Anal. Chem. 1994,66, 4074-4078
Fullerenes as Sorbent Materials for Metal Preconcentration Mercedes Gallego, Yaneira Petlt de Peiia,t and Miguel Valc6rcel' Department of Analytical Chemistry, Faculty of Sciences, University of Chrdoba, E- 14004 Chrdoba, Spain
The analytical potential of Cm fullerenes as sorbent materials for preconcentration of metal traces by formation of neutral chelates was studied for the first time in this work. The model system used for this purpose was the determination of lead tracesin waters by using ammonium pyrrolidinedithiocarbamate as ligand. The chelate is formed in a continuous-flow system, sorbed on a Cm fullerene minicolumn, and subsequently eluted for transfer to an atomic absorption spectrometer. Two other simultaneous batchesof experimentswere performed in parallel by using c 1 8 bonded silica and activated carbon as sorbents in order to study the features of the new sorbent material and its advantages. The primary assets of Cm fullerenesin this respect are a high sensitivity arising from efficient adsorptionand also high selectivity derived from the special features of this new type of sorbent material. Fullerenes, which are solids consisting of carbon atoms clustering in spherical structures, are bringing about a strong impact on materials science and technology. This is a result not only of their significance to basic science but also of their exciting prospective applications. Fullerenes represent the third allotropic form of pure carbon (theother two arediamond and graphite). The archetypal fullerene, named C60, is the roundest possible molecule. The history of fullerenes started in 1985,when Kroto et ale1produced carbon aggregates by laser-induced vaporization of graphite; the product analyses revealed the presence of abundant species containing 60carbon atoms. The original parent molecule was named buckminsterfullerene. Reports on this type of compound have steadily grown in number after the results of Kratschmer et a1.*were published in 1990. Two reviews on the discovery, stability, structure, chemical properties, reactions, and astrophysical implications of c 6 0 have been published so far.3*4Reactivity studies showed the molecule to possess inertness that was consistent with a closed structure and the absence of dangling bonds5 Unique physical and chemical properties of c 6 0 and its cousins have fueled speculation of many possible uses, but no products have yet materialized. Foreseeable applications include the development of optical devices based on fullerene photoconductivity or photovoltaic properties; chemical sensors; gas separation devices;diamonds; batteries; catalysts; hydrogen storage media; polymers (additives or new polymer blocks); medical uses; etc.6 In developing applications, one should
'
Permanent address: Department of Chemistry, University of Los Andes, Mtrida, Venezuela. (1) Kroto, H. W.; Heath, J. R.; OBrien, S.C.; Curl, R. F.; Smalley, R. E. Nature (London) 1985, 318, 162-163. (2) KrBtschmer, W.; Lamb, L. D.; Fostiropulos, K.; Huffman, D. R. Nature (London) 1990, 347, 354-358. (3) Kroto, H. W.; Allaf, A. W.; Balm, S. P. Chem. Rev. 1991, 91, 1213-1235. (4) Special Issue on Buckminsterfullerenes. Acc. Chem. Res. 1992, 25(3). ( 5 ) Weiss, F. D.; Elkind, J. L.; OBrien, S. C.: Curl, R. F.; Smalley, R. E. J . Am. Chem. SOC.1988, 110, 44644465.
