samples containing fissionable materials such as uranium or thorium if one group separation is made prior to irradiation. For example, t o determine dysprosium or europium in a samjde of uranium or thorium, a known amount of lanthanum may be added as carrier and a group separation from urar ium or thorium accomplished by stzndard methods (e, 6, 14). (The reagents and lanthanum used should not contain europium or dysprosium.) The final solutions can then be irradiated, one aliquot taken for ion exchange separation and another foi. chemical yield determination by colorimetric determination of lanthanum. Sensitivity will varj’ among the rare earths, depending upcln the activation cross section of the element, the decay schemes, and half lives of the isotopes produced. The minirium amounts of ytterbium, dysprosium, europium, and praseodymium detectable by the method outlined in this paper are given in Table 11. This is based on a 25minute irradiation in a thermal neutron flux of 2 x lo** neutrons em.+ second-’. Examples given in this paper are representative of different portions of the rare earth region r n d illustrate the applicability of the method to any rare earth element. Any specific laboratory application will, however, require prior work with known elements to standardize the procedures for the particular batch of resin available.
-
ACKNOWLEDGMENT
The authors thank William Dunbar and the staff of the Ford Nuclear Reactor for their help in making the irradiations. LITERATURE CITED
(1) Alstad, J., Pappaa, A. C., J. Inorg. Nucl. Chem. 15,222 (1960). (2) Choppin, G. R., Silva, R. J., U. S. Atomic Enerav Comm. Rept. UCRG 3265 (1956); -J. Inorg. Nuci. Chem. 3, 153 (1956). (3) Cornish, F. W., Brit. Atomic Energy Research Establishment Rept. AERE C/R 1224 (1956). (4) Gindler, J. E., Nuclear Science Series Rept. NAS-NS-3050, “The Radiochemistry of Uranium,” Office of Technical Services, Department of Commerce, Washington 25, D. C., 1962. (5) Hyde, E. K., Nuclear Science Series Repd. NAS-NS-3004, “The Radio-
chemistry of Thorium,” Office of Technical Services, Department of Commerce, Washington 25, D. C., 1960. (6) Kawashima, T., Osawa, M., Mochieuki, Y., Hamaguchi, H., Bull. Chem. SOC.Japan 34,701 (1961). (7) Kohn, H. W., Tompkins, E. R., U. S. Atomic Energy Comm. Rept. ORNG390 f 1949).
(8j-Mei;lke1W. W., Nucleonics 17, No. 9,
86-9 (1959). (9) Meinke, W. W., U. S. Atomic Energy Comm. Rept. AECU 3887 (1958). (10) Okada. M.. ANAL.CHEM.33, 1949 (1961). . l ) Phillips, G., Cornish, F. W., Brit. I
.
Atomic Energy Research Establishment Rept. AERE C/R 1276 (1953). -2) Preobraehensky, B. K., Kalyamin, A. V., Lilova, 0. M., Soviet J . Inorg. Chem. 2, 1164 (1957); Translation of
Table 11. Experimental Sensitivity for the Determination of Ytterbium, Dysprosium, Europium, and Praseodymium
Amount, Isotope Pg.” 2 . 0 h Yb177 1.5 Yb 4 . 2 d Yb116 0.4 Dy 2.32 h Dy1s6 0.004 Eu 9 . 2 h EuIs2 0.001 Pr 1 9 . 1 h Prlra 3 5 Based on 25-minute irradiation in a flux of 2 X IO1*neutrons cm.-* sec.-l Element Yb
Brit. Atomic Energy Establishment Rept. AERE-IGRL-T/R-81 (1958). (13’1 SDeddina. F. H . , Daane, A. H., “The ‘ Rare Eart&,” Wiley, New York; 1961. (14) Stevenson, P. C., Nervik, W. E., Nuclear Science Series Rept. NAS-NS3020, “The Radiochemistry of the Rare Earths, Scandium, Yttrium and Actinium,” Office of Technical Services, Department of Commerce, Washington 25. D. C.. 1961. (15)’Stewa;t, D. C., ANAL. CHEM. 27, 1279 (1955). (16) Vickery, R. C., “Analytical Chemistry of the Rare Earths,” Pergamon Press, Kew York, 1961. (17) Wong, K. M., Voigt, A. F., U. S. Atomic Energy Comm. Rept. IS 376 (1961). RECEIVEDfor review June 25, 1963. Accepted August 30, 1963. Division of Analytical Chemistry 144th Meeting, ACS, Los Angeles, balif., April 1963. Work supported in part by the U. 8. Atomic Energy Commission. Part of the stay of one of us (K. R.) wm supported by a training fellowship from the International Atomic Energy Agency.
