Anal. Chem. 1988, 60, 596-600
596
contain components with boiling points outside of the distillate temperature ranges. This possibility is consistent with the observed presence of butane (bp -0.5 “C) in cut A as seen by the resonances at 13.95 and 25.61 ppm. (2) The distillates may contain heteroatom compounds. In the spin-echo spectra, even signals that are separated by less than the solvent shift may be assigned if their phases are different as shown in Figure 1B: The signals at 20.75 and 20.88 ppm can be unambiguously assigned to the C5 methylene carbon in 3-methylhexme and the methyl carbon in methylcyclopentane, respectively. Also the signals at 35.19 and 35.22 can be assigned to the C1 methine carbon and the Cz/C5 methylene carbons in methylcyclopentane, respectively. These two signals are observed as one broadened peak in the corresponding one-pulse spectrum. The peak heights of, e.g. methylcyclohexane (MCHX), 2methylhexane (PMHX), 3-methylhexane (BMHX), 2-methylpentane (2MP), and 3-methypentane (3MP) reflect correctly the structures of these molecules. Also the intensity ratios between 2- and 3-methylhexane (BMHX, 3MHX) going from distillate B to distillate C are as would be expected from the boiling points of the two compounds (90and 92 “C, respectively). The aromatic signals from toluene (MB) are reduced in intensity relative to its methyl signal because the evolution time, T,is matched to fit the aliphatic 13C-lH coupling constants. With Tag>> T2* the factor (1- exp(-Taq/T2*))T2*in eq 3 suggests a more than 10-fold gain in peak height for ultrahighresolution 13C NMR (T2* 0.1 Hz) as compared to routine methods (T2*2 1.0 Hz). With still higher magnetic fields, more complex mixtures and smaller volumes can be analyzed, and 13C NMR may thus provide an alternative to chromatographic methods in oil analysis. However, it seems necessary to develop and optimize methods where a combination of multiplicity, intensity, and chemical shift data are used in the spectral assignment. Further work on the use of multivariate techniques in the analysis of 13C NMR spectra of hydrocarbon mixtures is in progress. Registry No. 1, 111-65-9;2, 75-83-2; 3, 79-29-8; 4, 107-83-5;
-
5 , 591-76-4;6,565-75-3; 7, 540-84-1;8, 108-87-2;9,496-11-7; 10,
119-64-2;11,2189-60-8;12,90-12-0;BU, 106-97-8;ZMBU, 78-78-4; P, 109-66-0;CP, 287-92-3; 3MP, 96-14-0; H, 110-54-3;MCP, 9637-7; 22MP, 590-35-2;CHX, 110-82-7;24MP, 108-08-7;B, 71-43-2; 3MHX, 589-34-4; HP, 142-82-5; ECP, 1640-89-7; 25DMHX, 592-13-2; MB, 108-88-3; 24MHX, 589-43-5; 234MP, 565-75-3; 23DMBU. 79-29-8.
