Atomic Fluorescence Study on Iron, Cobalt, and Nickel Jaroslav Matous'ek and Vaclav Sychra Department of Instrumental Analysis, Technical University, Prague 6, Czechoslocakia
The resonance fluorescence of iron, cobalt, and nickel atoms in air-hydrogen, air-acetylene, and hydrogenoxygen-argon flames has been measured with a modified Techtron AA-4 atomic absorption spectrophotometer. High intensity, hollow cathode lamps are used as primary excitation sources. Fluorescence spectra are studied in detail and the strongest flu?rescence emission is observed at 2483,2407, and 2320 A, for iron, cobalt, and nickel, respectively. The effects of source parameters, optical arrangement, uptake of the sample solution, organic solvents, inorganic acids, and of 13 other elements are investigated. A comparison was made between the total consumption atomizer-burner and the Techtron premix system. The premixed hydrogen flame was used for atomic fluorescence measurements of methyl isobutyl ketone extracts of iron, cobalt, and nickel chelates with ammonium pyrrolidine dithiocarbamate. Practically the same detection limits of 0.02, 0.01, and 0.003 ppm of iron, cobalt, and nickel, respectively, were found for aqueous solutions both in premixed hydrogen-oxygenargon and turbulent air-hydrogen and hydrogenoxygen-argon flames. Using extraction method, detection limits of 0.0002, 0.0004, and 0.0001 ppm of iron, cobalt, and nickel, respectively, were obtained.
DETERMINATION of iron, cobalt, and nickel by atomic fluorescence spectrometry has already been described using a continuous source, such as the xenon arc lamp (I-5), a line source, such as mercury discharge lamp (6), and hot hollow cathode lamp (7, 8) for the excitation of fluorescence. However, detection limits did not reach those obtained by atomic absorption spectrometry. Recent works in atomic fluorescence have shown that for a number of elements, high intensity, hollow cathode lamps are sufficiently intense light sources for the excitation of atomic fluorescence spectra. Armentrout (9) described the determination of nickel in a n air-hydrogen flame using a total consumption burner. He reported a detection limit 0.3 ppm of Ni in consequence of some imperfections of the apparatus used, especially of insufficient power supply for the secondary discharge of the high intensity hollow cathode lamp. Manning and Heneage (5) obtained detection limits of 0.1, 0.04, 0.003, 0.02, 0.001, and 1.0 ppm for Fe, Co, Ni, Pb, Cu, and As, respectively. West and Williams (IO) studied the fluorescence of silver atoms in an air-propane flame and achieved detection limits of 1.7 X ppm and 4 X 10-5 ppm without and with extraction as di-n-butylammonium silver salicylate, respectively. (1) C. Veillon, J. M. Mansfield, M. L. Parsons, and J. D. Winefordner, ANAL.CHEM., 38, 204 (1966). (2) D. W. Ellis and D. R. Demers, ibid., 38,1943 (1966). (3) R. M. Dagnall, K. C . Thompson, and T. S. West, Anal. Chim. Acta, 36, 269 (1966). (4) D. W. Ellis and D. R. Demers, Paper presented at 153rd Meeting, ACS, Miami Beach, Fla., April 1967. (5) D. C. Manning and P. Heneage, Atomic Absorption Newsletter, 6, 124 (1967). (6) N. Omenetto and G. Rossi, Anal. Chim.Acta, 40, 195 (1968). (7) J. I. Dinnin, ANAL.CHEM., 39, 1491 (1967). (8) J. I. Dinnin and A. W. Helz, ibid., 39, 1489 (1967). (9) D. N. Armentrout, ibid., 38, 1235 (1966). (10) T. S. West and X. K. Williams, ibid., 40, 335 (1966). 518
0
ANALYTICAL CHEMISTRY
This study was undertaken to evaluate the usefulness of iron, cobalt, and nickel high intensity, hollow cathode lamps for the excitation of atomic fluorescence of iron, cobalt, and nickel and to find optimum experimental conditions for atomic fluorescence determination of the elements mentioned. EXPERIMENTAL
Apparatus. A Techtron AA-4 atomic absorption spectrophotometer equipped with a FE-1 emission burner, Beckman medium bore hydrogen burner, A.S.L. iron, cobalt, and nickel high intensity, hollow cathode lamps, HTV type R 106 photomultiplier and a (2-2 mV KBT chart recorder was used. The radiation from the lamps and the amplifier were square wave modulated at 285 cps. Reagents. Aqueous stock solutions containing 1000 ppm of the desired metals were prepared by dissolving spectral pure metals in nitric acid (1 :1) and diluted as required with twice distilled water. Solutions containing less than 10 ppm of the desired metal should be prepared immediately before use because of the marked adsorption (especially of iron) on glass surface. Ammonium pyrrolidine dithiocarbamate (APDC) was synthesized according to the method of Malissa and Schoeffmann (11). Water solution of APDC ( 2 S Z ) for use in methyl isobutyl ketone (MIBK) extractions was always freshly prepared. Conditions and Optical Arrangement. Experimental variables, such as lamp operating currents, monochromator slitwidths, region of the flame viewed, etc. were adjusted to give the maximum fluorescence signal and were the same for all the elements tested. The highest signal-to-noise ratio for a given solution was obtained by running the lamp at