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Retention behavior of polycyclic aromatic hydrocarbons in microcolumn liquid chromatography with acridine derivative as stationary phase
Analytica Chimica Acta 370 (1998) 125±130
Retention behavior of polycyclic aromatic hydrocarbons in microcolumn liquid chromatography with acridine derivative as stationary phase Jianhua Chu, Toyohide Takeuchi*, Tomoo Miwa Department of Chemistry, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Received 12 November 1997; received in revised form 17 April 1998; accepted 19 April 1998
Abstract Retention behavior of polycyclic aromatic hydrocarbons (PAHs) on an acridine derivative stationary phase was examined in microcolumn liquid chromatography. 3,6-Bis(dimethylamino)-10-dodecylacridinium was electrostatically introduced into a cation-exchanger, and its selectivity was compared with that of octadecylsilyl-bonded silica gel. The former stationary phase provided smaller retention for non-planar PAHs than that achieved by the latter stationary phase. The results suggest that interaction between PAHs and the acridinyl ring dominates the retention of PAHs, and preferential retention of planar PAHs is attributed to the fact that they have more chance to interact with the acridinyl ring of the stationary phase than non-planar PAHs. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Reversed-phase liquid chromatography; Retention behavior; Dodecylacridinium stationary phase; Polycyclic aromatic hydrocarbons
1. Introduction Chemically bonded stationary phases with various functional groups have been used in reversed-phase liquid chromatography (RPLC). The chain length, size or type of the bonded phase can affect the selectivities obtained [1±8]. A number of papers have reported that chemically bonded silica gel packings show different shape selectivities, e.g., recognition of planarity of molecules owing to differences in morphology of bonded phases [1±8]. Wise and Sander [3] proposed a slot model to *Corresponding author. Tel.: 0081 58 293 2806; fax: 0081 58 293 2806; e-mail: [email protected] 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00281-5
describe the retention of polycyclic aromatic hydrocarbons (PAHs) on polymeric octadecylsilyl-bonded phases (ODS) and showed that planar PAHs are preferentially retained than non-planar PAHs. It was also shown that planar PAHs are retained in preference on charge-transfer phases such as pyrenylethyl silica to corresponding non-planar PAHs [2,7]. It was demonstrated that stationary phases introduced by ion exchange could function in RPLC [9,10]. PAHs and dansyl amino acids were separated by anion- and cation-exchangers modi®ed with alkyl sulfates, alkane sulfonates or quaternary ammonium ions [9,10]. Ion-exchange-induced stationary phases have an advantage that various functional groups can be easily introduced, and the modi®ed ions bound by
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ion exchange can be replaced by passing other reagent ions through the column. It should be noted that various ions having hydrophobic moieties and wide range of polarity can be candidates for the stationary phase in RPLC. This paper describes retention behavior of PAHs on an acridine derivative stationary phase introduced by cation exchange. 2. Experimental 2.1. Apparatus The microcolumn liquid chromatograph was constructed from an MF-2 microfeeder (Azumadenki Kogyo, Tokyo, Japan) as a pump, an ML-522 microvalve injector with an injection volume of 0.13 ml (Jasco, Tokyo, Japan), a UV-970 intelligent UV/vis detector (Jasco) with a laboratory-made ¯ow cell, a 1500.35 mm i.d. microcolumn, and a Model 1022 Personal Integrator (Perkin Elmer, Norwalk, CT, USA). The microcolumn was prepared from fusedsilica tubing with 0.35 mm i.d. (GL Science, Tokyo, Japan), and TSKgel IC-Cation SW (5 mm; Tosoh, Tokyo, Japan) was used as the cation-exchanger. The cation-exchange capacity of the stationary phase is (0.30.1) milliequiv gÿ1. Develosil ODS-5 (5 mm monomeric ODS; Nomura Chemical, Seto, Japan) was also employed for comparison. The eluent ¯ow rate was 2.8 ml minÿ1. The detection wavelength was 254 nm, and the experiments were carried out at room temperature (25±308C). 2.2. Reagents 3,6-Bis(dimethylamino)-10-dodecylacridinium bromide (AO-10-DAB) was from Dojindo Laboratories (Kumamoto, Japan). The structures of PAHs and AO-10-DAB employed in this work are illustrated in Fig. 1. Reagent-grade naphthalene (1), ¯uorene (2), anthracene (3), phenanthrene (4), ¯uoranthene (5), pyrene (6), benzo[a]pyrene (BaP; 8), biphenyl (12), p-terphenyl (13), m-terphenyl (14), o-terphenyl (15), and toluene (19) were from Nacalai Tesque (Kyoto, Japan). Perylene (9), 1,3,5-triphenylbenzene (16), 9phenylanthracene (17), 9,10-diphenylanthracene (18) were from Tokyo Chemical Industry (Tokyo, Japan).
