Determination of Nickel by Atomic Fluorescence Flame Spectrometry SIR: Recently it has been demonstrated that for a limited number of metallic elements, atomic fluorescence flame spectrometry is a very sensitive analytical procedure (6, 9). For these metals it has been possible to obtain very intense discharge lamps with unreversed emission lines. A primary requirement for the production of atomic fluorescence emission is the availability of an appropriate high-intensity light source. Also, it is advantageous if the element of interest possesses in the ultraviolet region an atomic resonance line with a high transition probability. Below 280 mp, where flame background emission and atomic emission from the flame are relatively weak, considerably better fluorescence signalto-noise ratios can be obtained. Even if the radiation from the light source is modulated and the resulting modulated fluorescence signal is selectively amplified, a portion of the unmodulated emission from the turbulent flame will have frequencies that are within the bandpass of the tuned amplifier, and this emission will cause signal noise. Kickel has a satisfactory resonance line a t 232.0 mp and a sensitive emission line at 352.5 mp, but previous attempts to observe the fluorescence emission with conventional nickel hollowcathodes (1, 6) were unsuccessful. This was undoubtedly due to the weak emission intensities from the hollowcathodes then available. The recent development of high intensity hollowcathodes (?') has now made it possible to obtain intense unreversed line sources for amuch wider variety of metals. This study was undertaken to evaluate the usefulness of the new lamp for atomic fluorescence flame spectrometry. Strong nickel fluorescence was observed when aqueous solutions containing nickel nitrate were aspirated into an air-hydrogen flame. An oxygen-hydrogen flame was also tested for comparative purposes and found to be less satisfactory for nickel fluorescence work. Included in this paper are the analytical working curves, the detection limits, and a series of other wavelengths a t which nickel fluorescence was observable in the air-hydrogen flame. EXPERIMENTAL
Apparatus. The experimental arrangement is shown in Figure 1. The source was a high-intensity nickel hollow-cathode manufactured by Atomic
M
L
-IAl Mo
Figure 1. Optical arrangement for atomic fluorescence studies S: source; I: quartz lens; C: chopper; F: flame; M : mirror; Ma: monochromator
Spectral Lamps Pty. Ltd. It has two separate electrical discharges, and therefore two stabilized d.c. power supplies were required. The primary discharge produces atoms by cathode sputtering and some excitation while the secondary discharge causes the main excitation of the atomic vapor. The maximum recommended currents are 20 ma. for the primary discharge and 400 ma. for the secondary discharge. Besides the two power supplies, it was also necessary to have a current source capable of supplying about 5 amp. a t 1.5 volts to heat the cathode filament in the secondary discharge circuit. This heater current was turned off after the discharge was sustained and so did not have to be stabilized. The radiation from the source was first modulated a t 50 C.P.S. with a mechanical chopper to discriminate between the fluorescence emission and the other forms of emission from the flame. After being modulated, the source radiation was passed through the flame to the front-surfaced mirror, ill, and then directed back through the flame to a focus as shown. The addition of the concave mirror increased the resulting fluorescence intensity about three-fold, undoubtedly because it permitted the source radiation to be concentrated in the observed portion of the flame. The 50-C.P.S. fluorescence emission from the flame was focused on the entrance slit of a Hilger Watts Uvispek monochromator equipped with a quartz prism and was detected by a selected RCA 1P28 photomultiplier tube operated a t 1250 volts. The resulting signal was amplified with a Model RJB lock-in amplifier manufactured by Electronic, Missiles, Communications, Inc., Mount Vernon, N. Y.
