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
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
35
Table I. Experimental Conditions for Determination of Some Alkaloids and Drugs by AAS and Visible Spectrophotometry
substance
PH
papaverine atropine sparteine avacan quinine pilocarpine amy1ocai ne procaine codeine
2.4 5.4 5.7 6.2 5.4 6.4 3.0 4.5 1.7
concn of CO(SCN)~?M
shaking time, min
linear range mg.rnL-l in organic phase" AAS visible
extraction no.
0-0.63 1.25-3.75 0-0.38 0-0.15 0.2-0.7 0.05-0.3 0-0.24 0.6-2.0 1.0-4.0
0.30 0.45 0.30 0.60 0.45 0.06 0.24 0.45 0.30
0.1-0.3b 0.05-0.3*
0.1-0.3'
"Phase ratio aqueous:organic = 5:l. Reference 22. CReference19. butylscopolammonium bromide and tannate in blood, urine, and feces (17), and hyoscine-N-butyl bromide in pharmaceutical preparations (18))have all been determined. What is surprising is that although C O ( S C N ) ~has ~ - been used for many years in the colorimetric determination of alkaloids (19-22) and more recently for their heterometric determination (23),it has not yet been used for their determination by AAS, since, as has already been mentioned, this last technique allows the sensitivity of the determination to be considerably increased. The purpose of this paper is to study the application of the reagent C O ( S C N ) ~for ~ - determining alkaloids and drugs by AAS, choosing a series of substances which have not, up t o now, been determined by this technique. Furthermore, since the conditions of ion pair formation and extraction of some of the products studied are different, the use of this reagent allows the analysis of some of these in the presence of others, which has not so far been mentioned in the literature. EXPERIMENTAL SECTION A Perkin-Elmer 370 A spectrophotometer (10 cm slit burner) with an air/acetylene (oxidizing-lean,blue) flame was used with the following settings: wavelength 241.0 nm; lamp current, 37.0 mA; burner length, 10 cm; burner height, 100 mm; slit, 0.2 nm; air/acetylene ratio, 3.75. A Crison 505 pH meter was used for pH measurements. Whatman P/S paper (12.5 cm) was used to separate the phases. Solutions. Standard tetrathiocyanate cobalt(I1) solution was prepared by dissolving 450 g of NHISCN and 218 g of C O ( N O ~ ) ~ in distilled water to give 800 mL of solution. Aqueous solutions of the following alkaloids and drugs were used: papaverine hydrochloride (Sigma), quinine hydrochloride (Sigma), sparteine sulfate (Sigma), pilocarpine hydrochloride (Sigma), atropine sulfate (Sigma), ephedrine hydrochloride (Sigma), amylocaine hydrochloride (Sigma), avacan (isoamyl N-(0-(diethylaminoethyl)-a-(aminophenylacetatedihydrochloride) (Sigma), procaine hydrochloride (Sigma), and codeine. All other reagents were of analytical reagent grade. Procedures. The optimal conditions for determination of each alkaloid (CO(SCN),~concentration,pH, shaking time, phase ratio, and number of extractions) have been established as follows: In separatory funnels of 100 mL volume with Teflon taps, a fixed amount of alkaloid (1 mg for sparteine, avacan, pilocarpine, amylocaine and papaverine; 2 mg for quinine; 10 mg for procaine; 25 mg for atropine and codeine), increasing amounts of stock solution of Co(SCN)?-, solution of 0.1 M HCl or NaOH up to a determined pH, and distilled water up to 50 mL are added. Ten milliliters of 1,2-dichloroethane is added, the mixture is shaken, and the organic extract is filtered through P/S paper. The atomic absorption of Co in the organic phase is measured. To determine the optimal pH, the same procedure was carried out but each time with different pH value in the range 1-8. pH values were measured with a pH meter. Working with the previously determined optimal pH and Co(SCN),2- concentration, and modifying the shaking time of the two phases, we determined the optimal time for the maximum extraction of the ion pair.
OCH, PA PAVERlNE
AJR OP'hE
9bI V I N E
8 P A R TEINE
A,YY.SCA/NE
PILOCARPINE
PRSCA I N E
3H
CGDEINE
Figure 1. Formulas of some alkaloids.
