Atomic Fluorescence Spectrometry with a Continuum Source, Graphite Atomization and Photon Counting Stephen A. Clyburn,’ Betty R. Bartschmid,2and Claude Veillon3 Department of Chemistry, University of Houston, Houston, Texas 77004
A simple atomic fluorescence system, consisting of a low intensity continuum source, a novel graphite atomization system, a small monochromator and photon counting electronics, is used to obtain detection limits in the ppb range for 13 elements. These elements and detection limits (in ng/ml) are: Zn (5), Cd (i), Ni ( 3 0 ) , Fe (20), Mn (5), Pb (io), Mg (31, Sn (20), In (io), BI (io), Cu (2), Ag (0.7), and Cr (40). The heated graphite atomization system employs continuous sample introduction for good reproducibility (1.3%), and no background emission is observed. The furnace is operated continuously, and a continuous pyrolysis treatment of the furnace tube permits essentially indefinite operating life. The furnace is capable of operation at tube temperatures of up to 3000 OC at power inputs of up to 2.5 kW.
One of the principal limitations of atomic absorption spectrometry is the need for a separate line source for each element to be determined. This is not an important limitation when only a few elements are to be determined but it becomes quite significant when many elements must be determined, from the standpoints of both speed and cost. Multielement sources have not adequately overcome this limitation, because of the limited number of elements and combinations that can be employed. The use of a continuum source for atomic absorption spectrometry offers ii means of overcoming this limitation, and this possibility has been investigated by several workers i 1-3) employing conventional atomic absorption apparatus. Because the absorption linewidths of the analyte atoms are in general considerably narrower than the spectral bandwidth of the monochromator, the analytical sensitivity with continuum sources is lower than that obtained with narrow line sources, due to unabsorbed source radiation reaching the detector. In addition, absorbance us. concentration calibration curves are often nonlinear under these circumstances. Some of this sensitivity loss is recovered when ac scanning or modulation methods are used ( 4 , 5 ) . Carried to an extreme in the opposite direction, employing wide monochromator spectral bandwidths, continuum sources can be used in “background correction”
where scattering and nonspecific absorption by the sample are compensated for (6). This lower sensitivity can be eliminated by employing high dispersion systems having a spectral bandwidth narrower than the absorption linewidth. However, with conventional grating monochromators, long focal length systems would be required, with their resultant large size, high cost, and low aperture. Other high dispersion systems having higher aperture, such as echelle spectrometers ( 7 ) and Fabry-Perot interferometers ( 8 ) , offer ways of overcoming this problem. In atomic fluorescence spectrometry, this inherent loss of sensitivity due to unabsorbed source radiation does not occur, since the source is not viewed directly. The use of a continuum source for atomic fluorescence spectrometry was first investigated by Veillon et al. (9). They employed a 150-W Xe continuum and obtained detection limits in the 0.08-7.5 wg/ml range for 10 elements with flame atomization. Ellis and Demers ( 1 0 ) employed a 450-W Xe continuum and obtained detection limits for Zn, Ag, and Mg about a factor of 20 lower than those observed by Veillon et al. for these 3 elements. Dagnall et al. (11 ) used a 150-W Xe continuum with a modified atomic absorption instrument and obtained comparable detection limits for the 6 elements common to the Veillon et al. study. For most of these elements, the detection limits were comparable to those obtained by atomic absorption with line sources on the same instrument. Omenetto and Rossi (12) used a mercury arc continuum for atomic fluorescence studies of several elements. Cresser and West ( 1 3 )used a 500-W Xe continuum and obtained detection limits for several elements that were higher than those reported previously by others. Bratzel et al. ( 1 4 ) using a 150-W Eimac Xe lamp for atomic fluorescence observed comparable or higher detection limits than literature continuum source values for 14 elements. While the use of a continuum excitation source does not result in an inherent sensitivity loss, the source intensity is of primary importance in atomic fluorescence spectrometry. Although the overall spectral intensity of a Xe continuum may be high, the intensity over the absorption linewidth may be considerably lower than that of a line source, particularly in the UV region where the continuum decreases rapidly in intensity with decreasing wavelength. For this reason, detection limits obtained with continuum
Present address, Varian Instrument Div., Los Altos, Calif. 94022.
