2598
Anal. Chem. 1986, 58,2598-2602
(24) Bear, B. R., private communication, 1983. (25) Win@, R. K.; Peterson, v. J.; Fassei, v. A. APPl. sPectKJsc. 1979, 33, 206.
RECEIVED for review November 18,1985. Resubmitted June
13,1986. Accepted June 13,1986. The Ames Laboratory is operated for the U.S. Department of Energyby State University under Contract No. W-7405-Eng-82. This research was supported by the Division of Chemical Sciences, Office of Energy Research.
Laser-Excited Atomic Fluorescence of Atoms Produced in a Graphite Furnace D. Goforth and J. D. Winefordner* Department of Chemistry, University of Florida, Gainesville, Florida 3261 1
Laser-excited atomlc Huotescence in a graphne furnace gives detection IbnHs for Pb, Cu, Mn, Sn, AI, In, LI, and Pt, in the pkogram to sub-picagram range. The linear dynamk range for these elements varles from 3 to 7 orders of magnnude. A graphlte rod, a plain graphite cup, and a slotted graphite cup are compared as the cuvette In the fluorescence system. Detection lhnlts for a pyrolytic coating, a tantalum foil liner, and a tantalum carbide coating of the graphlte cuvette are compared. A hydrogen-argon atmosphere, a low-pressure atmosphere, and an argon atmosphere are compared as the atmosphere s u r r o w the graphite cuvette. Lastly, Cu and Yn are determined In several standard reference materials.
Laser-excited atomic fluorescence using a graphite furnace has been shown to give very low detection limits (1-9). Bolshov et al. ( 5 ) achieved excellent detection limits for Pb, Fe, Na, Pt, Ir, Eu, Cu, Ag, Co, and Mn, using a Nd:YAGpumped dye laser system. These results ranged from 1.5 fg for P b to 300 pg for Eu. Tilch et al. (7)utilized a nitrogenpumped dye laser system to obtain similar detection limits for some of the same elements as well as a few others. Probably the most impressive reaulb have been the femtogram detection limits for Pb. Besides Bolshov and Tilch (5, 7) Human et al. (6) also obtained a detection limit in the femtogram range for Pb. Pulsed lasers offer several advantages for atomic fluorescence such as wide tunability and high peak powers. However, these lasers are less than optimum when using electrothermal atomization because of the low repetition rate (e.g., 20 Hz) of the lasers. The pulsed laser must be synchronized so that there is a laser pulse during the time when atoms are present in the laser path. This problem can be overcome somewhat by using a computer to synchronize the laser pulse with the formation of atoms during the atomization stage or by confining the atomic vapor within a tube atomizer to increase the residence time. An alternative is to operate the pump laser at higher frequencies if possible, e.g., by use of a copper vapor laser pumped dye laser system a t ca. 6 kHz. The atomizer requires a special design for atomic fluorescence since the fluorescence is viewed at 90’ to the laser beam. A graphite cup (5, 7),a Massmann cup (IO), a graphite tube with holes cut in the sides ( 9 ) ,and a graphite rod ( I , 6), are several designs that have been tried. The simplicity of the graphite rod makes it attractive for use with volatile elements. On the other hand, the graphite cup offers an improvement over the graphite rod because the sample is in a semienclosed
environment, which gives more of a “furnace” effect. However, the atoms still must emerge from the furnace into the cooler atmosphere before being excited by the laser. As the atoms vaporize out of the hot furnace they have more of a chance to react with interferents, such as 02.In principle, the best atomizers are those that contain the atoms in a hot environment while they are being excited by the laser. The graphite tube with holes cut in the sides (9) and the Massmann cup (10) are both examples of this type. Several studies in graphite furnace atomic absorption have utilized coatings to inhibit reactions between the anal* and the graphite. A pyrolytic coating has been the most popular (11-13). Some alternate coatings include a tantalum-treated a molybdenum-treated graphite tube ( E ) , graphite tube (I4), tantalum foil liners (16-I8),and metal atomizers (8,9,20). Much of the work that has been done with laser-excited atomic