266
Anal. Chem. 1981, 53, 266-271
Matrix Isolation Site Selection in Fluorescence Spectrometry of Polar Derivatives of Polycyclic Aromatic Hydrocarbons Jon R. Maple‘ and E. L. Wehry” Department of Chemistty, University of Tennessee, Knoxville, Tennessee 379 16
The analytical utillty of site-selection techniques for the characterizationof multlcornponent, matrix-isolated mixtures of polar polycyclic aromatic hydrocarbon derlvatlves is explored by employlng hydroxyl derivatives of naphthalene as prototypes. The use of laser excltation for the selective excltatlon of polar molecules resldlng In different “sites” or environments results in much greater spectral resolution than can be achieved with conventlonal lamp sources, especially when an argon or fluorocarbon matrix is utlllzed. Quantltatlve aspects (precision and linear dynamic range) of slte selectlon matrix-isolation fluorometry are also discussed.
Recently, there have been a number of intensive efforts directed toward the spectroscopic characterizationof complex mixtures of polycyclic aromatic hydrocarbons (PAHs). Because many real samples, such as airborne particulate matter and liquefied coal products, contain many PAHs (some of which are carcinogenic or mutagenic) at trace levels, the analytical methodology which is chosen for their analysis must be highly selective and sensitive. For this reason, low-temperature fluorometric procedures (1-12), coupled with laser excitation (1-3, 7,lO-12),have proven to be especially useful for the analysis of PAHs. Despite the recent advances in the analysis of complex mixtures of PAHs, there have been no reports (to our knowledge) of a successful spectroscopic technique for the characterization of complex mixtures of polar PAH derivatives. Obviously, an analytical procedure which can be successfully exploited for the analysis of multicomponent mixtures of both polar and nonpolar aromatic compounds is needed. One difficulty, which can immediately be anticipated if a sample containing polar aromatic compounds is prepared in conventional liquid or frozen solution media, is aggregation of the polar compounds, especially at lower temperatures. Both the appearance and the intensity of the resulting emission spectra will then be highly dependent on the concentration and constituency of the sample. This highly undesirable feature can be avoided, however, by using matrix isolation sample preparation techniques, wherein a vaporized sample is mixed with a large excess of “inert” gas and deposited on a cryogenic surface (1-3). As a result of the low temperature and the isolation of sample molecules, energy transfer and quenching phenomena are suppressed. Consequently, the wide linear dynamic range and high degree of quantitative precision characteristic of matrix isolation fluorometry of PAHs should also be observed for polar derivatives of PAHs. Previous investigations in our laboratory have demonstrated that the fluorescence spectra of matrix-isolated polar PAH derivatives are characterized by very broad emission bands (ca. 3 nm FWHM) and relatively high detection limits (greater than 50 ppm) when a broad-band excitation source (Hg-Xe ‘Present address: Department of Chemistry, The University of New Mexico, Albuquerque, NM 87131. 0003-2700/81 /0353-0266$01.OO/O
lamp) is utilized. Thus, even for simple two- or three-component mixtures of the dihydroxynaphthalenes (DHNs), it was found that resolution of individual components from their isomers was quite difficult, if not impossible (13). It has, however, become apparent that highly resolved spectral bands can be obtained for many low-temperature solutions of aromatic molecules if laser, rather than lamp, excitation is employed (2, 7 , 11, 12, 14-18). Because guest molecules occupy many different microenvironments or “sites” in a low-temperature matrix, the purely electronic energy levels of