Atomic Fluorescence Spectrometry as a Means of Chemica I Analysis J. D. WINEFORDNER and T. J. VICKERS Department o f Chemistry, Universify of Florida, Gainesville, Fla.
b A new method of flame spectrometric analysis i s intrcduced in which the intensity of fluorescent emission i s measured when atoms in a flame are excited b y the absorption of radiation of the proper frequency. Theoretical principles are discussed, and equations are derived relating )he intensity o f fluorescent emission to the concentration o f atoms. Experimental requirements are considered, and the method i s compared to atomic ubsorption and atomic (thermal) emission methods. Fluorescence technique: are shown to have several advantciges over a b sorption and emission techniques which should make the method a valuable tool for chemical analyzis.
I
N ATOMIC FLUORESCE,NCE SPECTROMETRY atoms are excited by the
absorption of radiation of the proper
frequency and then are deactivated by the emission of radiation of the same or lesser frequency. The frequency of the emitted radiation is characteristic of the absorbing atoms, and the intensity of the emission may be used as a measure of their concentration. The intensity of the emitted radiation should be dependent on the fraction of excitation radiation absorbed, on the efficiency of conversion of absorbed radiation to emitted radiation, on the fraction of emitted radiation self-absorbed by similar ground state atoms, and, just as in all fluorescence techniques, on the intensity of the radiation from the source of excitation. Thus, atomic fluorescence spectrometry has characteristics which resemble atomic emission (thermal emission) and atomic absorption, as well as molecular fluorescence techniques, but, as will be pointed
out in this paper, it offers several advantages uniquely its o m . The experimental arrangement used in atomic fluorexence spectrometry is basically similar t o that of molecular fluorescence spectrometry. Holyever, the individual components are qiniilar to those used in atomic emission and atomic absorption flame spectrometry. A typical experimental arrangement Yould consist of an intense source of radiation with its emitted beam focused on a suitable sample cell containing atomic vapor of the element of interest. The fluorescent radiation at right angles t o the exciting beam would be focused on the entrance slit of a monochromator, and the amplied signal from a photomultiplier detector would be displayed on a meter or recorder. In the event the sample emits thermal radiation of the same frequency as the fluorescent VOL. 36, NO, 1, JANUARY 1964
0’
161
radiation, then, just as in atomic absorption spectrometry, the incident radiation from the excitation source would be chopped and an amplifier tuned to the chopping frequency would be used. A number of studies involving the measurement of the fluorescence of atoms have been reported and summarized by Mitchell and Zemansky ( 6 ) . Most of these studies were performed in the early 1900's. In addition to these studies, atomic fluorescence spectrometry, just as atomic absorption spectrometry, has found its primary use in astrophysical work for the study of the composition of various stellar and solar atmospheres. All of the early studies of atomic fluorescence utilized special quartz or glass cells for confinement of the atomic vapor. This method is extremely inconvenient, and it Rould seem that a stable flame would be a much more convenient "sample cell" for atomization of the sample. Fluorescence of species in flames were first reported by Nichols and Howes (6) who obtained weak fluorescencefrom sodium, lithium, calcium, strontium, and barium when present in high concentrations in a flame. More recently Robinson (8) noted weak fluorescence of the Mg 2852 A. line when a 1000-p.p.m. solution of magnesium was atomized into a HJOz flame and was excited at right angles with a magnesium hollow cathode discharge tube. However, under