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bear in mind that fullerene molecules are thoroughly oxidized by oxygen at moderately low temperatures; in fact, the temperature for the air-burning peak for fullerene (744 K) is similar to that of activated carbon on account of its large surface area.7 Future developments in fullerene chemistry and its applications rely partly on two achievements, namely, (1) optimization of the synthesis variables and (2) development of analytical methods for obtaining individual pure fullerenesS8 Synthetic selectivity is of paramount significance owing to the large variety of stable, closed polyhedral structures possible from C20 to C240.~Thermal analysis is a convenient means of characterizing the soot used as the cluster source, the extracted mixture of fullerenes, and the individual cluster itself.1° These complex mixtures are usually subjeted to highperformance liquid chromatography (HPLC) in order to purify useful amounts of the different clusters. Thus, HPLC with polystyrene gel' and monomeric and polymeric octadecyl~ilica-bonded,~~ y-cyclodextrin-bonded,I4and tetraphenylporphyrin-silica gel" stationary phases have been employed for selective separation of fullerenes (especially c 6 0 and C70). The low solubility of c 6 0 and C ~ fullerenes O in the organic solvents commonly used as eluents (e.g., hexane, n-pentane, dichloromethane, and acetonitrile) makes preparative fullerene separations troublesome. Fullerenes, especially C60, have so far been investigated mainly by physicists and physical chemists and, to a lesser extent, organic chemists. No analytical chemical work on fullerenes other than their characterization has so far been carried out. Two geometric features of these compounds, viz. their high molecular surface area and volume, make them potentially useful for interacting with solvents and solutes. This paper is the first to discuss the analytical potential of c 6 0 fullerenes for preconcentration of metal traces; their large surface areas make them potentially useful sorbents. Because the material should be handled with great caution as very little is known about it and fullerenes might be carcinogenic,3 the experiments were carried out in a continuous-flow manifold. For this purpose, c 6 0 fullerene-packed minicolumns were inserted into a flow manifold in such a way that the (6) Cuesta, A. I.; Marthez-Alonso, A.; Tasc6n. J. M. D. In Carbon '92; Deutsche Keramische Gesellschaft: Essen, 1992; p 804. (7) Baum, R. M. Chem. Eng. News 1993, Nov 22, 8-18. (8) Diack, M.; Hettich, R. L.; Compton, R. N.; Guiochon, G . Anal. Chem. 1992, 64, 2143-2148. (9) Klein, D. J.; Seitz, W. A.; Schmalz, T. G . Nature 1986, 323, 703-707. (10) Gallagher, P. K.; Zhong, Z.; Twu, J.; Kane, S.M. Trends Anal. Chem. 1991, 10, 279-282. (1 1) Giigel, A.; Miillen, K. J. Chromatogr. 1993, 628, 23-27. (12) Gtigel, A.; Miillen, K. Chromarographia 1993, 37, 387-391. (13) Jinno, K.; Uemura, T.; Ohta, H.; Nagashima, H.; Itoh, K. Anal. Chem. 1993, 65, 2650-2654. (14) Cabrera, K.; Wieland, G.; Schaefer, M. J . Chromarogr. 1993,644,396-399. (15) Kibbey, C. E.; Savina, M. R.: Parseghian, B. K.; Francis, A. H.; Meyerhoff, M. E. Anal. Chem. 1993, 65, 3717-3719.
0003-2700/94/0366-4074$04.50/0
@ 1994 American Chemlcal Society
sorbent was kept in a closed, air-tight system, out of contact with the operator, and was consumed in very small amounts, which is especially advantageous, owing to its high cost.
EXPERIMENTAL SECTION Equipment and Columns. A Perkin-Elmer 380 atomic absorption spectrometer equipped with a bead impact system in the burner chamber and a hollow cathode lead lamp was used. The wavelength and lamp current used were 283.3 nm and 10 mA, respectively; deuterium arc background correction was employed throughout. The acetylene flow rate was 2.0 L/min, and an air flow rate of 21.5 L/min was employed to obtain a clean blue flame. The spectrometer output was connected to a Radiometer REC-80 Servograph recorder. The flow manifold comprised a Gilson-Minipuls 2 peristaltic pump furnished with poly(viny1 chloride) tubes, two Rheodyne 5041 injection valves, and laboratory-made adsorption minicolumns packed with C60 fullerene, silica RP-Clg, or activated carbon. The minicolumns were made from poly(tetrafluoroethy1ene) capillaries and sealed on both ends with small cotton wool beds to prevent materials losses. The minicolumns were initially flushed with 0.1 N nitric acid; subsequent useof methyl isobutyl ketone as eluent in each operational cycle was sufficient to make the minicolumns ready for reuse. Reagents. A 1000 mg/L lead solution was prepared by dissolving 1.598g of lead nitrate (Merck, Darmstad, Germany) in 1% (v/v) nitric acid and diluting to 1 L with 1% (v/v) nitric acid. A 0.1% (w/v) ammonium pyrrolidinedithiocarbamate (APDC) (Aldrich, Barcelona, Spain) aqueous