Present address, Department of Chemistry, Colorado State University, Fort Collins, Colo. 80521. Present address, Biophysics Research Laboratory, Harvard Medical School, Peter Bent Brigham Hospital, Boston, Mass. 02115. Author to whom reprint requests should be sent. (1) V. A . Fassel, V. G. Mossotti, W. E. L. Grossman, and R. N. Kniseley, Spectrochim. Acta, 22, 347 (1966). (2) C. W. Frank, W G. Schrenk, and C. E. Meloan, Anal. Chem. 39, 535 (1967). (3) W. W. McGee and J. D. Winefordner, Anal. Chim. Acta, 37, 429 (1967). (4) W. Snelleman. Spectrochim. Acta, Part B, 23, 403 (1968). (5) V. Svoboda, Anal. Chem., 40, 1384 (1968).
(6) S. R. Koirtyohann and E. E. Pickett, Anal. Chem., 37, 601 (1965). (7) M. S. Cresser, P. N. Keliher, and C. C. Wohlers, Anal. Chem., 45, 111 (1973). (8) C. Veillon and P. Merchant, Jr., Appl. Spectrosc. 27, 361 (1973). (9) C. Veillon, J. M. Mansfield, M. L. Parsons, and J. D. Winefordner. Anal. Chem., 38, 204 (1966). (10) D. W. Ellis and D. R. Demers, Anal. Chem., 38, 1943 (1966). (11) R. M. Dagnall, K. C. Thompson, and T. S. West, Anal. Chim. Acta, 36, 269 (1966). (12) N. Omenetto and G. Rossi, Anal. Chim. Acta, 40, 195 (1968). (13) M. S. Cresser and T. S. West, Spectrochim. Acta, Pari E, 25, 61 (1970). (14) M. P. Bratzel, Jr., R. M. Dagnall, and J. D. Winefordner, Anal. Chim. Acta, 52, 157 (1970).
A N A L Y T I C A L CHEMISTRY, VOL. 46, N O . 1 4 . DECEMBER 1 9 7 4
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ASBESTOS G A S K E T
Flgure 1. Graphite furnace
sources in atomic fluorescence are usually higher than with intense line sources, such as electrodeless discharge lamps. Scattering of source radiation by particulate matter in the flame can be more of a problem with continua when large monochromator spectral bandwidths are employed, and large bandwidths are usually employed to take advantage of the simplicity of the atomic fluorescence spectrum. On the other hand, scattering can be easily ascertained and corrected for by scanning over the line. With a line source, the detection of, and correction for, scattering is not so straightforward. In a previous study ( 1 5 ) of atomic fluorescence spectrometry, employing photoelectron counting techniques, a low intensity continuum source, and a nonflame atomization system, detection limits were obtained for Zn and Cu which were comparable to the lowest reported detection limits by conventional atomic absorption spectrometry. These encouraging results prompted the present investigation. The high sensitivities obtained with relatively simple apparatus and a single low intensity continuum source are due in large measure to a combination of several factors. Analytical sensitivities and detection limits are obtained with heated graphite atomization systems that are clearly superior to those observed with chemical flame atomization, by about 2 orders of magnitude for many elements. The graphite furnace system employed in this investigation atomized sample continuously for improved precision, and exhibited no background emission. This absence of background emission is very important in atomic fluorescence and represents an ideal situation in that the sample atomic fluorescence appears above an essentially zero background. Even extremely weak fluorescence signals can then be measured by employing low-light-level measurement techniques such as photon counting.
