Laser-excited Shpol'skii spectrometry - Analytical ... - ACS Publications

Instrumentation

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Arthur P. D’Silva Velmer A. Fassel Ames Laboratory and Department of Chemistry Iowa State University A M s , Iowa 50011

$hp$lbkii Spectrometry

Although comparisons of the mutagenic or carcinogenic potency of various chemical substances are subject to considerable uncertainty, there is little doubt that selected compounds in the polynuclear aromatic hydrocarbon (PAH) series are among the most potent known. For example, the higher than normal incidence of cancer among chimney sweeps and workers in the coal tar, creosote, coal gas, coke, and cutting oil industries, where exposure to PAHs is localized, has been extensively documented in the past ( I ) . In the years ahead, man seems destined to experience an increasing burden of exposure to low concentrations of PAHs from more extensive and diffuse sources, e.&, aerosol emissions from the stacks of coal-fired utility boilers or fugitive emissions from coal gasification or liquefaction and from oil shale recovery operations. The assessment of the potential health effects of this increased environmental loading by the PAHs is rendered difficult by the following: the large range of potency found among structural isomers of compounds with the same number of condensed rings and among the various alkylated derivatives (2,3);the enhancement of carcinogenicity through synergistic effects (4); and the absence of reliable data bases. These three factors impose several stringent requirements on ana0003-2700/84/0351-985AS01.50/0

@ 1984 American Chemical Society

l

1

lytiral methodologies for PAHs. especially if a comprehensive health effects assessment is the goal. Among these requirements are adequate powers of detertion. the capability to distinguish between structural isomers and alkylated species of the same parent rompound. acceptable sample throughput, and acceptable accuracy. During the past decade, there have been significant advances in developing analytical methodologies addreJsed to meeting these needs. These methodologies are usually based on lirit isolating the PAHs as a com. pound class, followed by isolation of the individual PAHs or their structur. al isomers or alkylated derivatives. Techniques used include thin-layer rhromatography. capillary column gas chromatography with flame ionization detertion or mass spectrosropic characterization, gas-liquid chromatography using nematic liquid crystal columns, and high-performance liquid chromatography with fluorescence detection. A critical test for any of these techniques is their capability of achieving effirient and adequate resolution of individual, high-potency. structural isomers and alkylated species at the concentration ratios found in environmental samples. For many of these samples, staw-of-theart methodologies do not satisfy all of the requirements stated above. As a

ANALYTICAL CHEMISTRY, VOL. 56, NO. 8. JULY 1984

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3wndensad Ammat +Heplane &2-4-a

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Naphthalene

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n-Nonane & d + 4 %

Anthracene

Naphtha-

Pericondensad Aromatics yrene

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Hexane

Heptm

Coronei

n-o*ane

Figure 1. Geometric relationships of walkane and PAH molecules, which attempt to rationalize the Shpol'skii "key and hole" rule

a

--__

Figure 2. Schematic representation of the orientation of (a) paraffin chains in an +heptane single crystal and (b) substitution of benzo[a]pyrene for n-heptane molecules in a lamellar plane The labeling a1 i bcoordinates follows conventions indicated in Reference 25. The planar +heptane mOleculeS occur as parallel rig-zag chains. With the chain axis making a 71' angle with the crystal growth axis. a

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ANALYTICAL CHEMISTRY. VOL. 56. NO. 8. JULY 1984