solution was also made that remained stable for at least three days. Methyl isobutyl ketone was of analytical reagent grade or better and obtained from Romil Chemicals (Loughborough, England). A commercially available sample O f C60 (gold grade, >99.4%) was obtained from Hoechst (Frankfurt, Germany). Darco 20-40 activated carbon and polygosyl-bonded silica reversedphase sorbent with octadecyl functional groups (RP-Clg), 60100 pm, were purchased from Aldrich and Millipore (Madrid, Spain), respectively. Solubility of Cm. The solubility of c 6 0 in 47 solvents was recently determined.16 c 6 0 is essentially insoluble in polar and H-bonding solventssuch as acetone, acetonitrile, methanol, and ethanol, sparingly soluble in alkanes, and quite soluble in aromatic solvents such as benzene, toluene, and naphthalene. However, the solubility of c 6 0 in methyl isobutyl ketone and aqueous solutions had not yet been determined; therefore, in undertaking this work, we first evaluated the solubility of c 6 0 in methyl isobutyl ketone and 0.1 N nitric acid. For this purpose, an amount of ca. 10 mg of C6o was placed in a 1-dram glass bottle (2.5 cm long X 1 cm diameter), and 1 mL of solvent was added; a 5 mm X 2 mm Teflon-coated stirring bar was used toagitate thesolution in thedarkat rmm temperature for 24 h to ensure equilibration. The resulting saturated solution was filtered through a 0.5 pm poly(tetrafluor0ethylene) (PTFE) filter (FHLP, Millipore) or paper (Watman no. 1) filter for organic and aqueous solutions, respectively. The solid residue was dried with N2 and then weighed. The solubility was established by difference between the weights of the glass bottle with and without solid residue. (16) Ruoff, R. S.;Tse, D. S.;Malhotra, R.; Lorents, D. C. J. Phys. Chem. 1993, 97,3379-3383.
300cm
,COLUMN
I W
Flgure 1. F I system for the preconcentration determination of lead. IV, injection valve: W, waste: APDC, ammonium pyrroliiinedkhiocarbamate; MIBK, methyl isobutyl ketone; AAS, atomic absorption spectrometer.
Procedure. The continuous-flow manifold used is depicted in Figure 1. In the preconcentration step, 6-1 5 mL of sample containing 5-250 ng/mL of Pb(1I) in 0.1 N nitric acid was continuously pumped into the system and mixed thoroughly with a 0.1% APDC solution. The chelate was adsorbed on the c 6 0 minicolumn, placed in the loop of the injection valve (IV1). During this step, a water carrier was pumped to the instrument and the loop of the injection valve (IV2) filled with eluent (methyl isobutyl ketone, MIBK). In the elution step, both injection valves were switched simultaneously in such a way that 200 p L of MIBK solvent was injected into the water carrier, which was then passed through the minicolumn in order to elute the adsorbed chelate and sweep the Pb to the detector. A blankof 200 p L of MIBKinjected prior to sample preconcentration was used ( A = 0.020 unit). Peak height was used as the analytical measurement. RESULTS AND DISCUSSION The separation and preconcentration of trace elements using ion-exchange columns17 or sorbent e x t r a c t i ~ n l ~has - ~ ~effectively opened up new prospects for on-line procedures. Microcolumns packed with bonded silica containing octadecyl functional groups (c18),18-20amberlite XAD-4,21activated alumina,22 and activated carbon (AC)23,24as sorbents are typically used for this purpose. Recently, our team developed an on-line sorbent extraction system for preconcentration and determination of lead by flame atomic absorption spectrome t r ~ By . ~ using ~ ammonium pyrrolidinedithiocarbamate or dithizone as the complexing agent, AC sorbent as the column material, and methyl isobutyl ketone as the eluent, an enrichment factor of 50 relative to direct sample introduction was obtained with 2 min of preconcentration and 6.0 mL of sample. In order to investigate the potential of C60 as a sorbent material, the above-mentioned system24 was selected on account of its simplicity; also, APDC was chosen as the chelating reagent in preference over dithizone because it is more selective toward lead. Optimization of the Working Conditions. First, the solubility of C60 fullerene in 0.1 N HN03 and MIBK was ValcArcel, M.; Luque de Castro, M. D. Non-Chromatographic Continuous Separation Techniques; The Royal Society of Chemistry: Cambridge, 1991. Fang, Z.; Sperling, M.; Welz, B. J. Anal. At. Specfrom. 1990, 5, 639-646. Welz, B.; Sperling, M.; Sun, X. Fresenius' J. Anal. Chem. 1993, 346, 550CCL
JJJ.
(20) Ma, R.; Van Mol, W.; Adams, F. Anal. Chim. Acta 1994, 285, 33-43. (21) Lee,M. L.;TBlg, G.Anal. Chim. Acta 1993, 272, 193-203. (22) Sperling, M.; Xu, S.; Welz, B. A n d . Chem. 1992, 64, 3101-3108. (23) Santelli, R.; Gallego, M.; ValcArcel, M. Talanta 1994, 41, 817-823. (24) Petit de Peiia, Y.; Gallego, M.; ValcBrcel, M. Talanra, in press.