different molecules of the same solute are shifted to different extents (14-18). Alternatively, it can be said that guest molecules reside in different “sites” if their “0-0”(purely electronic) absorption frequencies are different (17). The bandwidth over which these different 0-0 absorption frequencies are found is a measure of the extent of “inhomogeneous broadening”, which may be contrasted to the intrinsic or homogeneous bandwidth of a single molecule (14-18). Upon monochromatic excitation near the 0-0 absorption band (where the density of vibronic states is small), only those molecules whose absorption bands overlap the laser line will absorb and, consequently, fluoresce (7,14-18). The resulting emission will therefore exhibit much narrower bandwidths than if a conventional broad-band lamp excitation source was used. This effect is often called “site selection”, “energy selection”, or “fluorescence band narrowing” (7,11, 14-20). The purpose of the present investigation is to inquire into the analytical utility of site selection techniques in the matrix isolation fluorescence (MIF)analysis of polar PAH derivatives, using mono- and dihydroxynaphthalene isomers as prototypes. The resolution of complex mixtures of DHNs can be expected to pose a stringent test for any spectroscopic analysis, because of the similarity of the emission and absorption characteristics of the DHN isomers. In experiments of this type, the choice of matrix material may be crucial, for the following reason. Upon laser excitation within about 2000 cm-’ of the 0-0 transition region (from the ground electronic and vibrational level to the ground vibrational level of the first excited singlet state), the resulting emission spectra often consist of a number of narrow bands (corresponding to radiative transitions to various vibrational levels of the ground electronic state), each of which is accompanied by a much broader, longer wavelength phonon wing (7,14-18). The narrow bands are often called “zero-phonon lines”, because no changes in the matrix phonon states accompany the emission process (18,21,22).On the other hand, the broad phonon wings, which indicate the simultaneous creation or annihilation of matrix phonons during the emission process (14-18, 21, 22), negate the advantages of the linenarrowing phenomenon. When the interactions between the guest molecule and the host (matrix) are strong, the change in size of the guest (due to a change in electronic states) during the emission process creates (or annihilates) phonon modes in the matrix (14, 17, 21, 22). The term “electron-phonon coupling” is often used to describe this process. Increasing temperatures also cause an increase in the intensity of the phonon wing at the expense of the zero-phonon line (14,21, 0 1981 Arnerlcan Chemical Society
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
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Table I. Analytical Wavelengths for Hydroxynaphthalenes
compound 1-naphthol 2-naphthol 1,3-DHN 1,4-DHN 1,5-DHN 1,6-DHN 1,7-DHN 2,3-DHN 2.7-DHN
source Aldrich Aldrich Aldrich Pfaltz and Bauer Aldrich Pfaltz .and Bauer Aldrich Aldrich Fisher
“best” excitation wavelength (nm) in in argon perfluorohexane 312.0 313.0 317.7 312.2 327.0 329.0 327.5 330.0 322.3 312.1 320.1 321.0 327.9 328.0 313.0 314.7 314.5 307.8
22), causing narrow zero-phonon lines to be completely obscured at temperatures as low as 40 K (14). Consequently,
electron-phonon coupling is both temperature and solvent dependent (17, 19, 21,22). The problem, then, is to find a solvent for the DHNs which is characterized by a minimal electron-phonon interaction, preferably without having to resort to use of liquid-helium temperatures. In this investigation it was observed that two quite different types of matrix materials (argon and perfluorohexane) were suitable “solvents” for the hydroxynaphthalenes.