the maximum operating currents recommended by the manufacturers (20 mA for the primary current and 400 mA for the secondary current). This strongly decreases as both the primary and secondary current is decreased. Variation of the mono$hromator slit-width from 10 /.L up to 300 /.L (band-width 9.9 A) produced a nearly linear increase in the signal-to-noise ratio. Slit-widths greater than 300 p were not available with the instrument used. Further opening of the slit is expected to increase the signal-to-noise ratio because of the increase of the incident radiant flux and contributions of fluorescence emission from other lines included in the effective spectral band width. These facts have already been confirmed and reported by Armentrout (9) for nickel. As very specific line sources were used for the excitation of fluorescence and, therefore, there is no danger of spectral interferences, the slit-width was set for all measurements to its maximum value of 300 F . Fluorescence responses of the elements were measured at various heights in the flame. The height of measurement in the flame was not a critical factor with regard to the fluorescence signal. For all tvpes of premixed laminar flames, measurements were performed at a height of 3.0 cm above the burner top. With the Beckman total-consumption burner, the best signal-to-noise ratio was obtained for all the elements investigated in the highest region of the flame about 9.0 cm above the top of the burner. As the instrument was fitted with a variable nebulizer, the dependence of the fluorescence signal on the solution uptake (11) H. Malissa and E. Schoeffmann, Mikrochim. Acta, 1, 187 ( 1955).
Flame H2-air H2-02-k C2H2-air H2-air H2-02- Ar
Burner type
Table I. Burner Operating Conditions Flow rate in liters/minute Hz CzHz Air
Specially manufactured Meker type burner Specially manufactured Meker type burner Standard Techtron FE-1 emission burner Beckman medium bore burner Beckman medium bore burner
4.1 (1.8p
0 2
Ar
2.9
5.0
1.8
2.2
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5.9
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7.0
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24.0
MIBK extracts.
rate could be followed. The dependence has a maximum corresponding to the value 4.0 ml/min at which all the measurements with the premix system were carried out. The Beckman burner was operating a t the solution uptake rates 2.5 and 6.0 ml/min for air-hydrogen and hydrogen-oxygenargon flames, respectively. The instrument was adapted to fluorescence work employing the conventional right angle illumination system as described in previously reported studies of atomic fluorescence spectrometry. The quartz window of the lamp was placed as close as possible to the flame (3.5 cm and 5.5 cm using the premixed flame and the turbulent flame, respectively). A 5.5-cm focal length spherical aluminum mirror was positioned 5 cm behind the flame to enable double-pass of the exciting beam through the flame. The lamp and the mirror were mounted on an optical rail to facilitate optical alignment. The fluorescence emission from the flame was focused with a 6.4-cm focal length condensing quartz lens behind the entrance slit of the monochromator. The optimum optical set-up was achieved when the burner and the condensing lens were situated 24.0 cm and 16.0 cm from the entrance slit, respectively. With this arrangement, introduction of the second mirror in the optical path of the fluorescence radiation (at the opposite side of the flame from the entrance slit) was not found to be advantageous. Flames. Table I summarizes data on the operation of the burners for all types of flames used. Besides the conventionally used flame, the argon diluted stoichiometric hydrogen-oxygen flame recommendtd by Jenkins (12) was also studied. For fluorescence measurements in premixed flames, except for air-acetylene flame, a specially manufactured Meker type brass burner head with 25 holes (1.0 mm in a diameter) was substituted for the standard Techtron FE-1 emission burner head. G a s flow rates were optimized individually for each element of interest. Optimum conditions corresponding to the maximum fluorescence signal-to-noise ratio for a given element were practically the same. In spraying MIBK extracts, the hydrogen flow rate was reduced until a lean flame was obtained. Atomic Fluorescence Lines. T o choose the most sensitive fluorescence lines and to evaluate quantum efficiencies of the fluorescence process, relative intensities of lines emitted by the high intensity, hollow cathode lamps and of corresponding fluorescence lines in the air-hydrogen flame were determined. In scanning the fluorescence spectra, 20 ppm solutions of Fe, Co, and Ni were sprayed into the air-hydrogen flame under optimum conditions and at a slit-width of 25 p. Emission spectra of the lamps were scanned a t the maximum lamp operating currents and at a slit-width of 10 p. The relative intensities corrected for detector response are listed in Table 11. (12) D. K.Jenkins, Spectroclzim. Acta, 23B, 167 (1967).