similar experimental conditions, Robinson reported no detectable fluorescence from the Na 5890 A. line, the Ni 3524.5 A., or the Ni 3414 A. lines. In general most of the above studies were performed to gain information on excitation mechanisms of atoms in a confined gas or a flame and were not used for analytical purposes. At the 1962 Spectroscopy Colloquium, alkemade ( 1 ) gave an excellent paper on the mechanisms of excitation and deactivation of atoms in flames. He described the use of an atomic fluorescence method for determination of quantum yield and indicated its possible analytical application. A brief comparison of atomic fluorescence flame spectrometry with atomic absorption flame spectrometry was made, and a description was given of the experimental equipment used to measure the quantum efficiency of the Na 5890 A. line as a function of flame gas composition. Prior t o this work, Boers, illkemade, and Smit (2) used atomic fluorescence techniques to evaluate the fluorescence quantum efficiency of the Na 2.90 A. line in a propane-air flame. In this paper the theoretical and experimental aspects of atomic fluorescence spectrometry as applied to chemical analysis are considered. An equation is derived relating the intensity of the fluorescent emission to experi-
162
0
ANALYTICAL CHEMISTRY
FREQUENCY,
U , SECi
Figure 1. Intensity-frequency distribution of an ideal absorption lineGaussian and triangular distribution
mental and chemical factors, and the sensitivity of the method is discussed in the light of this expression. The advantages of atomic fluorescence spectrometry using a flame sample cell over atomic emission (thermal emission) and atomic absorption flame spectrometry will be considered. Preliminary calculations and experimental work indicate that in many cases the sensitivity of atomic fluorescence flame spectrometry is as great or greater than either atomic emission or atomic absorption flame spectrometry. These results will be made clear in two subsequent papers, one dealing with the calculation of the limit of detection of the method, and the second covering the experimental results and the application of the method to the determination of several metals. It will be evident that atomic fluorescence flame spectrometry, just as atomic absorption flame spectrometry, is especially useful for the analysis of elements such as Zn and Cd which have resonance lines in the ultraviolet. THEORY
Atomic fluorescence spectrometry is an emission method based on radiational, rather than thermal, activation of an atomic vapor. In the case where the absorbed and emitted radiation are of the same frequency-i.e., resonance fluorescence-the energy emitted as fluorescent radiation per unit time by the sample is proportional to the energy of the resonance radiation absorbed by the sample per unit time, and so PF = $Paba (1) where P p is the radiant power emitted as fluorescent radiation, P & is the radiant power absorbed by the sample, and the proportionality factor, 9, is the quantum efficiency, which accounts for the loss of energy through other processes than the fluorescent radiative transition. I n a practical case where the fluorescent radiation originates throughout a finite body such as a flame, an additional term must be included in
Equation 1 to account for the selfabsorption of radiation. Figure 1shows an idealized absorption line, Gaussian in shape, on which is superimposed a triangle of the same area. The area under either curve represents the total power absorbed by the line. Because the area of the triangle is given by the peak absorption times the half-width of the triangle, it follows that P&# = PO(l- e - k o L ) A v watts (2) where P o is the incident radiant power, in watts, per unit frequency interval, in sec.-l, k" is the atomic absorption coefficient in em.-' a t the line center, L is the average path length, in cm., of radiation passing through the flame, and Av is half the base width of the triangle in see.-' Willis (10)has shown that the half width of the triangle, Av, is related to the half intensity width of the Gaussian curve, AVO,by the expression ,-