EXPERIMENTAL Measurement System. The photoelectron pulse counting system and operating conditions have been described previously ( 1 5 ) , except the detector dark count rate was about 40 sec-' a t 18 OC and 600 V. Continuum Source. A single, low-intensity 150-W Xe continuum source was used for all of the measurements reported in this study, and has been described previously ( I 51. Optical System. The optical arrangement described earlier (15 ) was employed, except that the monochromator used in this investigation was a 0.25-m,f/4, 32 A/mm Czerney-Turner, with a 1200groove/mm grating blazed for 3000 A. An unmagnified image of the source was focused in the observation region (about 2 mm above furnace) of the atomization system, and an unmagnified image of this region was focused on the entrance slit. Atomization System. The atomization system consisted of a graphite tube furnace operated continuously, powered by a simple (15) M. K. Murphy, S. A. Clyburn, and C. Veillon, Anal. Chem. 45, 1468 (1973). 2202
high-current, low-voltage ac supply, and fed with desolvated sample aerosol. The essential design features of the furnace are illustrated in cross-sectional view (not to scale) in Figure 1. The material of construction is copper, and the graphite parts are machined from spectrographic-purity rod stock (Grade UF-IS, Ultra Carbon Corp., Bay City, Mich.). The essential dimensions are: holder (22.2-mm 0.d.); internal chamber (50.8-mm dia X 63.5-mm height); graphite tube i.d. (6.35 mm); lower holes (four) in graphite tube (3.18 mm); coolant tubes and sample introduction tube (6.35-mm 0.d.). The sample introduction tube enters the chamber interior tangentially. The interior surfaces of the chamber and the upper end of the holder.were Ni-plated and polished, to reduce the radiant heat load on the water cooling system. A sliding fit between the holder and the bottom plate allowed for thermal expansion of the graphite tube and assured a constant electrical contact force, since the weight of the chamber was supported by the graphite tube element. Electrical connections were made to the holder and the upper part of the chamber, which were electrically isolated by a thin asbestos gasket between the bottom plate and chamber and the use of nonconductive screws. The graphite inserts a t each end of the tube element were machined from 25.4-mm 0.d. rod stock, and the tube was machined from 12.7-mm 0.d. rod stock. The external surface of the tube was machined to approximately the shape shown in Figure 1, and smoothed with S i c abrasive paper. This shape, with a larger crosssectional area at the middle than a t the ends, results in a relatively long heated zone of uniform temperature. The exact shape is not critical, since the pyrolysis treatment (see below) forms the tube into the correct shape. Surface temperatures of the graphite tube were measured within It%% with an optical pyrometer (Model 8632-C, Leeds and Northrup, Philadelphia, Pa.) and measurement of gas temperatures was attempted with a chromel-alumel thermocouple, as reported elsewhere ( 1 6 ) . P o w e r Supply. Since the furnace is operated continuously, only a simple ac power supply is needed. This supply consisted of a 0240 V, 10-A variable transformer (Superior Electric Co., Bristol, Conn.) connected to the input of a 220-V primary, 12-V secondary, 3-KVA transformer (No. 216-1131, Jefferson Electric, Bellwood, Ill.). Secondary voltage was monitored with a 0-10-V ac voltmeter and secondary current was monitored with a 0-5-A ac ammeter connected to a 1OO:l ratio current transformer (No. 1-111301, Simpson Electric Co., Chicago, Ill.) on one of the secondary leads. Secondary power leads to the furnace consisted of doubled 2-AWG stranded, insulated cable (No. 1389/2, Alpha Wire, Elizabeth, N.J.). Sample Introduction System. The sample was introduced as a desolvated aerosol in argon, using the Veillon-Margoshes system ( 1 7 ) .The Friedrichs condenser was modified by the addition of an external water jacket around the body of the condenser. A t a nebulizer gas pressure of 3.5 kg/cm2, the argon flow rate was 2.25 l./min and the solution aspiration rate was 2.72 ml/min. Heated chamber power was 288 W, resulting in a temperature just sufficient to completely evaporate the sample aerosol under continuous nebulization. A measured overall efficiency of 33% was obtained, and 97.5% of the solvent (water) was removed, at a condenser water temperature of 15 "C. This is a partial pressure of water of about 26 Torr and corresponds to the exit gas temperature of 26 'C. This indicates essentially complete desolvation, down to the point where the residual water vapor in the gas stream is due only to its saturation vapor pressure. All samples used in this study were prepared from metals dissolved in an appropriate acid. Pyrolysis T r e a t m e n t . This residual water vapor from the sample introduction system results in a gradual deterioration of the heated graphite tube, decreasing the cross-sectional area and thus altering the voltage-current-temperature relationships. By operating the furnace a t a high temperature and introducing a mixture of argon and methane. the methane is pyrolyzed and a coating of pyrolytic graphite is built up on the tube surfaces. This hard, dense, nonporous coating is much more resistant to oxidation than the conventional graphite, and the tube life was greatly extended. After eventual deterioration, the process could be repeated and the coating restored. However, a cunstant, small amount of methane could be introduced continuously, at a rate such that the oxidation by the residual water vapor was exactly compensated for, permitting indefinite operation of the furnace without having to replace (16) S. A. Clyburn, T. Kantor, and C. Veillon, Anal. Chem., 46, 2213 (1974). (17) C. Veillon and M. Margoshes, Spectrochim. Acta, Part E, 23, 553 (1968).