EXPERIMENTAL SECTION Commercially available, “research grade” nitrogen and argon gases, as well as “distilled in glass” heptane (Burdick and Jackson) and perfluorohexane (PCR, assay 99% perfluorohexanes, 85% “n-hexane”) were used as matrix materials without further purification. The hydroxyl derivatives of naphthalene used in this investigation are listed in Table I, along with the commercial source; these compounds were vacuum sublimed twice prior to use. Although the vacuum sublimation treatment appeared to result in sufficient purity for most of the compounds, 1,4-DHN, Ib-DHN, 1,7-DHNwere grayish white, grayish black, and brown, respectively, before and after each vacuum sublimation. Since the DHNs are expected to have a white color, it is thought that the dark color of these three compounds is due either to their oxidation or to the formation of oligomers. Even though there were undoubtedly significant concentrations of impurities in the samples of these compounds, it was found that the same fluorescence spectra were obtained both before and after vacuum sublimation and that time-resolved fluorescence spectra were also identical. Thus, it appears that either the impurities fluoresce very weakly or that the DHNs of interest have been selectively excited. Because of these considerations, the spectra which were obtained are thought to be identical with pure-compound spectra in all cases. All samples were deposited on a gold-plated copper surface, which was maintained at 15 K and which was mounted in the head of a closed-cycle helium cryostat (“Spectrim”, CTI Cryogenics), by using a Knudsen effusion vacuum sublimation apparatus which has been previously described (1, 23). Fluorescence spectra were obtained by using both broad-band (lamp) and narrow-band (laser) excitation sources. For continuum-source excitation, the output of a 2.5-kW mercury-xenon lamp was dispersed by a Schoeffel GM-250 grating monochromator (spectral band-pass = 7 nm FWHM). The fluorometer associated with this source is described elsewhere (23). Narrow-band excitation was achieved with a Molectron DL-14 dye laser which was pumped by a Molectron UV-24 nitrogen laser. In normal operation the dye laser produced pulses of ca. 6 ns FWHM and 0.02 nm bandwidth at a pulse repetition rate of 10 Hz. Dye laser output in the wavelength ranges 300-315,315-320, and 320-330 nm was obtained with rhodamine B, a mixture of (2.5 X M) rhodamine B and (6 X M) cresol violet perchlorate (CVP), and a mixture of (2.5 X W3M)rhodamine 6G and (3.3 X 10” M) CVP dyes, respectively, followed by frequency doubling with an angle-tuned ADP crystal. Following dye laser excitation,the fluorescence from the sample was filtered with an “32 polarizer (Polaroid), oriented with a
wavelength (nm) of maximum fluorescence in in argon perfluorohexane 319.9 326.4 333.8 332.9 327.0 328.6 333.7 322.5 322.7
318.6 325.5 332.0 331.5 326.2 327.8 332.7 320.7 322.3
polarization direction perpendicular to the laser polarization direction. The polarizer was useful for eliminating laser scatter into the emission monochromator, because the deposita were sufficiently thin that the laser scattering retained its polarization. In addition, it was found that significant improvements in the resolution of DHNs could be obtained when a polarizer was used in this manner, because the emission of longer wavelength vibronic bands is often polarized in a different direction than the 0-0 emission band (24, 25). The resulting fluorescence was dispersed by a 1-m grating monochromator (Jobin-Yvon HR-1000, equipped with an 1800 groove/mm holographic grating blazed at 300 nm) having a reciprocal linear dispersion of 0.8 nm/mm, and detected with an RCA 8850 photomultiplier. The photomultiplier signal was fed to a Tektronix 5440 cwcilloscope, equipped with a 5S14N dual-trace delayed-sweep sampling plug-in (rise time = 350 ps), prior to plotting by an X-Y recorder. Thus, time-resolved spectra could easily be obtained.