Chrysene (7) and benzo[ghi]perylene (10) were from Aldrich Japan (Tokyo, Japan). Triphenylene (11) was from Kanto Chemical (Tokyo, Japan). Alkylbenzenes with hexyl- (C6) to octadecyl (C18) groups were from Tokyo Chemical Industry. 1,2:3,4:5,6:7,8-tetrabenzonaphthalene (TBN; 20) and phenanthro[3,4-c]phenanthrene (PhPh;21) were kindly offered by Prof. K. Jinno, Toyohashi University of Technology, Toyohashi, Japan. All the standard solutions and eluents were prepared from HPLC-grade distilled water (Nacalai Tesque) and acetonitrile (Nacalai Tesque). 2.3. Preparation of AO-10-DAB-modified columns Aqueous acetonitrile (50% (v/v)) including AO-10DAB in 0.14 mM was passed through the cationexchange column until breakthrough was observed, followed by washing with acetonitrile. 3. Results and discussion 3.1. Retention behavior Retention times of PAHs on the AO-10-DAB-modi®ed stationary phase were measured by using acetonitrile as the mobile phase. Retention factors (k) of each analyte achieved by the AO-10-DAB-modi®ed stationary phase are listed in Table 1. The data obtained with the ODS stationary phase using acetonitrile±water (90:10) as the mobile phase are also shown in the table. It is found that the selectivity obtained with the former stationary phase is different from that obtained with the latter stationary phase. In Table 1, planar PAHs are denoted by , whereas non-planar PAHs are denoted by ÿ. As for the compounds 12±21 except for toluene phenyl rings are not on one plane due to steric hindrance, whereas the compounds denoted by are planar. In addition, ¯uorene (2) and toluene (19) have hydrogen atoms which are not on the plane of the aromatic rings, whereas TBN is almost planar. Table 1 also compares the ratio of the retention factors obtained on the AO10-DAB-modi®ed stationary phase and ODS stationary phase (kAO-10-DAB/kODS). It is found that under the conditions as in Table 1 the kAO-10-DAB/kODS values are less than 0.4 for the non-planar analytes except for
J. Chu et al. / Analytica Chimica Acta 370 (1998) 125±130
127
Fig. 1. Structures of PAHs and AO-10-DAB employed in this work.
TBN, whereas values larger than 0.4 are observed for the planar analytes except for naphthalene. It is presumed that the acridinyl ring of the AO-10-DABmodi®ed stationary phase has less chance to interact with the non-planar PAHs and the selectivity of the non-planar PAHs on the stationary phase is poor.