Thus, if the absorption line is Gaussian in shape, which is generally a good approximation in flame spectrometry performed a t atmospheric pressure, Av can be evaluated from the half-intensity line width, and the total absorption of the line can be calculated from Equation 2. The value of ko is given by ( 5 )
where A v D is the Doppler half n-idth in set.-' of the absorption line, g1 and go are the statistical weights of the upper, 1, and the lower, 0, states which are involved in the absorption process, N o is the ground state concentration of the absorbing species in atoms/cc., is the wavelength of the center of the absorption line in cm., A t is the transition probability in sec.-I for the transition 1 to 0-Le., the transition from the atomic fluorescent state 1 to the ground state +and 6 is defined as 6 2 times the ratio of the Lorentz plus Holtsmark plus natural half widths in see.-' t o the Doppler half width in see.-' ( 5 ) . Because the total energy absorbed per second by an absorption line is given by Equation 2, it follows that the energy emitted per second as fluorescent radiation must be given by Pp
= $ P 0 4 v (1
- e-koL)e-k"L/2
cosh ( k 0 L / 2 )watts (4)
where the product e - k o L / 2 cosh (lc0L/2) approximately accounts for the selfabsorption of the fluorescent radiation. The self-absorption term has been derived in the article by Kolb and Streed (4). The quantum efficiency, 9,is the number of atoms which undergo the observed fluorescent transition per unit
N, , GROUND ST4'E
ATOM
CONCEMRATION
triangular absorption line, k 2 O is the atomic absorption coefficient for exciting radiation at the line center, and klo is the atomic absorption coefficient for emitted radiation a t the line center. I n the most general case, several absorption lines may contribute to the intensity of the fluorescent radiation. Thus, the general form of the fluorescent intensity equation is given by the sum of Equations 6 and 7 and higher terms similar to Equation 7 , corresponding to still other absorption lines which may contribute to the population of state 1, the fluorescent excited state, and so
Figure 2. Theoretical working curve plot of intensity of fluorescence vs. ground state atom concentration
time divided by the total number of atoms leaving state 1 per unit time. Equation 4 can be rewritten in terms of intensity by dividing PF by the area of the flame cell, A,, in from which the fluorescent radiation is emitted, and by 4n steradians. If this is done, then the fluorescent intensity, I F is given by
IF
= ( ~ 1 / 4 n A / ) 4 z ( v i / v z ) P z ~XA ~ z ( 1 - e-kZ0L)e-kio'L/2 cosh(kl"L/2)
(6)
in watts/crn.'%ter., ,vhere 41 is the quantum efficiency defined above-Le., the number of atoms which undergo the observed fluorescent transition per unit time divided by the total number of atoms leaving state 1 per unit time. The term cZ is the rumber of atoms which reach the excited state 1 per unit time from state 2 divided by the total number of atoms leaTring state 2 per unit time. The frequency of the emitted radiation is vl, and the frequency of the absorbed radiation is v 2 . Hence the factor v l / v z is the energy efficiency of the process and is defined as the energy emitted per energy absorbed. The term Piois the in:ident power per unit frequency interval of the exciting line, A V ~is the half width of the
Figure 3. Block diagram of experimental setup for atomic fluorescence measurements
When k"L is small, as when the limit of detection is approached, Equation 5 may be greatly simplified by expanding the (1 - e--koL) term and noting that e - - k o L / 2 cosh ( k OL/2) approximately equals 1. Then I F is given by IF =
4PoA~k"L/4~Af
watts/cm.%ter.
(9)
Substituting for IC" gives e--hoL/2
cosh
in watts/cm.2-ster. one obtains
kl"L 2
+ . . ..