fluorescence in a furnace has been carried out in an argon atmosphere. The inert atmosphere keeps out contaminants from the air. Amos et al. (21) have also found that having a hydrogen flame surrounding the graphite burns up any entrained oxygen, thus giving a much more reducing atmosphere for the atomization process. Improved detection limits and a lessening of spectral interferences were found. Low-pressure fluorescence (22, 23) has resulted in the detection of individual atoms as they pass through a continuous wave (CW) laser beam by using a photomultiplier-photon counter system. This study describes the application of laser-excited atomic fluorescence with a graphite furnace for the determination of In, Sn, Pb, Cu, Mn, Pt, Li, and Al, including estimation of limits of detection (LOD). A graphite rod, a plain graphite cup, and a slotted (Massmann) cup, were evaluated to determine the best atomization cell. The detection limits from a pyrolytic coating, a tantalum carbide coating, and a tantalum foil liner were compared. A furnace in an open atmosphere with an argon sheath and a small flow of hydrogen (H,-Ar) was compared with a furnace in an enclosed argon atmosphere (Ar) and at a low pressure (LP). Lastly, some standard reference materials, wheat flour, spinach, and steel, were measured for Cu and Mn.
EXPERIMENTAL SECTION Laser System. In Figure 1, a block diagram of the experimental setup is given. The nitrogen-pumped dye laser (Molectron Model DL-I1 dye laser, Molectron UV-24 nitrogen laser) was operated at 20 Hz for the experiments with the H,-Ar atmosphere and at 30 Hz for those experiments done in Ar and LP atmospheres. The laser had a spectral line width of 0.015 nm (fwhm), a pulse width of 5 ns, and a typical pulse energy of 5-40 pJ in
0003-2700/86/0358-2598$01.50/0Q 1986 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
2599
/rc"
- H,V
0.10" ID
A
PLAIN
MIRROR
DYE
6 1-
t
GAS NEBULIZER MANIFOLD
;z
ROD
LASER TRIGGFR
I N2
-
I
U LASER
k.34u4
SLIT WIDTHS I
S L O T T E D CUP
0.1"8 0.16"
Figure 1. Schematic diagram of the graphite furnace iaser-excited atomic fluorescence system.
Flgure 2. Schematic diagram of graphite cuvettes.
the frequency-doubled region (220-360 nm) and from 0.3 to 0.8 mJ in the fundamental region (360-1100 nm). The laser beam was allowed to diverge over a 2-m distance to the atomizer. The fundamental beam had a diameter of about 2 mm at the atomizer; the frequency-doubled beam had a diameter of 6 mm at' the atomizer due to optics associated with the frequency-doubling system. The radio frequency interference from the nitrogen laser was reduced to a negligible level by enclosing the laser in a well-grounded brass screen Faraday cage. Graphite Cuvette. Before any data were taken, a preliminary investigation was carried out to determine the best graphite cuvette design. In Figure 2, the designs evaluated are shown. For the plain cup and the rod, the laser beam was directed over the top of the cuvette, and the fluorescence was viewed at 90°. For the slotted cup, the laser was directed into the cup from above and the fluorescence viewed through the slots on the side of the graphite cup. The graphite cups were held between two spring-loaded graphite electrodes. These electrodes firmly held the cups and helped to give good electrical contacts. The plain cup was used for all of the reported results. Furnace System. The electrodes and cuvette were supported on a laboratory-built furnace system. The furnace was heated by a programmable power supply (Electronics Measurements, Model SCR 20-250,5 kW). The desired atomization temperature could be obtained by setting the voltage on the power supply and setting the heating time on a laboratory-built circuit. In order to study the different atmospheres surrounding the graphite, two different furnace systems were used. In the H2-Ar system, an argon sheath surrounded the graphite cup, and hydrogen was fed through a capillary burner directly below the cuvette. The hydrogen ignited during the atomization stage and burned off any entrained oxygen, providing a more reducing atmosphere for the atomization process (21). The capillary burner was also connected by tubing to a nebulizer, which allowed a flame (acetylene-air, nitrous oxide-acetylene) to be used for wavelength tuning of both the laser and the monochromator. The second furnace system was designed so that it was compatible with either an argon atmosphere or a low-pressure atmosphere. The base of the enclosure was made of phenolic. The two copper electrodes were connected to the leads of the power supply by steel bolts that were fed through holes in the phenolic. A brass plate with an array of small holes was press fit into the center of the phenolic base. This plate allowed argon to flow into the enclosure. Besides the benefits of the inert atmosphere, the argon also swept the atoms up into the laser beam. Two steel quarter-inch tubes were also press fit through the phenolic to let in cooling water for the electrodes. A groove was cut into the phenolic surrounding the electrodes and water lines. Using
vacuum grease, an O-ring in the groove formed a sufficient seal with the flat edge of a glass dome to allow a pressure down to 25 torr. Sealant was used around all of the fixtures (brass plate, electrodes, bolts, steel tubes) to keep air from entering the chamber. The glass top had three 1-in. quartz windows attached with epoxy. One window allowed the laser beam to enter while the opposite window allowed the laser beam to leave the enclosure. The exit window was used to minimize air entrainment and scatter from the glass dome (the curvature of the glass dome caused considerable scatter). There was also a quartz window at 90' to these windows so that the fluorescence could pass through to the monochromator. There was a small outlet in the glass so that the Ar could escape; a Cajon connector was used to connect this outlet with a vacuum pump (General Electric, Model 1400) when the LP atmosphere was used. In this system, the laser was tuned by using a nearby flame-monochromator system and the monochromator used with the furnace was tuned by using a hollow cathode lamp. Detection System. A lens (1.5 in. diameter, 2 in focal length) and aperture system was used to project a 1:l image of the fluorescence into the monochromator (0.35-m Heath monochromator; stray light, 0.1%). The monochromator was rotated on its side (90°) so that the slit height was parallel to the laser beam. A tent of black felt placed around the furnace and monochromator helped to reduce stray light. The fluorescence was detected with a photomultiplier tube (Hamamatsu, Model R1414) mounted within a well-shielded laboratory constructed housing. The photocurrent pulse was stretched slightly by a 12004 load resistor (fwhm 100 ns) and connected directly to a boxcar averager (PAR Model 162 with a 164 plug-in) operated with a 5-ns gate. The boxcar was triggered by a photodiode which received a fraction of the nitrogen laser output. The boxcar output was displayed on a chart recorder for the data taken with the H,-Ar atmosphere; peak height measurements were made. For the Ar and LP atmospheres, the boxcar averager (Stanford Research, Model 250) was operated with a 90-ns gate, and the signal was output to a computer (IBM PC); in these cases, peak areas were used. Graphite Coatings. In an attempt to improve detection limits, the following coatings for the graphite were evaluated pyrolytic graphite; tantalum carbide; and tantalum foil. The graphite was pyrolytically coated in a quartz and graphite chamber. The "coating" gas used for this process consisted of 90% argon and 10% methane (P-10 gas). The graphite was heated to 2500 O C for 10 min while the "coating" gas was flowing through the chamber at about 13 L/min. The "coating" gas continued to flow during a 5-min cooling off period. The tantalum carbide treatment was the same as that of Zatka (14). The 6% tantalum soaking solution was prepared by weighing
2800
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
3 g of tantalum metal into a 100-mL PTFE beaker and adding 10 mL of 1/1, v/v hydrofluoricacid, 3 g of oxalic acid dihydrate, and 0.5 mL of 30% hydrogen peroxide. The solution was carefully
