Extraction of Volatile PAHs from Air by Use ofSolid Cyclodextrin

Michelle T. Butterfield, Rezik A. Agbaria, and Isiah M. Warner*

Department of Chemistry, Louisiana State University, Baton Rouge, Louisiana 70803

An approach for extraction of polycyclic aromatic hydro-carbons (PAHs) from air using solid cyclodextrin ispresented. A comparison study in which â-cyclodextrinis replaced by r-cyclodextrin provides evidence thatâ-cyclodextrin extracts vapor phase PAHs by formationof inclusion complexes rather than by association oradsorption interactions. Thus, solid cyclodextrin com-plexes with vapor phase PAHs and thereby reduces theirvolatilities. The gas-solid interaction of the PAHs withâ-cyclodextrin and the effect of â-cyclodextrin on thevolatilities of these compounds are discussed. Fluores-cence and absorbance spectroscopies are used to examinethe variables that affect the formation of the PAH com-plexes with the solid cyclodextrin. The use of this systemfor improved ambient air sampling is proposed.

There has been a continuing interest in the ambient airsampling detection of polycyclic aromatic hydrocarbons (PAHs).1

The identification and quantification of PAHs are complex, time-consuming, and often inaccurate. Therefore, there is a need foran air sampling method in which the above-mentioned processesare minimized.

PAHs are environmental pollutants that have received consid-erable attention because of their carcinogenic and mutageniceffects.2 Due to the extensive amount of data suggesting thehazards of these compounds, many PAHs are on the Environ-mental Protection Agency (EPA) priority pollutant list.3 Thus,analysis of these compounds in the atmosphere is an importanttask. Significant levels of PAHs are found in the atmosphere butare more prominent in polluted urban areas. These compoundsare often emitted into the atmosphere by way of combustionprocesses.2 For example, a partial combustion of fuel in aninternal combustion engine results in the formation of traceamounts of PAH compounds.4 Due to their wide range of vaporpressures, some atmospheric PAHs exist exclusively in the gasphase and others as adsorbed particulate matter. The volatilityof these organic compounds controls their transport in theworkplace and in the environment in general.

Atmospheric PAHs undergo oxidation and photochemicalreactions under high-volume air sampling conditions.5 For

example, destruction and chemical alteration of PAHs are possiblein the presence of oxidants such as HO•, O3, NO2, N2O5, andHNO3.2 Direct photolysis of PAHs is also possible.2 However,PAHs adsorbed to particles are much more resistant to reactions.2

Due to their reactivities and volatilities, losses of PAHs duringsampling are almost unavoidable. In addition, the concentrationsof PAHs in air are low, and many of these compounds are unstableand volatile. These phenomena add significant error to theaccurate detection of PAHs by air sampling. For this reason, mostair sampling methods tend to focus on sampling the less volatilePAHs. Therefore, an air sampling method that provides a moreaccurate and reliable detection scheme for quantification of PAHsin air is needed.

Several studies have reported the examination of differentsampling methods for the analysis of PAHs in air.6-9 Glass fiberfilters are often used since they allow high flow rates.8 Othersampling media include polyurethane foam, Chromosorb poly-mers, and carbonaceous adsorbents.6 To improve the collectionefficiency of the filters, Thrane and Mikalsen8 examined glass fiberfilters in combination with plugs of polyurethane foam. Anincrease in the collection efficiency of these filters was observedwith increased air pollution levels. It was observed that polyure-thane foam removes PAHs from the air through a trappingmechanism.8 It should be noted that many problems associatedwith the collection of PAH air samples have been reported.10 Inparticular, significant losses due to the volatility of PAHs duringthe collection time are often reported. Many low-volatility PAHsalso experience significant volatility losses during long collectiontimes on filters.