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Figure 1. Analytical working curves for iron/Fe 2483 A/ with various flames 0 Turbulent H2-02-Ar X Turbulent H2-air
Premixed H2-02-Ar Premixed Hs-air A Premixed C2H2-air 0
+
Procedure. Working curves for each metal were obtained by spraying 0.05, 0.1, 0.3, 0.5, 1 , 2, 3, 5 , 10, 20, 30, 50, 100, 250, and 1000 ppm solutions of the desired element, prepared by appropriate dilution of the stock solution. For the range of 0.05-1.0 ppm full gain and scale expansion (5X) were used. To study the effect of organic solvents miscible with water on the fluorescence signal, 2 ppm solutions of Fe, Co, and Ni, containing 10-70 grams of methanol, 20 grams of ethanol, propanol, acetone, dioxane, acetic acid, and glycerol in 100 ml were prepared and analyzed under the usual conditions. For the APDC-MIBK extraction, a 2-liter separating funnel was coated with a PTFE emulsion and annealed a t 450 " C in a furnace in order to obtain a homogeneous protective film. Extraction procedure was carried out individually for each element as follows: 1000 ml of 0.001-1.0 ppm solutions of Fe, Co, and Ni were prepared directly in the separating funnel. After the addition of 5 mi of 2 0 z sodium acetate solution (in the case of iron, the addition of 1 ml of glacial acetic acid) and of 5 ml of the 2 . 5 x A P D C solution, the A P D C chelates were extracted by 3-minute shaking with 20 ml of MIBK. The lower (aqueous) phase was discarded and the organic phase was transferred into a 25-ml volumetric flask, diluted to 25 ml with MIBK and sprayed directly into the air-hydrogen flame. Calibration VOL. 41, NO. 3, MARCH 1969
519
Table 11. Relative Intensities of Emission and Atomic Fluorescence Lines of Iron, Cobalt, and Nickel Relative Relative emission fluorescence intensity, Oscillator intensity, strength Element Line, A Transition (13, 14) corrected corrected co 2407.25 u 'F41/2-~'G51/20 2.2 91 100 38 2.7 59 2411.62 u 'F31/2-~'G41/20 2414,46* u 4F21/2-~ 'G31/2 3.2 15 3.5 2415.3ob u 'F1 1/2-~'G~zl/~" 2424.93 a4F41/2-~'F41/," 61 50 1.9 42 2432.21 u'F~I/~-X~F~I/? 16 2.2 100 2521.36 U ' F ~ ~ / 'D31/~0 %-X 1.9 45 58 1.6 2528.97 u 4F31/2-~4D~~/2 7 Ni 2289.98 u3F4-x3F3O 5 5 0.39 2310.96 U~F~-W~F~O 19 0.60 52 2312.34 u~F~-w~F~O 8 12 0.76 2313.98 u3FZ-w3Fz0 13 7 0.82 32 2320.03 a3F4-y3Gg0 100 0.86 2325.19 12 6 u 3F3-Y 3G40 0.72 13 2345.54 u 3Fa-~3D30 0.46 3002. 49b u 3D,3-~ 3D30 0.90 13 3003. 63b a3D2-y3Dz0 22 0.61 3050.82 a3D3-y3F40 74 12 3101. 55a u 3Dz-y3Fa" 57 9 3101.88= u 'Dz-y1F30 0.68 25 3134.11 a 3DI-y3Fz0 0.86 I 3414.76 a3D3-zaF40 95 13 1 .o 3461.65 a3D3-z5Fa" 0.57 99 4 3515.05 a3Dz-z3F3" 58 0.83 6 3524.54 u 3D3-~ 3P20 0.85 100 20 Fe 2483.27 u'D~-x'F~" 49 3.1 100 2488.15 u~D~-x'F~" 29 46 2489. 15a aSDo-xSFi" 25 23 2490.64" u~D~-x'FF~~ 2.4 34 2522.85 aSDa-x5Da" 33 2.7 12 2527.43 a5D3-x5D30 1.5 12 2719.02 ~'Da-y'P3~ 1.4 15 36 3020.495 a6Dz-y'Dz0 12 3020.64a u'D~-Y'D~~ 84 3021, 07a a6D3-ySD30 0.48 62 6 3734.87 a5F5-ysFsO 4.2 100 7 3137.13 a5DDa-z5F40 0.32 a Lines unresolved in emission of fluorescence spectra. b Lines unresolved in fluorescence spectrum.