Analytical Chemistry, Vol. 66, No. 22, November 15, 1994
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Table 1. Influence of the Amount of Sorbont and Mlnkolumn Dimendons
sorbent amount type (mg) C60
CIS AC C60
CIS 1
3
5
PH
Flgure 2. Effect of pH on the absorbance of lead retained on Cso fullerene, RP-Cle, and activated carbon (AC) minicolumns.
AC C60
CIS AC
C60
studied since both solvents were to be passed through the (&-packed minicolumn during the preconcentration and elution steps. Fullerene proved to be essentially insoluble in both solvents; the solubility was lower than 0.1 mg/mL, so a minicolumn packed with c 6 0 should be reusable in continuous systems. The effect of chemical variables was studied by continuously introducing a standard solution of 100 ng/mL lead into the flow system for 2 min (sample flow rate, 3 mL/ min; sample volume, 6.0 mL) and merging it with a 0.1% APDC stream. The minicolumn (0.7 cm length X 3 mm i.d.) was constructed from PTFE capillary and packed with 50 mg of c60. For comparison, similar minicolumns packed with 50 mg of AC (1 .O cm X 3 mm i.d.) or RP-Clg (1.8 cm X 3 mm i.d.) were also tested. Thus, the three types of minicolumn were loaded with the same amount of material, which, however, occupied a different volume in each. The eluent used was MIBK, which had previously proved to be the most suitable for elution of the lead-APDC chelate.24 The effect of pH on the chelate adsorption was studied over the range 0-8 by adjusting the lead sample with dilute nitric acid or ammonia as required. The results obtained for the three minicolumns are shown in Figure 2. As can be seen, the optimum range was different in each case. Thus, the maximum chelate adsorption was achieved at pH 0-5, 0.81.1, and 0.5-3.0for C60, AC, and RP-Clg, respectively. The optimum pH range was wider for c 6 0 fullerene because its adsorption constant is greater than those of the other two sorbents; thus, the chelate formation prevailed over the ligand protonation and the Pb(I1) hydrolysis below pH 0.5 and above pH 3, respectively. Theother two systems exhibited narrower pH ranges as the likely result of a smaller adsorption constant. Basically, the sorption of metal chelates is effected by the complexing ligands. This was confirmed by experiments in which the chelating reagent stream (pH 7) was replaced with a water stream (pH 7); in all cases, the sample was also circulated. The metal ion was not sorbed in the absence of chelate over the pH range 0-10; the absorbance obtained in the elution step was similar to that provided by injecting 200 pL of MIBK solvent (blank) and using the three minicolumns (c60, Clg, and AC). To simplify the operating procedure, 0.1 N nitric acid was selected for sample preparation with the three minicolumns. The effect of the APDC concentration was studied over the range 0.00 1-0.5%; the three minicolumns provided similar results, the absorbance remaining constant above a 0.05% concentration. A 0.1% APDC concentration in water was thus chosen for subsequent experiments. No blank was required, and the sample absorbance was obtained 4076
Analytical Chemistry, Vol. 66, No. 22, November 15, 1994
CIS AC
CSO Cl8
AC CSO CIS AC
50 50 50 80 80 80 110 110 110 50 30 20 110 70 40 180 120 70
dimensions length (cm) X i.d. (mm)
0.7 X 1.0x 1.8 x 1.1 x 1.6 X 3.0 X 1.5 X 2.3 X 4.0X 2.5 X 2.5 X 2.5 X 1.5 X 1.5 X 1.5 X 2.5 X 2.5 X 2.5 X
3 3 3 3 3 3 3 3 3 1.5 1.5 1.5 3 3 3 3 3 3
sensitivitya (mL/ng) 1.0 x 5.0 X 4.5 x 1.1 x 7.0X 3.0 X 1.3 x 1.0 x 1.3 X 1.0x 2.6 X 2.0 x 1.3 x 6.5X 4.0X 1.5 x 1.0 x
1v3
10-4 10-4 lo-’ l(r
10-4 10-3 1
~
3
10-4 10-3
10-4 l ( r 1v3
10-4 10-4 10-3 1v3
8.0 X IO-‘
“ n = 7.