By maintaining the volume of the aqueous phase constant (50 mL) in the previously determined optimal conditions and modifying the volume of the organic phase, we established the optimal phase ratio. For all these already fixed parameters the number of extractions necessary to achieve maximum extraction has been established. Recommended Procedure: Transfer a sample solution of alkaloid or drug into a separatory funnel. Add cobalt tetrathiocyanate solution and adjust the pH by adding an appropriate amount of NaOH (0.1 M) or HCl(O.1 M). Then, make up to 50 mL with distilled water. Extract with 10 mL of dichloroethane, shake for 1 min and filter through Whatman P/S paper. Measure the absorbance at the analytical conditions above mentioned. R E S U L T S A N D DISCUSSION The results of the optimal conditions for the determination of each alkaloid and drug are shown in Table I. E x t r a c t i o n of the Ion Pair. The weak complex cobalt tetrathiocyanate ion is stabilized by the formation of an ion pair with organic bases as follows:
-
2(alkaloid"+) + ~ C O ( S C N ) , ~ - (alkn+)2(Co(SCN):-)n The stabilization is caused by precipitation and extraction of the formed compound. The formulas of the alkaloids and drugs are shown in Figure 1. The extraction of the cobaltothiocyanates of the alkaloids and drugs mentioned with different solvents was investigated.
36
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 ~~
Table 11. Recovery Efficiency of Some Alkaloids and Drugs N
amt added,
amt recovered,
mg
mg
N
Avacan 1
2 3 4 5 6 7 8 9 10 mean std dev re1 std dev
1.005 1.005 1.005 1.005 1.005 1.005 1.005 1.005 1.005 1.005
2 3 4 5 6 7 8 9 10 mean std dev re1 std dev
12.00 12.00 12.00 12.00 12.00 12.00 12.00 12.00 12.00 12.00
0.994 1.011 0.982 1.017 1.028 0.994 0.998 0.982 0.994 0.970 0.996 0.018 0.018
1
2 3 4 5 6 7 8 9 10 mean std dev re1 std dev
12.50 11.60 12.17 12.11 11.63 11.72 11.82 12.07 11.79 12.04 11.90 0.208 0.017
1 2
3 4 5 6 7 8 9 10 mean std dev re1 std dev
2 3 4 5 6 7 8 9 10 mean std dev re1 std dev
25.08 25.08 25.08 25.08 25.08 25.08 25.08 25.08 25.08 25.08
35.12 35.12 35.12 35.12 35.12 35.12 35.12 35.12 35.12 35.12
34.93 35.77 35.35 33.66 34.78 33.94 35.49 36.05 35.21 34.22 34.94 0.79 0.023
1
2 3 4 5 6 7 8 9 10
1 2 3
4 5
6 7 8 9 10 mean std dev re1 std dev
std dev re1 std dev
1.445 1.457 1.451 1.377 1.394 1.491 1.474 1.510 1.394 1.388 1.438 0.047 0.033
Sparteine
1.49 1.49 1.49 1.49 1.49 1.49 1.49 1.49 1.49 1.49
1.43 1.50 1.49 1.36 1.47 1.53 1.45 1.46 1.51 1.48 1.47 0.048 0.033
1
2 3 4 5 6 7 8 9 10 mean std dev re1 std dev
1.26 1.26 1.26 1.26 1.26 1.26 1.26 1.26 1.26 1.26
1.26 1.16 1.30 1.28 1.30 1.22 1.15 1.28 1.24 1.23
1.277 0.089 0.070 Quinine
2.530 2.530 2.530 2.530 2.530 2.530 2.530 2.530 2.530 2.530
Ketones and among them methyl isobutyl ketone extracted the salts but cobaltothiocyanic acid interfered by being extracted as well. On the other hand, slightly polar solvents such as CHC13or 1,2-dichloroethane extracted only the ion pair of alkaloids. Among the last solvents dichloroethane has given better results due to the fact that chloroform produces a higher amount of HC1 in the flame. Moreover, dichloroethane increased the flame temperature. For this reason the chosen solvent was dichloroethane. The ion pairs of the alkaloids studied are slightly soluble in water and freely soluble in dichloroethane, except those of sparteine and avacan, whose solubility is low. Because of this, the distribution constants of these last two in the dichloroethane/water system at the optimal pH of extraction have been determined. A value of D = 127 was found for the ion pair sparteine-Co(SCN)42- and a value of D = 181 was found for the system avacan-Co(SCN)42-. Consequently, the yields of a single extraction in optimal conditions with an 0rganic:aqueous phase ratio of 1:5 are 96.29% and 97.32% for sparteine and avacan, respectively. The yield in the extraction of the other ion pair studied is practically 100%. Influence of pH. Figure 2 shows the results obtained with eight alkaloids and the avacan studied.