heated to dissolve the metal. More peroxide was added if the reaction became too slow. When dissolution was complete, 4 g of oxalic acid and approximately 30 mL of water were added. The acid was dissolved, and the solution diluted to 50 mL. The solution was stored in a plastic bottle. The graphite cuvettes were then immersed in the 6% tantalum soaking solution in plastic vials. The vials were placed in a dissicator and then evacuated by a water pump for 20-30 s. Atmospheric pressure was then restored in the desiccator and the graphite cuvettes were removed from the soaking solution and dried first in air (30 min) and then at 105 "C (1h). Each cuvette was then placed in the furnace system and heated gradually (30 s) to 1000 "C and then for a few seconds to 2500 "C. The treatment was repeated again but the tubes were soaked for only 10 s under reduced pressure. The third method consisted of lining the inner surface of the graphite cup with 0.025 mm thick tantalum foil. The liner was shaped around a smaller metal rod and then allowed to expand in the cup. The cup was then taken through several heating cycles which caused the liner to expand so that there was no clearance between the liner and the cup. Standard Reference Materials. Wheat flour (SRM 1567), spinach (SRM 1570), and steel (SRM 364) were all placed in a desiccator for 48 h prior to weighing out any sample. After drying, the samples were weighed out above a given minimum sample size for each particular sample. The specific sample size was to ensure homogeneity. The dissolution technique for the wheat flour and spinach consisted of placing each sample in a round-bottomed (RB) flask. Nineteen milliliters of purified nitric acid and 1 mL of sulfuric acid were added to the flask. The distilling apparatus included a nitric acid preserver, which consisted of a sidearm fitted with a stopcock. Gentle heating evaporated the nitric acid up into the condensing apparatus. As the nitric acid vapor recondensed, it would collect in the nitric acid preserver. When all of the nitric acid had evaporated out of the RB flask, the heat would be turned off for a minute, and then the nitric acid would be allowed to slowly drain from the nitric acid preserver back into the RB flask. The entire process would then be repeated. After several hours, the solution became clear. At this point, the dissolution was complete. The solution was transferred to a volumetric flask and then diluted up to 50 mL. The steel sample was put into solution by placing it in 30 mL of nitric acid along with 2 mL of sulfuric acid. The solutions were diluted up to 250 mL. Further dilutions were carried out as needed to bring the concentrations to a workable level. Reagents. The stock solutions were prepared in accordance with the directions of Smith and Parsons (24). The standards were prepared from serial dilution of the stock solutions. Procedure. For the H2-Ar atmosphere, the gas flow rates were ca. 3 L/min of argon sheath gas, 2 L/min of argon flame diluant, and 1 L/min of hydrogen. In the Ar atmosphere, the argon flow rate was 5 L/min. For the LP atmosphere, the enclosure was pumped down to 25 torr. Solution samples of 5 pL were deposited with an Eppendorf micropipet. The analyses were carried out with a drying stage for 20 s at 100 "C. During the atomization stage, the temperature was set for the specific element up to 2700 O C . For the SRM samples, a charring step was included with temperatures of 500 "C for Mn and 600 "C for Cu. When necessary, neutral density filters were placed between the furnace and the monochromator to avoid saturating the detection system. The monochromator slit width was varied between 300 pm and 1500 pm. After optimization of the experimental setup and conditions, analytical calibration curves were obtained for each element. The limits of detection, LODs, were estimated as concentrations of analyte-producing signals that were 3 time the standard deviation of the blank.
RESULTS AND DISCUSSION Choice of Graphite Cuvette. From the preliminary study concerning graphite cuvettes, the plain graphite cut was found to give the best results. The graphite rod worked well with
Table I. Limits of Detection (pg) by Laser-Excited Atomic Fluorescence for Various Coatings of a Graphite Furnacen coating
A1
Mn
cu
pyrolytic Ta foil Ta carbide
5 x 102 3 X lo2 1 x 102
7 x 100 1 X lo2 1 x 10'
7 x 100 6 X 10' 5 x 10'
5-pL aliquots.