Cyclodextrins (CDs) are widely used for many purposes and,in particular, as organized media for many types of chemistries.11,12

The CDs are cyclic oligosaccharides formed by an R-(1,4) linkageof glucopyranose units. The most commonly used oligosaccha-rides are R-, â-, and γ-cyclodextrins with six, seven, and eightglucopyranose units, respectively. These compounds possess ahydrophilic exterior, which makes them soluble in water, and aninterior cavity which is less polar than water. The R-, â-, andγ-CDs have approximate inner cavity diameters of 5.0, 7.8, and9.5 Å, respectively. These properties enable CD to incorporate

(1) Bjorseth, A.; Dennis, A. J. Polynuclear Aromatic Hydrocarbons: Chemistryand Biological Effects; Battelle Press: Columbus, OH, 1980.

(2) Manahan, S. E. Environmental Chemistry; Lewis: Chelsea, MI, 1991.(3) Brouwer, E. R.; Hermans, A. N. J.; Lingeman, H.; Brinkman, U. A. Th. J.

Chromatogr. A 1994, 669, 45-57.(4) Bjorseth, A. Handbook of Polycyclic Aromatic Hydrocarbons; Marcel Dek-

ker: New York, 1983.(5) Pitts, J. N.; Van Cauwenberghe, K. A.; Grosjean, D.; Schmid, J. P.; Fitz, D.

R.; Belser, W. L.; Knudson, G. B.; Hynds, P. M. Science 1978, 202, 515-519.

(6) Wong, J. M.; Kado, N. Y.; Kuzmicky, P. A.; Woodrow, J. E.; Hsieh, D. P. H.;Seiber, J. N. Anal. Chem. 1991, 63, 1644-1650.

(7) Cothan, W. E.; Bidleman, T. F. Environ. Sci. Technol. 1992, 26, 469-478.(8) Thrane, K. E.; Mikalsen, A. Atmos. Environ. 1981, 15, 909-918.(9) De Raat, W. K.; Bakker, G. L.; Meijere de, F. A. Atmos. Environ. 1990,

24A, 2875-2887.(10) Cheremisinoff, P. N. Air/Particulate Instrumentation and Analysis; Ann Arbor

Science: Ann Arbor, MI, 1981.(11) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer: New York,

1978.(12) Sjetli J. Cyclodextrins and Their Inclusion Complexes; Academiai Kiado:

Budapest, 1982.

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guest molecules on the basis of size and hydrophobicity.12 It iswell established that CDs will size-selectively complex with PAHsthrough host-guest interactions.13 Cyclodextrins are also usefultools in providing protective environments for PAHs.14-16 Inaqueous solutions, various inclusion complexes are formed withPAHs.17 In addition, solid CDs have been used to enhance thestability of volatile pharmaceuticals.12 However, most of thesecomplexes were initially prepared in aqueous solutions.

Clathration18 is a special type of complexation that occurs inthe solid state, in which the guest molecule is retained in the hostby crystal lattice forces. However, CDs exhibit complexation inboth the solution phase and the solid state. The structures ofsolid CDs have been described as cage- or channel-type struc-tures.12,18 In the channel-type structure, the CD molecules arevertically stacked, with the guest molecules embedded into thesechannels. In contrast, the cavity of one CD in a cage-type structureis blocked off on both sides by adjacent CD molecules, therebyleading to isolated cavities. A considerable amount of workregarding the use of solid CDs as host matrices has involved room-temperature solid surface luminescence analyses.19-21 Theseanalyses have explored cyclodextrin-salt mixtures as solidmatrices. Otherwise, complex formation studies of CDs in thesolid state have not received much attention from researchers.18

Smolkova et al.22-24 reported on the use of CDs in a gas-solidchromatographic system. They verified the existence of theformation of inclusion compounds with sorbates in the gaseousstate. However, their measurements were performed at temper-atures from 50 to 80 °C. More recently, Armstrong et al.25,26 usedCDs as stationary phases for the gas-solid chromatographicseparation of light hydrocarbons and inorganic gases at ambientto elevated temperatures. They showed that these CD stationaryphases provided a practical and efficient means for separating awide variety of gases. Although Smolkova provided the evidencefor the existence of the formation of inclusion complexes of CDswith substances present in the gaseous phase, our study is thefirst example of the use of solid CD and gaseous analytes at roomtemperature.

