Radiotracer study of the preatomization behavior of lead in the

30

Anal. Chem. 1985, 57,30-34

Radiotracer Study of the Preatomization Behavior of Lead in the Graphite Furnace Walther Schmid and Viliam Krivan*

Sektion Analytik und Hochstreinigung, Universitat Ulm, Oberer Eselsberg N-26, D 7900 UlmJDonau, Federal Republic of Germany

The behavior of lead In the graphite furnace during the Individual steps of the temperature program was Investigated by means of 203Pbas a radiotracer. The matrlx systems Included HCI and HNO, solutions, 1% NaCl soiutlon, blood, serum, urlne, and the solid samples bovlne h e r and orchard leaves. Without matrlx rnodlfication, a pyrolysls temperature of up to 500 OC can be used for all samples. Signlflcant stablllzatlon effects can be achleved In the preatomlzation steps by uslng NH,H,PO, and ammonla as matrlx modlflers and by addltion of hydrogen to the flowing argon gas. I t was found that the use of a L'vov platform also helps to ellmlnate the matrlx effects. WRh respect to lead stablllzatlon and eilmlnation of matrlx effects, optlmum experlmental condltlons are achleved by slrnultaneous use of a L'vov platform and NH4H,P0, as the matrix modlfler.

Table I. Operating Parameters Used in the Radiotracer Experiments

investigated step drying charring

heating to atomization

conditions temp, 120 "C ramp time, 20 s hold time, 40 s drying as above temp, 500-1000 "C ramp time, 20 s hold time, 40 s drying as above charring temp, 500 "C," 700 " C b temp, 2100 "C ramp time, 1-4 s hold time, 0 s int. flow, 0 mL/min

Without addition of NH4H2P04.

With addition of

NH4H2P04.

Flameless atomic absorption spectrometry belongs to the most important methods for the determination of lead in biological and environmental materials (1-4). However, frequently problems arise in connection with the matrix-dependent behavior of lead in the graphite tube during the execution of the temperature program. The literature data published on this topic are rather inconsistent. Strong interferences were observed with matrices containing chloride ions but these effects varied in extent. Fernandez and Manning (5) found that sodium chloride a t a concentration of 100 mg/L depressed the lead signal by 70%. A similar effect was observed for MgClz and HC1 (6-8). Barnard and Fishman obtained a signal depression of 70% with only 10 mg of NaCl per liter (9). Signal depressions at this level were reported by other authors for CaC12,NaC1, MgClZ,and HC1 solutions (10-15). For the determination of lead, maximum ashing temperatures between 700 and 860 "C have been used (16-19). Frech and Cedergren did not observe any losses from a 0.2% NaCl solution up to 650 "C (20). On the other hand, Czobik and Matousek reported losses of lead in NaCl solutions a t temperatures higher than 470 "C (21,22). In the present work, we investigated the losses of lead during the individual steps of the temperature program in the graphite furnace by the radiotracer technique using z03Pb. This technique allows the direct detection of losses and is therefore superior to the absorption measurements in which the information on the preatomization behavior is obtained from absorption signals in the atomization step. The radiotracer technique has already been successfully used for the investigation of copper (23),mercury (24),and chromium (25) in the graphite tube. EXPERIMENTAL SECTION Chemicals and Apparatus. Reagents used in the tracer experiments were of "pro analysi" grade and were obtained from Merck, Darmstadt, F.R.G. The acids were additionally purified by subboiling distillation. Thallium of a 99.9999% purity grade, supplied by Fluka Chemicals, Neu-Ulm, F.R.G., was used for the