Conventional chemically bonded phases also recognize the planarity of PAHs to some degree [1±8]. It is well known that ODS stationary phases discriminate on the basis of the planarity of PAHs. The shape selectivity of ODS stationary phases is explained by a model that non-planar PAHs are restricted to have
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Table 1 Retention factors of PAHs for AO-10-DAB-modified stationary phase and ODS PAHs
Planaritya
Retention factor b
(1) Naphthalene (2) Fluorene (3) Anthracene (4) Phenanthrene (5) Fluoranthene (6) Pyrene (7) Chrysene (8) Bap (9) Perylene (10) Benzo[ghi]perylene (11) Triphenylene (12) Biphenyl (13) p-Terphenyl (14) m-Terphenyl (15) o-Terphenyl (16) 1,3,5-Triphenylbenzene (17) 9-Phenylanthracene (18) 9,10-Diphenylanthracene (19) Toluene (20) TBN (21) PhPh a b c
() ÿ ÿ ÿ ÿ ÿ ÿ ÿ () (ÿ) ÿ
kAO-10-DAB/kODS c
AO-10-DAB
ODS
0.40 0.52 0.83 0.77 1.20 1.50 2.46 8.08 7.14 24.10 2.41 0.34 0.59 0.47 0.39 0.64 0.72 0.63 0.31 3.60 1.94
1.11 1.52 1.92 1.74 2.25 2.68 3.23 5.70 4.50 8.05 2.86 1.25 2.51 2.16 1.92 3.87 3.55 7.05 0.92 8.60 5.05
0.36 0.34 0.43 0.44 0.53 0.56 0.76 1.42 1.59 2.99 0.84 0.27 0.23 0.22 0.20 0.17 0.20 0.09 0.34 0.42 0.38
, Planar; ÿ, non-planar; (), contains hydrogen atoms existing out of the plane of the aromatic ring; (ÿ), almost planar. Mobile phase: 100% acetonitrile. Mobile phase: acetonitrile±water90:10.
access to the stationary phase due to the steric hindrance, leading to less interaction with the stationary phase [3]. As for planar compounds 8, 9 and 10, the AO-10DAB-modi®ed stationary phase gave larger k values than the ODS stationary phase. Considering the fact that a stronger mobile phase, i.e., pure acetonitrile, is used in the former case, it can be concluded that the former stationary phase retains the above planar PAHs more strongly than the latter stationary phase. The AO-10-DAB-modi®ed stationary phase possesses an acridinyl ring and a dodecyl group, but the effect of the latter group is not clear. When alkylbenzenes with various alkyl groups, C6±C18, were injected on the AO-10-DAB-modi®ed stationary phase, the retention times of these analytes were nearly the same, e.g., 4.6±4.9 min. This means that the present stationary phase cannot recognize the length of the alkyl group. Therefore, it is presumed that the main functional group of the present stationary
phase is the acridinyl ring and the dodecyl group has less interaction with the PAHs and alkylbenzenes. Selectivities of PAHs are therefore considered to arise from the ± interaction between the acridinyl ring and PAHs. The ± interaction is in¯uenced by the planarity of the analyte. Planar PAHs can be strongly held in the acridinyl ring as a result of ± interaction. On the other hand, non-planar PAHs cannot interact well with the acridinyl ring due to their structures. There was no bleeding of the modi®ed stationary phase when pure acetonitrile was used. However, when aqueous acetonitrile was used as the mobile phase, bleeding of the stationary phase was observed. By chemically bonding the acridinyl ring into silica gel, the bleeding problem will be overcome. 3.2. Separation of PAHs Fig. 2 demonstrates the separations of o-terphenyl (15) and triphenylene (11) on the AO-10-DAB-
J. Chu et al. / Analytica Chimica Acta 370 (1998) 125±130
129
Fig. 2. Separation of o-terphenyl (15) and triphenylene (11) on the AO-10-DAB-modified stationary phase (A) and ODS stationary phase (B). Columns: 1500.35 mm i.d., AO-10-DAB-modified stationary phase (A); Develosil ODS-5 (B). Flow rate: 2.8 ml minÿ1. Wavelength of UV detection: 254 nm. Analytes: o-terphenyl (15) and triphenylene (11). Other operating conditions as in Table 1.