by subtracting the eluent signal recorded before the sample was preconcentrated. The influence of the sample flow rate on the signal was examined over the range 0.6-4.0mL/min by using an overall samplevolumeof 6.0 mL. The signal increased with increasing flow rate up to 1.5 mL/min, above which it remained constant up to 3 mL/min for c 6 0 , CIS,and AC. Above this last value, the signal decreased as a result of the contact time between the chelate and sorbent decreasing with increasing flow rates. A sample flow rate of 3.0 mL/min was thus chosen. A reagent flow rate of 0.3 mL/min was selected because higher flow rates resulted in increased sample dilution and hence decreased atomic signals. The optimal length of the preconcentration coil ranged between 300 and 350 cm for all the minicolumns tested. The effect of the eluent (MIBK) volume was studied between 50 and 300 pL. The desorption process was found not to depend on the type of column used: elution was instantaneous and complete for volumes above 200 CLL.Some carryover was observed below such a volume; above this volume, the signal decreased through dispersion of the analyte in the organic solvent. Based on the above results, the same flow system could be used with all the minicolumns packed with each sorbent material. Effect of the Amount of Material and the Minicolumn Dimensions. Different minicolumns packed with the solid sorbents tested were prepared in order to study the effect of the amount of C6o fullerene, RP-Cls, or AC used and the minicolumn dimensions on the adsorption of the lead-APDC chelate. For this purpose, a calibration graph was run for each minicolumn by using at least seven standards of lead containing between 10 and 1000 ng/mL and a sampling time of 2 min (sample volume, 6 mL); the slopes of the calibration graphs (sensitivity) for each minicolumn are listed in Table 1. These experiments allowed the following conclusions to be drawn. (1) The amount of C ~ fullerene O used as sorbent was scarcely influential; in fact, the sensitivity increased by only 50% from 50 (sensitivity, 1.0 X mL/ng) to 180 mg (sensitivity, 1.5 X mL/ng). The minicolumn dimensions
had a similar effect, wherein the sensitivity increased with increasing column length and inner diameter. (2) The sensitivity achieved by using 110 mg of as sorbent was twice that obtained with 50 mg; below 50 mg of C1g, the chelate was inadequately adsorbed and the sensitivity decreased as a result (sensitivity for 30 mg, 2.6 X lo4 mL/ng). The effect of the minicolumn dimensions was more marked on c18 than it was on c60. (3) The sorbent particle size was a critical variable as it determined the column compactness. Of the three sorbents assayed, activated carbon featured the smallest particle size and C60 fullerene the largest. AC columns were tightly packed, so interstitial spaces were minimal; as a result, the flow rate of the emerging eluate was lower than that of the incoming stream for columns longer than 2.5 cm. The effect was occasionally (viz. for 3 cm X 3 mm and 4 cm X 3 mm columns) strong enough to stop the solution flow altogether and disengage the system connections, thereby resulting in increased dispersion of eluted analytes and decreased sensitivity to lead. The optimal dimensions for the AC minicolumn were 2.5 cm X 3 mm i.d., which held 70 mg of sorbent. Adsorption Isotherms. The recently determined adsorptivity of N2 and 0 2 molecules on fullerene is consistent with the presence of micropores in c 6 0 crystals.2s C60 molecules have a diameter of 1 nm, and their crystals have large intermolecular interstices (exactly four octahedral interstices per unit cell); accordingly, c 6 0 crystals must possess a high physical adsorption capacity enhanced by the overlapped surface field. Polany's theory interprets the nonspecific adsorption isotherms for organic vapors on AC but is inapplicable to monomolecular and polymolecular adsorption on uneven sorbents and liquid phases. The Polany equation for monomolecular adsorption of a liquid phase onto a solid phase can be simplified to q = KC", where K and n are constants. This empirical equation for the adsorption isotherm is known as the Freundlich equation.26 In order to calculate the two constants in the equation ( K and n), a plot of log q against log C (log q = log K n log C) is constructed, q being the amount adsorbed (ng/g) and C that remaining in solution (ng/mL). By using the flow system depicted in Figure 1, under the above-described optimal conditions, different lead solutions between 5 and 220 ng/mL were continuously introduced into the system at a flow rate of 3 mL/min for 5 min. This sampling time was selected in order to determine the adsorption capacity of the different sorbents at low lead concentrations. The three types of minicolumn were packed with the same amount of material, viz. 80 mg O f C60 (1.1 cm X 3 mm i.d.), Clg (1.6 cm X3mmi.d.),andAC(3.0cmX 3mmi.d.), butwereobviously of different dimensions. A volume of 200 pL of MIBK was used as extractant in all instances. A calibration graph was constructed by using standards of 1-1 0 pg/mL lead in MIBK containing 0.1% APDC in order to determine the amount adsorbed. These organic solutions were injected (200 pL) into a water carrier via the second injection valve shown in Figure 1. For each lead concentration in the solution (5-220
+
(25) Kaneko, K.; Ishii, C . ;Arai, T.; Suematsu, H. J . Pfiys. Cfiem.1993,97,67646766. (26) Guerasimov, Y. A.; Dreving, V.; Eriomin, E.; Kiseliov, A,; Lebedev, V.; Panchenkov, G . ;Shliguin, A. Curso de Qulmica Fisica, 2nded.; Mir: Moscow, 1977.
