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Analysis of oxygen-containing polycyclic aromatic compounds by gas chromatography with atomic emission detection
Fuel 88 (2009) 348–353
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Fuel journal homepage: www.elsevier.com/locate/fuel
Analysis of oxygen-containing polycyclic aromatic compounds by gas chromatography with atomic emission detection K.D. Bartle a, S.R. Hall b, K. Holden b, S.C. Mitchell b, A.B. Ross a,* a b
Energy and Resources Research Institute, University of Leeds, Woodhouse Lane, Leeds, Yorkshire LS2 9JT, UK Department of Chemistry, University of Leeds, Leeds LS2 9JT, UK
a r t i c l e
i n f o
Article history: Received 3 March 2008 Received in revised form 21 August 2008 Accepted 21 August 2008 Available online 17 September 2008 Keywords: Gas chromatography Atomic emission detection Polycyclic aromatic furans Coal-derived oils
a b s t r a c t Atomic emission detection (AED) is a convenient method for the selective detection of oxygen-containing polycyclic aromatic compounds (O-PAC), especially furan derivatives, in the gas chromatographic (GC) analysis of coal-derived oils, although the specific response for oxygen compounds is much less than those of compounds detected from carbon emission. The use of very high purity helium as GC carrier gas is preferred, since interferences are fewer; sensitivity is improved by optimisation of cavity and transfer line temperatures. The variation of response factor with mass chromatographed is linear in the working range, but is not compound independent for O-PAC. Applications are shown of the use GC-AED in the analysis of benzo-, dibenzo- and benzonaphthofurans at low concentration in coal liquids. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Knowledge of the distribution of heteroatoms in coal-derived oils is necessary in the context of their stability and further processing, e.g. by hydrotreatment. Oxygen is the predominant heteroatom in low-rank and bituminous coals, with phenolic hydroxyl usually the principal functional group; there is also evidence for the presence of ether structures [1–3]. Polycyclic aromatic furans, both present in the original coal and formed from phenols via condensation reactions, are the principal oxygen-containing compounds in coal-processing products such as coal tar [3– 6]. While fractions containing any acidic, phenolic and carbonyl compounds can be separated by liquid chromatography (LC), oxygen-containing polycyclic aromatic compounds (O-PAC) such as aromatic ethers and furans are more difficult to isolate, and accumulate in the polycyclic aromatic hydrocarbon (PAH) fraction separated by open-column LC [7]. Gas chromatography–mass spectrometry (GC–MS) has, of course, a major role in the analysis of coal derivatives for O-PAC [3], but valuable complementary information can be provided by an element specific GC detector, especially in cases where the GC peak of a major hydrocarbon overlaps that of an analyte of interest. The atomic emission detection (AED) is the element-selective GC detector with the widest range; it allows, in principle, the detector of all elements except helium [8–10]. The GC eluent passes from the end of the capillary column via a heated transfer * Corresponding author. Tel.: +44 1133432459. E-mail address: [email protected] (A.B. Ross). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.08.014
line into a microwave-induced plasma (MIP) where compounds are atomised; the resulting atoms are in electronically excited states from which they relax with emission of radiation at a wavelength, characteristic of the element, in the visible or near-visible region, monitored by photodiode array (PDA). The PDA consists of pixels in a flat-focal plane spectrometer, and allows simultaneous detection of up to four elements. By monitoring at an appropriate wavelength selective chromatograms may be recorded for chosen elements. A computer controlled spectrometer and PDA generate real-time or background-subtracted chromatograms via automated wavelength focusing and measurements of intensity and linewidth. In this paper, we discuss the application of the atomic emission detector (AED) in the analysis of O-PAC in coal-derived oils, with emphasis on optimisation and interferences, and on detector response. GC-AED has been found particularly useful as a selective detector for sulphur, nitrogen [6,11–15] and organometallic [16] constituents of fuels, but applications to oxygen compounds have been less widespread, and mostly restricted to biomass pyrolysis products [17], transport fuels [18,19] and to phenols in coal liquids [20]. 2. Experimental 2.1. Chromatography The neutral PAC fraction was separated from coal derived oils and tars by liquid chromatography on a 20 cm 1.1 cm open column of neutral alumina (80–200 mesh, activated at 450 °C) using