LITERATURE CITED (1) Allerhand. A.; Maple, R. M. Anal. Chem. 1987, 59, 441A-452A. (2) Laude, D. A.; Wilklns. C. L. Anal. Chem. 1986, 58, 2820-2824. (3) Gillet, S.; Deipuech, J.J.; Valentin, P.; Escaiier, J.-C. Anal. Chem. 1980, 52, 813-817. (4) Thiault, B.; Mersseman, M. Org. Magn. Reson. 1976, 8 , 28-33. (5) Levy, G. C.; Cargioli, J. D. J. Magn. Reson. 1973, 10, 231-234. (6) Siibernagei, B. G.; Gebhard, L. A.; Dyrkacz, G. R. Magnetic Resonance /Introduction Advanced Topics and Applications to Fossil En ergy; Petrakis, L., Fraissard, J. P., Eds.; Reidel: Dordrecht/Bostonl Lancaster, 1984; pp 645-653. (7) Schlick, S.; Kevan, L. Magnetic Resonance /Introduction, Advanced Toplcs and Applications to Fossll Energy; Petrakis, L., Fraissard, J. P., Eds.; Reidel: Dordrecht/Boston/Lancaster. 1984; pp 655-665. (8) Duber, S.; Wieckowskl, A. B. Magnetic Resonance /Introduction, Advanced Topics and Applications to Fossil Energy; Petrakis, L., Fraissard, J. P., Eds.; Reidei: Dordrecht/Boston/Lancaster, 1984; pp 667-676. (9) Hajec, M.; Sklenar, V.; Sebor, I.; W e b e r , 0. Anal. Chem. 1978, 50, 773-775. (IO) Mareci, T. H.; Scott, K. N. Anal. Chem. 1977, 4 9 , 2130-2136. (11) Forsyth, D. A.; Hedlger, M.; Mahmoud, S. S.; Glessen, 8. C. Anal. Chem. 1982, 5 4 , 1896-1898. (12) FormBEek, V. Thesis, Wurzburg, 1979. (13) Kvalheim, 0. M.; Aksnes, D. W.; Brekke, T.; Eide, M. 0.; Sietten, E.; Telnaes. N. Anal. Chem. 1965, 57, 2858-2864. (14) Rummens, F. H. A. NMR-Basic Prlnclples and Progress; Diehl, P., Fluck, E., Kosfeldt, R., Eds.; Springer-Verhg: Berlin, 1975; Vol. 10. (15) Ernst, R. R.; Morgan, R. E. Mol. Phys. 1973. 26, 49-74. (16) Bremser, W.; Ernst, L.; Frank, 6.; Gerhards, R.; Hardt, A. Carbon-13 Spectral Data ; Verlag-Chemie: Weinheim, 1981.
-
RECEIVED for review February 24,1987. Accepted November 7, 1987.
Selective Enhancement of Room Temperature Phosphorescence Using Cyclodextrin-Treated Cellulose Substrate Ala M. Alak and Tuan Vo-Dinh* Advanced Monitoring Development Group, Health and Safety Research Division, Oak Ridge National Laboratory, Oak Ridge, Tennesse 37831 -6101
The selectlve enhancement of room temperature phosphorescence (RTP) for a wlde varlety of pdynuclear aromatlc hydrocarbons (PNAs) on cyclodextrin-treated Her paper was Investigated. Fllter paper substrates were treated by simple lmpregnatlon In saturated solutions of CY-, 0-, and ycyclodextrins. The resllfts show dmerenl RTP enhancement factors for PNAs depending on the type of cyclodextrln (CD) used and on the binding strength between the cyclodextrln and the PNAs. A two-component mlxture of PNAs was analyzed by RTP and synchronous RTP In order to Illustrate the Improved sensltivlty of the RTP technique and demonstrate the “selectlve enhancement by cyclodextrln” (SECD) methodology In multicomponent analysls.
Room temperature phosphorescence (RTP) is attracting considerable interest among analytical spectroscopists as a
practical and efficient tool for trace analysis of organic compounds (1-12). This technique is based on measuring the phosphorescence emitted, a t room temperature, by organic compounds adsorbed on solid substrates or stabilized in special liquid media. Filter paper is one of the most commonly used solid substrates for inducing RTP. Other solid substrates such as silica gel, asbestos, and sodium acetate have also been investigated (1,2). Sensitized RTP (11) and micelle-stabilized R T P (12) are two methods that have been used to measure RTP in liquid media. Recently, R T P has been reported from liquid samples containing cyclodextrin (13-15). Cyclodextrin-NaC1 mixtures were also reported to induced R T P from a variety of organic compounds (16, 17). Cyclodextrins have been used in fluorescence spectroscopy where increased intensities are observed when the fluorophor is induced into the cyclodextrin cavity (18). Cyclodextrin also enhanced the fluorescence densitometry of polynuclear aromatic hydrocarbons (PNA)
0003-2700/88/0360-0596$01.50/0 0 1988 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