RESULTS AND DISCUSSION Qualitative Aspects of MI Site-Selection Spectroscopy. Because site-selection spectra can be solvent dependent (17, 19), careful consideration should be given to the choice of matrix for the analysis. In general, increasing guest-host interactions result in decreasing probabilities for a zero-phonon transition (14, 15, 17, 18,21,22). That is, electron-phonon coupling manifests itself in laser-induced fluorescence spectra by very weak zero-phonon peaks and relatively large phonon wings (14, 15, 17). The severity of this effect, which is deleterious to the analysis, can be quantitatively expressed by the Debye-Waller factor, which is merely the fraction of the total integrated emission intensity (14, 15, 17, 18, 26) or emission peak height (19) of the zero-phonon line relative to the phonon wing. For example, Cunningham et al. found that the Debye-Waller factor was 1.3 for 1-naphthol in an ethanol glass a t 10 K, indicating that, under these conditions, the zero-phonon line was just barely observable (19). Since hydrogen bonding between the 1-naphthol and ethanol was assumed to be responsible for the weak zero-phenon line (19), a more “inert” matrix material would seem highly desirable for the analysis of the DHNs. In an attempt to find a reasonable matrix material for a laser-induced MIF analysis of the DHNs, 1-naphthol, 2naphthol, and a few representative DHNs were deposited in the relatively “inert” matrices argon, nitrogen, heptane, and perfluorohexane (PHI. The latter two materials were also chosen in the hope of obtaining the quasi-linear fluorescence spectra characteristic of the “Shpol’skii effect” (2, 6, 8, 12, 27, 28) via broad-band excitation, although this latter hope did not materialize. The improved resolution in MIF spectra typically obtained by using laser excitation is illustrated in Figure 1, which compares the results of broad (lamp) and narrow (laser) band excitation for 2-naphthol and 2,3-DHN isolated in argon. A similar comparison is made in Figure 2 for 1-naphthol and
268
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981 2-
Naphthol In Argon
I-Naphthol
In Perfluoro-n-hexane
a
a
Dl
Hg-Xe Lamp
Laser
nm
312
312.0 n m
nrn
nrn
--
,
330
,
rio
370
320
2,3-DihVdrOXVnaPhthaiene
360
400
nm
rim
in Argon 2,7-Dihydroxynaphthalene a
,,
313 nrn
nrn
in
Laser
1.
314.1 nm
Hg-Xe Lamp
d
1
Laser
nrn
Figure 1. Matrix isolation fluorescence (MIF) spectra of 420 ng of 2-naphthol in argon with (a) lamp (A,, = 312 nm) and (b) dye laser (A,, = 312.2 nm) excitation and of 530 ng of 2,3diiydroxynaphthalne In argon wkh (c)lamp (Aexc = 313 nm) and (d) dye laser (A,, = 314.7 nm) excitation. 2,7-DHN in a P H matrix. The narrow emission lines (15-35 cm-’ FWHM for the 0-0 bands) for these compounds result from different vibronic transitions from the fiit excited singlet state to various vibrational levels of the ground electronic state. The congestion a t longer wavelengths than the 0 4 emission (lowest wavelength) band is apparently due both to overlapping bands from different (closely spaced) vibronic transitions, as will be discussed later, and to overlapping phonon wings from each emission band. Because the broad phonon wings overlap to a much greater extent than the narrow zero-phonon lines the contribution of the phonon wings to the emission spectra is much greater at longer wavelengths where many more vibronic transitions occur. This effect can be observed in Figures 1 and 2 for the laser-excited spectra but is much more pronounced when broad-band excitation is employed (since many more “sites” are excited). Similar spectra were also obtained in a nitrogen matrix, although the base-line resolution was not as good, presumably because of stronger electron-phonon coupling and, therefore, larger phonon wings. No zero-phonon bands were observed in an heptane matrix, indicating that the electron-phonon coupling was very strong (14, 15). A careful inspection of the site-selection spectra of any of the compounds in Table I is needed in order to observe a distinct phonon wing, indicating very weak electron-phonon coupling for DHNs in argon and P H media. This situation can be contrasted to the results of previous investigations of
(In,
Perf luoro-n-hexane
:
330
370
+
-c--c
410
3?‘a-
360
400
nm nm e 2. MIF spectra of 500 ng of l-naphthol in PH with (a)lamp (b,, = 312 nm) and (b) dye laser excitation and of 660 ng of 2,7dC hydroxynaphthalene In PH with (c) lamp (A,, = 308 nm) and (d) dye laser (A,, = 314.5 nm) excitation. m