consequence, there is a continuing interest in alternative approaches, especially those that would allow the direct determination of specific high-potency species without prior isolation of the individual compounds. Luminescence Spectrometry of PAH Compounds Although the characteristic ultraviolet and laser-excited optical luminescence of PAHs has been advocated and utilized for the sensitive detection of PAH compounds (5-7),the fluorescence and phosphorescence spectra of these compounds are quite broad a t room temperature, having full widths a t half-maximum (FWHM) of the order of 300 em-'. As a result, ambient temperature luminescence techniques have been utilized only to a limited extent for the analysis of complex mixtures of PAHs (6).To overcome this limitation, the sharpening of the luminescence spectra observed when these molecules are incorporated in appropriate matrices and solidified at low temperatures has been utilized in the development of several ultraviolet, X-ray, and laser-excited techniques. These techniques have been based on the following: the Shpol'skii effect in the frozen solution mode (814) and the vapor-phase, matrix isolation mode (15,16); matrix isolation (MI) ( 17,281;and fluorescence line narrowing spectrometry (FLNS) in organic glasses (19-22). All of these techniques have been utilized for the analysis of mixtures of PAH compounds. In this paper we will he concerned with the conventional Shpol'skii effect observed in frozen solution matrices. The Shpol'skii Effect To understand the Shpol'skii effect, it is instructive to perform the following simple experiments. Let us rapidly freeze a dilute solution of benzo[a]pyrene (B[a]P) in ethanol to temperatures of -10 to 15 K. The absorption and luminescence spectra of the analyte B[a]P in this frozen solid would consist of broad hands having widths of -500 cm-I (FWHM). If the experiment is repeated with an n-heptane solvent, the spectra of B[o]P are observed to be quasilinear (linelike) with bandwidths of -10 cm-' (FWHM). This dramatic sharpening of the spectra of the dissolved PAH is a direct manifestation of the Shpol'skii effect. Historically, the physical interpretation of the Shpol'skii effect has invariably been based on some dimensional end geometrical correlations between the aromatic solute molecules and the n-alkane hosts. These correlations are readily visualized for catacondensed (linearly fused benzene rings) aromat-

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Figure 3. Absorptionspectrumof dibenzofuran in n-hexane at 4 K ic molecules, as shown in the top half of Figure 1.A match in the linear dimensions, geometric tit, and bond angles for these solute-solvent or impurity-host systems is clearly apparent.

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For pericondensed (nonlinearly fused benzene rings) molecules, these similarities are not &s obvious, as can be seen in the lower half of Figure 1.The solvents indicated are those that have

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been experimentally determined to give quasilinear spectra. Because of the strong correlation of development of the Shpol’skii effect with dimensional and geometric similarities between the solvent host and solute molecules, the explanations of the Shpol’skii effect generally have been based on either the “lock and key” principle (22) or the Shpol’skii “key and hole” rule (23).For either of these the implication of a highly structured orientation of the solute molecules in the solid host solvent has been quite clear. In the n-heptane solvent we are considering,the frozen matrix will consist of microcrystallites in which the paraffin chains are arranged in lamellar planes, as illustrated in Figure 2 for an n-heptane crystal (24). At first glance it would be assumed that the B[a]P molecules would be randomly dispersed among the lamellar planes of the solvent microcrystallites. But this is not so. The suhstitutional solid solution formed on rapid cooling results in the B[a]P molecule replacing two or more solvent molecules with the long axis of the solute molecule parallel to the length of the n-heptane chain, as has been predicted for coronene and pyrene (25,26). In Figure 2 the suhstituted n-heptane chains are shown as dashed lines in the left Lamellar plane. The orientation of all the impurity molecules in the solid solution will not be identical. In fact, substitution in a limited number of perhaps two to four crystallographically different sites is observed. For dibenzothiophenein n-heptane, a6 many as 40 sites have been observed, but only a few are prominent (27). The important consideration is that all of the PAH molecules that occupy specific types of crystallographic sites are in strictly oriented positions, experience identical molecular fields, and behave as isolated molecules. The vibronic components should naturally be sharp, and because the spectra are observed a t 15K or less, thermal broadening is minimized. As a consequence, the characteristic absorption or luminescence spectra will be quasilinear in character, having an FWHM of approximately 10 cm-I or less. The preceding three sentences constitute an exDerimenta1definition of the Shpoi’skii effect. The typical Shuol’skii effect ahsorption spectrum shown in Figure 3 for dibenzofuran in n-hexane illustrates the sharpness of the spectral linen that can be obtained 128). Although this effect has been observed for hundreds of solute-solvent comhinations, only certain solute-solvent combinations provide ideal development of the effect. as would he predicted by the necessity of establishing a geometricldimensional match. A

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ANALYTICAL CHEMISTRY, VOL. 56. NO. 8, JULY 1984