and dansylated amino acids when used as a spray reagent over silica gel and alumina thin-layer chromatography plates (19). Cyclodextrins have the structure of doughnut-shaped sugar molecules with a hollow, hydrophobic cavity. The relative stabilities of cyclodextrin inclusion complexes are governed by factors such as hydrogen bonding, hydrophobic interaction, and solvation effects as well as the size of the guest molecule (20-22). Although cyclodextrins containing as many as 1 2 glucose units have been identified, only cyclodextrins consisting of six, seven, and eight glucose units (with internal diameters of -5.7, -7.8, and -9.5 A, respectively) (22) are most commonly used. Cyclodextrins have been shown to form inclusion complexes with compounds that can fit into their hydrophobic cavity. In a previous study we have reported that the R T P signal of anthracene, which is usually very weak, can be enhanced by simple treatment of the filter paper with 8- and y-cyclodextrin. a-Cyclodextrin did not show any noticeable enhancement in the anthracene R T P signal. Quantitative explanation for the enhancement in R T P signal of anthracene and measurement of the binding constants of anthracene to the cyclodextrin molecules were reported previously (23). In this study, we further investigated the effect of cyclodextrins on the RTP emission of a wide variety of polynuclear aromatic (PNA) compounds. The results demonstrate that the simple treatment of filter paper substrate with cyclodextrins can selectively enhance the R T P signal of certain PNAs. Quenching of the R T P signals were also observed for other PNA's. The R T P enhancement effect of CY-, p-, and y-cyclodextrin was determined and discussed in detail. These enhancementlquenching features are the basis of the selective-enhancement-by-cyclodextrin(SECD) concept that can be used to improve the specificity of R T P analysis of complex mixtures. The SECD methodology was illustrated with the analysis of a two-component model mixture using conventional fied-excitation and synchronous RTP. The binding constants of three PNAs were also investigated to illustrate that binding to cyclodextrins is an important factor for the enhancement of the R T P signals. EXPERIMENTAL SECTION Apparatus. All measurements were carried out with a Perkin-Elmer spectrofluorometer (Model 43A, Perkin-Elmer, Norwalk, CT), equipped with a phosphoroscope attachment. A 150-W xenon-arc lamp was used for excitation. The detector was a photomultiplier (Hammamatsu Co., Middlesex, NJ; Model R-777) with a photocathode spectral response from 185 to 850 nm. Warm, dry air was passed through the sample compartment during the measurements to prevent moisture from quenching the phosphorescence. For synchronous luminescence, both excitation and emission monochromatorswere locked together and scanned simultaneously. Special laboratory-constructed sample holders were used for RTP measurements (24). Reagents. All the PNA compounds tested were commercially available, purchased at their highest purity, and used as received without further purification. The solvent used in the preparation of all samples was ethanol of spectroscopic grade (Aaper Alcohol and Chemical Co.). The heavy-atom salts for RTP analysis were thallium/lead acetate (1 M/1 M) and used in ethanol-water mixture (volume ratio 1:l). Cyclodextrin (a,p, y) was purchased from Aldrich Chemicals Co. and used as received. Whatman filter paper (Whatman 1)was used as the substrate material for all RTP samples. Procedure. The various steps to prepare the samples for RTP measurements have already been described in previous works (24-26). The filter papers were cut into 1cm diameter disks and impregnated in saturated aqueous solutions of CY-, @-, and y-cyclodextrin for 2 h. The analyte solutions (3 FL) were spotted on the filter papers followed by delivery of 3 pL of heavy-atom (thallium/lead acetate) solution, and the samples were dried with an infrared heating lamp; the samples were then transferred to the sample compartment for spectroscopic measurement.
a) NOCD
b) WITH ALPHA CD SENSITIVITY 1
SENSITIVITY 1
c) WITH BETA CD SENSITIVITY 0.3
420
460
5w
597
d) WITH GAMMA CD
540
580
420
460
500
540
4
580
WAVELENGTH (nm)
Figure 1. RTP spectra of phenanthrene (hex= 300 nm): (a) on filter paper without CD; (b) on filter paper treated with a-CD; (c) on filter paper treated with PCD; (d) on filter paper treated with y C D .