The focus in this work is on the use of solid CDs to stabilizevolatile PAHs. Our ultimate goal is to develop an air samplingsystem based on this phenomenon. A better understanding ofthis system is a prerequisite for developing a system in whichcyclodextrins can be used as a stabilizing agent for PAHs.

EXPERIMENTAL SECTIONApparatus. The design of our laboratory air sampling system

is shown in Figure 1. In this system, a gas cylinder of compressedair is attached to the air inlet. Solid cyclodextrin was spread overa glass fiber filter, through which air flows. The PAHs examinedin the initial phases of this study were sprinkled directly onto thefilter with solid cyclodextrin. Later studies used naphthalene ina cup, which was placed onto a platform raised over the filter withcyclodextrin. This latter approach prevented solid-solid contactof PAH and cyclodextrin. To capture vapor, which is not extractedby the filters, the air passes through two additional 1-L liquid trapsof organic solvent. Fused silica frits were used to create smallbubbles in order to increase the interacting surface area of theair with the solvent in the traps. Flow rates between 200 and 300mL/min were used.

Materials. Cyclodextrins were obtained from American MaizeProducts (Hammond, IN) and used as received. Naphthalene(99%), phenanthrene (98%), acenaphthene (99%), and acenaphth-ylene (75%) were obtained from Aldrich Chemical Co. (Milwaukee,WI) and were used as received. Glass fiber filters were obtainedfrom Sierra Instruments (Carmel Valley, CA). Cyclohexane(HPLC grade) was used without further purification.

Method. The glass fiber filter was cut and weighed initially,and an amount of â-cyclodextrin ranging from 200 to 1200 mgwas uniformly spread onto the filter. A known amount of PAHwas placed on the filter as described above. The first and secondtraps were filled with 400 and 250 mL of cyclohexane, respectively.The vaporous PAH is carried with the flowing air and passesthrough the solid cyclodextrin, which can extract the PAH fromthe air stream through formation of an inclusion complex.Naphthalene, the most volatile of the PAHs examined, wassampled for 10 h. After sampling, the filter content was placed

(13) Elliott, N. B.; Prenni, A. J.; Ndou, T. T.; Warner, I. M. J. Colloid InterfaceSci. 1993, 156, 359-364.

(14) Patonay, G.; Warner, I. M. J. Inclusion Phenom. Mol. Recognit. Chem. 1991,11, 313-322.

(15) Patonay, G.; Fowler, K.; Shapira, A.; Nelson, G.; Warner, I. M. J. InclusionPhenom. 1987, 5, 717-723.

(16) Nelson, G.; Patonay, G.; Warner, I. M. Appl. Spectrosc. 1987, 41, 1235-1238.

(17) Blyshak, L. A.; Dodson, K. Y.; Patonay, G.; May, W. E.; Warner, I. M. Anal.Chem. 1989, 61, 955-960.

(18) Ramamurthy, V. Photochemistry in Organized and Constrained Media; VCH:New York, 1991.

(19) Bello, J. M.; Hurtubise, R. J. Anal. Chem. 1987, 59, 2395-2400.(20) Richmond, M. D.; Hurtubise, R. J. Appl. Spectrosc. 1989, 43, 810-812.(21) Richmond, M. D.; Hurtubise, R. J. Anal. Chem. 1989, 61, 2643-2647.(22) Smolkova E.; Kralova, H.; Krysl, S.; Feltl, L. J. Chromatogr. 1982, 241, 3-8.(23) Mraz, J.; Feltl, L.; Smolkova-Keulemansova, E. J. Chromatogr. 1984, 286,

17-22.(24) Koscielski, T.; Sybilska, D.; Feltl, L.; Smolkova-Keulemansova, E. J. Chro-

matogr. 1984, 286, 23-30.(25) Reid, G. L., III; Monge, C. A.; Wall, W. T.; Armstrong, D. W. J. Chromatogr.

1993, 633, 135-142.(26) Reid, G. L., III; Wall, W. T.; Armstrong, D. W. J. Chromatogr. 1993, 633,

143-149.