iPlo*ul(l
-
$z yf P z o A v 2 ( 1 Y2
PI $3 Y J
(7)
Collecting terms
cosh ( k 0 L / 2 )m:ttts/cm.z-ster. ( 5 )
Equation 5 is an elact expression for the intensity of fluclrescent radiation only in the special cmes in which the absorbed and emitted radiation are of the same frequency-i.e., resonance fluorescence. In practice, the absorbed radiation is frequently of higher frequency than the emitted radiation. For example, the Na E1890 A. doublet fluorescent radiation may be produced by exciting Na atoms with the 3303 A. doublet ( 5 ) . If subscxipt 1 refers t o the excited state from which the fluorescent transition originates, and subscript 2 designates the excited state resulting from the absorptionof Iexciting radiation, then the fluorescent in1;ensity is given by
- -
e-ki*L)
+
- e-boL) +
PaoAvs(l - e - k a o L )
+ . . . .I (8)
in watts/cm.2-ster., where subscripts 3, 4, . . . refer to higher excited states which may contribute to the population of state 1. Although Equation 8 undoubtedly represents a case of practical importance, simplicity and clarity suggest the use of Equation 5 in subsequent discussion. The results obtained in this manner may be extended to the general case of Equation 8 without difficulty. From Equation 5 it should be noted that, just as in molecular fluorescence of liquids, a working curve plot of IF us. concentration No should give a curve of the general shape shown in Figure 2. The curve is nearly linear a t low concentrations and is curved a t high values of No-i.e., high values of ICOL. The curvature at high values of No is a result of two factors becoming significant. As Noincreases (1 - e - k " L ) approaches 1 and e - - k o L / 2 approaches 0, and so the curve of I F us. No goes through a maximum and then decreases, In addition the quantum efficiency may also decrease as No increases because of self-quenching (5, 7). However, the concentration a t which 4 begins to decrease with N o will probably be a t an appreciably higher concentration than is normally present in flames a t atmospheric pressure.
PONo watts/cm.2-ster. ( 1 0 )
For any given spectral line of a metal vapor in a flame of constant composition and temperature, the term in brackets will be a constant, designated C, and so la = C P o N owatts/cm.2-ster.
(11)
If the incident source power is constant, then IF will be linear with ground state atom concentration-Le., number of atoms in ground state per cc. of flame gases. An increase in sensitivity can be obtained by increasing P o or C or both. Of course, just as in all flame spectrometric methods the value of NOdepends on the rate of the processes of droplet dispersion, solvent evaporation, and compound dissociation. POSSIBLE EXPERIMENTAL ARRANGEMENT
The experimental arrangement required for atomic fluorescence flame spectrometry has been briefly outlined in the introduction. One possible arrangement is shown in Figure 3. Any monochromator adequate for thermal emission methods should be suitable for atomic fluorescence, provided that it is capable of operations in the ultraviolet because this is the region in which atomic fluorescence will most likely find its major application. Total consumption atomizer-burners are convenient and may be used for atomic fluorescence measurement, although light scattering due to small solvent droplets and small flame size ( L ) makes such burners far from optimum. Chamber type atomizers should reduce light scattering due to solvent droplets and should increase the flame size ( L ) . I n many cases light scattering problems may be avoided by exciting a t one wavelength and observing the fluorescence a t a second, longer wavelength. VOL. 36, NO. 1 , JANUARY 1964
163
Thus, if the line a t the longer wavelength is filtered from the excitation beam, light scattering cannot interfere. The excitation source may be a hollow cathode discharge tube, a broad unreversed line source such as an Osram, Phillips, or Wotan lamp, or possibly even a high-intensity continuous source. 9 s will be reported in a subsequent paper, Osram lamps are adequate excitation sources. The proper choice of entrance optics-i.e., a suitable arrangement of lenses and mirrorscan provide a simple means of increasing the sensitivity of the method. Further increases in the sensitivity may be obtained by choice of suitable flame conditions so as to increase the quantum efficiency. Alkemade (1) has already discussed methods by which the quantum efficiency of the Na 5890 A. line can be increased and similar methods should apply to other metals. An ideal detection system should consist of a high sensitivity a.c. amplifier tuned to the frequency of the modulation of the radiation source. Light sources are usually operated from d.c. and so in Figure 3 a chopper is used to modulate the light beam a t the proper frequency. This is especially important if the species in concern thermally emits under the flame conditions being used. A possible circuit has been described by Jones, Fisher, and Kelley (3). If thermal emission is negligible, as is usually the case for atoms with spectral lines below 3000 A., then an ordinary high-gain, high-stability d.c. amplifier can be used. DISCUSSION