The binding constants of acridine, dibenzofuran, and 2,6-dimethylquinoline to cyclodextrin molecules were measured by the TLC method (27,28). All TLC developments were conducted with a 22 cm high and 7 cm diameter developing chamber. Cyclodextrin mobile-phase concentrationswere 0.05,0.04,0.03,0.02, and 0.01 M. All solutes were developed using HPLC grade water as the mobile phase. Polygram polyamide-6 TLC plates (Machery-Hagel, West Germany) were used for all binding M measurements. All measurements used 1-pL samples of PNA solutions. The TLC spots were visualized by fluorescence quenching. R E S U L T S AND DISCUSSION R T P S p e c t r a on CD-Treated Cellulose Substrate. Figure 1 shows the R T P spectra of phenanthrene on filter paper with and without cyclodextrin. The spectrum on Figure l a corresponds to phenanthrene adsorbed on Whatman 1 filter paper with no cyclodextrin treatment. With a filter paper substrate treated with a-CD, the R T P spectrum of phenanthrene showed a little enhancement (Figure l b ) . The enhancement became more dramatic when @-CDand y C D treated papers were used as substrates (Figure 1,parts c and d). The results of RTP measurements using CD-treated paper for another PNA compound, triphenylene, are shown in Figure 2. In the presence of cyclodextrin (mainly p- and r-CD), the excitation spectra of most of the compounds studies showed a small blue shift of approximately 5 nm, suggesting that the inclusion process was associated with chemical interactions between the phosphors and cyclodextrin molecules. We
598
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
I
400
440
400
440
L , , L - , l 400
440
400
440
WAVELENGTH (nm)
Figure 2. RTP spectra of triphenyiene: (Aex = 268 nm): (a) on filter paper without CD; (b) on filter paper treated with a-CD; (c) on filter paper treated with P-CD; (d) on filter paper treated with T-CD.
Table I. RTP Measurements of Different PNAs Using Filter Paper Treated with Cyclodextrin ICDl1OU
compounds
wCD
p-CD
Y-CD
A,,
A,,
10 1.5 2 2
398
626 698
3.0
300 284
Polyaromatic Hydrocarbons benzo[ghi]perylene benzo[a]pyrene chrysene coronene
dibenzoanthracens fluoranthene phenanthrene pyrene
triphenylene
1 1 1 1 1 1.5 1.5 1 1
2
1.5
1.8 2 2 2 3 1 2
2 4.0 1 3
395
325 310 300 343 268
525 520
565 555 478
596 595
Heterocyclic Aromatic Compounds acridine 7,8-benzoquinoline 4-bromobiphenyl dibenzofuran 2,6-dibenzoquinoline dibenzothiophene 2,6-dimethylquinoline indole isoquinoline 1-naphthol phenazine quinoline
2 2 1.4 1 1 2.5 2 1 1 1 1
0.5
8 2 1
6 3 3 3 4
8
360
640
1.5
355
509
1 6 3
325
511
295
440
305 329
448
3.5 3
505
0.6
0.6
324 288 330
1 1 1.5
2.5
310
530
1 1.0
317
590
320
510
4
470
436 520
"The IcD/Io values represent the RTP intensity on treated filter paper to that of nontreated filter paper. Relative standard deviation = &Is%. conducted RTP investigations for a wide variety of homocyclic and heterocyclic PNA species. Table I gives the results of R T P measurements of 21 PNA compounds on filter paper treated with a-, p-, and -y-CD. The values of intensity enhancement (ZcD/Zo) represent the ratio of R T P intensity of the analyte on the filter paper treated with cyclodextrin (ZcD) over that obtained with the untreated filter paper substrate (lo).All measurements were conducted under the same experimental conditions. The heavy-atom agents were thallium/lead acetate; without heavy-atom agents no R T P was observed with or without cyclodextrin. It is noteworthy that the sole effect of cyclodextrins on phosphorescence would not be investigated since a heavy-atom agent was necessary to induce RTP. This is to be expected since the phosphorescence quantum yields of polynuclear aromatic systems are extremely weak and a heavy-atom perturber is normally required to enhance R T P (IO). Therefore, the photophysical process studied is the effect of cyclodextrins on the PNA-heavy atom