The output from the narrow frequency bandwidth amplifier was then displayed on a recorder. A voltage stabilizer placed between the line voltage and the electrical components ensured the maximum instrumental stability. The burner employed was a Beckman (No. 4020) medium-bore hydrogen burner operated with flow rates of 4.8 liters per minute for air and 13.6 liters per minute for hydrogen. Procedure. In practice, the primary discharge current was set a t 18 ma. After the cathode of the secondary discharge circuit was heated for 5 minutes with 3 amp., the secondary discharge voltage was applied and the secondary discharge current was slowly increased to 80 ma. This was the maximum output of the available power supply. Once the secondary discharge was obtained, it was unnecessary to continue heating the cathode filament, and therefore that voltage was switched off. The subsequent steps for measuring the atomic fluorescence intensity have already been presented in detail (9). RESULTS AND DISCUSSION
First, the experimental variables were adjusted to give the optimum nickel fluorescence emission a t 232 mp. A study of the dependence of fluorescence intensity upon the secondary discharge lamp current showed a direct proportionality between them. The hollow-cathode lamp was therefore operated a t the highest current possible. The gas flow rates and height of observation in the flame were not critical factors in regard to the fluoresVOL. 30, NO. 9, AUGUST 1966
1235
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NICKEL CONCENTRATION, p.p.m.
Figure 2. Analytical working curves for nickel fluorescence intensities with the air-hydrogen flame 0
X
cence intensity. However, the water droplets in the flame scattered the radiation from the hollow-cathode and caused an appreciable background signal. For that reason the lowest flow rate of air at which maximum fluorescence could be attained was used. Also, measurements were made in the highest portion of the flame where dispersion of droplets and evaporation of water were more complete. The region of the flame observed was 50 to 70 mm. above the burner tip. An investigation of the nickel fluorescence signal and the noise level at 232 mp as a function of the monochromator slitwidth showed that the widest slitwidth possible, which was 2.0 mm., gave the highest signal-to-noise ratio. This unexpected result can be attributed to the following three factors. Most important was the relative absence of signal noise caused by the nonfluorescence radiation from the air-hydrogen flame. Although the fluorescence signal wm modulated, comparable data taken with the same burner using oxygen instead of air gave a prohibitively high noise level a t slitwidths greater than 1.0 mm., even when only pure water was aspirated into the flame. Thus it was apparent that modulation did not allow complete isolation of the fluorescence signal from the unmodulated emission. A second reason for the improved results using a wide slitwidth was the contribution of fluorescence emission from nickel lines other than the 232.0-mp line. As the slit was opened from 0.4 to 2 mm. and the bandwidth increased from 0.4 to 2.0 mp, the intensity increased approximately to the 1.4 power of the slitwidth. Had only one line been observed, the intensity would have been proportional to the first power of the slitwidth, as was the case for slitwidths below 0.4 mm. On the other hand, if a continuum were observed, the intensity would have been proportional to the second power of the slitwidth. The 231.1-mp resonance line and the 231.2-mp, 231.4-mp, and 231.7-mp 1236
ANALYTICAL CHEMISTRY
232 rnp 352 rnp
atomic nickel lines all have high transition probabilities and low lying ground state levels (2), and undoubtedly fluorescence emission from these transitions also contributed to the total measured signal. When the nickel fluorescence spectrum was scanned with a slitwidth less than 0.4 mm., two broad peaks, one a t 232.0 mp and the other a t 231.1 mp, could be resolved. A third reason for the observed advantage of a wide slitwidth was that as the slit was opened and the fluorescence signal increased, the noise components from the amplifier and recorder became less significant. In Figure 2 are given the experimental working curves for the nickel fluorescence intensity us. the nickel concentration at 232 mp and 352 mp. Aqueous nickel nitrate solutions were used to obtain these results, The slitwidth at 352 mp was 0.5 mm. All the experimental points shown have been corrected to account for the signal from the scattered hollow-cathode radiation. Although the measured fluorescence intensity at 232 mp was not