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Figure 4. Schematic diagram of the experimental apparatus utilized for the observation of lasersxcited Shpol’skii spectroscopy tabulation listing the optimum solventk) for observing the quasiline spectra of more than 20 compounds has been published (29). For a solubsolvent system where multiple sites are observed, each site provides slightly shifted subspectra. The composite spectra or multiplets sitnultaneously observed from all the sites formed is often a unique signature of a given solutRsolvent system. Such a signature can often be utilized to detect the presence of a compound in a complex mixture. For analytical purposes the relative intensities of the multiplets should be reproducible from sample to sample. That this can be accomplished has been documented (30). Although the means are now available to obtain sharp line absorption or fluorescencespectra of the PAHs, a complex mixture of several PAHs or their derivatives, occupying several types of crystalline sites in the Shpol’skii host, will still present the analyst with hopeless spectral overlaps. An additional refinement must be added, and this is the challenging task of selectively exciting only those molecules of a specific compound occupying the same crystallographic site in the host. As indicated earlier, the narrow ahsorption handwidths (FWHM) of -10 cm-1 or less for PAHs in n-alkane solvents are a unique feature of the Shpol’skii effect. These narrow absorption bandwidths allow the selective “tuning in or out” of specific site spectra. Thus, site-specific excitation of a given PAH species present in a complex mixture can be obtained through the utilization of SSOA

narrow-bandwidth tunahle dye laser excitation. An additional advantage of high-intensity lasers is the enhanced detection sensitivity that can he achieved, an approach that has been suggested in the past (31). instrumentation A schematic diagram of the experimental arrangement is shown in Figure 4. A XeCl excimer laser operating at 308 nm and with an approximately 10-ns pulse width was utilized to pump the tunable dye laser in the fundamental tuning range of 33L970 nm. For a KIF excimer pump laser (248 nm), tunable dye laser output in the 311-385-11111 region could be obtained. The dye laser bandwidth was approximately 0.01-0.03 nm. To obtain tunable laser output in the 217365-nm region, a second harmonic generation (SHG) system, which consists of a doubler (crystal assembly) and an auto-tracker was utilized. The autotracker sensed the UV output of an angle-tuned SHG crystal and adjusted to the phase match angle so as to maximum conversion from the visible into the ultraviolet. The active feedback design of the system accommodated crystal temperature changes produced either by ambient or laser-induced heating. The SHG system was required to excite the fluorescenceof compounds such as carbazole, dibenzothiophene, and dibenzofuran, and one-, two-, and three-ring PAH compounds (e.g., benzene, naphthalene, phenanthrene), and their multialkylated homologues, which have their SI states a t energies greater than 31,000 em-’.

ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JVLY 1984

When the SHG system was required, the dye laser output was diverted to pass through the doubler by a system of quartz prisms, as shown in Figure 4. The two quartz prisms positioned inside the dye laser were mounted on a stage that could be reproducibly moved in and out of the optical path of the dye laser output. The dye laser or SHG output beam was focused onto the sample contained in a multiple cell holder. Each cell of the multiple cell holder was designed to be loaded with samples of approximately 20 pL and could be rapidly attached to the cold finger of a closed-cycle helium refrigerator normally operated to cool the samples to 15 K. The refrigerator assembly was designed to he reproducibly raised or lowered to position anv one of the four cells in the OPtical path. The luminescence eenerated in the sample was focused by a fused-silica lens onto the entrance slit of a 0.64-m monochromator equipped with two interchangeable detectors, a photomultiplier (PMT), and a photodiode array detector (PAD). Data acquisition with the PMT was through a laboratoryfahricated gated-detection system, which provided the output to an X-Y recorder. Signals from the PAD were processed through an optical multichannel analyzer (OMA). The experimental facilities and operating conditions utilized have been tabulated elsewhere (32). Procedures

To assure reproducible site population from sample to sample it is essential to reproduce the freezing-cooling cycle. Our procedure for reproducing these cycles was to immerse the cell holder containing the samples in liquid Nz,which achieved solidification in 10to 15 s. At this stage the cell holder was rapidly attached to the cold finger maintained a t 100 K. Cooldown from 100 K to 15K was completed in -10 min. Confirmation of reproducible site population for this cooldown procedure has been presented (30). For the analysis of liquid samples, the latter were diluted with an appropriate n-alkane solvent by factors varying from 102 to 104 with frequent agitation by a mechanical shaker. If insoluble sediments remained, they were allowed to settle to the bottom of the container overnight. The insoluble sediments were not isolated from the sample before a clear aliquot of the sample was removed for subsequent injection into the cell. Site-Selective Excitation Historically, the first demonstration of site-selective excitation of the lumi-