systems. The presence of heavy atoms in the environment greatly increases the rates of singlet-triplet processes. This is achieved by an increase in spin-orbit coupling between the two states in question and leads to enhancement of phosphorescence (IO). The addition of cyclodextrin molecules on the filter paper could lead to formation of a complex between the cyclodextrin and the heavy atomjanalyte system and minimize the effect of phosphorescence quenchers, including oxygen, and enhance R T P emission. The results in Table I showed that 0-CD and 7-CD enhanced the RTP intensity of most PNA compounds. The p-CD and y C D , which have cavity internal diameters of 7.8 and 9.5 A, respectively (20, 221, could form inclusion complexes with most of the PNA compounds depending on the size and shape of the PNA molecules. Thus inclusion complex formation could prevent or minimize intramolecular and intermolecular radiationless deactivation processes and could, therefore, enhance the RTP emission. Some PNA compounds, such as benzo[ghi]perylene, phenanthrene, acridine, dibenzofuran, and indole showed large enhancement in the R T P emission intensity (Table I). On the other hand, other PNA compounds including pyrene, 4-bromobiphenyl,and phenazine showed little or no significant change in the R T P intensity. The a-CD molecules, which consists of six glucose units and have a cavity internal diameter of 5.7 A (20,22), are, therefore, not sufficiently large to trap the larger PNA molecules and will induce no or only little RTP enhancement. It is noteworthy that the size of the guest molecule is only one of the many factors that affect the relative stability of CD inclusion complexes. Other factors including hydrogen bonding, hydrophobic interaction, solvent effect, etc., play an important role in the stability of the cyclodextrin complex (20-22). Although further investigations are required to explain the interaction mechanisms of cyclodextrins in detail, the results show that the selective enhancement of cyclodextrins on R T P can be used to further improve the specificity of the R T P technique. PNA Selective Enhancement for M i x t u r e Analysis. The selectivity aspect of cyclodextrin to induce different phosphorescence enhancements with PNA's can be advantageously exploited in order to improve the selectivity of RTP measurements of mixtures. Figure 3 illustrates the selective enhancement by CD (SECD) method for a two-component mixture containing dibenzofuran (DBF) and 2,6-dimethylquinoline (DMQ). Although the excitation wavelength at 303 nm is not the best possible for each individual component, it was chosen because it lies in the absorption range of these two compounds. Without CD treatment, the R T P spectrum of the model mixture (Figure 3a) using the 303-nm excitation exhibited emission bands characteristic of dibenzofuran (412 and 440 nm) and 2,6-dimethylquinoline (470, 510, and 540 nm). Measurements with individual compounds indicated that a-CD did not enhance the R T P emission of DBF and DMQ (Figure 3b). However, p-CD and 7-CD preferentially increased the R T P signal of DBF in the model mixture as demonstrated in Figure 3c,d. In these two figures the enhanced RTP signal of DBF provided the major contribution of the total signal. On the other hand, RTP emission of DMQ barely emerged from the total spectrum, which is dominated by the DBF emission. In some situations, the SECD methodology can provide a practical means in order to enhance the R T P of the compound of interest in the presence of other interfering compounds. Improved Selectively by Synchronous RTP. Synchronous phosphorimetry has been previously developed and proposed as a means to further enhance the specificity of RTP analysis (25, 26). In Figure 4 synchronous R T P (SRTP) spectra of the DBF and DMQ were generated by scanning both the emission and excitation wavelengths simultaneously
ANALYTICAL CHEMISTRY, VOL. 60, NO. 6, MARCH 15, 1988
Table 11. Binding Constant Measurement for Different PNA-Cyclodextrin Systems Using TLC
b) WITH ALPHA C D
a) N O C D
SENSITIVITY 1
SENSITIVITY 1
599
K , binding constant a-CD P-CD 7-CD
compounds acridine dibenzofuran 2,6-dimethylquinoline
10.8 0 0
29.43 24.62
32.84 19.86
9.78
18.7
tectable (Figure 4c,d), whereas the fixed-excitation R T P emission of DMQ was severely overlapped by DBF emission (Figure 3c,d).