modi®ed and ODS stationary phases. It is clear that the AO-10-DAB-modi®ed stationary phase provides better selectivity of o-terphenyl (15) and triphenylene (11) than the ODS stationary phase as shown in Fig. 2. The separation factor of the analytes were 6.2 and 1.5 for the former and the latter stationary phase, respectively. o-Terphenyl (15) and triphenylene (11) are used as the analytes for evaluating the planarity recognition of the stationary phase [2,6]. It is reported that separation factors of the above pair of PAHs achieved by ODS stationary phases are at most 2.3 [6], whereas the pyrenylethyl stationary phases achieved a separation factor of around 3 [2]. It was found that the AO-10DAB-modi®ed stationary phase achieved much larger separation factor of o-terphenyl (15) and triphenylene (11) than conventional stationary phases. Fig. 3 demonstrates the separations of three PAHs designated by standard reference material (SRM) 869, column selectivity test mixture for LC available from the Standard Reference Materials Program, National Institute of Standards and Technology, Gaithersburg, MO, USA [11]. This material was speci®cally designed to assess shape selectivity characteristics of alkyl phases. Baseline separations of PhPh (21), BaP (8) and TBN (20) are achieved on the ODS and AO-10-DAB columns using acetonitrile±water (90:10) and pure acetonitrile as the mobile phase, respectively. It is seen that quite different selectivities are observed between these two columns. According
Fig. 3. Separation of SRM 869 on the AO-10-DAB-modified stationary phase (A) and ODS stationary phase (B). Columns: 1500.35 mm i.d., AO-10-DAB-modified stationary phase (A); Develosil ODS-5 (B). Mobile phases: 100% acetonitrile (A); acetonitrile±water: 90:10; flow rate: 2.8 ml minÿ1. Wavelength of UV detection: 254 nm. Analytes: BaP (8), TBN (20) and PhPh (21).
to the description of the SRM 869, in case the values for the separation factor of TBN and BaP ( TBN/BaP) obtained by using 85% acetonitrile aqueous solution as the mobile phase at 258C are larger than 1.7, the ODS phases are monomeric, whereas values for TBN/BaP lower than 1 re¯ect polymeric ODS phases. It is estimated that the Develosil ODS-5 is intermediate because TBN/BaP for 85% acetonitrile was 1.48. Under the conditions of Fig. 3 TBN/BaP values for the Develosil ODS-5 and the AO-10-DAB phases were 1.51 and 0.45, respectively. It is expected that the AO-10-DAB phase has a shape selectivity similar to polymeric ODS phases. It should be noted again that the AO-10-DAB phase recognizes the planarity of PAHs. Acknowledgements The authors wish to thank Prof. Kiyokatsu Jinno, Toyohashi University of Technology, for his kind offer of TBN and PhPh.
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References [1] N. Tanaka, K. Sakagami, M. Araki, J. Chromatogr. 199 (1980) 327. [2] N. Tanaka, Y. Tokuda, K. Iwaguchi, M. Araki, J. Chromatogr. 239 (1982) 761. [3] S.A. Wise, L.C. Sander, J. High Resolut. Chromatogr. Chromatogr. Commun. 8 (1985) 248. [4] L.C. Sander, S.A. Wise, Anal. Chem. 59 (1987) 2309. [5] K. Jinno, T. Nagoshi, N. Tanaka, M. Okamoto, J.C. Fetzer, W.R. Biggs, J. Chromatogr. 392 (1987) 75. [6] T. Kimata, K. Iwaguchi, S. Onishi, K. Jinno, R. Eksteen, K. Hosoya, M. Araki, N. Tanaka, J. Chromatogr. Sci. 27 (1989) 721.
[7] L.C. Sander, R.M. Parris, S.A. Wise, P. Garrigues, Anal. Chem. 63 (1991) 2589. [8] K. Jinno, K. Yamamoto, T. Kuwamoto, H. Nagashima, T. Ueda, S. Tajima, E. Kimura, K. Itoh, J.C. Fetzer, W.R. Biggs, Chromatographia 34 (1992) 381. [9] T. Takeuchi, R. Hu, T. Miwa, Chromatographia 39 (1994) 597. [10] T. Takeuchi, T. Miwa, R. Hu, J. Chu, J. High Resolut. Chromatogr. 18 (1995) 745. [11] W.P. Reed, Standard Reference Material 869; Column Selectivity Test Mixture for Liquid Chromatography, National Institute of Standards and Technology, 28 March 1990.