preparation of '03Pb. In the tracer experiments, a well-type NaI (Tl) detector (2 X 2 in.) coupled to a single-channel analyzer was used for counting the 279-keV y ray of 203Pb. A Perkin-Elmer HGA 500 graphite tube furnace with power supply and programmer without optical system was used for the radiotracer studies. In order to prevent contamination of the environment, the graphite tube furnace was placed in a closed chamber made of Plexiglas which was connected to an exhaust hood of the radiochemical laboratory. The behavior of lead was investigated in the following systems: 0.01 M "0,; 0.01 M, 0.1 M, 1 M HC1; 1% NaCl solution (chloride content: 0.172 mol/L), two untreated urine samples of healthy persons; one urine standard obtained from the Behring Institute, Marburg, F.R.G.; human serum; human whole blood; blood standard supplied by Behring Institute, Marburg, F.R.G.; bovine liver NBS SRM 1577;and orchard leaves NBS SRM 1571. Preparation of 203Pb.203Pbwas produced via the reactions 203Tl(p,n)and 20ql(p,3n)by irradiation of natural thallium with 22-MeV protons at 0.3-pA current for 30 min in the isochroneous cyclotron of the Nuclear Research Center, Karlsrhe, F.R.G. The irradiated thallium targets were dissolved in 1 mL of 7 M "OB. The solution was neutralized with a 2 M NaOH solution and filled up with water to 5 mL. From this solution, the lead traces, containing 203Pb,were separated from the thallium matrix by extraction with 0.05 M dithizone in CC14 followed by reextraction with 1 mL of 1 M "OB. The carrier concentration of the lead tracer was about 0.1 pg/mL. One day after the irradiation, the specific activity was about 4 X lo7 Bq/pg. the radioactive purity was checked by a high resolution y-ray spectrometer. After a cooling time of 24 h, only the y rays of '03Pb were found. Performance of the Tracer Experiments. Liquid samples were labeled by adding 10-100 pL of the tracer solution to 1mL of the sample solution. A 10 p L portion of the labeled samples was injected into the graphite tube and counted with the well-type detector being in the horizontal position. The tube was fitted into the furnace, and the program selected was started. After the execution of the given temperature program (Table I), the activity of the tube was measured again. In the case of solid samples (orchard leaves and bovine liver), 100 mg of sample was added to 10 mL of the 1 : l O diluted tracer solution, the mixture was left to stand for 24 h at room temperature, and it was then filtered and dried under an IR lamp. The samples were then

0003-2700/85/0357-0030$01.50/00 1984 American Chemical Society

ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985

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I

I

PRETREATMENT TEMPERATURE (OC) Flgure 1. Losses of lead from the graphite tube vs. pretreatment temperature for various matrices: (a-c) graphite tube; (&e) L'vov platform: (f) graphite tube, solid samples; (g,h) graphlte tube and 1% hydrogen in the flowing gas; (I) graphite tube and addition of ammonia.

brought into the graphite tube using the Perkin-Elmer powder sampler. Each measurement was executed at least 4 times with different graphite tubes.

t

RESULTS AND DISCUSSION During the drying step up to 120 "C, no detectable loss was measured in any of the sample systems. The results for the thermal pretreatment in uncoated graphite tubes are illustrated in Figure la-c. As can be seen, a pretreatment up to 550 OC can be applied to all investigated matrices. The maximum possible pretreatment temperature@,however, depend on the matrix. As expected, they are lower for media containing higher chloride concentration, i.e., for HCl and NaCl solutions and urine, than for blood standard and "03 solutions. This is attributed to the high volatility of lead chlo:ide, the formation of which is promoted by increasing chloride concentrations. In Figure 2, the pretreatment temperatures at which lead losses of 1 5 % occur are plotted vs. the chloride content of the different samples. It is evident that, almost independent of the kind of sample, the chloride concentrations decisively affect the behavior of lead in the graphite tube. Varying chloride concentrations in the same matrix can strongly alter the retention of lead in the graphite tube as can be seen in the case of different urine samples. The use of the L'vov platform was recommended by several authors to reduce signal depressions and to increase the sensitivity (26-29). As can be seen in Figure ld,e, the L'vov platform does not lead to higher loss-free pretreatment temperatures. With some matrices even the opposite effect occurs, This is apparently caused by different surface properties of the normal graphite tube and the platform which is made of massive pyrolytic graphite having less porosity. Often higher pretreatment temperatures are required for the elimination of the matrix constituents. For the stabilization of lead at higher temperatures, matrix modifiers have been used of which NH4H2PO4is of greatest importance (2,

4

E w

""Or

u

m c

I

I

001

I

I

+

01 1 CHLORIDE C O N T E N T ( m o l / L )

Flgure 2. Effect of the chloride content on the pretreatment temperature at which lead losses of 5% occur: (0)HCI, 0.01 M; (0)blood St; serum; (0) urine St; (A) HCI, 0.1 M;(A)urine 2; (e)NaCI, 1%; (Pa) HCI, 1 M.