Thin-Layer Chromatography Method for Measuring the Binding Constants. The TLC method for measuring
d) WITH GAMMA CD SENSITIVITY 0.3
the binding constants of organic compounds with cyclodextrin introduced by Armstrong et al. (27,28) was used to estimate the binding of acridine, dibenzofuran, and 2,6-dimethylquinoline with a-, p-, and y-cyclodextrin. The primary and secondary binding constants of cyclodextrin-substrate were determined from the equation
1
Rf 1- R f
- or-=-
h’
A 420
460
500
Y O
5
WAVELENGTH (nm)
Figure 3. RTP spectra of a two-component mixture (dibenzofuran and 2,6dimethyiquinoiine): (a) on filter paper without CD; (b) on filter paper treated with a-CD; (c) on filter paper treated with @-CD;(d) on filter paper treated with r-CD. I
NDCD SENSlTlVlTY t
j WITH ALPHA CD
SENSITIVITY 1
o! WITH B E T A C O SENSITIYITIO 1
I
d) WITH GAMMA CD SENSITIVITY 0 5
l
l
Figure 4. Synchronous RTP spectra of a two-component mixture (Aex = 303 nm, AA = 150 nm): (a)on filter paper without C D (b) on filter paper treated wlth a-CD; (c) on filter paper treated with p-CD; (d) on filter paper treated with y-CD.
while keeping the spectral interval AA between them at a constant value (150 nm). In the SRTP technique, a phosphorescence peak occurs only when the excitation and emission wavelengths correspond to spectral positions where both absorption and phosphorescence occur. The methodology introduces an additional degree of selectivity, which is based on the energy separation between the singlet and triplet state, known as the singlet-triplet splitting. The AA value of 150 nm was not the optimal value for either DBF or DMQ but was selected such that the SRTP spectrum of the mixture exhibited two well-resolved bands of comparable intensities when no CD was used (Figure 4a). With the use of a-CD treated substrate, the S R T P spectrum showed no enhancement. When fl-CD or r-CD treated substrate was used, the DBF peak was significantly enhanced as expected (Figure 4c,d). It is noteworthy that the improvement in selectivity provided by the synchronous technique was illustrated by the fact that the DMQ band remained clearly de-
Kl[CDI @K[A]
+
K1K2[CDI2 @mAI
where k’is the capacity factor, R, is the retardation factor of a solute in thin-layer chromatography, a parameter denoting the ratio of the distance traveled by the mobile phase over the distance traveled by the solute, K is the equilibrium constant between the solute and the stationary phase, K, and Kz are equilibrium constants for the substrate and cyclodextrin (K, for a 1:l solute-cyclodextrin complex, K2 is a 1:2 solutecyclodextrin complex), @ is the phase ratio, [CD] is the concentration of cyclodextrin, and [A] is the concentration of the solute. When the cyclodextrinsolute stoichiometry is 1:l (i.e., K2 = 0), straight line plots are obtained. In these studies the stoichiometry was 1:l for p- and y-cyclodextrin. Plots of l l k ’ o r Rf/(l- R f )vs cyclodextrin concentration give a straight line in which the slope corresponds to K,/ @K[A]and the intercept l/@K[A]. From the ratio of the slope over the intercept values, one can calculate K,. Table I1 shows the values of K, for acridine, dibenzofuran, and 2,6-dimethylquinoline with a-, p-, and y-CD. As we can see from Table 11, the K, value of acridine with a-CD is 10.8 whereas dibenzofuran and 2,6-methylquinolie have zero value for their binding constants with a-CD. The enhancement in R T P signals of acridine with a-CD treated substrates (see Table I) confirms that binding to the cyclodextrin molecules is an important parameter. There was no or only little enhancement in the R T P signals of DBF and DMQ when the filter paper was treated with a-CD; this feature is in agreement with the absence of interaction between a-CD and DBF and DMQ (see Table 11). In general, the values of the binding constants in Table I1 can be qualitatively correlated with the enhancement in the RTP signals in Table I. The larger the value of K,, the higher the enhancement of the R T P signals. One should mention that the binding constant measurements dealt only with the interactions between the PNAs and the cyclodextrins and did not include the effect of heavy atoms. The detailed study of interactions between cyclodextrins and multiple guests (analyte/havy atoms) on solid substrates is beyond the scope of this work.