1.449 1.449 1.449 1.449 1.449 1.449 1.449 1.449 1.449 1.449
mean
Pilocarpina 24.93 24.56 24.69 24.69 24.93 25.00 25.12 25.25 25.00 24.81 24.90 0.21 0.008
amt added, amt recovered, mg mg Amylocaine
Papaverine
Atropine 1
N
Codeine
Procaina 1
amt added, amt recovered, mg mg
2.539 2.478 2.608 2.513 2.417 2.583 2.504 2.669 2.417 2.469 2.519 0.082 0.032
1
2 3 4
5 6
7 8 9 10 mean std dev re1 std dev
3.99 3.99 3.99 3.99 3.99 3.99 3.99 3.99 3.99 3.99
3.97 4.13 4.10
4.02 3.95 3.92 4.10 4.04 4.00 4.05 4.03 0.069 0.017
0.3 0.2
0.1
Figure 2. Influence of pH in the determination of alkaloids and drugs by AAS: (1) sparteine; (2) pilocarpine: (3) papaverine; (4) avacan; (5) amylocaine; (6) quinine; (7) atropine; (8) codeine; (9) procaine.
It can be seen that the extraction of cobaltothiocyanates of sparteine, pilocarpine, quinine, and amylocaine are strongly
ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985
37
Table 111. Analysis of Binary Mixtures of Alkaloids and Drugs by AAS substance
amt added, mg
procaine
amylocaine
1.93 1.93 1.93 1.93 1.93 1.93 2.01 2.01 2.01 2.01 2.01 2.01 2.01 2.02 2.02 2.02 2.02 2.02 5.04
2.00 3.00 4.00
avacan
papaverine
substance added, mg pilocarpine quinine
1.00 2.00 1.00
2.00 3.00 4.00 2.03 3.04 4.04 10.15 1.99 2.98 4.00 10.00
1.00 5.05 7.5 10.1
5.04
5.04 5.04 5.04 5.04 5.04 5.04
pilocarpine sparteine
5.07 5.07 5.05
sparteine
5.07 10.14 20.28 2.50 2.50 2.50 2.5 2.5
5.05
2.50
amt found
f
std dev"
1.92 f 0.042 1.93 f 0.038 1.94 f 0.051 2.026 1.60b 2.21b 2.02 f 0.020 2.05 f 0.023 2.05 f 0.016 2.04 f 0.025 2.010 f 0.029 2.03 f 0.019 2.05 f 0.021 2.03 f 0.018 2.02 f 0.022 2.04 f 0.017 2.06 f 0.029 2.62' 5.05 f 0.038 5.02 f 0.045 5.00 f 0.039 5.02 f 0.042 5.00 f 0.033 5.07 f 0.048 5.38b 5.1Ib 5.46' 5.376 7.46' 9.04b
The uncertainties represent the standard deviation of the mean for ten replicate samples. Average of three replicate samples. affected by the pH of the solution. This influence is negligible in the extraction of the other ion pairs. The ion pair of ephedrine is not extracted in the complete range of pH. All the compounds studied are not ionized above pH 7.0. Hence, the formation of the ion pair is not possible. Optimum Concentration of Cobalt Thiocyanate. The effect of the concentration of C O ( S C N ) ) ~on ~ -the formation of ion pairs is shown in Figure 3. I t can be observed that a great excess of the reagent is not favorable to the extraction of the ion pairs, with exception of amylocaine. For instance, pilocarpine is not extracted when the concentration of Co(SCN)42-is over 0.60 M.