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Figure 5. Site-selective excitation of the fluorescence of 1i-methyl-benz[a]anthracene in moctane nescence of pyrene, B[a]P, and coronene, narrow-band excitation was obtained by dispersing xenon lamp radiation through a spectrometer (33). Although the bandwidth of this radiation was far too broad for effective site-selective excitation, such excitation was observed for coronene, pyrene, and B[o]P. The site-selective excitation of coronene was later confirmed with single-frequency argonion laser radiation having a bandwidth of less than one cm-1(34). The fortuitous coincidence of the 0-0 absorption line of the SO-Sp transition in the long-wavelength subspectrum of coronene a t 350.7 nm and the argon-ion 992A

laser emission a t 351.1 nm made this demonstration possible. Similar observations were reported for perylene (35)and phenazine (36). The capability of only exciting the fluorescence of molecules trapped in specific crystallographic sites via the use of tunable laser radiation is illustrated in Figure 5. In this fgure, A represents the nonselective excitation a t 364.6-nm of 11-methyl benz[a]anthracene (ll-M-B[a]A). Nonselective excitation into the congested region of the upper vibronic manifold of the excited state leads to spectral emission of ll-M-B[a]A molecules occupying at least three different crystallographic

ANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984

sites. The wavelengths of 0-0transitions from molecules of ll-M-B[a]A occupying these sites are a t 384.8, 385.2, and 386.0 nm and are identified in the figure as sites 1, 2, and 3, respectively. If the spectrometer is now set to monitor the emission a t 384.8 nm while the tunable dye laser output is scanned through the wavelength intervals shown for spectrum B , the excitation spectrum of molecules occupying site 1may he obtained. The resulting excitation spectrum (Figure 5B') clearly shows that the best wavelength to excite fluorescence emission of ll-M-B[a]A molecules occupying site 1is 374.8 nm, i.e., a t the wavelength of the most intense peak in Figure 5B'. If the laser radiation is now tuned to 374.8 nm, the fluorescence spectrum of site 1molecules as shown in Figure 5B may he recorded. In an analogous fashion, the best laser wavelengths for exciting analyte molecules that occupy cbrystallographic sites 2 and 3 were determined to be 375.1 and 375.8 nm, respectively (the most intense peaks in Figures 5C' and D'). The resulting fluorescence spectra from these sites are shown in Figures 5C and 5D. The cauabilitv of exciting these site-specific speetra not oniy enhances specificity but also provides added flexibility in the event that spectral interferences by other sample constituents invalidate the use of one sitespecific line. Should such interferences occur, other interference-free, site-specific lines may usually be selected. The high degree of selectivity achieved by this approach has been demonstrated by several examples cited in our earlier publication (30). The consequences of the above observations to the direct analysis of complex mixtures of organic compounds present in samples such as coal liquids, shale oil, and organic extracts from particulate samples have been documented (37,38). A few examples are also provided herein. Selective Detection of PAHs in Complex Samples An elegant demonstration of the selective excitation inherent in laser; excited Shpol'skii spectrometry (LESS)is the site-specific, compoundselective excitation of the individual components of a mixture consisting of benz[a]anthracene and 11alkylated derivatives in the in n-octane hostsolvent system. At the top of Figure 6 is a schematic representation of the multiplet structure of the 0-0 transitions of a mixture of benz[a]anthra. cene (B[a]A) and 11alkylated and multialkylated isomers of B[a]A. The compounds known to be mutagenic are identified by an asterisk.

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993 A

Alkylated Benzlelanthracene Mixture in n-octane

Flgure 6. Spectral positions of the 0-0 multiplets (top) and site-specific laser-excited fluorescence spectra of individual compounds In a mixture of B[a]A and alkylated benz[a]anthracenes The excltatiw wavelengths for the spectra shown we as follows: A. 376.9nm;8,373.6nm:C,374.7 nm:D,375.0nm:E.375.4nm:F.372.4nm:G,380.0nm:H.381.4nm:1,389.7nm:J.384.6nm.The compourds known Io be mutagenic are indicated by asterisks.

The site multiplets for the 12 compounds studied (top of figure) cover a spectral region of -20 nm, with each compound exhibiting doublet, triplet, or auartet structures. The waveleneth interval covered by the 0-0 transit& from molecules in nonequivalent sites can be as narrow as 0.3 nm (2-MB[a]A) or as wide a t 1.8 nm (7-MB b l A). From the figure it is clear that the proximity of the multiplet structures of several of the compounds may re994A

sult in spectral interferences when the mixture is excited by broadband UV excitation. In the “worst case” example, it is seen that the multiplets of 6,8,12-TM-B[alAare subject to spectral interferences from the multiplets of 7-M-B[a]A and lZ-M-B[a]A. Thus, an ultrahigh resolution spectrometer would be required to resolve the fluorescence of 6,8,12-TM-B[a]Ain the presence of the other two compounds when nonselective excitation is utilized.