ANALYTICAL CHEMISTRY, VOL. 46, NO. 14, DECEMBER 1974
the tube and without changes in the voltage-current-temperature relationships. An additional benefit of these pyrolysis treatments (initial and continuous) is that the optimum tube shape for a long, uniform-temperature zone is formed since the methane is pyrolyzed most rapidly where the tube is hottest. This is analogous to the tungsten cycle in quartz-halogen lamps. The details and data for this pyrolysis treatment are given in a separate publication
330c
r
(16).
RESULTS AND DISCUSSION Minimum detectable concentrations, expressed as detection limits in pg/ml, were determined for 16 elements in aqueous solutions employing the apparatus previously described. All of these atomic fluorescence measurements were made with a single continuum source under the same atomization conditions, namely, a graphite tube temperature of 2000 "C. No attempt was made to optimize the atomization conditions, nor were matrix effects investigated. For each element, an optimum temperature of the furnace system exists ( 1 5 ) a t that point where complete atomization occurs or a t the maximum operating temperature of the furnace. The graphite tube temperature is not as important as the gas temperature achieved, but reliable means of measuring the latter were not available and are under investigation at the present time. Matrix effects will no doubt 'be dependent on atomization temperature, and our preliminary investigations have confirmed this. These matrix effects are more serious with samples having a high solids content (especially inorganic), and it should be pointed out that high-solids samples may present problems for the desolvation-type sample introduction system used, in that frequent cleaning may be required. Temperature Measurements. Measurement of gas temperatures was attempted by placing a thermocouple in the analytical region immediately above the exit orifice of the furnace. The temperatures obtained were about half the graphite tube temperatures (optical pyrometer measurements) and are undoubtedly too low because of the thermal conductivity of the thermocouple leads. Placing the thermocouple junction inside the furnace tube presents the possibility of erroneously high gas temperature readings a t the higher temperatures, due to radiation from the tube element. Catalytic reactions on the thermocouple could also cause errors. Typical temperature curves are illustrated in Figure 2 for the optical pyrometer tube temperatures and the thermocouple gas temperature measurements described above. The former are accurate, while the latter are believed to be too low. However, the two curves illustrate a definite change of slope beginning a t about 1000 "C (tube temperature). This change of slope in the temperature us. power curve was somewhat of a surprise, in that similar curves of temperature us. voltage and temperature us. amperes were both linear over the entire range, while the product of these curves (temperature us. power) shows a change of slope. We feel that this apparent anomaly can be explained in the following way. The fact that the temperature us. voltage and temperature us. current curves are essentially linear merely indicates that the resistance of graphite changes in an approximately linear fashion over this temperature range. The fact that the temperature us. power curve shows a change of slope is an indication of the mechanism of heat loss by the system: below about 1000 "C, the primary mode of heat loss is cooling by the gas flowing through the tube, while above 1000 "C, the primary mode of heat loss is radiational. At very low temperatures, another mode of heat loss is dominant, which may not be apparent in Figure 2. These curves do not go through the origin ( i e . , room temperature, or about 18 "C) and, in this low temperature region, the primary mode of heat loss is initially the thermal conductivity of the graphite, conduct-
530
1,"s
3 FonrQ
200"
"+T
Temperature vs power curves for the furnace. The circles are optical pyrometer tube temperatures, while the dots are thermocouple gas temperatures and are too low (see text)
Figure 2.