site-selection spectroscopy (see, for example, ref 7, 11, 14-17, 19, 29-32) in frozen solutions, wherein relatively intense phonon wings could easily be observed, even at liquid-helium temperatures. In matrix isolation, virtually any material which exists as a gas or liquid a t room temperature can be used as a matrix material (2,3),allowing a much greater solvent selection than is available for conventional frozen-solution spectroscopy. For example, neither argon nor P H could be used for a frozen-solution analysis of the DHNs (the DHNs are insoluble in liquid PH). The greater selection of possible matrix materials is an important and heretofore unappreciated advantage for matrix isolation sample preparation, and in this case allows the analysis of the DHNs by fluorescence band narrowing without the use of liquid-helium temperatures. The narrow-band excitation dependence of the naphthalene hydroxyl derivatives is quite similar to the behavior of other organic molecules in low-temperature glasses, as originally discussed by Personov (14,15). No fluorescence line narrowing was observed for the DHNs except when the laser excitation frequency was within about 3000 cm-’ of the 0-0 absorption band. However, when the excitation wavenumber setting for
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
Nlne component Mixture In Perf luoro-n-hexane
1,5-DHN
in Argon
269
Nine component Mixture in Argon
314.7
I
nrn
I
I
i11 350
’
360
’
460
nm
Dye laser excited MIF spectra of (left) 730 ng of 1,5dihydroxynaphthalene in argon (A,, = 312.1 nm) and of an equimolar (ca.200 ppm) ninecomponent mixture of hydroxynaphthalenes in argon at excitation wavelengths of (mule) 312.1 nm and (right)314.7 nm. Note the similarity of the left and middle spectra and also of the spectrum on the right to Figure Id. These results demonstrate the selective excitation of fluorescence from 13- and 2,3dlhydroxynaphthalene in the nine-component mixture. Figure 4.
320
360
nm
400
320
360
400
nm
Dye laser excited fluorescence spectra of an equimolar (ca. 200 ppm) ninecomponent mixture of hydroxynaphthalenes (compounds listed In Table I) matrix isolated in PH with excitation wavelengths of (left) 314.5 nm and (right) 320.1 nm. The spectrum at right 1s the “worst” (in terms of spectral resolution)of any spectrum obtained for this mixture, yet a strong feature of one sample constituent (1,6dihydroxynaphthalene) Is clearly visible in the spectrum. Figure 3.
the laser was within 2000 cm-’ of the 0-0 absorption band, additional changes in the laser frequency of less than ca. 300 cm-’ (the bandwidth due to inhomogeneous broadening) produced identical shifts in the emission band positions. This phenomenon is characteristic of site-selection spectroscopy and can be regarded as evidence that the line-narrowing effect in Figures 1 and 2 is indeed due to reduction of inhomogeneous broadening (14, 15, 17, 18). Another effect which was usually observed upon laser excitation into low-energy vibronic bands was the appearance of numerous additional sharp bands in the fluorescence spectra. As discussed in detail by Personov et al., when the extent of inhomogeneous broadening is large compared with the spacings of vibronic levels in the first excited singlet state, complex “multiplets” appear in the laser-induced fluorescence spectrum, with the (energy) distances from the laser line to individual “multiplet” components corresponding to the vibrational frequencies of the guest molecule in the excited electronic state (29,30). The appearance of this vibrational fine structure is detrimental to analytical applications because of the undesirable complexity of the resulting site-selection spectra. Thus, the “best” wavelength for excitation should avoid this effect as much as possible. Although this problem could be entirely avoided by direct excitation into the 0-0 absorption band (29,30),we were often limited to excitation at shorter wavelengths because of very low laser power at the appropriate M) absorption frequencies. The “best” excitation wavelengths listed in Table I (obtained for pure samples of the compounds) were observed to minimize the amount of fine structure in the fluorescence spectrum, to result in maximum base-line resolution (i.e., minimal contribution from phonon wings), and (within these constraints) to produce maximal fluorescence intensity. In a real analytical situation, however, the optimum wavelength for a given compound may differ from that in Table I, as discussed below. The “optimum” emission wavelength for each compound in Table I is simply the most intense fluorescence band observed when excitation at the wavelength specified in Table I is employed. However, in multicomponent samples, band overlaps might necessitate use of another wavelength as the analytical wavelength for a particular compound. A complex mixture, nearly equimolar (ca. 200 ppm) in each