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a method adapted from that of Later et al. [7]; after elution of the aliphatic with hexane, the neutral PAC fraction containing O-PAC was obtained by elution with toluene. Gas chromatography was carried out on SGE (Melbourne, Australia) BPX-5 (5% phenylmethylsiloxane) 25 m and 30 m long fused silica capillary columns with internal diameter (i.d.) 0.22 lm and 0.32 lm, 0.25 or 0.5 lm film thickness in a Hewlett Packard 5890 Series II instrument with generally 1 ll splitless injections at 260 °C by an HP 7673 autosampler. The carrier gas was 1 ml min 1 helium. The oven was held at 40°C for 2 min, then programmed at 4 °C/min to 280 °C, held for 30 min. 2.2. Atomic emission detection The GC column was coupled via a heated transfer line to an HP 5921A AED with data handling by an HP 382 ChemStation with HP 35920 GC-AED software and an HP 650 optical disk drive for data storage. The AED utilised a helium MIP contained within a polyimide-coated silica tube (42 mm long, 1.25 mm outer diameter, 1.0 mm i.d., Metlab, Liverpool, UK). The MIP was cooled with water at 65 °C and doped with reagent gases (see Section 3) to reduce chemical interference, carbon deposition and the formation of refractory oxides. The spectrometer was purged with 26 ml min 1 nitrogen (BOC). 2.3. GC–mass spectrometry Confirmatory GC–MS analyses were made on a Finnegan MAT GCQ ion-trap mass spectrometer with an A2005 autosampler. TM
Table 1 Specifications of helium GC carrier gas Impurity
5.0 grade helium (ppb)
6.0 grade helium (ppb)
UHP helium (ppb)
O2 H2O THC CO + CO2 N2
2000 1000 500 500 5000
<500 <500 <100 <100 <500
<10 <20 <50 <50 <100
studied. The 5.0 and 6.0 supplies were passed through oxygen scrubbers and regenerative moisture traps, but no on-line purification was applied to the UHP/GC+ helium other than the use of a high-purity regulator which allowed gas lines to be purged of air prior to connection. After a 12 h equilibration period, selected emission lines were monitored; these included the carbon 193 and 165 nm lines, and the 777 nm primary oxygen line, which should ideally register a background value below 500 K. Hydrocarbon impurities were monitored from the ratio of intensities of the carbon lines and showed that the 6.0 and UHP/GC+ gases contained the lowest levels; relatively high moisture levels were also found in the 5.0 grade helium. The lowest background levels of oxygen were found in the UHP/GC+ helium (Fig. 1), thus reducing the limit of detection (LOD); confirmation of the value of this highest purity gas in oxygen-selective GC/AED analysis was obtained from tests in which the 777 nm line was monitored over a nine-day period for the complete system. Signal variation was less than that of the background helium emission (318 nm) (Fig. 2), and baseline noise lev-
2.4. Standards, samples, gases and solvents O-PAC and other standard compounds were either purchased from Aldrich (Dorset, UK) or the gift of Professor Milton Lee, Brigham Young University, Provo, UT. Coal derived oils and tars were provided by Drs. K. Kubica and M. Sciazko of the Institute for the Chemical Processing of Coal, Zabrze, Poland and Dr. G. Harrison of the University of Staffordshire. These were firstly, the product oil from the treatment of Samca (Spanish) coal with a process-derived hydrogen donor solvent, and, secondly, tars from the flash pyrolysis of Belchatow and Wieczorek (Polish) coals in a circulating fluidised-bed reactor, prepared as part of a programme to develop smokeless fuels; other tars were generated from the hot briquetting of Belchatow and Wieczorek coal chars with coking Anna (Polish) coal. Solvents (Aldrich, and Riedel de Haan, Seelze, Germany) were HPLC grade. Gases were supplied by Air Products, (Crewe, UK) and BOC, (Manchester, UK).
Fig. 1. Monitoring of intensity of oxygen (777 nm) spectral emission line for different carrier gas purities.
3. Results and discussion 3.1. AED operational considerations 3.1.1. Gas supplies Manufacturer recommendations were followed in the use of hydrogen (Grade 5, Air Products), oxygen (Grade 4.5, Air Products) and 10% methane in nitrogen (Air Products) as the reagent gases during AED operation in oxygen-selective mode to minimise degradation and devitrification of the discharge tube and to avoid carbonaceous build-up. Special attention was paid to the GC mobile phase since atomisation and excitation should occur in an inert chemical environment to minimise baseline disturbance and extraneous signals from chemical interference and background molecular (e.g. CO, CN, N2) spectral emissions; three commercially available (Air Products) helium supplies (5.0, 6.0 and ultra-high purity UHP/GC+) with the specifications shown in Table 1 were
Fig. 2. Nine day variation of the helium (318 nm) and oxygen (777 nm) emission line intensities for UHP/GC+ grade helium carrier gas.