ing the heat to the cooled ends of the tube. In any event, the actual gas temperature is probably somewhere between the two curves shown in Figure 2. When the graphite tube temperature was increased slightly (to about 2100 "C), a small blue flame appeared above the exit orifice. This flame is supported by entrained air and is primarily due to combustion of the H2 from the methane pyrolysis, with some contribution from CO combustion. Measurements were also made within the flame region and compared to those results obtained in the absence of the flame. In the wavelength region above about 3000 A, the background emission increased considerably due to the flame, while below 3000 A, the system was still essentially background-free. In addition to this, the presence of the flame altered the fluorescence intensity of several of the elements. For Ni, Fe, Sn, In, and Cr, the detection limits in the presence of the flame were improved by factors of between 5- and 10-fold. One possible explanation of this is that these elements may be incompletely atomized in the furnace and are further atomized in the flame, and/or quenching by molecular species (e.g., CO) may be important. For other elements, like Zn, Pb, Mg, Bi, Cu, and Ag, the detection limits in the presence of the flame were higher (6- to over lOO-fold), indicating that compound formation had decreased the atomic concentrations. For Cd and Mn, essentially the same detection limits were obtained in the presence or absence of the flame. The flame can be eliminated a t operating temperatures above 2100 "C by sheathing the furnace exit orifice with argon to prevent air entrainment. Analytical Results. The results for 13 of the 16 elements investigated are shown in Table I. For 3 of the elements, namely Ba (3501 A), A1 (3082 A), and As (1972 A), analytically useful results were not obtained under the conditions used. The signals for Ba and A1 were predominantly scatter (detection limits > 10 pg/ml) and indicate that these elements were incompletely atomized at 2000 "C. For As (1972 A), the source intensity was insufficient to detect 10 pglml. It is interesting to note that several of the elements which can form carbides, namely, Fe, Ni, Cr, and Mn, can be determined a t low concentrations with this atomization system. For the other 13 elements (Table I), the detection limits (S/N = 2 ) obtained compared quite favorably with those obtained by conventional atomic absorption spectrometry employing flame atomization and individual line sources
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 1 4 , DECEMBER 1 9 7 4
2203
LEAD CONCENTRATION.
2
6
10
~G/HL
1.
18
Table I. Detection Limits Obtained, a n d Literature Values for Absorption (Line Sources), Emission a n d Fluorescence (Continuum Sources) Detection limits, iig/rnla AA, line
Line, A
This stud)
sources
AEC
Zn Cd
2139 2288 3261 2320 3415 2523 2483 3720 2795 403 1 2833 2170 3635 2852 2863 2246 2840 3034 3039 4511 3068 223 1 3247 3274 3281 3579 4254
0.005 0.001 ... 0.03 ... 0.02 ...
0.002 0.005
50*
0.005
...
Ni
Fe 0 2
0.6
1.0
1.4
1.8
CRDMlUN CONCENTRATION, p d ? ~
Figure 3.
Analytical curves for Pb and Cd.
