Anal. Chem. l98B, 6 1 , 2328-2330
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TECHNICAL NOTES Room-Temperature Phosphorescence of Polynuclear Aromatic Hydrocarbons on Matrix-Modified Solid Substrates L. M. Perry, A. D. Campiglia, a n d J. D. Winefordner* Department of Chemistry, University of Florida, Gainesville, Florida 32611 INTRODUCTION Room-temperature phosphorescence spectrometry (RTP) is an established analytical technique used for the analysis of trace components in biological, pharmaceutical, and environmental samples ( I ) . Although RTP analyses have been performed with analytes in the solid, liquid, and gaseous states, the simplest method of analysis is done with the analyte in the solid state on a solid support. Reports on solid surface RTP (SSRTP) appeared in the literature in 1896, but until the work of Roth, Schulman, and Walling, the full potential of this technique was not recognized (2-4). Since then, numerous studies have been reported, and recent publications by Vo-Dinh and Hurtubise have reviewed the RTP phenomena on solid substrates in detail ( 1 3 ) . Silica gel, sodium acetate, polymer salt mixtures, and filter paper are some of the supports that have been used for SSRTP (1,6-9). Although filter paper is the most commonly used support because of low cost and commercial availability, it suffers from two important drawbacks. It has a broad RTP emission band from 400 to 600 nm, the region where many organic compounds also phosphoresce, and its surface is not uniform. Both conditions can affect the precision and limit of detection (LOD) of the analytes. In addition, filter paper is susceptible to moisture, which causes quenching of the analyte signals and additional problems with reproducibility. Analysis of polynuclear aromatic hydrocarbons (PAHs) has been accomplished by RTP on solid substrates ( I , 9, IO). Selectivity, sensitivity, and small sample requirements make this technique conducive for trace analysis. In general, satisfactory analysis of nonpolar PAHs requires the use of a heavy atom or enhancement of the R T P signal. Phosphorescence enhancement by use of a heavy atom has been reported by several investigators (9, 11-13). Winefordner and Lue-Yen Bower investigated a series of heavy-atom enhancers for a variety of PAHs and determined Tl(I), overall, to be the most effective enhancer (9). RTP emission for PAHs depends not only on the presence of a heavy atom but also on the environment for the phosphors. Immobilization in a rigid medium, association by hydrogen bonding, or entrapment of the phosphor are believed to be required to minimize collisional deactivation and loss of the phosphorescence signal ( I , 5). Recent literature reports investigations of PAHs on matrix-modified filter paper (9, 14-1 6). Diethylenetriaminepentaacetic acid, cyclodextrin, and various inorganic salts, i.e., sodium citrate and malonate, are just a few of the many compounds used to modify the surface of filter paper for the inducement or enhancement of phophorescence. Previously, we reported nanogram detection of anthracene, a weak phosphor, by SSRTP (17). The solid substrate, Whatman No. 1, was pretreated with Tl(1). Measurements
* Author to whom correspondence should be addressed. 0003-2700/89/0361-2328$01.50/0