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curves were plotted in the co-ordinates fluorescence signal us. concentration taking into account blank solution. Scale expansion (5 X) was used. RESULTS AND DISCUSSION Calibration Graphs. Shapes of the analytical working curves obtained for different fuel gases and flame types for primary fluorescence lines of each of the three elements are shown in Figures 1-3. The analytical working curves are linear with concentration over a range of more than three orders of magnitude with curvature appearing for the turbulent flame at lower values of concentrations. For comparison, analytical working curves for the secondary Fe, Co, and Ni fluorescence lines measured in the premixed air-hydrogen flame are shown in Figures 4 and 5 . Some of these curves exhibit curvature at higher values of concentrations compared to the primary fluorescence lines and, therefore, they are more suitable for the determination of higher concentrations. Shapes and slopes (approximately equal to 1) of (13) C. E. Moore, "Atomic Energy Levels as Derived from the Analysis of Optical Spectra," Vol. 11, National Bureau of Standards, Washington, D. C., 1952. (14) C . H. Corliss and R. Bozman, "Experimental Transition Probabilities for Spectral Lines of Seventy Elements," National Bureau of Standards, Washington, D. C . , 1962. 520
ANALYTICAL CHEMISTRY
all the analytical curves are in a good agreement with those, derived by Winefordner et al. (15) for the case when absorption line half-width is much greater than source line halfwidth. Table I11 summarizes limits of detection of the primary fluorescence lines obtained both in the premixed and in the turbulent flames and of the secondary lines in the premixed air-hydrogen flame. The detection limit is considered as the concentration for which the fluorescence signal is equal to twice the peak-to-peak fluctuations that appear during nebulization of solvent only. For the premix system, the stoichiometric hydrogen-oxygen flame diluted with argon (15) which was expected to raise the fluorescence signal compared to the air-hydrogen flame because of much smaller quenching cross-section of argon than of nitrogen, gives better detection limits than air-hydrogen and air-acetylene flames. However, the improvement in detection limit for nickel in the hydrogen-oxygen-argon flame as against the air-hydrogen flame is not so significant as reported by Manning and Heneage (5). Therefore, all further measurements were performed in the air-hydrogen flame with respect to the simple operation with this flame type. Practically the same detection (15) J. D. Winefordner, M. L. Parsons, J. M. Mansfield, and W. J. McCarthy, Spectrochim. Acta, 23B, 37 (1967).
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Figure 4. Analytical working curves for iron and cobalt secondary fluorescence lines with premixed air-hydrogen flame
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'imits for each of the three elements were obtained both in the premixed hydrogen-oxygen-argon and in the turbulent air-hydrogen and hydrogen-oxygen-argon flames. For the hydrogen-oxygen-argon flame, the optimum operating conditions with the Beckman burner used did not correspond t o the stoichiometric hydrogen-oxygen flame diluted with argon. Although the fluorescence signal in this flame is slightly higher than that in the turbulent air-hydrogen flame, the higher noise of the hydrogen-oxygen-argon flame causes levelling of the detection limits in both flames. Besides that, the assumption can be made that in the turbulent flame the effect of argon is probably less pronounced in comparison with the premixed flame owing t o entrainment of nitrogen from surrounding air.