compound in Table I, was prepared in order to demonstrate the analytical utility of laser-induced MIF techniques for the analysis of polar PAH derivatives. Laser excitation was employed a t each wavelength in Table I for the appropriate matrix material. In Figure 3, a typical and a “worst case” result are shown. Even in the “wont case” result, the 0 4band of 1,6-DHN is clearly distinguishable, allowing a determination of this compound. In some instances, it was possible to selectively excite one component from the mixture and to obtain a fluorescence spectrum very similar in appearance to the corresponding pure compound spectrum. This effect is demonstrated in Figure 4, wherein the pure compound (left) and mixture (middle) spectra for 1,BDHN in argon (at 312.2-nm excitation) are compared. (An anticipated interference from 2-naphthol is not significant because it absorbs much less strongly than 1,8DHN at 312.2 nm). Likewise, the right-hand side of Figure 4 (314.7-nm excitation) should be compared with Figure Id. The latter comparison demonstrates that, in argon, 2,3-DHN has been selectively excited in the nine-component mixture. The resolution attainable for a given DHN depends, to some extent, on whether a fluorocarbon or argon matrix is used. However, the differences are slight and the results presented could have been attained by use of only one matrix. Timeresolution techniques were also found to be useful for resolving some DHNs in the synthetic mixture, although time-resolution capabilities are not necessary to obtain these results. Rather, it appears that the use of different matrices, as well as of time-resolution techniques, is quite complementary to the site-selection and selective excitation techniques that have been exploited for the DHNs and may not be needed except for exceedingly difficult cases. The “optimum” excitation wavelength for any particular compound may depend upon the composition of the sample. In real samples (such as synthetic fuels), the analyte may occur in the presence of a much larger concentration of an interferent. For example, pure 2-naphthol and 1,5-DHN have similar “optimum” excitation wavelengths (Table I), and an excess of either compound therefore would interfere in the determination of the other. In such a situation, the “optimum” excitation wavelength obviously would be that which produced the least interference by the undesired compound. Excitation at either 322.3 or 324.0 nm is sufficient to determine 1,5-DHN in the presence of a large excess of 2-naphthol (which does not absorb appreciably a t either
270
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
wavelength); similarly, excitation at 317.3 nm could be used to determine 2-naphthol in the presence of excess 1,5-DHN. Likewise, the optimum wavelength for determination of 1,6DHN in a mixture (the “worst case” mentioned above) depends upon which other hydroxynaphthalenes are present in greatest quantities. Such choices (necessitating sensitivityselectivity tradeoffs) must be made empirically after the identity of the interferent in question has been established. An alternative procedure to minimize interferences would be to perform the spectroscopic measurements at liquid-helium temperatures (not employed in this work). Because the Debye-Waller factor is known to increase with decreasing temperature ( 1 4 , 2 1 , 2 2 ) ,lower temperatures would produce increased intensities of the zero-phonon lines in Figure 3 at the expense of the broad phonon wings which limit the spectral resolution at 15 K. Lowered temperatures may also have the desirable effect of decreasing the observed zero-phonon bandwidths. Such line narrowing can be expected if pure vibrational dephasing (Ti process), caused by large amplitude elastic collisions between host molecules and quasi-localized