CONCLUSION This preliminary investigation has demonstrated that a simple treatment of filter paper with CD can enhance the sensitivity of the R T P technique. Also, the R T P measurements with CD-treated filter paper do not require removal of oxygen from the sample as in liquid samples. The ability of cyclodextrins to enhance the R T P of PNA compounds
Anal. Chem. 1988, 60,600-605
600
selectively has great potential for analytical applications. The limits of detection of the compounds investigated are in the subnanograms range and the linear range of the analytical curve covered approximately 2 orders of magnitude, which is often observed for a surface emission technique ( I O ) . The SECD method can be combined with other selective enhancement approaches such as the selective heavy-atom perturbation method ( 4 ) in order to improve the specificity of the simple but powerful R T P technique for the analysis of complex mixtures. Registry No. Benzo[ghl]perylene,191-24-2;benzo[a]pyrene, 50-32-8;chyrsene, 218-01-9; coronene, 191-07-1;dibenzoanthracene, 53-70-3;fluoroanthracene,206-44-0; phenanthrene,85-01-8; pyrene, 129-00-0;triphenylene, 217-59-4; 2,6-dimethylquinoline, 877-43-0; acridine, 260-94-6;7,&benzoquinoline, 230-27-3;4-bromobiphenyl, 92-66-0;dibenzofuran, 132-64-9;2,6-dibenzoquinoline,257-89-6; dibenzothiophene, 132-65-0;indole, 120-72-9;isoquinoline, 11965-3; 1-naphthol, 90-15-3; phenazine, 92-82-0;quinoline, 91-22-5; a-cyclodextrin, 10016-20-3; i3-cyclodextrin, 7585-39-9; y-cyclodextrin, 17465-86-0.
LITERATURE CITED Roth, M. S.J. Chromatogr. 1967, 30, 276. Parker, R. T.; Freedlander, R. S.;Dunlap, R. B. Anal. Chim. Acta 1980, 120, 1. Vc-Dinh, T.; Lue-Yen, E.; Winefordner, J. D. Anal. Chem. 1976, 4 8 , 1186. Vo-Dinh, T.; Hooyman, J. R. Anal. Chem. 1979, 51, 1915. Ford, C. D.; Hurtubise, R. J. Anal. Chem. 1982, 739, 315. Wellon, S.L.; Paynther. R. A,; Winefordner, J. D. Spectrochim. Acta, Part A 1974. 30, 2133. Paynther, R. A.; Wellon, S.L.; Winefordner, J. D. Anal. Chem. 1976, 4 6 , 736. Aaron, J. J.: Kaleel, E. M.: Winefordner, J. D. J. Agric. Food Chem 1979, 27, 1233.