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els for the 777 nm channel were also reduced by 40% in comparison with operation with the 5.0 grade helium. A corresponding increase in signal-to-noise (S/N) ratio and decrease in LOD might be expected during oxygen compound analysis with ultra high purity helium. Similar tests (not reported in detail here) for detection of the nitrogen emission line at 174 nm revealed an even greater reduction in the background emission when operating with ultra high purity helium to only 15% of that with 5.0 grade helium; an even more significant improvement in S/N ratio and LOD is therefore expected for nitrogen than for oxygen. 3.1.2. Operating temperatures A variation of AED response with cavity temperature might be expected because the laminar-flow torch which comprises the MIP inevitably has different temperatures in different regions and this may affect residence time in the plasma. The temperature of the transfer line between GC column and the cavity may also influence response, allowing analyte condensation if too low, but resulting in decomposition of thermolabile compounds if too high. The effects of cavity and transfer line temperatures on oxygen element response in the AED were studied by varying the former between 320 and 400 °C at a fixed transfer line temperature of 320 °C, and the latter between 260 and 360 °C for a fixed cavity temperature of 360 °C. Increasing cavity temperature up to 380 °C slightly increases response for O-PAC, as has been observed for nitrogen-containing PAC [21]; increasing transfer line temperature up to 320 °C also increases O-PAC response. RSDs of peak areas from repeat injections were not significantly affected by either cavity or transfer-line temperatures. For applications in this work cavity and transfer-line temperatures of respectively 380 °C and 320 °C were employed.
Fig. 4. Spectral ‘‘snapshot” at 777 nm during elution of benzofuran.
3.2. Selectivity, elemental response and calibration In Fig. 3 carbon and oxygen-specific chromatograms are compared for a mixture of 14 PAC. Carbon (193 nm), sulphur (181 nm) and nitrogen (174 nm) traces could be recorded for the same injection with hydrogen/oxygen scavenger gas, but a second injection with hydrogen/10% methane in nitrogen as reagent gas but otherwise identical conditions was necessary for oxygen. Excellent selectivity and absence of cross-interference through ‘carryover’ is evident. The presence of oxygen in the eluted compound was unambiguously confirmed by a spectroscopic ‘snapshot’ (Fig. 4) recorded during elution of benzofuran. Replicate
Fig. 3. Element specific chromatogram for (a) carbon (193 nm) and (b) oxygen (777 nm) for 14 PAC: 1. Benzofuran, 2. Benzothiophene, 3. Quinoline, 4. Acenaphthene, 5. Dibenzofuran, 6. Diphenylsulphide, 7. Dibenzothiophene, 8. Anthracene, 9. 5,6-Benzoquinoline, 10. Acridine, 11. Diphenyldisulphide, 12. Carbazole, 13. Fluoranthene, 14. Pyrene.
K.D. Bartle et al. / Fuel 88 (2009) 348–353
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Fig. 6. Variation of oxygen response for benzofuran (BF) and dibenzofuran (DBF). Fig. 5. GC-AED element specific response for carbon (193 nm) and oxygen (777 nm) for a range of PAC over the concentration range 0.01–150 ng/ul.
analysis (n = 5) gave peak-area RSDs of 0.9–5.4% for benzofuran and dibenzofuran injections for the range 0.2–190 ng oxygen. Graphs of peak area against element concentration showed a sequence of response carbon > sulphur > oxygen > nitrogen, in the approximate ratios 1: 0.33: 0.07: 0.02; the variation of response of carbon and oxygen in PAC are compared in Fig. 5. In principle, the very high temperatures in the MIP plasma should result in the complete atomisation of analyte molecules entering from the GC column so that AED response should be independent of molecular structure. In fact, compound-independent calibration has not proved reliable [22,23] for electron-rich aromatic structures with large dissociation energies because of incomplete decomposition and/or the formation of small molecular species by recombination reactions [24]. Similar results were observed here: the response per unit mass of oxygen for benzofuran and dibenzofuran determined under the same analytical conditions differed slightly (Fig. 6) and for relatively high (>20 ng oxygen) masses injected graphs of response versus mass chromatographed
deviated from linearity. Similar results were observed for the AED response of sulphur and nitrogen isosteres of the above compounds, and it follows that calibration of the detector with a compound of similar structural type is necessary during the analysis of O-PAC and other heteroatom-containing PAC by GC/AED [24]. Limits of detection (signal/noise = 3) were determined for oxygen-selective GC/AED as 10 ng of both benzofuran and dibenzofuran (1 ng oxygen). 3.3. Applications of GC/AED in the analysis of O-PAC Illustrative examples of the use of oxygen-selective GC/AED in of the analysis of coal derived oils containing low concentrations of O-PAC include: (a) the O-PAC in a coal-liquefaction product, and (b) identification and quantitation of O-PAC in tars from the flash pyrolysis of coal. The O-PAC in the neutral PAC fraction (containing PAH, PASH and O-PAC) separated from a hydrogen-donor solvent coal liquefaction product oil by open-column chromatography were identified from GC/AED chromatogram retention data of standards and literature retention indices of Vassilaros et al. [25] on a 5% phenyl
Fig. 7. Oxygen (777 nm) specific chromatograms of the oil derived by treating Samca (Spanish) coal at 400 °C with a process-derived hydrogen donor solvent (for conditions see text).