Atomic fluorescence flame spectrometry has several advantages and disadvantages when compared with atomic absorption flame spectrometry. In atomic fluorescence flame spectrometry i t should be possible to increase the sensitivity of analysis by increasing the source intensity until scattering becomes excessive. In many cases scattering can be avoided by excitation with filtered radiation and measurement of fluorescence a t longer wavelengthse.g., excitation of T1 with 3776 A. radiation and emission of T1 at 5350 A. In the latter case increasing the intensity of the excitation source should lead to large increases in sensitivity. No comparable phenomenon exists in atomic absorption spectrometry. By proper choice of flame conditions the term C in Equation 11 can also be increased to give better sensitivity of analysis. The parameters, except for L and #, comprising the term C are either constants or vary slightly for any given atom and line and for almost any flame condition. Both L, the flame diameter, and #, the quantum efficiency, can be increased by proper choice of flame composition. In atomic absorption methods, the source intensity is limited by the requirement of using a narrow line source unless a high quality spectrometer is
164
ANALYTICAL CHEMISTRY
available. In atomic fluorescence the use of a wide unreversed line source of excitation is possible. The use of such a source gives many more applications and results in the possibility of constructing an extremely simple and appreciably less expensive setup than used in atomic absorption methods. Because atomic line widths are essentially unimportant, it should be possible to excite the fluorescence of a line of one element with a line of a different element as long as there is some overlap between the source line and the absorption line-e.g., the Cs 8521.1 A. and 3888.6 A. lines can be excited by the He 3888 A. line (6). Because of the large number of lines in the spectra of certain elements-e.g., Fe, Nil etc., it is highly possible that hollow cathode discharge tubes of these elements can be used to analyze other elements. The self-absorption of fluorescent radiation by ground state atoms in the flame cell and the possible susceptibility to interelement effects, which probably influence the value of $, comprise the major disadvantages of atomic fluorescence when compared with atomic absorption flame spectrometry. Self-absorption of radiation is only significant a t high concentrations and so is not a serious disadvantage. As long as the flame temperature and composition are maintained approximately constant and the partial pressures of foreign metal and nonmetal atoms are relatively small compared to the total pressure of the flame gases, the effect of interelement effects will probably be small, and the # values will be nearly constant. Alkemade (1) found that the quantum efficiency for the resonance fluorescence of Na did not change when the flame temperature of a CJ€s/Ol/Ar flame was raised from 2000" K. to 2100° K. However, changing the Ar to CO, resulted in a large (about six-fold) decrease in the # value. Atomic fluorescence flame spectrometry has several advantages and disadvantages when compared with atomic (thermal) emission flame spectrometry. Because atomic fluorescence spectrometry is based partially on the absorption of radiation by ground state atoms, it should be much less a function of flame temperature than thermal emission (9). However, the parameter certainly varies to some extent with flame composition and temperature ( I ) , and so the variation of flame conditions probably has a slightly greater effect on atomic fluorescence than atomic absorption spectrometry. Atomic fluorescence spectrometry, like atomic absorption spectrometry, should be particularly sensitive to elements having their resonance lines in the ultraviolet where thermal emission methods are particularly insensitire. .-\tomic $J