ANALYTICAL CHEMISTRY, VOL. 56. NO.

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With site-specific excitation, the spectral signatures of the compounds in the mixture are substantially simplified, as shown by the spectra labeled A 4 in Figure 6. The site-specific excitation wavelengths are tabulated in the figure caption. Spectra of more than one compound are observed simultaneously in Figures 6B, I, and J. In Figure 6B it is seen that the site-specific excitation of 9-M-B[a]A also leads to the simultaneous excitation of fluorescencefrom all four sites of 5-M-B[a]A. No attempt was made to obtain site-specific fluorescenceof 5-M-B[a]Abecause the multiplets of the compound are not subject to spectral interferences from the multiplets of any other compound in the mixture. The complex spectra shown in I and J are the manifestations of a less-than-optimum choice in excitation wavelengths; the most selective excitation wavelengths for the compounds involved were located outside the laser output of the dyes used in this study. Even so, the three compounds involved, 7,lZ-DMB[a]A; 6,7,8-TM-B[a]A,and 5,7,12TM-B[a]A, could be readily identified because the multiplets of the individual compounds were not subject to spectral interferences. The fluorescencespectra shown in Figure 6 provide for the unequivocal identification and potential quantification of the individual components of complex mixtures of alkylated benz[a]anthracenes. In a similar fashion unequivocal identification of individual alkylated and multialkylated phenanthrenes present in a seven-component mixture has been achieved (39). To demonstrate the direct detection of selected PAHs in very complex mixtures, the site-selective luminescence of four individual PAHs in a solvent-refined coal liquid sample are shown in Figure 7. The spectra were obtained after a 1000-fold dilution of the sample in n-octane. The selected excitation wavelengths are shown in the figure. It should he noted that the excitation wavelength selected for B[a]P and benzo[k]fluoranthene (B[k]F)caused weak emission from B[k]F and B[a]P, respectively, because of partial overlap of primary absorption lines of these two compounds a t the excitation wavelength. However, the emission lines are sufficiently resolved to permit quantitative determination of these compounds in mixtures. Ouanlitative Analysis Because of the complex composition of many PAH-containing samples, “inner filter” and enhancement effects may bias the analytical resulta. The bias that may be occasioned by these effects can usually be eliminated

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Flgure 7. Site-specific, laser-axcited fluorescence spectra of pyrene, 4-methylr . rene (CMP), B[a]P. and B[k]F in a solvent-refined coal liquid (SRC11) sample

through either the “standard addition” or the “internal reference’’ approach. Although the “standard addition” approach has been shown to provide accurate analytical data (14), the necessity of establishing an analytical curve for each analyte and for each sample renders this approach impractical for routine analysis. Thus, the

“internal reference” approach is preferahle. As reference compounds, the deuterated analogues of the analyte possess several very desirable characteristics. These characteristics are spectroscopic properties that are very similar to those of the analyte, ensuring a high degree of internal compensation of

n Flgure 8. Fluorescence spectrum of B[a]Pd12 in IMCtane. hex= 379.5 nm, superimposed on the reference spectrum of B[a]P, he, = 380.2nm 9S6A


emission variations associated with intermolecular interaction, and inner filter effects; proximity in wavelength of the isotopically shifted internal reference line to the analyte line; availahility of the compound in high purity with respect to the analyte being determined; and unlikelihood of occurring as a sample constituent. Operationally, both components of the analyteheference line pair should be excitable by the same excitation wavelength. The line pair components should also be free from self-absorption effects and from spectral interference by other constituents in the sample. It has been possible to achieve these conditions to a high degree (32). An example of the isotopic shift observed in quasilinear Shpol’skii effect spectra is shown in Figure 8 for B[a]T and B[a]P-dl*.In thisease,the h t o p ic shift was -1 nm (-55 em-’) for the 0-0 line. The appearance of these isotopically substituted spectra when excited at 380.2 nm in a shale oil sample is shown in Figure 9.Spectral line shifts resulting from isotopically suhstituted solutes have been reported for several Shpol’skii systems ( 4 M 3 ) . The internal reference line pair intensity values typically have responded linearly to analyte concentrations covering three to four orders of magnitude. The addition of deuterated analogues in constant amounts to each sample solution and the measurement of the analytelreference compound luminescence intensity ratios have effectively compensated for quenching or enhancement effects (32,381. The analytical results for B[a]P and several other PAHs obtained hy the internal reference approach and those reported by other investigators who