Retention behavior of polycyclic aromatic hydrocarbons in microcolumn liquid chromatography with acridine derivative as stationary phase Jianhua Chu, Toyohide Takeuchi*, Tomoo Miwa Department of Chemistry, Faculty of Engineering, Gifu University, 1-1 Yanagido, Gifu 501-1193, Japan Received 12 November 1997; received in revised form 17 April 1998; accepted 19 April 1998
Abstract Retention behavior of polycyclic aromatic hydrocarbons (PAHs) on an acridine derivative stationary phase was examined in microcolumn liquid chromatography. 3,6-Bis(dimethylamino)-10-dodecylacridinium was electrostatically introduced into a cation-exchanger, and its selectivity was compared with that of octadecylsilyl-bonded silica gel. The former stationary phase provided smaller retention for non-planar PAHs than that achieved by the latter stationary phase. The results suggest that interaction between PAHs and the acridinyl ring dominates the retention of PAHs, and preferential retention of planar PAHs is attributed to the fact that they have more chance to interact with the acridinyl ring of the stationary phase than non-planar PAHs. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Reversed-phase liquid chromatography; Retention behavior; Dodecylacridinium stationary phase; Polycyclic aromatic hydrocarbons
1. Introduction Chemically bonded stationary phases with various functional groups have been used in reversed-phase liquid chromatography (RPLC). The chain length, size or type of the bonded phase can affect the selectivities obtained [1±8]. A number of papers have reported that chemically bonded silica gel packings show different shape selectivities, e.g., recognition of planarity of molecules owing to differences in morphology of bonded phases [1±8]. Wise and Sander [3] proposed a slot model to *Corresponding author. Tel.: 0081 58 293 2806; fax: 0081 58 293 2806; e-mail: [email protected] 0003-2670/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII S0003-2670(98)00281-5
describe the retention of polycyclic aromatic hydrocarbons (PAHs) on polymeric octadecylsilyl-bonded phases (ODS) and showed that planar PAHs are preferentially retained than non-planar PAHs. It was also shown that planar PAHs are retained in preference on charge-transfer phases such as pyrenylethyl silica to corresponding non-planar PAHs [2,7]. It was demonstrated that stationary phases introduced by ion exchange could function in RPLC [9,10]. PAHs and dansyl amino acids were separated by anion- and cation-exchangers modi®ed with alkyl sulfates, alkane sulfonates or quaternary ammonium ions [9,10]. Ion-exchange-induced stationary phases have an advantage that various functional groups can be easily introduced, and the modi®ed ions bound by
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ion exchange can be replaced by passing other reagent ions through the column. It should be noted that various ions having hydrophobic moieties and wide range of polarity can be candidates for the stationary phase in RPLC. This paper describes retention behavior of PAHs on an acridine derivative stationary phase introduced by cation exchange. 2. Experimental 2.1. Apparatus The microcolumn liquid chromatograph was constructed from an MF-2 microfeeder (Azumadenki Kogyo, Tokyo, Japan) as a pump, an ML-522 microvalve injector with an injection volume of 0.13 ml (Jasco, Tokyo, Japan), a UV-970 intelligent UV/vis detector (Jasco) with a laboratory-made ¯ow cell, a 1500.35 mm i.d. microcolumn, and a Model 1022 Personal Integrator (Perkin Elmer, Norwalk, CT, USA). The microcolumn was prepared from fusedsilica tubing with 0.35 mm i.d. (GL Science, Tokyo, Japan), and TSKgel IC-Cation SW (5 mm; Tosoh, Tokyo, Japan) was used as the cation-exchanger. The cation-exchange capacity of the stationary phase is (0.30.1) milliequiv gÿ1. Develosil ODS-5 (5 mm monomeric ODS; Nomura Chemical, Seto, Japan) was also employed for comparison. The eluent ¯ow rate was 2.8 ml minÿ1. The detection wavelength was 254 nm, and the experiments were carried out at room temperature (25±308C). 2.2. Reagents 3,6-Bis(dimethylamino)-10-dodecylacridinium bromide (AO-10-DAB) was from Dojindo Laboratories (Kumamoto, Japan). The structures of PAHs and AO-10-DAB employed in this work are illustrated in Fig. 1. Reagent-grade naphthalene (1), ¯uorene (2), anthracene (3), phenanthrene (4), ¯uoranthene (5), pyrene (6), benzo[a]pyrene (BaP; 8), biphenyl (12), p-terphenyl (13), m-terphenyl (14), o-terphenyl (15), and toluene (19) were from Nacalai Tesque (Kyoto, Japan). Perylene (9), 1,3,5-triphenylbenzene (16), 9phenylanthracene (17), 9,10-diphenylanthracene (18) were from Tokyo Chemical Industry (Tokyo, Japan).
