line at 180.7 nm. It was possible to scan the wavelength over a narrow range on either side of the line (ca. 1 nm), however, by movement of the profiling control. In this way it was possible to check the signal : background intensity ratio observed at 180.7 nm for the EDL source. This was found to be greater than 100: 1. The fuelflow rate, burner height, shielding gas flow rate, EDL operating power, and PMT voltage were optimized to produce the highest 1 % absorption sensitivity and lowest detection limits for sulfur, both as an aqueous solution of sulfate, and for a solution of thiourea in ethanol. Flame Transparency. Figure 2 shows typical recorder tracings of the effect on the absorption of the source radiation with and without the flame ignited and with and without nitrogen separation of the flame. Using the assembly illustrated in Figure 1, and setting the amplifier gain to 100% transmission while the flame is unignited and only nitrogen gas is flowing, it is evident from Figure 2 that the transmission of the atmosphere is only 25 relative to that of nitrogen (2 in Figure 2). The transmission of the unshielded air-acetylene and nitrous oxide-acetylene flames is even less, being ca. 3 and 7%, respectively (3 and 4 in Figure 2), while that of the nitrogen separated nitrous oxide-acetylene flame is very substantial ( 5 in Figure 2). The recorded transmission of the flame at the
SULFUR CONC.,
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Figure 4. Calibration curves for sulfur A . As thiourea in ethanol B. As potassium sulfate in water
optimum fuel-flow rate for sulfur is 64% of that of nitrogen alone and approximately 2.5 times greater than that of air. Effect of Acetylene Flow-rate. Figure 3 shows the effect of the variation in acetylene flow rate on the flame background transmission at 180.7 nm. It is apparent that the acetylene flow rate employed should be closely controlled. Maximum flame transparency occurs when a slightly fuel-rich flame is employed, between 4.3 and 4.5 liters/min of acetylene with a nitrous oxide flow rate of 8.2 liters/min. The decreased transmission in the leaner flame may be attributed to the presence of absorbing oxygen species in the interconal zone, while that of the richer incandescent flame may be due to absorption by excess gaseous or particulate carbon species in this zone. The fuel flow rate which was found here to produce the greatest sensitivity (for 1% absorption) was 4.2 liters/min of acetylene, slightly less than that necessary for maximum flame transANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
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B
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Figure 5. Recorder tracings of absorbance produced for 1000 ppm sulfur introduced into the flame in different forms A , C, and E . Potassium sulfate B. Potassium thiocyanate
D. Thiourea F. Sodium thiosulfate
Table I. Sensitivities and Detection Limits Obtained Sensitivity (ppm/ Detection Analyte 1 absorption) limit, pprn Sulfur as K2S04 in water 9.0 30 Sulfur as thiourea in ethanol 4.4 12
mission. As mentioned earlier, at this flow rate, the flame transmission was 6497, of that of nitrogen alone. Effect of Source Operating Power. The variation of the sensitivity obtained for the determination of sulfur with the operating power of the EDL was investigated. The lowest attainable operating power was found to produce the highest sensitivity; with the lamps used in this study this corresponded to approximately 10 Watts input to the 8/4-wavecavity. As the operating power was increased, the sensitivity decreased, an effect probably caused by excessive pressure and selfabsorption broadening of the 180.7 nm line at higher power. Sensitivity, Detection Limits, and Interferences. With the optimum operating conditions for the experimental assembly used in this work, the sensitivity values (for 197, absorption) and detection limits shown in Table I were obtained for sulfur as sulfate in aqueous solution and sulfur as thiourea in ethanolic solution. The 1 absorption sensitivity values are obtained from calibration curves in each case. These were found to be linear for sulfate in aqueous medium between 50 and 700 ppm, and for thiourea in ethanolic solution between 20 and 500 ppm (see Figure 4). The detection limits are defined to correspond to that concentration of sulfur in each medium which produces a signal equal to twice the standard deviation of the signal noise obtained near the limit of detection.
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No significant chemical or physical interference effect was observed in the absorbance signal produced by 200 ppm sulfur in aqueous medium at 180.7 nm in the presence of 50-fold weight excesses of the following ions: Al, Cu, K, Mg, Mn, Mo, Na, Ni, Zn, fluoride, phosphate, chloride, bromide, and iodide. As evident from Figure 5 , the introduction of the sulfur into the flame as thiosulfate, thiocyanate, or thiourea in aqueous solution resulted in similar 1 97, absorption sensitivities and detection limits to those obtained for sulfur introduced as sulfate in aqueous medium. The enhancement of sensitivity observed on introduction of the sulfur as thiourea in ethanol is fully accounted for by the improved nebulizer efficiency with the organic solvent.
ANALYTICAL CHEMISTRY, VOL. 44, NO. 7, JUNE 1972
The preliminary study described in this communication indicates that the nitrogen separated nitrous oxide-acetylene flame forms an efficient and transparent atom cell for the determination of sulfur, and that the determination at 180.7 nm may be made with freedom from chemical and physical interferences. Several factors limit the sensitivity and detection limits for sulfur observed with our apparatus in this study. The use of an ac system rather than the present dc system would probably result in improved analytical performance, as although no flame emission signal could be detected at 180.7 nm to necessitate this, narrower source line width and more effective amplification of the signal received at the detector should result under these conditions. The EDL source, even at the low power input used here, probably suffers appreciable self-absorption broadening. Greater sensitivity should be obtainable with further attenuation of the operating power to the EDL or by using a hollow-cathode discharge source. Although a vacuum monochromator has been employed here, a nitrogen or argon purged monochromator of greater luminosity could be usefully employed in its place. Additionally, as with the determination of many other elements in the nitrous oxide-acetylene flame, the sensitivity is limited by the dilution of the sample by the large volume of flame gases. Preliminary observations with other flames in which the dilution effect is less severe, however, such as the air-acetylene and the nitrogen-hydrogen diffusion flames, suggest that the transparency of these flames is too low at 180.7 nm to permit their use, and that chemical and physical interferences are more serious in these cooler flames. The present work suggests that phosphorus may also be determinable with our instrumental assembly at 178.3 nm, although the flame transmission is somewhat lower at this shorter wavelength. Even with the provisional analytical performance reported here, the determination of sulfur directly in crude and fuel oils is possible, and this will be reported, along with improvements to the apparatus to be undertaken, in a later publication. ACKNOWLEDGMENT
We wish to thank T. S. West for his interest in this work, and the British Steel Corp. for the loan of apparatus. RECEIVED for review September 27, 1971. Accepted January 7,1972.
