View metadata, citation and similar papers at core.ac.uk brought to you by CORE provided by Elsevier - Publisher Connector Reactions of Polycyclic Aromatic Hydrocarbon Radical Cations with Model Biological Nucleophiles Andy B. Whitehill and M. George University of Nebraska-Lincoln, Department of Chemistry, Lincoln, Nebraska, USA Michael L. Gross Washington University, Department of Chemistry, St. Louis, Missouri, USA The determination of gas-phase reactivity of a series of polycyclic aromatic hydrocarbons @AI+) with nucleophiles is directed at achieving isomer differentiation through ion-mole- cule reactions and collisionally activated decomposition spectra. A series of PAH isomers form gas-phase [ adduct - H] + ions with the reagent nucleophiles pyridine and N-methylim- idazole. Collisionally activated decomposition spectra of the [adduct - HI+ ions of the yridine/PAH systems are dominated by products formed by losses of C,H,N, CsHsN 4presumably neutral pyridine), and C,H,N. Collisional activation of PAH/N-methylim- idazole [adduct - H] ions causes analogous losses of C4H5N2, C,H6N, (presumably neutral N-methylimidazole), and C,H,N,. The relative abundances of the ions that result from these losses are highly isomer specific for A!-methylirnidazole but less so for pyridine. Furthermore, PAH/N-methylimidazole [adduct - HI+ ions undergo a series of metastable- ion decompositions that also provide hi hly isomer-specific information. The C,H,N, (from PAH/N-methylimidazole product ions 7 and C,H,N (from PAH/pyridine product ions) losses tend to increase with the AH, of the PAH radical cation. In addition, it is shown that the fragmentation patterns of these gas-phase PAH/nucleophile adducts are similar to fragmentation patterns of PAH/nucleoside adducts generated in solution, which suggests that the structures of products formed in gas-phase reactions are similar to those produced in solution. (1 Am Sot Mass Spectrom 1996, 7, 628-638) olycyclic aromatic hydrocarbons (PAHs) have cellar chromatography [4,6], capillary electrophoresis been known to be carcinogenic since the early [7], and supercritical fluid extraction [4,8] all have P 1900s. Because they are mutagenic and ubiqui- been successfully applied to PAH separation and de- tous in the environment, the need to detect and iden- termination. tify PAI-Is has continued to be an analytical challenge. Mass spectrometry has played an important role in Separation procedures and detection methods continue the determination of PAHs. In early applications, mass to be developed so that smaller and more complex spectrometers were employed as detectors after CC or samples can be analyzed and isomeric PAHs can be HFLC separations [2-41. For example, Wise et al. [9] distinguished. demonstrated the analysis of reference air-particulate Gas chromatography (GC) and packed column GC samples with liquid chromatography and GC sepa- coupled with UV absorption or fluorescence detection ration followed by electron ionization-mass spec- account for most of the methods for PAH deterrnina- trometry analysis. Another approach is GC with tion through the 1960s and early 1970s [ll. Books by pulsed-photon ionization and time-of-flight (TOF) mass Bjorseth [2], Lee [3], and Vo-Dinh [4] and the articles in spectrometry, for the analysis of coeluting PAH iso- reference 1 provide reviews of the history of PAH mers [lo]. determination by GC and other methods. The most effective and often used mass spectromet- The low volatility of some PAHs and their occur- ric techniques for the determination of PAHs, includ- rence as complex mixtures provide the driving force ing the differentiation of PAH isomers, are electron for the development of analysis techniques. High- ionization (EI) and chemical ionization (CI) with performance liquid chromatography U-IPLC) [2-51, mi- quadrupole and magnetic sector mass spectrometers. PAH determination also was shown to be possible with matrix-assisted laser desorption ionization-TOF Address reprint requests to Dr. Michael L. Gross, Washington Uni- versity, Department of Chemistry, 1 Brookings Drive, St. Louis, MO [ll] and ion mobility spectrometry [12]. 63130. Isomeric PAHs are known to isomerize through 0 1996 American Society for Mass Spectrometry Received December 4,1995 1044~0305/96/$15.00 Revised March 12,1996 PII SK&l-0305(96)00012-8 Accepted March 13,1996 J Am Sot Mass Spectrom 1996,7,620-638 REACTIONS OF PAH RADICAL CATIONS 629 extensive rearrangements to produce common inter- Buchanan et al. [21] approached PAH isomer dif- mediates that give identical losses upon high-energy ferentiation with a variety of methods. They showed ionization or collisional activation (CA). As a result, EI that electron-capture negative ion CI is useful for PAH and collisionally activated dissociation (CAD) spectra determination because there are significant differences of PAH isomers are usually indistinguishable. To rem- in electron affinity. Furthermore, the authors showed edy this problem, Shushan and Boyd [131 and Shushan that CI reactions with oxygen can