Effect of the Shaking Time, Stability, Number of Extractions,and Calibration Graph. The study of shaking time for the extraction of the ion pair showed that the shaking time was not very important. Only 1 min was necessary to reach the maximum extraction, with the exception of papaverine. The ion pair of papaverine needed 5 min of shaking to obtain a stable value. This can be attributed to the high solubility of papaverine in dichloroethane. The papaverine, in equilibrium with its ion pair precipitated in the aqueous phase, will cross into the organic phase and a more prolonged contact between the two phases will result in the total formation of the ion pair in the organic phase. All the ion pairs studied are stable for a t least 8 h. The stability of each of the ion pairs was determined using a stock solution obtained extracting a known amount of alkaloid in optimal conditions and comparing its atomic absorption, a t controlled intervals, with the other solutions of ion pair obtained in the same way but measuring immediately after extraction. Only one extraction was necessary for the quantitative extraction of the complex salts of papaverine, atropine, pilocarpine, procaine, codeine, quinine, and amylocaine, while
0
Flgure 3. Influence of concentration of Co(SCN)?-: (1) papaverine: (2) sparteine; (3) avacan; (4) amylocalne; (5)procaine; (6) quinine; (7) atropine; (8) pilocarpine; (9) codeine.
two extractions were necessary for the extraction of sparteine and avacan. Table I shows the results obtained in the calibration graph by atomic absorption spectroscopy compared with those obtained with visible spectrophotometry (19-22). The accuracy of the method has been established for each substance by taking three samples of different concentration and carrying out 10 determinations of each. The standard deviation in each case and for each alkaloid was very similar. In Table I1 the raw data of one sample are shown. Interferences. Foreign substances such as sulfamides, B group vitamins, antibiotics, sugar, excipients, and guayacol glyceryl ether which normally accompany one or other of the alkaloids studied in pharmaceutical preparations, such as
38
Anal. Chem. 1985, 57,38-41
cough syrups, sedatives, etc., do not cause interference. This is due to the fact that no ion pair is formed with C O ( S C N ) ~ ~ or extracted with 1,2-dichloroethane. Analysis of Mixtures. due to the different experimental conditions in the formation and extraction of some of the ion pairs studied, it is possible to determine one single alkaloid or drug in the presence of others. Because of this, a study was carried out of the mutual interference produced by substances which in principle and according to the previous graphs are distinguishable. In Table I11 results are shown which confirm the previous predictions. Thus it is possible to determine amylocaine in the presence of procaine, avacan in the presence of pilocarpine, procaine, or quinine, and papaverine in the presence of pilocarpine or sparteine even in proportions as low as 15. Registry No. Amylocaine, 644-26-8; papaverine, 58-74-2; sparteine, 90-39-1; procaine, 59-46-1; quinine, 130-95-0;codeine, 76-57-3; atropine, 51-55-8; pilocarpine, 92-13-7; avacan, 54-30-8; ephedrine, 299-42-3; tetrathiocyanate cobalt, 18904-81-9.
LITERATURE CITED (1) Kidani, Y. Bunseki Kagaku 1981, 3 0 , 59. (2) m r d a Vargas, M.; Milla, M. and PBrez-Bustamante, J. Ana/ysf (London) 1983, 108, 1417. (3) Minami, Y.; Mitsui, T.; Fujimura, I. Bunseki Kagaku 1983, 3 2 , 206.
(4) AI-Zamil, I. 2.; Aziz-Abraham. A. M.; AI-Haijaji, M. A. J . Chem. SOC. f a k . 1982, 4 , 249. Minami, Y.; Mitsui, T.; Fujimura, I. Bunsekl Kagaku 1982, 3 7 , 334. Minami, Y.; Mitsui, T.; Fujimura, I . Bunsekl Kagaku W82, 3 1 , 604. Tan, B.; Melius, P. Anal. f r o c . (London) 1981, 78,364. Le Bihan, A.; Courtot-Coupez, J. Bull. SOC.Chim. Fr. 1970, 406. Le Bihan, A,; Courtot-Coupez, J. Analusls 1974, 2 , 695. Alary, J.; Rochat, J.; Villet, A.; Coeur, A. Ann. fharm. Fr. 1976, 3 4 , 345. (11) Yamamoto, Y.: Kumamaru. T.: Havashi. Y. Talanfa 1967. 74. 611. (12) Yamamoto, Y.; Kumamaru, T.; Hayashi, Y.; Otsuchi, M. BunsekiKagaku 198% 78,354. (13) Yamamoto, Y.; Kumamuru, T.; Hayashi, Y.; Otani, Y. Bull. Chem. SOC.Jpn. 1989, 4 2 , 1714. (14) Kidani, Y.; Nakamura, K.; Inagaki, K.; Koike. H. Bunseki Kagaku 1975, 2 4 , 742. (15) Minami, Y.; Mitsui, T.; Fujimura, I. BunsekiKagaku 1981, 30(12), 611. (16) Mlnami, Y.; Mitsui, T.; Fujimura, I. Bunseki Kagaku 1981, 30, 475. (17) Minamikawa, T.; Matsumura, K.; Kamei, A,; Yamamoto, M. Bunsekl Kagaku 1971, 20, 1011. (16) Park, M. K.; Shon, Ch. Y.; Shin, M. H. Soul Taekakkyo Yakhak Nomunjip 1979, 4 , 140. (19) Deltombe, J.; Leboutte, G. J . fharm. Bels. 1958, 348 (73), 38. (20) Mairovici, C.; Cojocaru, 2 . Rev. Chlm. 1980, 7, 411. (21) Propovici, V.; Schweiger, A.; Spitzer, A. Farmacla 1965, 6, 353. (22) Deltombe, J.; Leboutte, G.; Roster, N. J . fharm. Be@. 1962, 347(73), 236. (23) Shanine, S.;Khamis, S.Mlcrochem. J . 1983, 2 8 , 26, (5) (6) (7) (8) (9) (10)
RECEIVED for review October 17, 1983. Resubmitted July 26, 1984. Accepted August 22, 1984.