were accomplished by using the surfactant salt thallium lauryl sulfate (TlLS) and the inorganic salt TWO3. The RTP signal of anthracene in the presence of TlLS was approximately 2-fold greater than obtained for TlN03. Additionally, the precision for the measurement of anthracene on Whatman No. 1pretreated with TLS was observed to be 2 times better than the one observed for the TlN03system. It was suggested that the long alkyl chains of the TLLS protected the phosphor from collisional deactivation and possible moisture quenching. In this investigation, Whatman No. 1pretreated with TlLS was examined as a general surface for inducing R T P from several PAHs. EXPERIMENTAL SECTION Instrumentation. A Perkin-Elmer LS-5 luminescence spectrofluorometer (Perkin-Elmer, Norwalk, CT) coupled to a Model 3600 data station was used to collect all RTP spectra and intensity measurements. The spectrofluorometer was equipped with a xenon flash lamp as the excitation source. All phosphorescence data were collected by using a pulse delay time of 0.03 ms and a gate time of 9.0 ms. The excitation and emission monochromator slits were set at 10 and 5 nm, respectively. The sample compartment was continuously purged with dry nitrogen. A 360-nm cutoff filter was used where necessary to minimize second-order scatter. Reagents. Naphthalene (Mallinckrodt, St. Louis, MO), 4aminobenzoic acid (PABA) and carbazole (Eastman Kodak Co., Rochester, NY), and 1,2,3,4-dibenzanthracene(1,2,3,4-DBA)and pyrene (Aldrich Chemical Co., Milwaukee, WI) were of reagent grade and used as received. The PABA was used as a reference compound for comparison of RTP figures of merit. Thallium lauryl sulfate (TlLS) was prepared in our lab by a procedure previously described ( I 7). Absolute ethanol (Florida Distillers Co., Lake Alfred, FL) and “Nanopure” demineralized water (Barnstead System, Sybron Corp., Boston, MA) were used as the solvents. Procedures. TlLS was prepared in ethanol/water (8020 by volume) solutions. Concentrations of 0.016,0.033,0.065, an 0.13 M TlLS were prepared and the solutions spotted onto Whatman No. 1filter paper disks for the preparation of the ‘heavy-atom curves”. All PAH standards were prepared daily in absolute ethanol by serial dilution of lo00 pg/mL stock solutions. The preparation of Whatman No. 1 filter paper disks with TlLS was described previously (17). The RTP intensity of the substrate, Le., Whatmen No. 1 treated with 5 p L of the appropriate concentration of TlLS, was determined at the maximum excitation and emission wavelengths for each PAH; subsequently 3 p L of analyte was spotted onto the center of each substrate. Each PAH sample had its own blank whose intensity was subtracted from the total intensity measured for the analyte. All measurements were performed in triplicate in the presence of dry nitrogen. RESULTS AND DISCUSSION Whatman No. 1 filter paper treated with 0.065 M TlLS produced a broad band of background emission from 400 to 600 nm. The excitation and emission maxima were 253 and 504 nm, respectively. The R T P emission intensity is approximately 10 times that of untreated Whatman No. 1filter 0 1989 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 61, NO. 20, OCTOBER 15, 1989
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Table 11. Reproducibility of Blanks at Excitation/Emission' Maxima for Analytes
Table I. Room-Temperature Phosphorescence Spectral Properties of Several PAHs and PABA Examined on Whatman No. 1 Filter Paper Pretreated with TlLS compound
TlLS, M
Ah, nm
XEm,LI nm
naphthalene PFene 1,2,3,4-DBA carbazole PABA
0.065 0.065 0.065 0.033 0.033
275 336 292 295 286
480, 513, (547) 593,653 567, 615,670 416, 440, (465) 430
a Wavelength of maximum RTP emission is italicized when there is more than one band; the wavelengths of the shoulders are in parentheses; the precision for the wavelengths is 10.5 nm.
analyte
no. of substrates
av blank RTP response
RSD, %
PABA 37 11.8 f 0.7 carbazole 18 13.0 f 0.7 naphthalene 41 47.9 f 2.8 1,2,3,4-DBA 30 20.1 f 0.8 pyrene 18 6.9 & 0.3 Excitation and emission maxima reDorted in Table I.