volatile elements, but with the less volatile elements, atomization was inefficient. Also, the graphite rod could only hold 2-pL samples and had a tendency to crack a t high temperatures. The slots on the slotted cup were cut in opposite sides of the cup. These slots allowed the fluorescence to be viewed through one slot while the opposite slot reduced the emission from the hot graphite which was directly viewed by the spectrometer. In this way, atoms were still in the hot environment of the cup when they were excited by the laser. Even with the opposite slit for reducing the emission from the luminous graphite, the measured emission still swamped fluorescence signals in the visible region. Extra slits placed between the atomizer and the monochromator were of little help. Another problem was the scatter caused by the laser hitting the bottom of the cup. If the excitation and fluorescence lines were close together, the background scatter was very large. Therefore, the plain graphite cup was used; the best results were obtained even though there was a chance for interference because of cooling of atoms as they exited the graphite cup. Comparison of Graphite Coatings. Table I lists the limits of detection, LODs, obtained for the different graphite coatings. From these results, the pyrolytic coating gave the best overall results, which agreed with electrothermal atomizer atomic absorption spectrometry (25). The foil liner caused the graphite cup to degrade much faster than the other coatings. The pyrolytically coated and carbide coated cups gave analytically useful results for about 70 atomizations. On the other hand, the foil-lined cups would only last about 50 atomizations. All three cups gave a reproducibility of about 10%. Comparison of Atmospheres. The H2-Ar, Ar, and L P atmospheres are compared in Table 11. In general, the LODs for the low-pressure atmosphere were worse than for the other two atmospheres. The original hope in carrying out this low-pressure work was that the quantum efficiency for the fluorescence would be drastically improved because there would be no interferents to quench the atoms. Unfortunately, any improvement in quantum efficiency that may have occurred was more than nullified by the increased diffusion rate of the atoms. Since the analyte atoms were in the path of the laser beam a shorter period of time and because of the relatively slow repetition rate of the laser, there was a decrease in signal. Expanding the size of the laser beam might have improved the signal because of the increased transit time of the atom (22). An anomalous results occurred for Sn, which gave no signal under low pressure except at 5000 ng,the highest amount used. This probably occurred due to the alteration of the mechanism of atomization; Rayson and Holcombe (26) suggested that oxygen attenuated the Sn signal by reacting with the Sn when measurements were at atmospheric pressure. However, at this time, there is not a good explanation for the loss in signal at low pressures. The slopes of the log-log calibration curves for In and Li were also not unity, Le., linear. The low slope (0.62) for Li a t L P was probably largely a result of poor background correction. Lithium was the only element for which resonance fluorescence was used. There was much more scatter for this
ANALYTICAL CHEMISTRY, VOL. 58, NO. 13, NOVEMBER 1986
2801
Table 11. Comparison of Hydrogen-Argon (€12-Ar),Argon (Ar), and Low-Pressure (LP) Atmospheresa
H2-Ar 7x 7x 6X 2x 5x 4x
cu Mn Pt Sn In Li
limits of detection,b pg Ar 2x 1x 1x 1x 3x 4x
100 100
10' lo-'
10-2 102
100
100 100 10' lo-' 102
LP
H2-Ar
2 x 102 7 x 100 2 x 10'
4.5 3 3 >6 6 3
7 x 10-1 4 x 103
AUR' Ar 5 4 3.5
LP
3 3 >5
5
6 3
H2-Ar 0.91 0.98 0.93 1.00
5 2.5
1.10 0.90
sloped Ar 1.05 0.98 1.05 0.95 0.91 0.92
LP 0.95 1.05 0.90 1.50 0.62
'5-fL aliquots. bLimit of detection is defined as 3u/m, where u is the standard deviation of the blank and m is the slope of calibration of curve. 'Analytically useful range (orders of magnitude). dSlope of log signal vs. log mass calibration curve. Table 111. SRM Standards (ppm, except where noted)
sample
NBS No.