Flgure 9. Selectively excited fluwescence spectrum of B[a]P in a shale oil sample with 10 ppb B[a]Pdra added as the Internal reference

me two spectrs w

e 61rnuIlBnBously excited at 380.2 nm

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methodologies can be considered excellent in terms of present capabilities. It is evident that the LESS approach used in the internal reference mode provides for the direct determination of selected P A H s in complex mixtures in a relatively rapid and simple fashion, without requiring prior separations in most applications. In contrast t o other methodologies, only a sample dilution step, the addition o f the internal reference compound, and rapid cooling t o 15 K are required prior t o spectroscopic observations.

Table 1. Concentrations of PAH in Liquid Samples ( p g / g ) as Determined by LESS and Other Techniques a B1P .

Sample

Coalderued fuel oil Shale derived diesel oil

Pswlsne

BlhIF

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LESS data are underlined Reference Y B ueo are n parsnlhes~r and tnose macaled with a slash 111 were obla ned oy d llerenl analytica lechn que% as descrmed m the app~opriale a

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Relerence 38 Relerence 44. OReterenca 32. Reference 45, 'Relerence 46. 0 hBS data

PAHs

Benzo[a]pyrene

Coronene

N-Heterocyclics

@ & Ilsoquinoiine H

Carbazole

o-Phenanthroline

S-Heterocyclics

OHeterocyciics

Oibenzothiovhene

Dibenzofuran

Diphenyipoiyenes = Wn%

@-(CH

n

= 1-7

Scope of Application employed more classical analytical methodologies are compared in Table I. T h e analytical methodologies employed for obtaining the reference values have generally involved extensive and time-consuming fractionation and separation procedures. T h e level of agreement in the data obtained by the internal reference approach and those obtained b y other

Summary of Recent Studles on Shpol'skll Effect Spectra of Unusual Compounds CompoundlHosl Comments Hexahydrohexaheiicene (HHHL) in The nonplanar HHHL molecule has two 1.2*pentane at 77 K substituted naphthalene units linked by two hydrogenatedrings. The compound, dissolved in *pentane to satisfy the Shpol'skii key-and-hole rule, gives rise to quasiline phosphorescence spectra (50). 3-Methyl-lumiflavinand chlorophyilides in ndecane at 4.2 K

Derivatizationby the addition of an alkyl "tail" matching the linear dimensionof the mdecane host providedthe anchor to "lock' the molecule in an oriented manner in the host (51, 52)

Halogenated derivatives of anthracene in *hexane at 12 K.

Laser-induced, site-selected fluorescence of various compounds observed (53).Conventional fluorescence studies are indicated in Reference 49.

Pericondensed thiophenes in R hexane at 63 K

Ultravioietexcited fluorescence and phosphwescence spectra observed. The compounds were detected in fractions isolated by HPLC from a carbon black extract (54. 55).

Amino- and hydroxy-substituted anthraquinones in *heptane at 12 K

Laser-induced excitation and emission spectra of parent and deuterated compounds were obtained (56. 57). Observations on amino compounds are significant, as such compounds are not known to give rise to quasiline spectra.

Chrys-a. a PAH derivative of triterpene. in Rheptane at 4.2 K

The quasilinear fluorescence of the compound, which is a biogeochemical marker. was utilized to detect its presence in marine sediments (58).

F'henyl-. methyl-, bromo-. chloro-, cyano-. methoxy-. hydroxy- and aminopyrenes In *hexane at 63 K.

Conventional UVexcited quasiline fluorescence was utilized to determine shins in the 0-0 band with the type of substituent (59).