6.1 5.4 5.7 3.9 4.3
Table 111. Room-TemperaturePhosphorescence of Several PAHs and PABA on Whatman No. 1 Pretreated with TILS compound PABA carbazole naphthalene 1,2,3,4-DBA pyrene
(e) ,
0.00
,
,
,
1
,
,
1
0.04
,
,
1
,
,
/
,
,
,
,
0.08
,
I
I
,
J
I
,
,
,
,
I
0 12
I
,
I
J
,
I
,
0 16
TILS Concentration ( M ) Figure 1. Ratio ( A / 8 )of net analyte RTP slgnal intensity to the substrate background RTP emlssion intensity at the wavelengths of the particular analyte plotted versus the molar concentration of TILS to determine the effects of heavyatom concentration on RTP emission of various PA& and PABA (a) 1,2,3,eDBA; (b) carbazole; (c) pyrene; (d) PABA; (e) naphthalene.
paper under identical experimental conditions. The effect of the high background emission on the measurement of various PAHs was examined and is discussed below. Table I lists the PAHs and their RTP excitation and emission characterisitics chosen for examination on Whatman No. 1 pretreated with TlLS, hereafter referred to as the substrate. These compounds were chosen because their emission maxima occurred on the broad background of the substrate and their excitation wavelengths were similar. PABA, carbazole, naphthalene, 1,2,3,4-DBA, and pyrene phosphorescence emission maxima were observed at 430,440, 513, 567, and 593 nm, respectively. The relative emission maxima represent different situations for the measurement of these compounds. Since the naphthalene emission maximum at 513 nm is very near the RTP emission maximum of the substrate, ita analytical figures of merit should be the most affected. Conversely, the other compounds should produce more favorable analytical results. Carbazole and PABA were specifically chosen because of their different polarities,in order to evaluate their interactions with the alkyl chains of the substrate. The influence of heavy-atom concentration for each PAH was determined from a plot of the ratio ( A / B ) of the net analyte RTP signal intensity ( A )to the substrate background RTP emission intensity ( B ) a t the wavelengths of the particular analyte versus the molar concentration of TlLS. From Figure 1,it can be seen that the A / B ratios for all the analytes changed little with Tl(1) concentration. However, for all further studies, we chose the TlLS concentrations given in Table I for the five analytes to achieve good signal to background ratios with little sensitivity to change in TlLS concentration.
slope LDR," ng log-log 3-450 1.5-300 15-900 1.5-75 1.5-750
0.98 0.99 1.02 1.03 0.97
corr coeff
LOD,*p'
0.997 0.996 0.998 0.996 0.998
1.3 0.8
ng
8.8
0.5 1.4
precisiond method, % 7.6 7.8 13.9 5.8 3.3
a Linear dynamic range resulting from triplicate measurements of each analyte on Whatman No. 1 pretreated with TlLS. *Limit of detection calculated by 3sbbk/m,where S b h k is the standard deviation of 16 blanks and m is the slope of the analytical calibration curve plotted on linear-linear coordinates. cValues are detection limits in nanograms. dPrecision for the method was obtained by the following formula: Pme*d= (SA+B' + SB*)~('/~A-B X 100. Sixteen determinations of 10 ppm analyte and their respective blanks were utilized to determine SA+B, standard deviation of analyte plus blank intensity; sB, standard deviation of blank RTP intensity; and ZA-B, average net analyte RTP intensity.