markedly greater than a t 352 mp, the signal a t the former wavelength was much more stable. The unmodulated emission a t 352 mp from the turbulent flame caused appreciable noise. The air-hydrogen flame does not have enough energy to produce appreciable nickel emission at 232 mp, but at 352 mp the atomic nickel emission is very intense. As the nickel concentration was raised from 1 to 50 p.p.m., the signal-to-r.m.s. noise ratio a t 352 mp increased to a maximum of 20:l. Above 50 p.p.m., the ratio became progressively less. At 232 mp the signal-tor.m.s. noise ratio attained a maximum of 70: 1 for 100 p.p.m. nickel, and then it remained relatively constant for more concentrated nickel solutions. The nickel detection limit a t 232 mp was calculated according to the procedure described by Herrmann and Alkemade (4). Twenty values of the scattered light intensity were obtained by aspirating twice-distilled water into
the flame. The mean intensity x B and the standard deviation UB of the values were calculated. Substitution of these two values into the equation gave the meter reading at the detection limit, X. From the working curve the nickel concentration at the detection limit was seen to be 0.3 p.p.m. Four separate fluorescence emission values for a 1-p.p.m. nickel solution had a relative standard deviation of 3.6%. The good analytical reproducibility near the nickel detection limit was the result of the relatively large signal for the pure water solution. Distilled water gave a signal of 1.6 units, and 1-p.p.m. nickel solution gave a signal of 3.8 units. The value 3.8 could be reproduced quite well, however a large portion of that value was due to the scattered light and not to fluorescence. Sufficient data were not taken to correctly calculate a detection limit a t 352 mp, but an estimation of 0.7 p.p.m. based on only a calculation with four values for distilled water can be given. For the 232-mp fluorescence line the nickel concentration necessary to give a signal-to-r.m.s. noise ratio of 0.75: 1 was calculated so that a comparison between the results of this work and earlier publications (6,9)could be made. When pure water was aspirated into the flame, the r.m.s. noise level of the signal was 0.3 unit. Thus a nickel concentration of 0.1 p.p.m. would have given the desired signal-to-noise ratio. A brief investigation of the fluorescence intensity a t 232 mp from an oxygen-hydrogen flame was performed with the same burner to determine if in the hotter oxygen-hydrogen flame more nickel atoms would be formed. This would result in additional fluorescence emission. As in the case of the airhydrogen flame, the fluorescence intensity was not critically dependent on the gas flow rates. A maximum emission was obtained with 4.3 liters per minute of oxygen and 15.5 liters per minute of hydrogen. At higher oxygen flow rates the fluorescence signal was not enhanced and the noise level was somewhat greater. Because the oxygen-hydrogen flame gave much more background radiation a t 232 mp than the air-hydrogen flame, the slit was closed 1.0 mm., and then the comparison between the two flames was carried out. A 100-p.p.m. nickel solution aspirated into the oxygen-hydrogen flame gave slightly less fluorescence emission than that obtained from the air-hydrogen flame with a 1.0-mm. slitwidth. Moreover the signal-to-r.m.s. noise ratio with the oxygen-hydrogen flame was only 20: 1, whereas it was 60: 1 with the air-hydrogen flame. The noise level from the oxygen-hydrogen flame was even greater than that observed with
the air-hydrogen flame and a slitwidth of 2.0 mm. It is evident that the cooler air-hydrogen flame is BS efficient for producing nickel atoms as the oxygen-hydrogen flame. Additional spectral regions where nickel fluorescence emission could be expected were investigated with the airhydrogen flame. The slitwidth was reduced to 0.2 mm. to better resolve the lines. A number of nickel fluorescence emission peaks were observed and are given in Table I with their relative intensities. None of the intensity measurements were corrected for the phototube response or the monochromator transmission characteristics. Since the bandwidth was about 1.0-1.5 mp in the spectral region between 300 and 360 mp, each peak consisted of more than one fluorescence emission line. The use of atomic fluorescence flame spectrometry for the analytical determination of nickel appears to be very promising. A severe limitation to the fluorescence technique with the apparatus used was the short range in which the analytical working curves were linear. However, it is believed that significant improvements can be made in the nickel