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Fig. 8. GC-AED Oxygen (777 nm) specific chromatogram identifying oxygen containing PAC in the neutral PAC fraction isolated from the flash pyrolysis tar from Belchatow (Polish) coal: (for conditions see text).
polymethylsiloxane stationary phase as dibenzofuran and its alkylated derivatives (Fig. 7). Product tars from the flash pyrolysis of Polish coals of different rank in a circulating fluidised-bed reactor were prepared [26], and the neutral PAC fractions of the pentane solubles were analysed by GC/AED. Typical oxygen-specific chromatograms (Fig. 8) show that flash pyrolysis of all the coals produced benzofurans, dibenzofurans and benzo[b]naphthofurans (BNF), with more alkyl derivatives in the lower-rank coals. Among the group of peaks in the oxygen chromatogram at approximately 48–51 min (Fig. 8) four were identified as having molecular weight 218 from the retention time of a BNF standard and from complementary GC–MS analysis. Retention indices (RI) [3,25] allowed identification of the individual peaks as, in sequence: benzo[b]naphtho[2,1-d]furan (BNF[2,1-d], RI 352.3); BNF[1,2-d] (RI 354.5); BNF[2,3-d] (RI 356.8) and benzo[kl]xanthene (RI 359.9). As has been observed for a number of other, diverse coal derivatives [3], but not emissions from coal combustion [27], the [2,1-d] BNF isomer predominates. Quantitation using one of the parent BNF as an external standard showed O-PAC to be present in trace amounts (0.03–0.3% in the tars, 0.2– 0.1.1% in the neutral PAC fraction of the pentane solubles). GC/AED is clearly a very viable analytical procedure for O-PAC in coal derivatives. Even lower contents of O-PAC were recognised in the neutral fraction of tars resulting from the hot briquetting of pyrolysis chars with coking coal; here, BNF isomers were present in the greatest proportion. Variations in the proportions of benzofurans and benzonaphthofurans in pyrolysis tars can be correlated with coal rank and oxygen content, and with processing conditions, and GC/AED is of particular value here given the expected [28] propensity of oxygen compounds to enter the gas phase rather than the char on pyrolysis, with consequently reduced concentrations. 4. Conclusions Although their specific response is lower than that of hydrocarbons, oxygen-containing PAC cyclic ethers may be selectively detected and determined in coal-derived oils by GC/AED.
The use of high purity helium as GC carrier gas considerably reduces interference and maximizes sensitivity, which is also improved by cavity and transfer-line temperatures of 380 and 320 °C, respectively. Detector response varies linearly with mass chromatographed, but is dependent on compound identity. The small concentrations of benzo-, dibenzo- and benzonaphthofurans in coal derived oils were determined by GC/AED. Benzo[b]naphtho[2,1-d]furan was the predominant BNF isomer in the oils analysed. Acknowledgements We are grateful to the European Union, the European Coal and Steel Community and the UK Engineering and Physical Sciences Research Council for support of this work, and to Air Products for providing supplies of high-purity helium. References [1] Unsworth JF. In: Unsworth JF, Barrett DJ, Roberts PT, editors. Coal quality and combustion performance: an international perspective. Amsterdam: Elsevier; 1991 [chapter 3]. [2] Murata S, Hosokawa M, Kidena K, Nomura M. Fuel Process Technol 2000;67:231. [3] Meyer zu Reckendorf R. Chromatographia 2003;58:103. [4] Lang KF. Forsch Chem Forschung 1966;7:172. [5] Lang KF, Eigen I. Forsch Chem Forschung 1967;8:91. [6] Meyer zu Reckendorf R. Chromatographia 2000;52:67. [7] Later DW, Lee ML, Bartle KD, Kong RC, Vassilaros DL. Anal Chem 1981;53:1612. [8] Lobinski R, Adams FC. Spectrochim Acta B 1997;52:1865. [9] Quimby BD, Dryden PC, Sullivan JJ. J High Resol Chromatogr 1991;14:110. [10] Quimby BD, Sullivan JJ. In: Uden PC, editor. Element-specific chromatographic detection by atomic emission spectroscopy. Washington DC: American Chemical Society; 1992 [chapter 4]. [11] Meyer zu Reckendorf R. Chromatographia 1997;45:173. [12] Lewis AC, Askey SA, Holden KM, Bartle KD, Pilling MJ. J High Resol Chromatogr 1997;20:109. [13] Schmid B, Andersson JT. Anal Chem 1997;69:3476. [14] Becker G, Colmsjo A. Anal Chim Acta 1998;376:265. [15] Moessner SG, Wise SA. Anal Chem 1999;71:96. [16] Quimby BD, Sullivan JJ. J High Resol Chromatogr 1991;14:110. [17] Ross AB, Junyapoon N, Bartle KD, Jones J, Williams A. J Anal Appl Pyrol 2001;371:58.