The percent relative standard deviations (RSDs) of the RTP background emission signals of blanks pretreated with the optimum TlLS concentrations and measured at the excitation/emission peaks for the five analyses (see Table I) are given in Table 11. The percent relative standard deviations of the RTP background emissions of the substrate for all analytes were observed to be less than 7%. Additionally, the RTP background emissions of 4 disks of the same batch of substrates used for the analysis of naphthalene were measured at the naphthalene excitation and emission wavelengths at approximately a 1-month interval. There were no significant differences in the reproducibilities of the RTP background intensity observed over the monthly period as opposed to the reproducibilities obtained over a few hours. The slopes (sensitivities)of the analytical calibration curves for the PAHs and the analytical figures of merit are summarized in Table 111. The slopes of the log-log calibration plots are all close to unity (k0.03) with excellent correlation coefficients. Pyrene, carbazole, naphthalene, and PABA show linearity over 2 decades, while 1,2,3,4-DBA was linear only over 1 decade. Nanogram detection limits were observed for all analytes. The precision of the method for each analyte was determined by using the 10 pg/mL standard solution for each PAH. With the exception of naphthalene, the method precision for all analytes was observed to be less than 8%, which is statistically the same as the precision of the background emission. The poorer method precision (13.9%) for naphthalene can be mainly attributed to the high standard deviation of the substrate emission and to the high background emission at the peak wavelength for naphthalene. It should also be mentioned that the time for the naphthalene RTP signal to stabilize was more than twice that of the other compounds tested. The RTP intensity peaked at a maximum value after 10 min of drying in the sample compartment with dry N2 and then rapidly degraded. Volatilization of the an-
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Anal. Chem. 1989, 61, 2330-2332
alyte during the drying time (18) was, most likely, responsible for the unfavorable precision observed. CONCLUSION TlLS was used as a matrix modifier for Whatman No. 1 filter paper. This new substrate was used successfully for nanogram detection of several PAHs. The analytical figures of merit compare favorably with those in literature reports (9,18-22),which reveal a variety of filter papers and matrix modifiers utilized for such molecules. This study suggests that Whatman No. 1 filter paper pretreated with TlLS can be used as a general method for trace analysis of different kinds of PAHs. The substrate was stable over a long period of time, meaning it could be prepared ahead of time and stored for future use. The reproducibility of the substrates' background was less than 7%. As previously reported, the Tl(1) cation and the analyte were protected from source irradiation damage by the long alkyl chains of the anion (17). An additional advantage of the nonpolar environment was the protection of the analyte against quenching during the measurement (17). All of the analyte signals, with the exception of that of naphthalene, reached a steady state in less than 3 min as long as a N2 purge was used. Although PABA and carbazole were chosen because of their polarity differences with respect to the other PAHs, these species were just as well behaved as the nonpolar PAHs. ACKNOWLEDGMENT A. D. Campiglia thanks the Coordenacao de Aperfeicoamento de Nivel Superior-Capes for the grant that is making possible his stay in Gainesville and the University of Brasilia (Brazil) for permission to leave. Registry No. PABA, 150-13-0;1,2,3,4-DBA,215-58-7; TlLS,
72925-49-6; naphthalene, 91-20-3; carbazole, 86-74-8; pyrene, 129-00-0,
LITERATURE CITED Vo-Dinh, T. Room Temperature phasphorknetry for Chemical Analysis; Elving, P. M., Winefordner, J. D., Eds.: John Wiiey 8 Sons: New York, 1984. Roth, M. J. Chromatogr. 1987, 30, 276-278. Schulman. E. M.; Wailing. C. J. Phys. Chem. 1973, 7 7 , 902-905. Schulman. E. M.; Walling, C. Sclence 1972, 778, 53-54. Hurtubise, R. J. Sola Surface Luminescene Analysis: Theory Instrumentation, Applkations; Marcel Dekker: New York, 1981. Dakerio, R. A.; Hurtubise, R. J. Anal. Chem. 1982, 5 4 , 224-228. Burrell. G. J.; Hurtubise, R. J. Anal. Chem. 1988, 60, 564-568. von Wandruszka, R. M. A.; Hurtubise, R. J. Anal, Chem. 1977, 49, 2 164-2 169. Lue-Yen Bower, E.; Winefordner, J. D. Anal. Chlm. Acta 1978, 702, 1-18. Vo-Dinh, T.; Gammage, R. B.; Martinez, P. R. Anal. Chem. 1981. 53, 253-258. SeyboM, P. G.; White, W. Anal. Chem. 1975, 4 7 , 1199, 1200. Vo-Dinh, T.; Lue-Yen, E.; Winefordner, J. D. Anal. Chem. 1978, 4 8 , 1186-1188. Suter, G. W.; Kallir, A. J.; Wild, U. P. Anal. Chem. 1987, 5 9 , 1644-1 646. Karnew, H. T.; Schulman, S. G.; Winefordner, J. D. Anal. Chim. Acta 1984, 764, 257-262. Alak, A. M.; Vo-Dlnh, T. Anal. Chem. 1988, 60. 596-600. Parker, R. T.; Freelander, R. S.; Schulman, E. M.; Dunlap, R. B. Anal. Chem. 1979, 57, 1921-1926. Perry, L. M.; Campiglia, A. D.; Wlnefordner, J. D. Anal. Chkn. Acta, in press. Ramis Ramos. G.; Khasawneh, I. M.; Garcia AlvarezCoque, M. C.; Winefordner, J. D. Takmta, 1988. 35, 41-48. VoDinh, T.; Gammage, R. 8. Anal. Chlm. Acta 1979, 707, 261-271. Vc-Dinh. T.; Hooyman, J. R. Anal. Chem. 1979, 57, 1915-1921. Ramis Ramos, G.; Garcia AlvarezCoque, M. C.; O'Reiily, A. M.; Khasawneh, I . M.; Winefordner, J. D. Anal. Chem. 1988, 60. 418-420. VeDin. T.; Lue-Yen, E.; Winefordner, J. D. Talenta 1977, 2 4 , 146-1 48.