detection limits and the linear range of the analytical curves. The hollow-cathode radiation can be easily increased when a power supply, capable of supplying greater current to the secondary discharge of the lamp, is available. In addition, the hollowcathode radiation could be passed through the flame an additional number of times. The elimination of the water droplets in and around the flame would also lead to improved results. The addition of a burning sheath of air and hydrogen that surrounds the flame would reduce the amount of scattered source radiation and increase the rate of nickel atom production. The sheath would more efficiently evaporate water droplets and salt particles that escape from the side of the flame without sacrificing the sample flow rate or worsening the problem of self-absorption. Also, the sheath would reduce the turbulence of the flame (3) and thereby stabilize the fluorescence emission. Using organic solvents would increase the solvent evaporation, but a disadvantage would be the resulting increased noise level
caused by the additional flame background emission. A detailed study of chemical interferences with the nickel fluorescence emission has not as yet been undertaken. It was noticed that nickel sulfate solutions between 10 and 50 p.p.m. nickel gave about 250/, less fluorescence emission than the corresponding nickel nitrate solutions. Also, the presence of 0.02M lanthanum nitrate enhanced the fluorescence emission of a 50-p.p.m. nickel nitrate solution 15%. Since the atomic fluorescence intensity is dependent upon the number of nickel atoms in the ground state, the chemical interferences encountered should be similar to those reported for comparable atomic absorption techniques. Although interferences arising from changes in the quenching of the fluorescence radiation were not investigated, it is believed that significant variation in the degree of quenching will occur only if the gases added to the flame or the type of solvent is altered. Under normal analytical conditions these variables are maintained constant. The author has studied many cation and anion interferences with the nickel absorption and emission in oxygenhydrogen and oxygen-acetylene flames (1). The results should be helpful for predicting chemical interferences in the air-hydrogen flame. A spectral interference with the fluorescence emission will occur only if two conditions are fulfilled. First, there must be an appropriate emission line of the interferent element present in the radiation source spectrum, and second, the fluorescence emission line of the interferent element must lie within the spectral bandwidth of the monochromator. This is likely to happen if a very intense, continuous radiation source is used to produce fluorescence emission, but not when line sources for individual elements are employed. Shortly after this work was submitted for publication an excellent paper appeared (8) in which a continuous source was successfully used to obtain fluorescence emission. The availability of high intensity hollow-cathodes for elements other than nickel should in the near future greatly increase the number of metals
Table 1. Relative Intensities of Observed Atomic Nickel Fluorescence lines
Wavelength, mr 352 342 346 349
362
300 305 339 232 337 304 357
Rel. intensity 10 5.2 4.4 4.0 4.0 3.0 2.6 2:4 2.0 2.0 1.4 1.2
that can be analyzed by atomic fluorescence flame spectrometry. ACKNOWLEDGMENT
The author is very grateful to R. Herrmann and W. Lang for their helpful suggestions and interest in this project. LITERATURE CITED
(1) Armentrout, D. N., Ph.D. thesis,
Cornel1 University, Ithaca, N. Y . , 1965.
(2) Corliss, C. H., Boeman, W. R., Nut. Bur. Std. (U.S.) Monoaravh - . 53
(1962). (3) Gilbert, P. T., Intern. Con$ Spectry., College Park, Md., June 1962. (4) Herrmann, R., Alkemade, C. T. J.,
“Chemical Analysis by Flame Photometry,,’ 2nd. ed., p. 257, P. T. Gilbert, trans., Interscience, New York, 1963. (5) Mansfield, J. M., Winefordner, J. D., Veillon, Claude, ANAL. CHEM. 37. 1049 (1965). ( 6 ) Robinson, J. W., Anal. Chim. Acta 24, 254 (1961). (7) Sullivan, J. V., Walsh, A., Spectrochim.Acta 21 ,721 (1965).
(8) Veillon, Claude, Mansfield, J. M., Parson. M. L.. Winefordner.’ J. D.. ANAL.CHEM.38, 204 (1966). (9) Winefordner, J. D., Stabb, R. A., Ibzd., 36,165 (1964). D. N. ARMENTROUT Abteilung fur Med. Physik Univ.-Hautklinik 63 Giessen, Germany
RECEIVED for review February 4, 1966. Accepted April 25, 1966. Investigation supported by Public Health Service Fellowship 1-F2-GM-28, 353-01 from the National Institute of General Medical Sciences.
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