DeLima, C. G.; de M. Nicoia, E. M. Anal. Chem. 1978, 50, 1658. Vo-Dinh, T. Room Temperature Phosphorimetry for Chemical Analysis; Wiley: New York, 1984. Donkerbroek, J. J.; Gooljer, C.; Velthorst, N. H.; Frei, R. W. Anal. Chem. 1982, 54, 891. Cline Love, I. J.; Skrilec, M.; Habarta, J. G. Anal. Chem. 1980, 52, 754. Seypinski, S.;Cline Love, L. J. Anal. Chem. 1984, 56, 322. Seypinski, S.;Cllne Love, L. J. Anal. Chem. 1984, 56, 331. DeLuccia, F. L.; Cline Love, L. J. Anal. Chem. 1984, 56, 2811. Bello, J.; Hurtubise, R. J. Appl. Spectrosc. 1986, 40, 790. Bello, J.; Hurtubise, R. J. Anal. Lett. 1986, 79(798) 775. Hoshino, M.; Imamura, M.; Ikehara, K.; Hama, Y, J. Phys. Chem. 1981, 85, 1620. Alak, A.; Heilweil, E.;Hinze, W. L.; Oh, H.; Armstrong, D. W. J. Liq. Chromatogr. 1984, 7, 1273. Szejfli, J. Cycldextrin and Their Inclusion Complexes; Academias Kiado: Budapest, Hungary, 1982. Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: New York, 1978. Hinze, W. L. Sep. Purif. Methods 1981, 70, 2. Vo-Dinh, T.; Alak, A. M. Appl. Spectrosc. 1987, 41(6), 963. Vo-Dinh, T.; Martinez, P. R. Anal. Chim. Acta 1981, 13, 125. Vo-Dinh, T.; Gammage, R. B. Anal. Chem. 1978, 50, 2054. Vo-Dinh, T.; Gammage, R. B. Anal. Chim Acta 1979, 107, 261. Armstrong, D. W.; Stine, G. Y. J. Am. Chem. SOC.1983, 705.2962. Armstrong, D. W.; Nome, F.; Spino, L.; Golden, T. J. A m . Chem. SOC. 1986, 108, 1418.
RECEIVED for review June 25,1987. Resubmitted October 28, 1987. Accepted November 19,1987. Research was sponsored by the Office of Health and Environmental Research, U.S. Department of Energy, under Contract DE-AC05-840R21400 with Martin Marietta Energy Systems, Inc. This research was also supported in part by an appointment of A.M.A. to the postgraduate Research Training Program under Contract No. DE-AC05-760R00033between the U.S. Department of Energy and the Oak Ridge Associated Universities.
Vaporization Kinetics for Solids Analysis with Electrothermal Atomic Absorption Spectrometry: Determination of Lead in Metal Samples Thomas M. Rettberg’ and James A. Holcombe* Department of Chemistry, University of Texas at Austin, Austin, Texas 78712
A method Is proposed for the direct analysis of species evolved from metaHurglcaisamples that Involves extrapolation of a function describing the evolution rate of the analyte. The second surface atomlrer was employed to determlne the tlme-dependent vaporization rate of an analyte from an lndMual sample over several heatlng cycles. These data were fit by using a flrst-order klnetic expression and were related to the total amount of analyte In the sample by uslng a callbratlon curve constructed wHh slmpie, aqueous standards. Results are shown for the determlnatlon of Pb In Sn, Cu, and steel.
The direct analysis by electrothermal atomization (ETA) of solid metal samples containing trace concentrations of other metals has been performed by numerous researchers (1-15). Present address: Varian Atomic Absorption Resource Center, 20 W. Touhy, Park Ridge, IL 60068.
Many of these previous studies elucidated some of the important factors relating to the direct analysis of these sample types, some of which are often an impediment to a successful, straightforward analysis. Items mentioned have included the slow vaporization rates of the analyte from the host metal (2, 8,14); problems caused by the host metal, such as background or the buildup of residue (2, 4, 8, 9, 11); and/or problems related to graphite interactions, such as carbide formation. Despite the unique nature of solid metal samples, the analyses mentioned above typically employed a protocol for heating and signal measurement similar to that used for aqueous samples. Direct sample heating using rapid heating rates to high furnace temperatures was employed to produce an absorbance “spike”, yet broad, tailed peaks were often still observed. While high atomization temperatures [one source listed 3200 “C (3)]maximize the partial pressure and evolution rate of the analyte, they also occasionally resulted in rapid vaporization of the bulk sample and high background levels. Complete volatilization of the analyte within a reasonable integration period w8s a prerequisite for accurate quantitative analysis, yet this often was difficult to attain ( 2 , 8, 12, 14).
0003-2700/88/0360-0600$01.50/0 0 1988 Amerlcan Chemical Society