Evaluation of Multiwavelength First- and Second-Derivative Spectra for the Quantitation of Mixtures of Polynuclear Aromatic Hydrocarbons Yahya R. Tahboub and Harry L. Pardue*
Department of Chemistry, Purdue Uniuersity, West Lafayette, Indiana 47907
Thls paper descrlbes the use of multiwavelength flrst- and second-derlvatlve spectra wlth matrlx least-squares data processing to resolve mlxtures of components wlth overlapplng absorptlon spectra. Procedures are evaluated for polynuclear aromatlc hydrocarbons In the presence of a llghtscatterlng component. Results show that the multlwavelength flrst- and second-derlvatlve spectra offer hlgher degrees of selectivlty than absorptlon spectra, permit the resolution of mixtures of components wlth overlapping spectra, and yleld an Improved slgnal-to-nolse ratlo relatlve to singlewavelength derivative data. Polynuclear aromatlc hydrocarbons In twoand three-component mlxtures are quantlfled In the range from 0.2 to 2.0 pg/mL wlth uncertalntles In the range of 0.005 pg/mL.
There are many situations in which first- or second-derivative spectra can offer significant improvements in selectivity relative to absorption (zeroth-derivative) spectra ( 1 , 2 ) . To date, however, virtually all applications of derivative spectroscopy have involved single-wavelegnth measurements. Multiwavelength applications of derivative spectra should offer significant advantages including improved signal-to-noise ratios via signal averaging and the ability to resolve mixtures with overlapping spectra. The recent advent of spectrophotometers based on the use of imaging detectors makes it relatively easy to obtain multiwavelength derivative spectra. Also, the matrix least-squares
data-processing methods developed for absorption data (3, 4) and incorporated as part of the software of the HewlettParkard line of diode-array-based spectrophotometers should be directly applicable to multiwavelength first- and secondderivative spectra. This paper presents results of a study designed to evaluate and compare relative merits of multiwavelength data-processing methods applied to zeroth-, first-, and second-derivative spectra for mixtures. Components chosen for study were polynuclear aromatic hydrocarbons (PAH’s). Mixtures of the P A H s were prepared in the presence of a suspension of finely divided porcelain slip (clay) to provide a controllable lightscattering background signal. Results are reported for twoand three-component mixtures of PAH’s with the lightscattering component.
EXPERIMENTAL SECTION Instrumentation. All experiments were performed with an HP 8450A UV/vis spectrophotometer (Hewlett-PackardCo., Palo Alto, CA). This is a general-purpose,microprocessor-controlled, parallel-detectoninstrument that scans wavelengths from 200 to 800 nm at a rate of one scan per second. The optical system uses a pair of photodiode arrays for simultaneous detection at all wavelengths. The system includes a fast 16-bit microprocessor with a 28K operating system and 32K random access memory for data storage. The operating system includes programs to compute first and second derivatives of absorbance with respect to wavelength and to fit spectral data for mixtures of up to 12 components to a model based on the spectrum for each of the components. This latter program computes the concentration of each component that, when combined with concentrations of
0003-2700/85/0357-0038$01.50/00 1984 American Chemical Society