RECEIVED for review March 24,1989. Accepted June 19,1989. This research was supported by NIH-GM11373-27.
Apparatus for the Fabrication of Poly(chlorotrifluoroethy1ene) Composite Electrodes Jeffrey E. Anderson,* Dale Hopkins, John W. Shadrick, and Yee Ren
Department of Chemistry, Murray State University, Murray, Kentucky 42071-3306 INTRODUCTION Since the introduction of Kel-F graphite (Kelgraf) electrodes (1, 2), a number of papers have appeared dealing with the application (3-7) of these electrodes, as well as the characterization of their behavior ( 4 9 ) . The application of Kelgraf electrodes has primarily been as voltammetric detectors in liquid chromatography and flow injection analysis (2-5). Characterization studies indicate that they exhibit behavior typical of microelectrode ensembles and as such have the advantage of enhanced current densities leading to a higher signal to noise ratio than observed for other carbon electrodes such as glassy carbon. Kelgraf electrodes that have been reported previously have been fabricated from Kel-F 81 brand plastic resin from 3M. The recommended fabrication techniques from the manufacturer (10) include compression molding of sheets and injection molding. In both cases, the physical properties of the resulting material are affected by the time and temperature of heating and cooling. For example, the degree of crystallinity is a function of the thermal history of the polymer. Rapid cooling of the Kel-F from above its crystalline melting point (212 "C) to below 150 "C yields a more amorphous material that is less dense, more elastic, more transparent, and tougher than its crystalline counterpart. The more amorphous material is also favored by minimizing the time that the plastic 0003-2700/89/0361-233080 1.50/0
is exposed to temperatures above 212 "C. This minimizes the thermal degradation of the polymer, which would lead to a lower molecular weight and more crystalline product. The denser translucent crystalline product is favored by slow cooling of the melt. It should be emphasized that under no conditions is the product purely crystalline or amorphous, which implies that the fabrication conditions must be strictly controlled to ensure a consistent product. An additional fabrication parameter that should be mentioned is the pressure. In the compression molding used to produce Kelgraf electrodes, it is important to maintain a pressure of a t least 1000 psi during the cooling phase of the fabrication. If the pressure is not maintained, shrink voids may develop caused by shrinkage of the plastic during cooling. Given the above considerations, an apparatus was designed to provide rapid heating of the Kel-F graphite mixture to above its melting point to decrease the fabrication time and minimize the time above 212 "C. In addition, provisions were made to simplify and provide control over the cooling time in an attempt to ensure a consistent product. Note that in the past, cooling was provided by squirting water on the fabrication die, a rather messy nonreproducible affair. This apparatus should be of interest given the more recent introduction of composite electrodes based on mixtures of Kel-F and silver (11)and anticipated extension of this work to other 0 1989 American Chemical Society