Chrysene (7) and benzo[ghi]perylene (10) were from Aldrich Japan (Tokyo, Japan). Triphenylene (11) was from Kanto Chemical (Tokyo, Japan). Alkylbenzenes with hexyl- (C6) to octadecyl (C18) groups were from Tokyo Chemical Industry. 1,2:3,4:5,6:7,8-tetrabenzonaphthalene (TBN; 20) and phenanthro[3,4-c]phenanthrene (PhPh;21) were kindly offered by Prof. K. Jinno, Toyohashi University of Technology, Toyohashi, Japan. All the standard solutions and eluents were prepared from HPLC-grade distilled water (Nacalai Tesque) and acetonitrile (Nacalai Tesque). 2.3. Preparation of AO-10-DAB-modified columns Aqueous acetonitrile (50% (v/v)) including AO-10DAB in 0.14 mM was passed through the cationexchange column until breakthrough was observed, followed by washing with acetonitrile. 3. Results and discussion 3.1. Retention behavior Retention times of PAHs on the AO-10-DAB-modi®ed stationary phase were measured by using acetonitrile as the mobile phase. Retention factors (k) of each analyte achieved by the AO-10-DAB-modi®ed stationary phase are listed in Table 1. The data obtained with the ODS stationary phase using acetonitrile±water (90:10) as the mobile phase are also shown in the table. It is found that the selectivity obtained with the former stationary phase is different from that obtained with the latter stationary phase. In Table 1, planar PAHs are denoted by , whereas non-planar PAHs are denoted by ÿ. As for the compounds 12±21 except for toluene phenyl rings are not on one plane due to steric hindrance, whereas the compounds denoted by are planar. In addition, ¯uorene (2) and toluene (19) have hydrogen atoms which are not on the plane of the aromatic rings, whereas TBN is almost planar. Table 1 also compares the ratio of the retention factors obtained on the AO10-DAB-modi®ed stationary phase and ODS stationary phase (kAO-10-DAB/kODS). It is found that under the conditions as in Table 1 the kAO-10-DAB/kODS values are less than 0.4 for the non-planar analytes except for
J. Chu et al. / Analytica Chimica Acta 370 (1998) 125±130
127
Fig. 1. Structures of PAHs and AO-10-DAB employed in this work.
TBN, whereas values larger than 0.4 are observed for the planar analytes except for naphthalene. It is presumed that the acridinyl ring of the AO-10-DABmodi®ed stationary phase has less chance to interact with the non-planar PAHs and the selectivity of the non-planar PAHs on the stationary phase is poor.