4 4
Fe 3735 Co 2401 A 0 Co 2425 0 Co252l A
A Fe2523 A A Fe2719 0 Fe3020 A
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Relative standard deviations calculated from analyses of 10 solutions of the same concentration in the premixed air-
hydrogen flame did not exceed 2.5 %, at the 1-ppm level. Effect of Organic Solvents. The effect of seven watermiscible organic solvents (20 grams/100 ml of methanol ethanol, propanol, acetone, dioxan, acetic acid, and glycerol) on the fluorescence signal of 2 ppm solutions of Fe, Co, and Ni in the air-hydrogen flame was investigated. In the premixed flame, all organic solvents studied, except for glycerol, enhance the fluorescence signal of all the three elements, whereas in the turbulent flame, almost in all cases the decrease of fluorescence signal was observed. For the premixed flame, this effect can be explained taking into account three factors: the increase of the amount of the solu10000
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+
Figure 5. Analytical working curves for nickel secondary fluorescence,lines with premixed air-hydrogen flame
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VOL. 41, NO. 3, MARCH 1969
521
Table 111. Atomic Fluorescence Detection Limits in ppm with Various Flames Premix system H2-OZ-Ar 0.02
Wavelength, A H2-air CsHz-air 2483 0.03 0.08 2523 0.1 2719 0.4 3020 0.5 3735 3.0 co 2407 0.02 0.01 0.05 2425 0.1 2521 0.2 Ni 2320 0.006 0.003 0.03 2311b 0.1 3414 0.2 2290 0.3 2345 0.3 3002 0.3 3524 0.3 3050 0.5 3461 0.6 a The values obtained with the same instrument and the same spectral sources. b Slit-width 200 p , Element Fe
tion reaching the flame owing to the improved efficiency of nebulization, the increase of efficiency of vaporization in the flame, and the decrease of the solution uptake rate caused by changes in the viscosity and surface tension of the solution containing organic solvent. The first factor is dominant and, therefore, the enhancement of the fluorescence signal occurs. On the other hand, for the total consumption atomizerburner, the increase of the efficiency of evaporation in the flame is not sufficient t o compensate the decrease of the fluorescence signal caused by decreased solution uptake rate. The effect of methanol in the premixed air-hydrogen flame was studied over a wide range of methanol concentration. It was found that 80% of methanol in water improves the detection limits measured in aqueous solution approximately by a factor 2-3-i.e., to the values of 0.01, 0.008, and 0.003 ppm for Fe, Co, and Ni, respectively. MIBK-APDC Extraction. The MIBK extracts of iron, cobalt, and nickel chelates with APDC sprayed into the airhydrogen flame gave a scale reading approximately 4-5 times greater than that of aqueous solutions of the same concentration. This result agrees well with that reported by Dagnall et a!. (3) for zinc MIBK extracts. Following the extraction procedure described, linear calibration curves were obtained over the concentration range of 0.001-1.0 ppm. The limits of detection were found t o be 0.0002,0.0004, and 0,0001 ppm for Fe, Co, and Ni, respectively. The simultaneous extraction and determination of iron, cobalt, and nickel at comparable concentrations can be performed without any mutual interferences. Interferences. The influence of inorganic acids (in concentrations of 0.01-1.OM) and of 100-fold excess each of Na, K , Mg, Ca, AI, V, Cr, Mo, W, Mn, Fe, Co, and Ni on the atomic fluorescence of iron, cobalt, and nickel excited in the air-hydrogen and air-acetylene flames was investigated. I n the air-hydrogen flame, fluorescence signals measured at the 5-ppm level are considerably affected by the presence of sulfuric, phosphoric, and hydrofluoric acid even at their low concentrations and also the suppressing effect of refractory elements is apparent. Both the effect of acids and the interelement effects were expected with regard t o relatively low temperature of the air-hydrogen flame. 522
ANALYTICAL CHEMISTRY
Total consumption atomizer-burner Hs-air H2-Os-Ar 0.02 0.02
Atomic absorptiona C2H2-air 0.03
0.01
0.01
0.03
0.005
0.006
0.02
In hoter air-acetylene flame, the interferences are practically eliminated. This agrees well with the results obtained by measuring the same interferences in atomic absorption with air-acetylene flame. Because of the effects of acids and of the other elements on the fluorescence signal of iron, cobalt, and nickel in the airhydrogen flame, it is recommended that all solutions measured by atomic fluorescence spectrometry using this flame type should be prepared to have the same concentrations of acids and that the matrix effect should be taken into account in preparing standards. N o scattering of the excitation radiation caused by unvaporized particles was observed in this study. It was confirmed by spraying the solution containing 5000 ppm of Na into the flame. CONCLUSIONS
This study has shown that iron, cobalt, and nickel high intensity, hollow cathode lamps are sufficiently intense spectral sources for exciting atomic fluorescence. Linear calibration curves were obtained over a range of more than three orders of magnitude. Table I11 shows that the detection limits obtained by the atomic fluorescence are 1.5, 3, and 7 times lower for Fe, Co, and Ni, respectively, than those obtained by the atomic absorption method with the same instrument and the same spectral sources. Further improvements in the apparatus, such as the possibility of running the high intensity, hollow cathode lamps at higher operating currents and further opening of the slit would have produced a still greater decrease of detection limits. Following the extraction procedure, subnanogram amounts of iron, cobalt, and nickel can be determined. To avoid interfering effects in the airhydrogen flame, care must be taken of the preparation of sample solutions and standards. ACKNOWLEDGMENT
The authors thank J. Mosteckg, Head of the Department of Synthetic Fuels, and F. StrifeIda, Head of the Department of Instrumental Analysis for support in their work. RECEIVED for review August 2, 1968. Accepted October 21, 1968.