K.D. Bartle et al. / Fuel 88 (2009) 348–353 [18] Quimby V, Granocco J, Sullivan JJ, McCleary KA. J High Resol Chromatogr 1993;15:705. [19] Goode SR, Thomas CL. J Anal Atom Spectrosc 1994;9:13. [20] Sumbogo Murti SD, Sakanishi K, Mochida I. Prepr Am Chem Soc Div Fuel Chem 2000;45(4):860. [21] Janak K, Ostman C, Carlsson H, Bemgard A, Colmsjo A. J High Resol Chromatogr 1994;17:135. [22] Jelink JT, Venema A. J High Resol Chromatogr 1990;13:447. [23] Schafer W. J High Resol Chromatogr 1993;16:674. [24] Janak K, Colmsjo A, Ostman C. J Chromatogr Sci 1995;33:611.
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[25] Vassilaros DL, Kong RC, Later DW, Lee ML. J Chromatogr 1982;252:1. [26] Holden KML, Bysh PJ, Bartle KD, Mitchell SC, Sciazko M, Kubica K, Czaplicka M. Final report: clean coal-derived solid fuels for domestic and power plant combustion. University of Leeds (UK) and Institute for Chemical Processing of Coal (Zabrze, Poland); 1996. [27] Grimmer G, Jacob J, Dettbarn G, Naujack KW, Fresenius Z. Anal Chem 1985;322:595. [28] Lazaro MJ, Ibarra JV, Moliner R, de Andres Gonzalez, Thomas KM. Fuel 1996;75:1014.
Contents lists available at ScienceDirect
Fuel journal homepage: www.elsevier.com/locate/fuel
Analysis of oxygen-containing polycyclic aromatic compounds by gas chromatography with atomic emission detection K.D. Bartle a, S.R. Hall b, K. Holden b, S.C. Mitchell b, A.B. Ross a,* a b
Energy and Resources Research Institute, University of Leeds, Woodhouse Lane, Leeds, Yorkshire LS2 9JT, UK Department of Chemistry, University of Leeds, Leeds LS2 9JT, UK
a r t i c l e
i n f o
Article history: Received 3 March 2008 Received in revised form 21 August 2008 Accepted 21 August 2008 Available online 17 September 2008 Keywords: Gas chromatography Atomic emission detection Polycyclic aromatic furans Coal-derived oils
a b s t r a c t Atomic emission detection (AED) is a convenient method for the selective detection of oxygen-containing polycyclic aromatic compounds (O-PAC), especially furan derivatives, in the gas chromatographic (GC) analysis of coal-derived oils, although the specific response for oxygen compounds is much less than those of compounds detected from carbon emission. The use of very high purity helium as GC carrier gas is preferred, since interferences are fewer; sensitivity is improved by optimisation of cavity and transfer line temperatures. The variation of response factor with mass chromatographed is linear in the working range, but is not compound independent for O-PAC. Applications are shown of the use GC-AED in the analysis of benzo-, dibenzo- and benzonaphthofurans at low concentration in coal liquids. Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction Knowledge of the distribution of heteroatoms in coal-derived oils is necessary in the context of their stability and further processing, e.g. by hydrotreatment. Oxygen is the predominant heteroatom in low-rank and bituminous coals, with phenolic hydroxyl usually the principal functional group; there is also evidence for the presence of ether structures [1–3]. Polycyclic aromatic furans, both present in the original coal and formed from phenols via condensation reactions, are the principal oxygen-containing compounds in coal-processing products such as coal tar [3– 6]. While fractions containing any acidic, phenolic and carbonyl compounds can be separated by liquid chromatography (LC), oxygen-containing polycyclic aromatic compounds (O-PAC) such as aromatic ethers and furans are more difficult to isolate, and accumulate in the polycyclic aromatic hydrocarbon (PAH) fraction separated by open-column LC [7]. Gas chromatography–mass spectrometry (GC–MS) has, of course, a major role in the analysis of coal derivatives for O-PAC [3], but valuable complementary information can be provided by an element specific GC detector, especially in cases where the GC peak of a major hydrocarbon overlaps that of an analyte of interest. The atomic emission detection (AED) is the element-selective GC detector with the widest range; it allows, in principle, the detector of all elements except helium [8–10]. The GC eluent passes from the end of the capillary column via a heated transfer * Corresponding author. Tel.: +44 1133432459. E-mail address: [email protected] (A.B. Ross). 0016-2361/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.fuel.2008.08.014
line into a microwave-induced plasma (MIP) where compounds are atomised; the resulting atoms are in electronically excited states from which they relax with emission of radiation at a wavelength, characteristic of the element, in the visible or near-visible region, monitored by photodiode array (PDA). The PDA consists of pixels in a flat-focal plane spectrometer, and allows simultaneous detection of up to four elements. By monitoring at an appropriate wavelength selective chromatograms may be recorded for chosen elements. A computer controlled spectrometer and PDA generate real-time or background-subtracted chromatograms via automated wavelength focusing and measurements of intensity and linewidth. In this paper, we discuss the application of the atomic emission detector (AED) in the analysis of O-PAC in coal-derived oils, with emphasis on optimisation and interferences, and on detector response. GC-AED has been found particularly useful as a selective detector for sulphur, nitrogen [6,11–15] and organometallic [16] constituents of fuels, but applications to oxygen compounds have been less widespread, and mostly restricted to biomass pyrolysis products [17], transport fuels [18,19] and to phenols in coal liquids [20]. 2. Experimental 2.1. Chromatography The neutral PAC fraction was separated from coal derived oils and tars by liquid chromatography on a 20 cm 1.1 cm open column of neutral alumina (80–200 mesh, activated at 450 °C) using