Conventional chemically bonded phases also recognize the planarity of PAHs to some degree [1±8]. It is well known that ODS stationary phases discriminate on the basis of the planarity of PAHs. The shape selectivity of ODS stationary phases is explained by a model that non-planar PAHs are restricted to have
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J. Chu et al. / Analytica Chimica Acta 370 (1998) 125±130
Table 1 Retention factors of PAHs for AO-10-DAB-modified stationary phase and ODS PAHs
Planaritya
Retention factor b
(1) Naphthalene (2) Fluorene (3) Anthracene (4) Phenanthrene (5) Fluoranthene (6) Pyrene (7) Chrysene (8) Bap (9) Perylene (10) Benzo[ghi]perylene (11) Triphenylene (12) Biphenyl (13) p-Terphenyl (14) m-Terphenyl (15) o-Terphenyl (16) 1,3,5-Triphenylbenzene (17) 9-Phenylanthracene (18) 9,10-Diphenylanthracene (19) Toluene (20) TBN (21) PhPh a b c
() ÿ ÿ ÿ ÿ ÿ ÿ ÿ () (ÿ) ÿ
kAO-10-DAB/kODS c
AO-10-DAB
ODS
0.40 0.52 0.83 0.77 1.20 1.50 2.46 8.08 7.14 24.10 2.41 0.34 0.59 0.47 0.39 0.64 0.72 0.63 0.31 3.60 1.94
1.11 1.52 1.92 1.74 2.25 2.68 3.23 5.70 4.50 8.05 2.86 1.25 2.51 2.16 1.92 3.87 3.55 7.05 0.92 8.60 5.05
0.36 0.34 0.43 0.44 0.53 0.56 0.76 1.42 1.59 2.99 0.84 0.27 0.23 0.22 0.20 0.17 0.20 0.09 0.34 0.42 0.38
, Planar; ÿ, non-planar; (), contains hydrogen atoms existing out of the plane of the aromatic ring; (ÿ), almost planar. Mobile phase: 100% acetonitrile. Mobile phase: acetonitrile±water90:10.
access to the stationary phase due to the steric hindrance, leading to less interaction with the stationary phase [3]. As for planar compounds 8, 9 and 10, the AO-10DAB-modi®ed stationary phase gave larger k values than the ODS stationary phase. Considering the fact that a stronger mobile phase, i.e., pure acetonitrile, is used in the former case, it can be concluded that the former stationary phase retains the above planar PAHs more strongly than the latter stationary phase. The AO-10-DAB-modi®ed stationary phase possesses an acridinyl ring and a dodecyl group, but the effect of the latter group is not clear. When alkylbenzenes with various alkyl groups, C6±C18, were injected on the AO-10-DAB-modi®ed stationary phase, the retention times of these analytes were nearly the same, e.g., 4.6±4.9 min. This means that the present stationary phase cannot recognize the length of the alkyl group. Therefore, it is presumed that the main functional group of the present stationary
phase is the acridinyl ring and the dodecyl group has less interaction with the PAHs and alkylbenzenes. Selectivities of PAHs are therefore considered to arise from the ± interaction between the acridinyl ring and PAHs. The ± interaction is in¯uenced by the planarity of the analyte. Planar PAHs can be strongly held in the acridinyl ring as a result of ± interaction. On the other hand, non-planar PAHs cannot interact well with the acridinyl ring due to their structures. There was no bleeding of the modi®ed stationary phase when pure acetonitrile was used. However, when aqueous acetonitrile was used as the mobile phase, bleeding of the stationary phase was observed. By chemically bonding the acridinyl ring into silica gel, the bleeding problem will be overcome. 3.2. Separation of PAHs Fig. 2 demonstrates the separations of o-terphenyl (15) and triphenylene (11) on the AO-10-DAB-
J. Chu et al. / Analytica Chimica Acta 370 (1998) 125±130
129
Fig. 2. Separation of o-terphenyl (15) and triphenylene (11) on the AO-10-DAB-modified stationary phase (A) and ODS stationary phase (B). Columns: 1500.35 mm i.d., AO-10-DAB-modified stationary phase (A); Develosil ODS-5 (B). Flow rate: 2.8 ml minÿ1. Wavelength of UV detection: 254 nm. Analytes: o-terphenyl (15) and triphenylene (11). Other operating conditions as in Table 1.