fluorescence, as atomic absorption methods, should also find use in isotopic analysis (9, 11). Some of the means of increasing sensitivity discussed in the above paragraph should still apply here. The experimental setup is necessarily somewhat more complicated than that used in atomic emission flame spectrometry. Atomic fluorescence spectrometry can offer few if any significant advantages to the analysis of elements such as the alkali metals which have their resonance lines in the visible. The large background continuum should give poorer sensitivities even with the use of elaborate detection equipment. Also even though the selection of sources should be considerably simpler and less expensive than in atomic absorption spectrometry, the use of sources which have intense unreversed broad lines over the region of absorption is still a disadvantage when compared with atomic emission flame spectrometry. Comparison of atomic fluorescence spectrometry with atomic emission spectrometry by an a.c. spark, d.c. arc, or other electrical means should be similar to the discussion given by Walsh (9) in which atomic absorption is compared to these methods. In the discussion above the term fluorescence was used to denote the radidional deactivation of atoms excited by the absorption of radiation. However, for future reference it will be convenient to divide atomic fluorescence into four basic types which may prove valuable to the analyst. Several of these have already been discussed. Resonance Fluorescence, in which atoms emit the same spectral lines used for excitation, will probably have the greatest analytical use. The other three types should, however, extend the range of application. The first of these will be called Direct Line Fluorescence, which consists of radiational deactivation of an excited atom to a metastable state above the ground state-e.g., emission of T1 5350 A. after excitation by 3776 -4.T1 line. The second will be called Stepwise Line Fluorescence n-hich consists of the emission of resonance radiation after excitation of the atoms to a state higher than the first (resonance) state and then deactivation to the first excited state-e.g., emission of the Na 5890 A. doublet after excitation a-ith the h'a 3303 A. doublet. It should be noted that Equation 8 is applicable to both types of line fluorescence, but for direct line fluorescence +z equals 1.00. The final method is called Sensitized Fluorescence. In this method the atom in concern emits radiation after collisional activation by a foreign atom which had previously been excited by absorption of resonance radiation (6). For example, if a gas contains a high pressure of Hg vapor and
a low pressure of T1 vapor, and if i t is excited by the 2537 A. Hg line, then a portion of the absorbed energy will be converted into T1 3776 and T1 5350 A. line emission (6). The analytical possibilities of atomic fluorescence spectrometry seem great. In many cases it may prove superior to either atomic emission or atomic absorption flame spectrometry and undoubtedly will prove complementary to both methods in theoretical and routine studies. Further evidence of this will be presented in subsequent papers on the application of the method to the
analysis of specific elements and on calculations of the limit of detection for various spectral lines of several elements under given experimental conditions. LITERATURE CITED
(1) Alkemade, C. T. J., International Conference on Spectroscopy, College Park, Md., June 1962. (2) Boers, A. L., Alkemade, C. T. J., Smit, J. A., Physica 22,358 (1956). (3) Jones, H. C., Fisher, D. J., Kelley, M. T., U . S. A t . Energy Comm. TID7629, 31 (1961). (4) Kolb, A. C., Streed, E. R., J . Chem. Phys. 20, 1872 (1952).
(5) Mitchell, A. C. G., Zemansky, M. W., “Resonance Radiation and Excited Atoms,” University Press, Cambridge, 1961. ( 6 ) Nichols, E. L., Howes, H. L., Phya. Rev. 23, 472 (1924). (7) Pringsheim, P. “Fluorescence and Phosphorescence,” Interscience, New York, 1949. (8) Robinson, J. W.,A n a l . Chim. Acta 24, 254 (1961). (9) Acta 7. 108 . , Waleh. A.., SDectrochim. . (1955). (IO) Willis, J. R., Australian J . Sci. Res. A4, 172 (1951). (11) Zaidel, A. N., Korennoi, E. P., Optzka. 1. Spectr. 10, 299 (1961). RECEIVEDfor review July 19, 1963. Accepted October 23, 1963. I
Determination of Zinc, Cadmium, and Mercury by Atomic Fluorescence Flame Spectrometry J. D. WINEFORDNER and R. A. STAAB Department of Chernisi’ry, University of Florida, Gainesville, Fla.
b
Atomic fluorescen1:e flame spectrometry is applied to the analysis of zinc, cadmium, and mercury in aqueous solutions. Solutions are aspirated by means of a total consumption atomizerburner into a H?/Oz or C?H2/O2flame positioned directly in front of the monochromator entrance $lit, and the resulting metal vapor is excited at right angles to the optical axis of the monochromator by means of an appropriate metal vapor lamp. The resulting fluorescence is measured by means of a compact monochromatordetector system. Working curves for Zn 2 1 3 9 A., Cd 2 2 8 8 A., and Hg 2537 A. lines are linear over a large concentration range. The limit of detection of Zn 21 39 A,, Cd 2 2 8 8 A., and Hg 2 5 3 7 A. lines are 0.04, 0.1, and 5 p.p.m., respectively, in a Hz/O? flame and 0.04, 0.05, and 1 p.p.m., respectively, in a CzHz/02 flame. Means of imlwoving the limit of detection are discussed. The advantages of atomic fluorescence flame spectrometry with respect to other atomic spectrometric methods, and a number of possible sources of excitation for metals other than zinc, cadmium, and mercury are also given.