Figure 1. Laboratory-designed air sampling system.

1188 Analytical Chemistry, Vol. 68, No. 7, April 1, 1996

in cyclohexane solution for extraction of the included PAH fromthe solid cyclodextrin. Since cyclodextrins are not soluble innonpolar organic solvents, this produces a cyclodextrin precipitatewith the PAH extracted into the organic solvent. The solvent fromeach trap was then removed in vacuo (Buchi RE 111 Rotavapor).Afterward, a solution was prepared that was 10% of its initialvolume. An appropriate volume of solution was pipetted into a10-mL flask, and the flask was filled to the mark with cyclohexanebefore measurement. All measurements were performed at roomtemperature. Fluorescence measurements were acquired usinga Spex Model F2T21I spectrofluorometer equipped with a ther-mostated cell housing and a water-cooled Hamamatsu R928photomultiplier tube. Excitation and emission bandwidths of 2.0and 8.0 nm, respectively, were used. Excitation wavelengths of275, 303, 252, and 303 nm were used for naphthalene, acenaph-thene, phenanthrene, and acenaphthylene, respectively. Absorp-tion measurements were obtained with a Shimadzu UV-3101 PCUV-vis-near-IR scanning spectrophotometer using a 1-cm-pathlength cell.

RESULTS AND DISCUSSIONThe formation of a solid CD-PAH complex is not always

straightforward. It should be noted that water molecules play anactive role in the driving force of CD complex formation withhydrophobic molecules in aqueous solution.12 The water mol-ecules included inside the CD cavity and those water moleculessurrounding the PAH molecule are not in a favorable energy state.Therefore, the CD-PAH complex formed in aqueous solutionsreleases these water molecules, which, in turn, stabilizes theinclusion complex.

The above considerations are not obvious when generallyspeaking about solid CD. However, studies which involved thegrinding of solid CD with solid pyrene or solid PPO reported theformation of such inclusion complexes.27 It should also be notedthat solid CD includes water molecules in its cavity.12 The numberof water molecules included in the solid CD depends on the kindof CD and ranges from 6 to 17 water molecules.12 Therefore, adriving force for the CD-PAH complex formation, in which watermolecules can be released from the CD cavity, is still possible,even in solid CD. However, the formation efficiency of such acomplex and the diffusion limitation over the solid CD moleculesare beyond the scope of this study. Based on the abovediscussion, solid CD would be expected to form complexes withgas phase PAHs.

Figure 2 displays the relative fluorescence emission intensitiesof the filter contents for several representative PAHs in theabsence and presence of solid â-CD. It is interesting to observethe increase in the fluorescence emission intensities with theaddition of solid â-CD on the filter, which confirms a gas-solidinteraction between the PAH and the solid CD. As expected,absorbance measurements support the gas-solid interaction. Thisgas-solid interaction led us to focus on one compound andexamine it more thoroughly. We chose naphthalene as our modelcompound because of its high vapor pressure. Therefore, thiscompound is very volatile in comparison to other solid PAHs. Asa result, it is easier to introduce gas phase naphthalene into theair stream than other PAHs. Naphthalene is allowed to interactwith the solid CD, and the formation of a CD-naphthalenecomplex is made possible.

Figure 3A shows the fluorescence emission intensity ofentrapped naphthalene from the air as the amount of solid CDincreases. The enhancement in the fluorescence emission inten-sity as the amount of solid CD increases supports the conclusionof gas-solid complexation of naphthalene with solid â-CD. Theincrease in the amount of solid CD on the filter correlates wellwith an increase of CD concentrations in aqueous solutions. Thus,higher concentrations yield more PAH-cyclodextrin complex.Using the same flow rate and time, larger amounts of solid CDon the filter should yield more naphthalene-CD complex.Therefore, an increase in the fluorescence emission intensity isexpected.

As a complementary measurement, the liquid trap solutionswere analyzed by use of fluorescence spectroscopy. Examinationof the fluorescence emission spectrum in Figure 3B reveals a(27) Yamamoto, K. Yakugaku Zasshi 1992, 112, 161-173.