(m)

28). We examined the effect of the addition of 0.2 mg of

NH4HzP04to a 20-bL sample solution. Figure 3a,b shows the percentage retention of lead in uncoated graphite tubes when NHIHZPO, was added. Up to 800 OC, no losses were detected with any of the examined matrices. When NH4HzP04is used as the matrix modifier, the loss-free pretreatment temperature can be increased by 200-400 O C but the relative differences between the individual matrices remain unchanged. Further improvement can be achieved by the use of NH4HZPO4in combination with the L'vov platform (Figure 3c,d), but also in this case the influence of the matrix on the behavior of lead cannot be elimin4ted completely. Volatilization of the lead during the heating phase from the pretreatment temperature to the atomization temperature can

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ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 W

m

3

I-

W

m 0 N

700

900

1100

1300

700

900 p R E T RE AT ME N T

1100

T E M PE R AT U R E

Figure 3. DeDendence of the losses of lead on the Dretreatment temDerature for various matrices to which NH,H,PO, tube; (c,d) L'iov platform.

also give rise to systematic errors, if peak height measurements are applied. i n addition, during the heating phase, lead can evaporate in molecular form as lead chloride and thus lower the amount detected in the absorption measurement. The volatilization of lead (in atomic as well as in molecular form) was measured at different heating rates by using the program listed in Table I. After the end of the ramp time, the temperature program was stopped and then the activity remaining in the graphite tube was measured. The results are illustrated in Figure 4a-f as the dependence of the z03Pb activity remaining in the graphite tube on the ramp time. With uncoated graphite tubes, a considerable part of the lead can be evaporated already at a ramp time of 1s: 52% with a 1% NaCl solution, 22% with urine 1,30% with urine 2,47% with urine St, 10% with 0.1 M HC1,30% with 1 M HCI, and no losses with 0.01 M "OB. The volatility of lead depends also in this stage on the chloride content of the matrix. Only the results for hydrochloric acid deviate from the general trend. The volatilization of HC1 already during the drying step is a possible explanation for this behavior. The measurements were carried out only at gas stop, but preliminary experiments showed that the extent of the losses does not significantly depend on the gas flow. It is likely that, in several instances, these losses have been the reason for signal depressions described in the literature (5, 6). Slavin and Manning reported that the signal depression caused by NaCl can be decreased or even completely avoided by using the L'vov platform. Therefore, we carried out tracer measurements under the same conditions with the L'vov platform. The results obtained are in good accordance with the finding of Slavin and Manning: the losses at 1-s ramp time are below 5%; only in 1 M HC1 are they about 10%. These results confirm the theory of the delayed heating using the platform. A similar improvement of the thermal stability of lead as in the pretreatment step can be achieved for the heating by addition of NH4H,P04 (Figure 4g,h). No losses of lead could be detected with any of the matrices at a 1-s ramp time. However, under other conditions, the curves differ considerably. For example, 52% of the lead i s lost at a ramp

1300

was added:

(OC)

(a,b)graphite

time of 2 s from 1 % NaCl solution but only 2% if 1 M HCl solution is processed. Again, this can be explained by the removal of HCl during the drying step. When the combination of NH4H2P04and the L'vov platform are used, the matrix effect can be largely eliminated as the results in Figure 5a,b show. Even at a ramp time of 4 s, the losses are below 5% for all matrices excluding blood, where they are 12%. The direct analysis of solid samples (24,30-32) is sometimes a meaningful approach for the solution of analytical problems, especially since the background compensation by Zeeman atomic absorption spectrometry is possible. For this reason, analogous radiotracer experiments were carried out with two NBS standard reference materials, Le., orchard leaves (NBS SRM 1571) and bovine liver (NBS SRM 1577) as solid samples. From the results obtained (Figures If and 49, it can be seen that lead behaves rather differently in the two solid samples. From bovine liver, lead losses begin to occur a t temperatures higher than 800 "C while from orchard leaves they do already at 600 "C. The differences between these two materials are significant in the heating stage, too. With bovine liver, no losses occurred at 1-9 ramp time, whereas 10% loss was measured in processing orchard leaves, and the different behavior remains also for longer ramp times. Although both are biological materials, a look a t their composition allows an explanation to be found for the different thermal behavior of lead. While bovine liver contains only 0.35 pg/g P b but 10 500 pg/g phosphorus, orchard leaves contain 46 pg/g P b and 1980 pg/g phosphorus. The chloride content in bovine liver and orchard leaves was 2680 and 720 p g / g , respectively. One can assume that the reason for higher lead stability in processing bovine liver is the much higher P / P b ratio in this material as compared with orchard leaves. The large excess of phosphate can probably lead to a similar stabilization effect as the addition of the matrix modifier NH4H2P04. Further radiotracer investigation included the influence of ammonium ions on the behavior of lead. For this purpose, 10 pL of 25% ammonia solution was added to the injected labeled sample solution in the graphite tube. From the com-

ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 B l o o d st Blood A Serum

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I

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4 (S)

Flgure 4. Influence of the ramp time in the heating step from charring to atomization temperature for various matices: (a-d) graphite tube; (e,f) L'vov platform; (g,h) graphite tube and addition of NH4H,P04; (i) graphite tube, solid samples; (k,l) graphite tube and 1% hydrogen added to the flowing gas; (m) graphite tube and addition of ammonia.

w

W

4I-

a m H N O 3 0.01M @HCi 1M rNaCl 1%

Z

w V

a

w n "

1

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@ U r i n e st

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Figure 5. Influence of the ramp time in the heating step from charring to atomization temperature for various matrices by using the L'vov platform as the matrix modifier. and NH,H,PO,

parison of the results with and without ammonia in Figures l i and 4m, the stabilization effect of ammonia is evident. It is obviously based on the formation of volatile NH4C1leading to the removal of chloride. However, the stabilization of lead with NH4HzP04is more effective. On the basis of thermodynamic calculations, Frech suggested the addition of hydrogen to the flowing gas (34). Therefore, we finally examined the losses during pretreatment

and heating using a mixture of 1% Hzand 99% argon as the flowing gas. Figures 1 g,h and 4 k,l show the results for 1% NaCl solution and urine. In both cases, the stability of lead in the graphite tube was improved by the addition of hydrogen. The high losses occurring at a 1-s ramp time (-50%) can be completely avoided by addition of hydrogen to the flowing gas, but, at higher ramp times, the stabilizing effect of hydrogen is not sufficient for quantitive retention.

34

Anal. Chem. 1905, 5 7 , 34-38

ACKNOWLEDGMENT We thank Kernforschungszentrum Karlsruhe GmbH., Karsruhe F.R.G., for providing irradiation facilities. Registry No. Lead, 7439-92-1. LITERATURE C I T E D Berman, E. “Toxlc Metals and Their Anaiysls”; Heyden Sons Ltd.: London, 1970. Welz, 8 . “Atom-Absorption-Spektrometrie”; Verlag Chemie: Welnheim, 3 Auflage, 1983. Tech. Rep. Ser.-I.A.E.A. 1979, No. 197, 141-165. Delves, H. T. Prog. Anal. A t . Spectrosc. 1981, 4 , 1-48. Fernandez, F. J.; Manning, D. C. A t . Absorpt. Newsl. 1971, 70, 65-72. Hodges, D. J. Analyst (London) 1977, 702, 66-69. Cruz, R. B.; Loon, J. C. van Anal. Chlm. Acta 1974, 72, 231-243. Ottaway, J. M. Proc. Anal. Div. Chem. SOC. 1976, 13, 165-191. Barnard, W. M.; Fishman, M. J. A t . Absorpt. New/. 1973, 72, 118-1 24. Hagemann, L. R.; Nichols, J. A,; Viswanadhan, P.; Woodrlff, R. Anal. Chem. 1979, 57, 1406-1412. Krasowskl, J. A.; Copeland, C. R. Anal. Chem. 1970, 57,1843-1849. Julshann, K. A t . Absorpt. Newsl. 1977, 76, 149-153. Rattonetti, A. Anal. Chem. 1974, 46, 739-742. Regan, J. G. T.; Warren, J. Analyst (London) 1978, 103, 447-451. Campbell, W. C., Ottaway, J . M. Talanta 1974, 27,837-844. Sturgeon, R. E.; Chakrabarti, C. L.; Langford, C. H. Anal. Chem. 1972, 4 8 , 1792-1807.