T
HE POSSIBLE APPLICATION of atomic fluorescence spectrometry using flame cells, to the qua9titative analysis of trace quantities of metals has already been discussed (1, 6). In this paper a simple atomic fluorcascence spectrometric setup is described and is applied t o the determination cf zinc, cadmium, and mercury in aqueous solutions. Working curves for :4n 2139 A., Cd 2288 A., and Hg 2537 A. lines and the
limit of detection for each spectral line are given. A discussion of the atomic fluorescence of metals other than zinc, cadmium, and mercury, a discussion of possible sources of excitation, and a comparison of atomic fluorescence with atomic absorption and atomic emission (thermal emission) flame spectrometry are given. For some metal analyses, atomic fluorescence flame spectrometry will have definite advantages over other spectrometric methods. This should make atomic fluorescence flame spectrometry a valuable method because it will then complement the methods of atomic absorption and atomic emission flame spectrometry. EXPERIMENTAL
Apparatus. The experimental components are arranged as described b y Winefordner and Vickers (6). All components, individually described below, were held in place by means of two triangular optical benches (each 0.5 meter long, No. 22-702, The Ealing Corp., Cambridge 38, Mass.), which were mounted a t right angles on a piece of 0.5 inch thick Bakelite. All components were connected to the optical benches by means of rods (13.5-mm. diameter), which were held in carriers (KO.22-728, The Ealing Corp.). The carriers could be positioned in place a t any point on the optical benches. The monochromator - detector combination and the burner were mounted on one optical bench, and the excitation source was mounted a t right angles to the optical axis of the monochromator (6). SOURCE. Zinc and cadmium Osram lamps (The Ealing Corp.), with holes in their outer soft glass envelopes, were operated on a.c. using a step-up transformer and ballast. The lamps were
placed in a socket (NO. 26-250, The Ealing Corp.) containing a mounting pin in the center of the base for positioning in a carrier. For mercury, a Hanovia mercury vapor lamp (No. 90012 Hanovia Chemical and Mfg. Co., Newark 5, N.J.) operated with a reactive transformer (No. 23634, Hanovia Chemical and R4fg. Co.) was used. The mercury lamp was held in place by means of a universal laboratory clamp. The radiation from the appropriate source was focused on the desired portion of the flame containing the desired metal by means of a quartz lens. The lens was mounted in a holder (No. 22-810, The Ealing Corp.) and then placed in a carrier and positioned on the optical bench. The use of a lens was not critical because essentially the same results could be obtained by moving the lamp as close to the flame as possible. However, in the latter case, it was necessary to shield the entrance slit of the monochromator from the incident radiation. This was done by placing flat black baffles along the sides of the lamp. ATOMIZER BURNER. A Beckman medium bore (No. 4020) total consumption atomizer-burner was used for all studies. It was mounted on a rod (6 inches long by 13.5-mm. diameter) and held in a carrier on the optical bench. ‘C-nder the atomizer capillary a small piece of aluminum sheet was positioned to hold the sample cuvettes. The positioner was mounted on a swivel so that i t could be moved out from under the capillary. Both Hz/Oz and C2H2/O2 flames were used for the studies in this paper. Provision was made to introduce a small flow of argon into any desired flame. This was done by premixing argon with oxygen before introduction into the atomizer-burner. MONOCHROMATOR - DETECTOR. A compact, low-priced Bausch and Lomb
-
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