be used to differen- et al. [141 demonstrated isomeric differentiation of tiate certain isomeric PAHs. Reactions of PAHs with PAHs with metastable-ion analysis. These lower- dimethyl ether [22] under hydroxylation conditions energy ions give spectra more indicative of the neutral [23] do produce products, but the information is lim- precursor than do the more energized ions formed by ited for isomer differentiation. high-energy EI or CA. This article describes a study of PAH isomer dif- Cooks and co-workers [151 studied gas-phase PAH ferentiation by means of CI in which reactions of PAH ions with a variety of techniques. They showed that radical cations with neutral pyridine or N-methylim- PAI-I ions abstract methyl, ethyl, and propyl groups idazole are used to produce adducts. Isomer differen- under ion-surface reaction conditions, and that sur- tiation is achieved by comparing CAD and metastable- face-induced dissociation spectra of PAHs vary with ion spectra of the gas-phase adducts of isomeric PAHs. collision energy. They also showed that with molecular The choice of neutral reagents is motivated by the secondary-ion mass spectrometry, PAI-Is that contain a work of Cavalieri and co-workers [25,26,28], who pro- “bay region” yield larger [PAH + Agl+:M+’ ion abun- posed that PAH radical cations, which are formed in dance ratios than PAHs without a “bay region.” More biological systems through enzymatic one-electron oxi- recently, they determined electron affinities of several dation or their decomposition products form PAH- PAHs by the kinetic method [ 161. Gross and co-workers DNA adducts [24]. PAH radical cations, formed both 1171 showed that PAH ions that are formed by EI, CI, electrochemically and enzymatically, do give adduct desorption, and photoionization methods and then formation in reactions with nucleosides [251. Most ad- fragmented by low- and high-energy collisional activa- ductions occur on the imidazole portion of.the purine tion as singly and multiply charged ions, provide valu- bases adenine and guanine. In these and other studies, able information in understanding PAH gas-phase ion mass spectrometry was shown to be effective in the processes. The difficulty of PAI-I isomer differentiation identification of PAH-nucleoside adduct structures was shown to be related to high internal energy capac- [26]. We also showed in a preliminary study [27] that ity of these ions as reflected by the large dissociation in an ionized mixture of pyridine and benzo[a]pyrene, energies, which allowed for ring opening and isomer- an ion-molecule reaction product forms, but the ization, and that resulted in the ultimate production of adduct was not characterized. In addition, 7,12-di- common fragments. methylbenz[ alanthracene/pyridine adducts also were The most effective mass spectrometric technique for shown to give structure-specific CAD spectra by using the differentiation of PAH isomers has been chemical fast-atom bombardment for desorption and tandem ionization-mass spectrometry in which PAH neutrals mass spectrometry for analysis [28]. These studies sug- are ionized by protonation or charge exchange [19-231. gest that neutral pyridine and N-methlyimidazole may Once created, the PAH ions can be compared or re- be suitable derivatizing reagents for PAI-I radical acted with a common neutral. A detailed study on the cations. gas-phase reactivity of pyrene showed that its radical The results presented here and in previously re- cation is unreactive, whereas the [M - HI+ aryl ions ported work on the reactions of benzene, naphthalene, participate in cycloadditions and alkyl-halide addition and PAHs of higher mass serve as first steps in a reactions [la]. The lack of reactivity of the radical program to establish whether there is a correlation cation with the neutrals studied was attributed to the between the gas-phase reactivity of PAHs and their rr-delocalization of charge and radical sites over the biological activity [29]. A similar approach was taken aromatic system. by Freeman et al. 1301 to determine whether the gas Simonsick and Hites [19] used an argon/methane phase reactions of ally1 reagents and pyridine can be reagent gas mixture to create M+ and [M + HI+ PAH used to screen complex environmental samples for ions. The ratio of [M + HI+ ion abundance to that of biologically active compounds. the radical cation CM+‘), GC elution times, and ioniza- tion energies were used in the analysis of standard references and carbon-black samples. Although trou- Experimental bled by coeluting isomers, the technique is shown to be effective in the analysis of complex samples and in Ma teriak the differentiation of isomers. Recently, Burrows [ZO] demonstrated the formation of adducts for a series of Anthracene, tetracene (2,3-benzanthracene), benz[ al an- PAHs reacting with dimethyl ether. In most cases, the thracene (1,2-benzanthracene), benzc4 ulpyrene, benzo- relative amount of adduct ions depends on the nature [ elpyrene, and N-methylimidazole were purchased of the PAH isomer. from Sigma Chemical Co.