(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34)

Pinta, M.; Riandley, C. Analusls 1975, 3 , 86-93. L’vov, B. V. Spectrochlm. Acta Part6 1978, 338, 153-193. Slavin, W.; Manning, D. C. Anal. Chem. 1979, 57,261-265. Frech, W.; Cedergren, A . Anal. Chim. Acta 1977, 88, 57-67. Czobik, E. J.; Matousek, J. P. Anal. Chem. 1978, 5 0 , 2-10. Matousek, J. P. Prof. Anal. A t . Soectrosc. 1981. 3. 247-310. Lendero, L.; Krlvan, V. Anal. C h e h . 1982, 5 4 , 579-581. Krivan, V.; Lang, M. Fresenius’ 2.Anal. Chem. 1982, 372, 324-330. Veilion, C.; Guthrie, B. E.; Wolfe, W. R. Anal. Chem. 1980, 52, 457-459. L’vov, B. V. Spectrochim. Acta 1961, 77, 761-770. Siavin, W.; Mannlng, D. C. Spectrochim. Acta Part 6 1980, 358, 701-714. Slavin, W.; Manning, D. C. Anal. Chem. 1979, 57,261-265. Fernandez, F. J.; Beaty, M. M.; Barnett, W. V. A t . Spectrosc 1981, 2 , 16-21. Marks, J. Y.; Welder, G. G.; Speiimann, R . J. J . Appl. Spectrosc. 1977, 37, 9-11. Backman, S.; Karlssson, W. R . Analyst (London) 1979, 704, 10 17-1 029. Grobenszki, 2.; Lehmann, R.; Welz, B. A t . Spectrosc. Appl. 1981, Study No. 667. Gladney, E. S. Anal. Chlm. Acta 1980, 778, 385-396. Frech, W.; Cedergren, A. Anal. Chim. Acta 1978, 82, 93-102.

RECEIVED for review July 18,1984. This project was financially supported by Bundesministerium fur Forschung und Technologie, Bonn.

Indirect Determination of Alkaloids and Drugs by Atomic Absorption Spectrometry Cristina Nerin* a n d A g u s t h Garnica

Departamento de Qulmica, Escuela TGcnica Superior de Ingenieros Industriales, Uniuersidad de Zaragoza, Zaragoza, Spain

Juan Cacho Departamento de Quimica A n a l h a , Facultad de Ciencias, Uniuersidad de Zaragoza, Zaragoza, Spain

A new procedure for determination of alkaloids and other pharmaceutical drugs is described. The method consists of extracting an ion pair between the organic base and the inorganic complex Co(SCN)t- and measurlng Co In the organic phase by AAS at 241.0 nm. The optimal experimental conditions pH, concentration of Co( SCN)t-, shaking time, phase ratio, number of extractions, and the lineal range of calibration are studied in the determination of amyiocaine, papaverine, sparteine, procaine, quinine, codeine, atropine, pilocarpine, and avacan. The organic phase used is 1,2=dichioroethane. The standard devlatlon of the method varies between IO-’ and IO-*, depending on the substance analyzed. The new method allows the deiermlnation of amyiocaine in the presence of procaine, avacan in the presence of pilocarpine, procaine, or quinine, and papaverine in the presepce of pliocarplne or sparteine. The interference of foreign substances which accompany these drugs In pharmaceutlcai preparations Is studled, and the method is pppiied to thelr quantitative determination In medlclnes.

The use of ion pair formation in analytical chemistry is being more and more extended mostly due to the fact that it permits the combination of different analytical techniques, such as extraction and spectrophotometry. The result is an

increase in the sensitivity and selectivity of the determinations. Most of the papers in the literature on the subject of determination of organic products by ion pair formation use the technique of molecular spectrophotometry and the ion pair is formed using acidic or basic dyestuffs. Analysis by ion pair formation using charged metal complexes, which has been known for many years, has not been developed as much since these ion pairs have less color than the former and thus the sensitivity of the determination by molecular spectrophotometry is also lower. However, the use of metal complexes allows the indirect determination of organic products by atomic absorption spectrophotometry; the increase in sensitivity which is achieved by this technique has aroused the interest of researchers, as is shown by the number of papers which have appeared in recent years (1-7). In this way, complexes of 1,lO-phenantroline have been used with copper to determine quaternary ammonium and anionic surfactants (8-10), with iron for the determination of pentachlorophenol and salicylic acid (11, 12), with nickel for the determination of 2hydroxynaphthoic acid in water (13), and with cadmium for the determination of penicillin (14). The complexes formed between thiocyanate and various cations are also the bases of indirect determination of organic products by AAS, the most notable being those with Cr (15) and Co. Thus, using C O ( S C N ) ~ ~aliphatic -, amines (16),N -

O 1984 American Chemical Society 0003-2700/85/0357-0034$01.50/0