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a method adapted from that of Later et al. [7]; after elution of the aliphatic with hexane, the neutral PAC fraction containing O-PAC was obtained by elution with toluene. Gas chromatography was carried out on SGE (Melbourne, Australia) BPX-5 (5% phenylmethylsiloxane) 25 m and 30 m long fused silica capillary columns with internal diameter (i.d.) 0.22 lm and 0.32 lm, 0.25 or 0.5 lm film thickness in a Hewlett Packard 5890 Series II instrument with generally 1 ll splitless injections at 260 °C by an HP 7673 autosampler. The carrier gas was 1 ml min 1 helium. The oven was held at 40°C for 2 min, then programmed at 4 °C/min to 280 °C, held for 30 min. 2.2. Atomic emission detection The GC column was coupled via a heated transfer line to an HP 5921A AED with data handling by an HP 382 ChemStation with HP 35920 GC-AED software and an HP 650 optical disk drive for data storage. The AED utilised a helium MIP contained within a polyimide-coated silica tube (42 mm long, 1.25 mm outer diameter, 1.0 mm i.d., Metlab, Liverpool, UK). The MIP was cooled with water at 65 °C and doped with reagent gases (see Section 3) to reduce chemical interference, carbon deposition and the formation of refractory oxides. The spectrometer was purged with 26 ml min 1 nitrogen (BOC). 2.3. GC–mass spectrometry Confirmatory GC–MS analyses were made on a Finnegan MAT GCQ ion-trap mass spectrometer with an A2005 autosampler. TM
Table 1 Specifications of helium GC carrier gas Impurity
5.0 grade helium (ppb)
6.0 grade helium (ppb)
UHP helium (ppb)
O2 H2O THC CO + CO2 N2
2000 1000 500 500 5000
<500 <500 <100 <100 <500
<10 <20 <50 <50 <100
studied. The 5.0 and 6.0 supplies were passed through oxygen scrubbers and regenerative moisture traps, but no on-line purification was applied to the UHP/GC+ helium other than the use of a high-purity regulator which allowed gas lines to be purged of air prior to connection. After a 12 h equilibration period, selected emission lines were monitored; these included the carbon 193 and 165 nm lines, and the 777 nm primary oxygen line, which should ideally register a background value below 500 K. Hydrocarbon impurities were monitored from the ratio of intensities of the carbon lines and showed that the 6.0 and UHP/GC+ gases contained the lowest levels; relatively high moisture levels were also found in the 5.0 grade helium. The lowest background levels of oxygen were found in the UHP/GC+ helium (Fig. 1), thus reducing the limit of detection (LOD); confirmation of the value of this highest purity gas in oxygen-selective GC/AED analysis was obtained from tests in which the 777 nm line was monitored over a nine-day period for the complete system. Signal variation was less than that of the background helium emission (318 nm) (Fig. 2), and baseline noise lev-
2.4. Standards, samples, gases and solvents O-PAC and other standard compounds were either purchased from Aldrich (Dorset, UK) or the gift of Professor Milton Lee, Brigham Young University, Provo, UT. Coal derived oils and tars were provided by Drs. K. Kubica and M. Sciazko of the Institute for the Chemical Processing of Coal, Zabrze, Poland and Dr. G. Harrison of the University of Staffordshire. These were firstly, the product oil from the treatment of Samca (Spanish) coal with a process-derived hydrogen donor solvent, and, secondly, tars from the flash pyrolysis of Belchatow and Wieczorek (Polish) coals in a circulating fluidised-bed reactor, prepared as part of a programme to develop smokeless fuels; other tars were generated from the hot briquetting of Belchatow and Wieczorek coal chars with coking Anna (Polish) coal. Solvents (Aldrich, and Riedel de Haan, Seelze, Germany) were HPLC grade. Gases were supplied by Air Products, (Crewe, UK) and BOC, (Manchester, UK).
Fig. 1. Monitoring of intensity of oxygen (777 nm) spectral emission line for different carrier gas purities.
3. Results and discussion 3.1. AED operational considerations 3.1.1. Gas supplies Manufacturer recommendations were followed in the use of hydrogen (Grade 5, Air Products), oxygen (Grade 4.5, Air Products) and 10% methane in nitrogen (Air Products) as the reagent gases during AED operation in oxygen-selective mode to minimise degradation and devitrification of the discharge tube and to avoid carbonaceous build-up. Special attention was paid to the GC mobile phase since atomisation and excitation should occur in an inert chemical environment to minimise baseline disturbance and extraneous signals from chemical interference and background molecular (e.g. CO, CN, N2) spectral emissions; three commercially available (Air Products) helium supplies (5.0, 6.0 and ultra-high purity UHP/GC+) with the specifications shown in Table 1 were
Fig. 2. Nine day variation of the helium (318 nm) and oxygen (777 nm) emission line intensities for UHP/GC+ grade helium carrier gas.