wheat fluor spinach steel
1567 1570 364
Mn expt given
cu expt
given
2.0 i 0.3 7.3 8.5 f 0.5 2.5 12 f 2 128 165 6 17 0.23% 0.25% 0.29% 0.24%
*
reason, and potentially a much poorer background correction may have occurred. The low pressure apparently magnified this effect. The reason for the extraordinarily high slope for In (1.50) at LP is not understood. Sturgeon and Chakrabarti (27)have shown that the atomization time can change with pressure. However, at a particular pressure, the atomization time as well as the transport of the atoms into the laser beam should remain constant. There was no apparent reason why the signal should increase so disproportionately with analyte concentration. The LODs in the Hz-Ar atmosphere and the Ar atmosphere were similar. The detection limits for Cu, Mn, and Pt, were improved in the Ar atmosphere. Although the hydrogen flame provided a reducing atmosphere, the entire system was still open to the air. Interferents may still have gotten through the argon sheath and the hydrogen flame to react with the analyte atoms, causing losses. The enclosed Ar atmosphere was much easier to control and much more reproducible. Both Sn and In had improved detection limits in the Hz-Ar atmosphere; Sn has previously been shown (26) to give improved detection limits when hydrogen was added to the argon sheath gas for atomic absorption spectrometry. Indium probably had improved detection limits due to its high volatility. Certainly, the cool hydrogen flame was hot enough to atomize In; keeping the In atoms in this atmosphere should have minimized losses due to side reactions. SRM Results. The results from the SRM samples are shown in Table 111. The results were good considering the SRM samples were determined with aqueous standards. One of the benefits of atomic fluorescence over atomic absorption is that, in many cases, accurate results can be obtained without
having to make standards in a matched matrix. Of course, there is still the problem of atomization from the graphite surface; these problems are inherent in any furnace technique and have to be dealt with by using matrix modifiers or some other technique. Comparison with Previous Studies. In Table IV, LODs obtained in this work are compared with other literature values for the same techniques and also furnace atomic absorption. The LODs for Sn and Pt in the Hz-Ar and Ar atmospheres, respectively, are considerably improved over the previous results, while the In LODs are slightly improved. Cu and Mn have similar LODs with the electrothermal atomic absorption spectrometry but are about an order of magnitude worse than by previous fluorescence spectrometric studies. The poorer LODs here could be due to the close proximity of the excitation and fluorescence wavelengths. There was an increase in the magnitude of the scatter and there was also an increase in the variability of this scatter. The limiting noise here was a result of particles coming off of the graphite which flowed into the laser beam and scattered light into the monochromator. Although the pyrolytic coating improved detection limits and helped to protect the graphite surface, further improvements may have been found if the graphite had been pyrolytically coated by a commercial company. This would have ensured the reproducibility of the coating from cup to cup. Another possible reason for the better LODs of Bolshov (5) is that he used a Nd:YAG laser as the pump laser. This laser is capable of giving more than an order of magnitude increased energy per pulse in the frequency doubled range of the dye laser, when compared to the nitrogen laser as a pump source. The increased pulse energy should have given an accordingly large fluorescence signal until saturation was achieved (28). The poor detection limit for P b was due to a large amount of contamination, both from the graphite cuvette and from the distilled-deionized water and reagents used to make up the standards. The low slope of the log-log calibration curve was due to a poor background correction. Even with the contamination, the detection limit was an improvement over the detection limit for electrothermal atomic absorption spectrometry,