Photolysis by-products (free radicals) of acenaphthene and monomethyl naphthalenes in R pentane at 20 K

Photolysis was performed with a 100-W Hg arc lamp. UV and laser excitation were utilized to observe the emission and excitation spectra

Partially hydrogenated hydrocarbons(e.9.. 7.8.9.10tetrahydrobenzo[a]pyrene) in R octane at 63 K

Quasiline spectra observed under UV excitation

Organometallics

NA

N

N-

Nickel Phthalocyanine Figure 10. Typical aromatic structures that can provide quasilines in appropriate n-alkane solvents 998A

T h e typical aromatic structures that should provide quasilines in appropriate n-alkane solvents are shown in Figure 10. T h e listing includes n o t only P A H s but also the N,S. and 0 heterocyclics, metallo-porphyrins, and diphenylpolyenes. For the heterocyclic compounds in n-alkane solvents, the phosphorescence spectra are far more intense than the fluorescence spectra. However, the phosphorescence spectra


160).

(61).

normally fall in the 300-450-nm region, where the fluorescence spectra of a variety of PAHs are usually observed. In complex mixtures consisting of PAHs and heterocyclic compounds, time-resolved detection of the phosphorescence of the heterocyclic compounds facilitates their identification in the presence of PAHs. The most widely utilized solvents for the observation of the quasilinear spectra of organic compounds are the n-alkanes. For the simple aromatics, such as benzene and its derivatives, cyclohexane is the preferred solvent (47). Cyclohexane undergoes a phase change on cooling and the low-temperature phase provides a framework to trap benzene molecules in well-oriented sites. Tetrahydrofuran has been shown to be an alternate solvent, but the wider bandwidth of the quasilines may limit the application of tetrahydrofuran for high-resolution applications (48). The wide array of organic compounds that exhibit quasilines is documented in the excellent review by Nurmukhametov (49).A summary of recent observations on the Shpol’skii effect spectra of unusual compounds is provided in the box on p. 998 A, which indicates that derivatization approaches are bound to expand the application of the technique to a wider variety of compounds. A current limitation to the widespread application of the LESS approach is the lack of well-documented excitation-emission spectra of a large number of compounds. Because quasiline excitation spectral data are especially important in achieving the needed selectivity, we are in the process of cataloguing such spectra to supplement the list of emission spectra published recently (62).This information will be published as an atlas.

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Conclusion LESS is clearly finding increasing application to the direct, selective, and sensitive detection and quantitation of a broad range of organic compound classes in complex mixtures. Our early slogan, “DO more with LESS,” seems to have been appropriately prophetic. Acknowledgment The successful initiation and continued success of this research was made possible by the laser expertise of the late M. Iles and the experimental ingenuity of Y. Yang. Enlightening discussions with L A. Nakhimovsky (University of Hartford), M. LaMotte (University of Bordeaux), and G. Small (Ames Laboratory and Department of Chemistry, Iowa State UniANALYTICAL CHEMISTRY, VOL. 56, NO. 8, JULY 1984

999A

versity) on the various facets of t h e Shpol’skii effect are gratefully acknowledged. T h i s research was supported b y the U.S. Department of Energy Contract No. W-7405-Eng-82, and Office of Health a n d Environmental Research Physical a n d Technological Studies, Budget Code HA-02-04-03,

(34) VbDinh.T.; Wild, P. U. J . Lumin. 1973,6,296. (35) Abram, 1.; Auerback, R. A,; Birge, R. R.; Kahler, B. E.;Stevensan, J. M. J. Chem. Phys. 1974,61,3857. (36) Dinse, K. P.; Winscom, C. J. J . Lumm. 1979,18/19,500. (37) Yang, Y.; DSilva. A. P.; Fassel, V. A,; Iles, M. In “Laser Spectroscopy for Sensitive Detection”; Proe. Soc. Photo-Opt. Inst& Eng. 286; Gelbwsehs, J. A,, Ed., 1 9 8 1 , ~126. . (38) Renkes, G. D.; WGters, S. N.; Woo, C. S.; Iles, M. K.; D’Silva, A. P.; Fassel, V. A. Anal. Chem. 19R3,55,2229. (39) Walker, R.; DSilva, A. P.; F a d . V. A.. unpublished material. (40) Cumin ham, K.; Siebrand, W.; Williams, D. F! Chem. Phys. Lett. 1973.20,

(47) Leach,S. J . Phys. 1967.28.134. (48) Kirkbright, G. F.;delima, C. G. Analvst 19‘14.99.338. ~, ~,~~~ (4$ N&nukhametov, R. N. Russ. Chem. Reu. (Engl. Trans/.) 1969.38,180. (50) Palewska, K.; Ruziemicz, 2.Chem. Phys. Lett. 1977.64,378. (51) Platenkamp, R. J.; Van Osnabrugge. H. D.: Visser.A.J.W.G. Chem. Phvs. Lett. 1 9 ~ 0 72,104. . (52) Platenkamp, R. J.; Den Blanken, H. J.: H0ff.A. J. Chem. Phvs. Lett. 1980.