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els for the 777 nm channel were also reduced by 40% in comparison with operation with the 5.0 grade helium. A corresponding increase in signal-to-noise (S/N) ratio and decrease in LOD might be expected during oxygen compound analysis with ultra high purity helium. Similar tests (not reported in detail here) for detection of the nitrogen emission line at 174 nm revealed an even greater reduction in the background emission when operating with ultra high purity helium to only 15% of that with 5.0 grade helium; an even more significant improvement in S/N ratio and LOD is therefore expected for nitrogen than for oxygen. 3.1.2. Operating temperatures A variation of AED response with cavity temperature might be expected because the laminar-flow torch which comprises the MIP inevitably has different temperatures in different regions and this may affect residence time in the plasma. The temperature of the transfer line between GC column and the cavity may also influence response, allowing analyte condensation if too low, but resulting in decomposition of thermolabile compounds if too high. The effects of cavity and transfer line temperatures on oxygen element response in the AED were studied by varying the former between 320 and 400 °C at a fixed transfer line temperature of 320 °C, and the latter between 260 and 360 °C for a fixed cavity temperature of 360 °C. Increasing cavity temperature up to 380 °C slightly increases response for O-PAC, as has been observed for nitrogen-containing PAC [21]; increasing transfer line temperature up to 320 °C also increases O-PAC response. RSDs of peak areas from repeat injections were not significantly affected by either cavity or transfer-line temperatures. For applications in this work cavity and transfer-line temperatures of respectively 380 °C and 320 °C were employed.
Fig. 4. Spectral ‘‘snapshot” at 777 nm during elution of benzofuran.
3.2. Selectivity, elemental response and calibration In Fig. 3 carbon and oxygen-specific chromatograms are compared for a mixture of 14 PAC. Carbon (193 nm), sulphur (181 nm) and nitrogen (174 nm) traces could be recorded for the same injection with hydrogen/oxygen scavenger gas, but a second injection with hydrogen/10% methane in nitrogen as reagent gas but otherwise identical conditions was necessary for oxygen. Excellent selectivity and absence of cross-interference through ‘carryover’ is evident. The presence of oxygen in the eluted compound was unambiguously confirmed by a spectroscopic ‘snapshot’ (Fig. 4) recorded during elution of benzofuran. Replicate
Fig. 3. Element specific chromatogram for (a) carbon (193 nm) and (b) oxygen (777 nm) for 14 PAC: 1. Benzofuran, 2. Benzothiophene, 3. Quinoline, 4. Acenaphthene, 5. Dibenzofuran, 6. Diphenylsulphide, 7. Dibenzothiophene, 8. Anthracene, 9. 5,6-Benzoquinoline, 10. Acridine, 11. Diphenyldisulphide, 12. Carbazole, 13. Fluoranthene, 14. Pyrene.
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Fig. 6. Variation of oxygen response for benzofuran (BF) and dibenzofuran (DBF). Fig. 5. GC-AED element specific response for carbon (193 nm) and oxygen (777 nm) for a range of PAC over the concentration range 0.01–150 ng/ul.
analysis (n = 5) gave peak-area RSDs of 0.9–5.4% for benzofuran and dibenzofuran injections for the range 0.2–190 ng oxygen. Graphs of peak area against element concentration showed a sequence of response carbon > sulphur > oxygen > nitrogen, in the approximate ratios 1: 0.33: 0.07: 0.02; the variation of response of carbon and oxygen in PAC are compared in Fig. 5. In principle, the very high temperatures in the MIP plasma should result in the complete atomisation of analyte molecules entering from the GC column so that AED response should be independent of molecular structure. In fact, compound-independent calibration has not proved reliable [22,23] for electron-rich aromatic structures with large dissociation energies because of incomplete decomposition and/or the formation of small molecular species by recombination reactions [24]. Similar results were observed here: the response per unit mass of oxygen for benzofuran and dibenzofuran determined under the same analytical conditions differed slightly (Fig. 6) and for relatively high (>20 ng oxygen) masses injected graphs of response versus mass chromatographed
deviated from linearity. Similar results were observed for the AED response of sulphur and nitrogen isosteres of the above compounds, and it follows that calibration of the detector with a compound of similar structural type is necessary during the analysis of O-PAC and other heteroatom-containing PAC by GC/AED [24]. Limits of detection (signal/noise = 3) were determined for oxygen-selective GC/AED as 10 ng of both benzofuran and dibenzofuran (1 ng oxygen). 3.3. Applications of GC/AED in the analysis of O-PAC Illustrative examples of the use of oxygen-selective GC/AED in of the analysis of coal derived oils containing low concentrations of O-PAC include: (a) the O-PAC in a coal-liquefaction product, and (b) identification and quantitation of O-PAC in tars from the flash pyrolysis of coal. The O-PAC in the neutral PAC fraction (containing PAH, PASH and O-PAC) separated from a hydrogen-donor solvent coal liquefaction product oil by open-column chromatography were identified from GC/AED chromatogram retention data of standards and literature retention indices of Vassilaros et al. [25] on a 5% phenyl
Fig. 7. Oxygen (777 nm) specific chromatograms of the oil derived by treating Samca (Spanish) coal at 400 °C with a process-derived hydrogen donor solvent (for conditions see text).