Figure 2. Fluorescence intensities of several PAHs: (0) withoutâ-CD and (9) with â-CD.

Figure 3. (A) Influence of increasing amounts of â-CD on fluores-cence intensity of naphthalene (filter): (a) 200, (b) 600, and (c) 1000mg of â-CD. (B) Influence of decreasing amounts of â-CD onfluorescence intensity of naphthalene (trap 1): (a) 1200, (b) 800, and(c) 200 mg of â-CD.

Analytical Chemistry, Vol. 68, No. 7, April 1, 1996 1189

decrease in the fluorescence intensities. This observation sug-gests that lesser amounts of PAH are extracted by the organicsolvent in the trap, which indicates that larger amounts of thePAH are entrapped by the increased concentration of CD. Thisis in good agreement with the data observed from the filter (Figure3A). Figure 4A shows that the fluorescence emission intensityof the solid extract increases linearly as the amount of cyclodextrinon the filter increases.

Figure 4B shows that the fluorescence intensity of the PAHin the organic solvent in the trap decreases linearly as the amountof cyclodextrin increases. It is clear that these complementarydata suggest a complexation between gaseous naphthalene andsolid cyclodextrin.

In an attempt to examine whether the â-CD-PAH complex isan association or an inclusion complex, a comparison study wasperformed with R-cyclodextrin. In the latter, weak complexationis expected because naphthalene is too bulky to fit into the 5.0 Åcavity of R-cyclodextrin. In addition, the surface area of R-cyclo-dextrin, which is proportional to the number of glucose units forthe same weight of R- and â-cyclodextrin, is similar to that ofâ-cyclodextrin. Therefore, if the interaction is merely association,the fluorescence intensities should be close to those measuredwith â-cyclodextrin. Figure 5 shows that there is a markeddecrease in the fluorescence intensities when using R-cyclodextrin.

This significant decrease in the fluorescence intensity of theextracted naphthalene suggests that the cavity of the R-cyclo-dextrin is too small to fully include the naphthalene. Therefore,solid â-cyclodextrin complexes with gaseous naphthalene byformation of an inclusion complex. It should be noted that thecavity of R-cyclodextrin is comparable to the size of a benzenemolecule. Therefore, R-, â-, and γ-cyclodextrin combined on onefilter or separated on different filters in a row can potentially beused as variations of the sampling methods proposed here.However, a thorough study of each cyclodextrin with differentPAHs is needed before such an approach can be used. Suchstudies are underway in our laboratory.

CONCLUSIONThe results reported herein suggest that gas-solid interaction

of gaseous PAHs with solid cyclodextrin can be used to decreasethe volatility of PAHs during air sampling. The enhancedfluorescence intensity of the extracted PAH due to the presenceof increasing amounts of solid cyclodextrin on the glass fiber filterprovides evidence for the formation of solid-gas complexes ofâ-CD-PAH. The complexation of cyclodextrins with volatilePAHs is significant and suggests a potential use for the improveddetection of these volatile compounds in air sampling methods.To explore the use of solid cyclodextrins with other PAHs,additional studies are crucial. The use of cyclodextrins incombination is being explored in our laboratory for the extractionof different volatile PAHs from air.

ACKNOWLEDGMENTThis work was supported in part by a grant from the National

Science Foundation (CHE 9224177). The authors are also gratefulto G. A. Reed of American Maize Products for providing thecyclodextrins used in this study.

Received for review October 9, 1995. Accepted January19, 1996.X

AC9510144

X Abstract published in Advance ACS Abstracts, March 1, 1996.

Figure 4. (A) Linear plot of fluorescence intensity versus amountof â-CD (filter). (B) Linear plot of fluorescence intensity versus amountof â-CD (trap 1).

Figure 5. Linear plot for naphthalene complex: (a) â-CD and (b)R-CD.

1190 Analytical Chemistry, Vol. 68, No. 7, April 1, 1996