or resonance phonon modes, is responsible for the 15-35-cm-’ FWHM homogeneously broadened absorption bands, because lowered temperatures reduce the phonon occupation number and density of states and, hence, the pure vibrational dephasing rate (33). On the other hand, if the observed bandwidths are primarily due to the vibrational relaxation (T, process) of the absorbing vibronic state, little reduction in the homogeneously broadened absorption line widths would be expected with decreasing temperature, since T1processes are generally considered to be temperature independent at low temperatures (33). In any event, liquid-helium experiments are costly and inconvenient, relative to use of closed-cycle refrigerators, and use of the latter is generally preferred in analytical spectroscopic measurements whenever feasible. Quantitative MI Site-Selection Fluorometry. Standard matrix isolation procedures were used to obtain quantitative results for 2,3-DHN in argon and for 2,7-DHN in PH, using benzo[a]pyrene (BaP) as the internal standard (I, 2). For each determination, two separate measurements were made of the peak heights of the 0-0 band of the internal standard (400.0 nm for BaP in argon and 400.9 nm for BaP in PH) and of the analyte (322.5 nm for 2,3-DHN in argon and 322.3 nm for 2,7-DHN in PH). The two measurements were then averaged in order to reduce the effect of a 10% variation in the laser pulse-to-pulse reproducibility. By use of the appropriate excitation wavelengths in Table I, the linearity of the fluorescence was found to extend from the detection limit (as defined in ref 2) of ca. 5 ng to greater than 5 pg (i.e., 3 decades of linearity) for both compounds. The precision of the measurements was estimated by five separate determinations of one point on the analytical calibration curve (451 ng of 2,7-DHN and 2.35 pg of 2,3-DHN). The relative standard deviations for these determinations were ca. 7% for 2,3-DHN in argon and 4% for 2,7-DHN in PH. The detection limits (ca. 5 ng) for the DHNs are significantly higher (about 1 order of magnitude) than for most PAHs when our dye laser was used for excitation, because of the low molar absorptivities (10&1000 L mol-’ cm-’ for the longest wavelength ‘A ‘Lb absorption band) of naphthalene and its derivatives (31). Excitation into higher energy electronic states, such as the ‘A ‘Bb transition (at about 220 nm), could presumably decrease the detection limits by a factor of as much as 500, because of the higher molar absorptivity (ca. lo5 L mol-’ cm-’) (31),if a suitable excitation source could be found. This situation is, in fact, characteristic of site-selection fluorometry of most PAHs and derivatives. That is, sensitivity must be sacrificed in order to obtain improved spectral resolution, because the molar absorptivities
-
-
(for many PAHs) of the longest wavelength absorption band are usually at least 1order of magnitude lower than those of other accessible higher energy bands 31). In any case, because the molar absorptivity for the longest wavelength absorption band is significantly lower (1 or 2 orders of magnitude) for naphthalene and its derivatives than for many other PAHs, significantly lower detection limits can be expected for polar derivatives of other PAHs. Another reason for the relatively high detection limits for the DHNs is that only a fraction of the DHN molecules can be excited by the laser pulses. Because the molecules occupy different “sites”, only those molecules having an absorption band which overlaps the laser line will fluoresce. For 1naphthol in PH, which has homogeneous and inhomogeneous absorption bandwidths (for the 333.8-nm peak) of ca. 20 and 300 cm-’, respectively, the fraction of molecules which could be excited in a site-selection experiment cannot possibly exceed 20/300 = 0.07 (i.e., only 7% maximum excitation efficiency), irrespective of the laser bandwidth. In contrast, use of a broad-band source having a bandwidth equal to the inhomogeneous absorption bandwidth (in this case, 300 cm-’) would provide an upper limit of 100% excitation efficiency. Obviously, such an experiment would forfeit the spectral resolution produced by site selection. Likewise, in any situation in which the homogeneously broadened bandwidth approaches that due to inhomogeneous broadening, the excitation efficiency approaches 10070, irrespective of the source bandwidth; in such a