Mn Pb
for each element. The detection limits were comparable in all but 3 cases: Mg and Cr yield somewhat better detection limits by conventional atomic absorption, while a somewhat better detection limit was obtained for Ag in this study. All of the atomic absorption data in Table I are those reported by Slavin (18). Essentially identical detection limits have been reported by Christian and Feldman (22) for these elements. For comparative purposes, detection limits reported for these elements by atomic emission in the nitrous oxide/ acetylene flame are also shown in Table I. All of these values are from the work of Pickett and Koirtyohann (19), except for the Zn and Bi values which are from Fassel and Golightly (20) in premixed oxy-acetylene using nonaqueous solvents and perchlorate salts. Over half of the elements exhibit significantly higher detection limits, and only In and Cr have lower detection limits by flame emission. The last column in Table I lists the lowest previously reported detection limits for atomic fluorescence with a continuum source. In every case, the detection limits obtained in this study are significantly lower. Analytical curves for these elements are linear over at least 3 orders, and begin to show curvature a t the higher concentrations at about the same point as similar atomic absorption curves. Typical analytical curves for P b and Cd are shown in Figure 3. The Cd curve has the expected slope of unity, while P b has a slope of about 0.82, indicating (as one possible reason) that the atomization efficiency may be a function of analyte concentration in this case. The system used in this investigation has important advantages over conventional atomic absorption and flame emission systems. It has the same advantage over flame emission as does atomic absorption, namely, elements with principal resonance lines below about 3000 A can be readily determined at high sensitivity. The principal advantage of this system over conventional atomic absorption systems is that comparable analytical sensitivities can be obtained with a single continuum source. The atomization system used in this study has proved to be extremely efficient, reproducible, and versatile, and has considerable potential as an atomization system for analytical atomic spectroscopy, especially for atomic fluorescence. The high overall atomization efficiency, use of an
(18) W. Slavin. in B. V. L'vov, "Atomic Absorption Spectrochemical Analysis," B. V. L'vov, Adam Hilger, London, 1970, p 166.
2204
AF, cont.
Element
Mg Sn
In Bi cu
Ag Cr
...
...
...
...
sourced
0.03 ( 1 0 ) 0.08 ( 1 4 )
2 8
(13)
0.03
...
...
0.0.05
...
1 (21)
...
...
0.05
...
0.005
0.002
...
...
0.2 ( 1 4 )
... ...
0.005
0.01
...
0.01
... 0.003 0.02
...
... ... 0.01
... ...
...
0.05
...
0.0003
0.005
0.01 ( I O ) ...
... 0.06
...
...
... ... ...
...
0.05 ...
0.01
...
...
...
0.02 0.005 ...
40*
...
0.005 0.005
...
...
0.3
... 0.002 ... 0.0007 0.04
3 (21)
0.005
... 0.01 0.02
...
0.005
5 (14) 15 ( 1 3 ) 2 (9)
... 0.2
(14)
0.001 ( I O ) 3 (13)
...
a Values based on S / N = 2. Values from reference (18); atomic absorption, line sources, air/acetylene flame. C Values from reference (19); flame emission, nitrous oxide/acetylene flame. Those indicated by asterisk are from reference (20); premixed oxy-acetylene flame, nonaqueous solvents. d Atomic fluorescence, continuum source; references indicated in parentheses.
inert atomic gas, and absence of background emission makes it an excellent choice in the application of atomic fluorescence spectometry to a variety of analytical problems. With high intensity line sources, such as thermostated electrodeless discharge lamps, high sensitivity, reproducible atomic fluorescence measurements can be made. Such a system is currently in use in a study of Zn and Cd in enzyme systems, with accurate determinations being made in the ng to pg range, and these results will be reported in the near future. RECEIVEDfor review February 11, 1974. Accepted August 23, 1974. Taken in part from the M.S. thesis of S.A. Clyburn, University of Houston, May 1974. Presented in part a t the 4th International Conference on Atomic Spectroscopy, Toronto, Ontario, October 1973. This work was supported in part by the National Science Foundation and in part by the Robert A. Welch Foundation. One of the authors (B.R.B.) was an AAUW-Margaret Lee Wiley Fellow. (19) E. E. Pickett and S. R. Koirtyohann, Specfrochim. Acta, Part 6,23, 235 (1968). (20) V. A. Fassel and D. W. Golightly, Anal. Chem., 39, 406 (1967). (21) D. C. Manning and P. Heneage, At. Absorption Newden. 7, 80 (1968). (22) G. D. Christian and F. J. Feldman, "Atomic Absorption Spectroscopy." Wiley-lnterscience, New York, N.Y., 1970, p 447.
A N A L Y T I C A L C H E M I S T R Y , V O L . 46, NO. 14, DECEMBER 1974