modi®ed and ODS stationary phases. It is clear that the AO-10-DAB-modi®ed stationary phase provides better selectivity of o-terphenyl (15) and triphenylene (11) than the ODS stationary phase as shown in Fig. 2. The separation factor of the analytes were 6.2 and 1.5 for the former and the latter stationary phase, respectively. o-Terphenyl (15) and triphenylene (11) are used as the analytes for evaluating the planarity recognition of the stationary phase [2,6]. It is reported that separation factors of the above pair of PAHs achieved by ODS stationary phases are at most 2.3 [6], whereas the pyrenylethyl stationary phases achieved a separation factor of around 3 [2]. It was found that the AO-10DAB-modi®ed stationary phase achieved much larger separation factor of o-terphenyl (15) and triphenylene (11) than conventional stationary phases. Fig. 3 demonstrates the separations of three PAHs designated by standard reference material (SRM) 869, column selectivity test mixture for LC available from the Standard Reference Materials Program, National Institute of Standards and Technology, Gaithersburg, MO, USA [11]. This material was speci®cally designed to assess shape selectivity characteristics of alkyl phases. Baseline separations of PhPh (21), BaP (8) and TBN (20) are achieved on the ODS and AO-10-DAB columns using acetonitrile±water (90:10) and pure acetonitrile as the mobile phase, respectively. It is seen that quite different selectivities are observed between these two columns. According
Fig. 3. Separation of SRM 869 on the AO-10-DAB-modified stationary phase (A) and ODS stationary phase (B). Columns: 1500.35 mm i.d., AO-10-DAB-modified stationary phase (A); Develosil ODS-5 (B). Mobile phases: 100% acetonitrile (A); acetonitrile±water: 90:10; flow rate: 2.8 ml minÿ1. Wavelength of UV detection: 254 nm. Analytes: BaP (8), TBN (20) and PhPh (21).
to the description of the SRM 869, in case the values for the separation factor of TBN and BaP ( TBN/BaP) obtained by using 85% acetonitrile aqueous solution as the mobile phase at 258C are larger than 1.7, the ODS phases are monomeric, whereas values for TBN/BaP lower than 1 re¯ect polymeric ODS phases. It is estimated that the Develosil ODS-5 is intermediate because TBN/BaP for 85% acetonitrile was 1.48. Under the conditions of Fig. 3 TBN/BaP values for the Develosil ODS-5 and the AO-10-DAB phases were 1.51 and 0.45, respectively. It is expected that the AO-10-DAB phase has a shape selectivity similar to polymeric ODS phases. It should be noted again that the AO-10-DAB phase recognizes the planarity of PAHs. Acknowledgements The authors wish to thank Prof. Kiyokatsu Jinno, Toyohashi University of Technology, for his kind offer of TBN and PhPh.
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References [1] N. Tanaka, K. Sakagami, M. Araki, J. Chromatogr. 199 (1980) 327. [2] N. Tanaka, Y. Tokuda, K. Iwaguchi, M. Araki, J. Chromatogr. 239 (1982) 761. [3] S.A. Wise, L.C. Sander, J. High Resolut. Chromatogr. Chromatogr. Commun. 8 (1985) 248. [4] L.C. Sander, S.A. Wise, Anal. Chem. 59 (1987) 2309. [5] K. Jinno, T. Nagoshi, N. Tanaka, M. Okamoto, J.C. Fetzer, W.R. Biggs, J. Chromatogr. 392 (1987) 75. [6] T. Kimata, K. Iwaguchi, S. Onishi, K. Jinno, R. Eksteen, K. Hosoya, M. Araki, N. Tanaka, J. Chromatogr. Sci. 27 (1989) 721.
[7] L.C. Sander, R.M. Parris, S.A. Wise, P. Garrigues, Anal. Chem. 63 (1991) 2589. [8] K. Jinno, K. Yamamoto, T. Kuwamoto, H. Nagashima, T. Ueda, S. Tajima, E. Kimura, K. Itoh, J.C. Fetzer, W.R. Biggs, Chromatographia 34 (1992) 381. [9] T. Takeuchi, R. Hu, T. Miwa, Chromatographia 39 (1994) 597. [10] T. Takeuchi, T. Miwa, R. Hu, J. Chu, J. High Resolut. Chromatogr. 18 (1995) 745. [11] W.P. Reed, Standard Reference Material 869; Column Selectivity Test Mixture for Liquid Chromatography, National Institute of Standards and Technology, 28 March 1990.