situation, the term “site selection” ceases to have meaning. Despite the inherent limits in sensitivity of the site-selection technique, the detection limits obtained for DHNs by using our present dye laser were approximately a factor of 10 lower than those obtained by use of a 2.5-kW mercury-xenon arc lamp source. In addition, a significant improvement in the detection limits would be effected by use of a laser capable of higher frequency doubled peak power in the UV (e.g., NdYAG-pumped dye laser). Nevertheless, i t is obvious that siteselection techniques are of greatest interest for complex samples where selectivity, rather than detectivity, represents the principal analytical challenge. While perfluorohexane is a highly unconventional matrix for matrix-isolation spectroscopy, only two disadvangates in using this material as a matrix were noted. First, the detection of fluorescence from the DHNs was limited both by background fluorescence and by laser scattering (Rayleigh and Brillouin) from the translucent deposits. Second, the DHNs underwent photodecomposition to a significantly greater extent in P H than in argon (wherein some DHNs exhibited no evidence of this effect). In the worst cases, the intensity of the 0-0band would decrease irreversibly by about 60% of the initial intensity after continuous irradiation of the sample for 20 min. The origin of this phenomenon is not well understood, but it has also been observed for other molecules (2, 14,32, 34-38). For perylene and 9-aminoacridine in ethanol, it has been suggested that reorientation of solute molecules in the matrix following electronic excitation is responsible (37). An alternative (nonphotochemical) model for this phenomenon assumes a double-well potential for the interaction between host and matrix molecules with a barrier height which is large enough to prevent site interconversion for molecules in the ground electronic state but low enough for molecules in an excited electronic state to undergo site interconversion via a thermally activated or phonon-assisted tunneling mechanism (38). Because “hole refilling” experiments did not restore the “lost” fluorescence intensity, this mechanism is not likely to be primarily responsible for the large and irreversible loss of fluorescence that has been observed. For quantitative work we minimized this effect by making relatively rapid measurements of fluorescence. Alternatively, lower laser power
ANALYTICAL CHEMISTRY, VOL. 53, NO. 2, FEBRUARY 1981
DlbenzIa,jlacrldlne
in Argon
H9-XB
Lamp
I
nm
1
I I1 I 1
I?\
280
I
c--
3SO
410
nm
430
390
Laser
317.8
410
nm
430
nm
Flgure 5. MIF spectra of 175 ng of dibenz[a,/]acrMlne in argon with (left) lamp excitation.
(A,,
= 298 nm) and (right) dye laser A(,,
=
377.8 nm)
can be employed, although at an obvious cost in detectivity. The lamp and laser-excited fluorescence spectra in Figure 5 of dibenz[aj]acridine in an argon matrix suggest that the site selection techniques employed for the characterization of mixtures of hydroxynaphthalenes can be readily extended to other classes of (fluorescing) polar aromatic compounds, such as nitrogen heterocycles. As before, much greater spectral resolution can be achieved with laser excitation and an argon (or fluorocarbon) matrix. In this case, the (250-cm-’ FWHM) bandwidth, due to inhomogeneous broadening, has been reduced to 20 cm-‘ by laser excitation at 377.8 nm. In conclusion, a highly effective analytical methodology, which does not require the use of liquid-helium temperatures, has been developed for the analysis of polar aromatic compounds. I t has been demonstrated that the key requirements for this method are an argon (or fluorocarbon) matrix, conventional matrix isolation sample preparation techniques, and (tunable) narrow-band excitation.
LITERATURE CITED Wehry, E. L.; Mamantov. G. Anal. Chem. 1979, 57, 643A. Maple, J. R.; Wehry, E. L.; Mamantov, G. Anal. Chem. 1980, 52, 920. Wehry, E. L.; Mamantov, G.; Hembree, D. M.; Maple, J. R. Paper presented at the Fourth International Symposium on Polynuclear Aromatic Hydrocarbons, Columbus, OH, Oct 4, 1979. Woo, C. S.; D’Sliva, A. P.; Fassel, V. A.; Oestrelch, G. J. Environ. Sci. Technol. 1978, 72, 173.
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RECEIVED for review July 14, 1980. Accepted November 12, 1980. This research was supported in part by National Science Foundation Research Grant CHE77-12542.