Table IV. Limits of Detection (pg) by Laser-Excited Atomic Fluorescence in a Graphtie Furnace"
element cu Mn Pt Sn In Li Pb A1
wavelength, nm exc fl 324.8 279.8 265.9 286.3 303.9 670.8 283.3 394.4
327.4 280.1 270.2 317.5 325.6 670.8 405.8 396.2
H2-Ar 7x 7x 6X 2x 5x 4x 2x 5x
100
Ar
2 x 100 7 x 100 2 x 10'
lit.' 2 x 10-1 2 x 10-1 1 x 102
GFAASd
2 x 100 4 x 10-1 10' 2 x 10' 10-1 5 x 102 2 x 10' 10-2 7 x 10-1 1 x lo-' NR' 102 3 x 101 4 x 103 NR' 10-1 2 x 10-3 5 x 100 102 NRe 1 x 100 5-pL aliquots. * Laser-excited atomic fluorescence with a graphite furnace. Data for In and Sn from ref 7; Pb, Mn, Cu, and Pt from ref 5. dData for (28). The Guide to Techniques and Application of Atomic Spectroscopy; Perkin-Elmer: Norwalk, CT, 1983. "R,no report. 100
2 x 100 1 x 100 1 x 100 1 x 101 3 x 10-1 4 x 102
LAFSb LP
2802
Anal. Chem. 1986, 58, 2602-2606
The aluminum LOD was 2 orders of magnitude worse than that for electrothermal atomic absorption spectrometry. Part of the reason was due to the closeness of the excitation and fluorescence wavelengths as previously discussed. Another problem with A1 was that it has a low volatility. The AA detection limits were carried out in a tube furnace, which provided much better atomization conditions, because of the semienclosed design. The fluorescence detection limits could have been improved if the atomization had been carried out in an enclosed furnace. In all of these cases, the detection limits may be improved by use of an enclosed cuvette. Dittrich and Stark (9) found an improvement in sensitivity of 1-3 orders of magnitude with a carbon tube rather than a carbon cup; this improvement was a result of the higher temperatures inside the carbon tube as well as to the confinement of the atomic vapor within the region of excitation. Also, more laser shots would occur while the atoms were present, resulting in a larger signal for a given amount of analyte. Registry No. Pb, 7439-92-1;Cu, 7440-50-8; Mn, 7439-96-5; Sn, 7440-31-5; Al, 7429-90-5; In, 7440-74-6; Li, 7439-93-2; Pt, 7440-06-4;Ta, 7440-25-7; tantalum carbide, 51680-51-4 graphite, 7782-42-5; steel, 12597-69-2.
LITERATURE CITED (1) Neumann, S.; Kriese, M. Spectrochlm. Acta, Part 8 1974, 298. 127. (2) Bolshov, M. A,; Zybin, 2. V.; Zybina, L. A,; Koloshnikov, V. G.Majorov, I.A. Spectrochlm. Acta, Part B 1978, 318, 493. (3) Hohimer, J. P.; Hargls, P. J., Jr. App/. Phys. Lett. 1977, 30, 344. (4) Bolshov, M. A.; Zybin, A. V.; Koloshnikov, V. G.;Vasnetsov, M. V. Specfrochim. Acta, Part 8 1981, 368,345.
(5) Boishov, M. A.; Zybin. A. V.; Smirenkina, I.I.Specfrochim. Acta, Part B 1981, 368, 1143. (6) Human, H. G. C.; Omenetto, N.; Cavalli. P.; Rossi, G. Spectrochim. Acta, Part8 t984, 398, 1345. (7) Tiich, J.; Falk, H.; Paetzoid, H. J.; Mon, P. G.;Schmidt, K. P. Colloquium Spectroscopium Internationale XXIV, FRG, Sept 15-21, 1985. (8) Arkhangeisky, 8. V.: Gonchakov. A. S.; Grazhuiene, S. S. Colloquium Spectroscoplum Internationale XXIV, FRG, Sept 15-21, 1985. (9) Dittrich, K.; Stark, H. J. Colloquium Spectroscopium Internationale XXIV, FRG, Sept 15-21, 1985. (IO) Massmann, H. Spectrochim. Acta, Part 8 1988, 238,215. (11) L'vov, B. V. Spectrochim. Acta, Part 8 1969, 248,53. (12) Manning, D. C.; Ediger, R. D. At. Absorpt. News/. 