I
10
15
20 Log C ( n g l m L )
Figure 3. Freundilch adsorption isotherms obtained by using Coo fullerene, RP-C18, and activated carbon minicolumns. C,amount of solute in the sample; q, amount of adsorbed solute.
ng/mL), the adsorbed lead was determined by using the above calibration graph for the three minicolumns; full desorption was assumed for the three sorbents assayed. Figure 3 shows the Freundlich adsorption isotherms obtained. The higher isotherm was obtained for C60 fullerene, which thus possessed the highest adsorption capacity. On the other hand, the adsorption isotherm for the activated carbon minicolumn showed lead to be inefficiently adsorbed below a concentration of 20 ng/mL. RP-Clg silica behaved more like c 6 0 than AC. The n value is usually less than 1; a value greater than 227is typical of hindered adsorption. The nvalues obtained for the three sorbent materials were 0.925 ( C ~ O ) , 1.05 (Clg), and 1.08 (AC), so all proved to be appropriate sorbents. Also, the adsorption capacity increased with increasing K (because flow systems reach no chemical equilibrium, this constant is an apparent or conditional constant). The apparent K values obtained for the three materials were 575.4 (C60), 155.0 (Clg), and 77.6 ng/g (AC). The large K value obtained with C60 suggests that van der Waals interactions between the ligand (chelate) and fullerene are stronger than those in the other two sorbents. Therefore, of the three sorbent materials tested, c 6 0 fullerene is the most effective for the preconcentration of trace lead thanks to its higher adsorption capacity. Features of the Method. Several calibration graphs were run by using the three types of minicolumns packed with c 6 0 (1.1 cmX3mmi.d.),Clg(1.6cmX3mmi.d.),andAC(2.5 cm X 3 mm i.d.); all contained 80 mg of sorbent except AC, which was loaded with 70 mg because, as noted earlier, large amounts gave rise to worse results. Aqueous standard solutions were processed along the preconcentration flow system depicted in Figure 1. The linear range for lead was 10-250, 20-400, and 15-365 ng/mL for c60, Clg, and AC, respectively, equivalent to a sample volume of 6.0 mL at a sampling time of 2 min. The detection limit was calculated as three times thestandarddeviationofthe peakabsorbancefor 15 injections of 200 pL of MIBK (blank), and turned out to be 5, 15, and 10 ng/mL for c60, CIS,and AC, respectively. The precision (as relative standard deviation) was checked on 11 standard solutions containing 50 or 100 ng/mL lead for c 6 0 or cl8 and AC, respectively; the precision was similar in all instances, namely 2.1,2.7, and 2.3% for c60, C18, and AC, respectively. (27) Okutani, T.; Tsuruta, Y.;Sakuragawa, A. Anal. Cfiem. 1993,65,1271-1276.
Analytical Chemistty, Vol. 66, No. 22, November 15, 1994
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Table 2. Tolerated Forrlgn Catlon/Lead Concentration Ratlo In the Detrrmlnatlon of 50 (C, Mlnlcolumn) or 100 ng/mL (Cq8and AC Mlnlcolumns) tolerated ratio
AC
CIS
1000
500
400
Zn2+
1000 500 400
300 200
Sn2+ cu2+ Hg2+ Bi3+ Fe3+ Ni2+ Cd2+
400 300 200 200 20-1000' 20- 1 oow 2-24
foreign cation
AI'+ Mn2+ co2+
C60
100
200 200 40 4 3 40 4 1
50 100 100 20 1 1 20
2