A%

(41) Meyer. B.; Metzger, J. L. Spectroehim. Acta A 1972,28,1563. (4‘2) I.eBel, G. L.; Lapwa, J. D. J. Mol. Speetrosc. 1972,24,1249. (43) Schettino, V. J . Mol. Spectrosc. 1970. .74 7R I_.

(44) Wise, S. A.; Bowie, S.L.; Chesler, S. N.; Cuthrell. W. F.; May, W. E.; Rebbert, R. E. In “Proceedings of the Sixth International Symposiun; on 1’olynuclear Aromatic Hydnrarlmns”; Conke, M.:Dennis. A. .I.: Firher. G. 1. .. Eda.; Rattelle Press: Columbus. Ohio, 1982; p. 014 ”.”

(45r Hertz, H. S.; Hrown. J. M.; Chesler. S. N ; Ruenther. F. R.; Hilpert. I.. R.; Mav. W E..l’arris. H.M : Wise. S. A. A d . Chem. 1980,’52,1650. ’ (4fi) Tomkins, B. A,; Kubota. H.; Griest, W.H.;Caton.J.E.;Clark.B.R.;Guerin, M. R. Anal. Chem. 1980.52,1332.

a

1891. (58) Ewald. M.; Moinet, A,; Saliot, A,; Albrecht, P. Anal. Chem. 1983.55,958. (59) Colms’o A L Zebuhr. U. Y.; Ostman. C. khek’ser. 1982.20.123. (60) Cofino, W. P.; Honrnweg, G.’Ph.;

4.

Gonijer. C.; MacLean, C.; Velthorst, N. H. Spectrochim. Acta A 1983.39, 283. (61) Colmsjo, A. L.; Zebuhr, U. Y.;Ostman, C. E.Chern. Scr., in press. (62) Colmsjo. A. L.; Ostman. C. E. “Atlas ofshwl’skii Soectra and Other Low Tem ‘erature Fluorescence Spectra of Pod”; University of Stockholm: Stockholm, Sweden, 1981.

. ;.A’’ \

I.”.

(27) Morantz, D. J. Chem. Phys. Lett. l970,5,20. (28) Nakimovsky, L. A.; Mamotte, M.. unpublished data. (29) !Ai, E. P:; Inman, E. L., Jr.; Winefordner. J. D. Talonta 1982,29,601. (30) Yang, Y.; DSilva, A. P.; Fassel. V. A. And. Chem. 1981,53,894. (31) Farocq, R.; Kirkbright, G. F. Analyst,

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A r t h u r P. D’Silva is a senior scientist in t h e Ames Laboratory, U S . Departm e n t of Energy. He received a BSc in chemistry and an M S c in physics from t h e Universities of Madras and Bombay, India, respectively. His current research interests involve laserexcited optical and ionization phenomena in inorganic, organic, and biochemical systems, and the analytic a l applications of atmospheric press u r e afterglows. He is t h e a u t h o r of more t h a n 60 p a p e r s and reports. Velmer A. Fassel is Senior Scientist in the Ames Laboratory, h basic research center for t h e US.Departm e n t of Energy o n t h e c a m p u s of Iowa S t a t e Uniuersity. During the period from 1969 to 1983, he held t h e


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position of D e p u t y Director of the Ames Laboratory and t h e Energy and M i n e r a l Resources Research Institute. He a l s o holds t h e academic rank of professor of c h e m i s t r y a n d t h e title of Distinguished Professor in Sciences and Humanities, bestowed in 1976. Fassel received his PhD in physical chemistry from Iowa State University in 1947. Previously he had received his BA at Southeast Missouri S t a t e University. He is the a u t h o r of approximately 200 publications on various aspects of atomic emission, absorption, and fluorescence spectroscopy, and particularly o n highresolution, high-sensitivity analytical techniques. H e has received thirteen major national and international awards.

Laser-excited Shpol'skii spectrometry - Analytical ... - ACS Publications

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