1976, 15, 421. (13) Sturgeon, R. E.; Chakrabarti, C. L. Anal. Chem. 1977, 4 9 , 90. (14) Zatka, V. J. Anal. Chem. 1978, 5 0 , 538. (15) Manning, D. C.; Slavin, W. Anal. Chem. 1978, 5 0 . 1234. (16) Renshaw, G. D. At. Absorpt. News/. 1973, 12, 158. (17) L'vov, 8. V.; Pelieva, L. A. Can. J. Spectrosc. 1978. 23, 1. (18) Gregoire, D. C.;Chakrabarti, C. L. Spectrochlm. Acta, Part 8 1982. 378, 11. (19) Bratzel, M. P.; Dagnali, R. M.; Winefordner, J. D. Anal. Chim. Acta 1969, 48, 197. (20) Cantle, J. E.; West, T. S. Talanta 1973, 20, 459. (21) Amos, M. D.; Bennet, P. A.; Brodie, K. G.;Lung, P. W, Y.; Matousek, J. P. Anal. Chem. 1971, 43, 211. (22) Pan, C. L.; Prodan, J. V.; Fairbank, W. M., Jr.; She, C. Y. Opt. Lett. 1680, 5 , 459. (23) Greenlees, G. W.; Clark, D. L.; Kaufman, S. L.; Lewis, D. A,; Tonn, J. F.; Eroadhurst, J. H. Opt. Commun. 1977, 23, 236. (24) Smith, 8. W.; Parsons, M. L. J . Chem. Educ. 1973, 50, 679. (25) L'vov, E. V. Atomic Absorption Spectrochemical Analysis; translated by Dixon, J. H.; Adam Hllger, Ltd.: London, 1970. (26) Rayson, G. D.; Holcombe. J. A. Anal. Chlm. Acta 1982, 136, 249. (27) Sturgeon, R. E.; Chakrabarti, C. L. Prog. Anal. At. Spectrosc. 1978, I , 11. (28) Wlnefordner, J. D. J . Chem. Educ. 1978, 55. 72.
RECEIVEDfor review November 18,1985. Resubmitted June 9,1986. Accepted June 26,1986. This research was supported by AFOSR-86-0015.
Determination of Chromium(I I I), Titanium, Vanadium, Iron(I I I), and Aluminum by Inductively Coupled Plasma Atomic Emission Spectrometry with an On-Line Preconcentrating Ion-Exchange Column Shizuko Hirata,* Yoshimi Umezaki, and Masahiko Ikeda' Department of Environmental Chemistry, Government Industrial Research Institute, Chugoku, 15000 Hiro-machi, Kure 737-01, Japan
A method utlilzlng a miniature lon-exchange column of Muromac A-1 (Muromachl Chemkals, Tokyo) has been developed to Increase the sensltlvfty for alumlnwn, chromium( I I I ) , Iron( I I I), tltanlum, and vanadium measurements by Inductively coupled plasma atomic emission spectrometry (ICPAES). A sample (pH 3.8) Is pumped through the column at 6.0 mL min-', mixing with the buffer sokrtkn, and sequentially eluted dkectty to the nebulizer of the ICP with 2 M nHrk acid at 3.0 mL mln-' by uslng a flow InJectlon analysis (FIA) system. This FIA-ICP method gave signal enhancements that were 34-113 times better than for a conventlonal continuously aspirated system for the metals studied here. A precision of the technique is better than 5 YOrelatlve standard deviatlon at the 10 gg L-' level for aqueous standards, and the sampling rate is 17 samples h-'.
Present address: Horiba Co., Ltd., 2 Miyanohigashi-cho,Kishoin, Minami-ku, Kyoto 601, Japan.
Inductively coupled plasma atomic emission spectrometry
(ICP-AES)is widely used for the determination of trace metals in environmental samples. Although it is very sensitive, it sometimes suffers from the lack of sensitivity for the determination of trace metals. Recently on-line ion-exchange preconcentrating methods for flame atomic absorption spectrometry (AAS) (1-6) or ICP-AES (7-10) have been reported, and they are capable of improving the detection limit by 1or 2 orders of magnitude, with the additional advantage of being rapid and separating the analyte from an interfering matrix. Olsen et al. (1)and Hartenstein et al. (7) report the reaction of heavy metals with Chelex-100 chelate resin in the pH 9-10 region. However, when a sample solution obtained by decomposition with strong acidsis adjusted to pH 9-10, chromium(III), titanium, iron(III), and aluminum form hydroxides and are precipitated, even if much buffer solution is added. Also, when a sample solution adjusted to pH 10 with ammonium hydroxide was aspirated continuously, the argon
0003-2700/86/0358-2602$01.50/00 1986 American Chemical Society