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Fig. 8. GC-AED Oxygen (777 nm) specific chromatogram identifying oxygen containing PAC in the neutral PAC fraction isolated from the flash pyrolysis tar from Belchatow (Polish) coal: (for conditions see text).
polymethylsiloxane stationary phase as dibenzofuran and its alkylated derivatives (Fig. 7). Product tars from the flash pyrolysis of Polish coals of different rank in a circulating fluidised-bed reactor were prepared [26], and the neutral PAC fractions of the pentane solubles were analysed by GC/AED. Typical oxygen-specific chromatograms (Fig. 8) show that flash pyrolysis of all the coals produced benzofurans, dibenzofurans and benzo[b]naphthofurans (BNF), with more alkyl derivatives in the lower-rank coals. Among the group of peaks in the oxygen chromatogram at approximately 48–51 min (Fig. 8) four were identified as having molecular weight 218 from the retention time of a BNF standard and from complementary GC–MS analysis. Retention indices (RI) [3,25] allowed identification of the individual peaks as, in sequence: benzo[b]naphtho[2,1-d]furan (BNF[2,1-d], RI 352.3); BNF[1,2-d] (RI 354.5); BNF[2,3-d] (RI 356.8) and benzo[kl]xanthene (RI 359.9). As has been observed for a number of other, diverse coal derivatives [3], but not emissions from coal combustion [27], the [2,1-d] BNF isomer predominates. Quantitation using one of the parent BNF as an external standard showed O-PAC to be present in trace amounts (0.03–0.3% in the tars, 0.2– 0.1.1% in the neutral PAC fraction of the pentane solubles). GC/AED is clearly a very viable analytical procedure for O-PAC in coal derivatives. Even lower contents of O-PAC were recognised in the neutral fraction of tars resulting from the hot briquetting of pyrolysis chars with coking coal; here, BNF isomers were present in the greatest proportion. Variations in the proportions of benzofurans and benzonaphthofurans in pyrolysis tars can be correlated with coal rank and oxygen content, and with processing conditions, and GC/AED is of particular value here given the expected [28] propensity of oxygen compounds to enter the gas phase rather than the char on pyrolysis, with consequently reduced concentrations. 4. Conclusions Although their specific response is lower than that of hydrocarbons, oxygen-containing PAC cyclic ethers may be selectively detected and determined in coal-derived oils by GC/AED.
The use of high purity helium as GC carrier gas considerably reduces interference and maximizes sensitivity, which is also improved by cavity and transfer-line temperatures of 380 and 320 °C, respectively. Detector response varies linearly with mass chromatographed, but is dependent on compound identity. The small concentrations of benzo-, dibenzo- and benzonaphthofurans in coal derived oils were determined by GC/AED. Benzo[b]naphtho[2,1-d]furan was the predominant BNF isomer in the oils analysed. Acknowledgements We are grateful to the European Union, the European Coal and Steel Community and the UK Engineering and Physical Sciences Research Council for support of this work, and to Air Products for providing supplies of high-purity helium. References [1] Unsworth JF. In: Unsworth JF, Barrett DJ, Roberts PT, editors. Coal quality and combustion performance: an international perspective. Amsterdam: Elsevier; 1991 [chapter 3]. [2] Murata S, Hosokawa M, Kidena K, Nomura M. Fuel Process Technol 2000;67:231. [3] Meyer zu Reckendorf R. Chromatographia 2003;58:103. [4] Lang KF. Forsch Chem Forschung 1966;7:172. [5] Lang KF, Eigen I. Forsch Chem Forschung 1967;8:91. [6] Meyer zu Reckendorf R. Chromatographia 2000;52:67. [7] Later DW, Lee ML, Bartle KD, Kong RC, Vassilaros DL. Anal Chem 1981;53:1612. [8] Lobinski R, Adams FC. Spectrochim Acta B 1997;52:1865. [9] Quimby BD, Dryden PC, Sullivan JJ. J High Resol Chromatogr 1991;14:110. [10] Quimby BD, Sullivan JJ. In: Uden PC, editor. Element-specific chromatographic detection by atomic emission spectroscopy. Washington DC: American Chemical Society; 1992 [chapter 4]. [11] Meyer zu Reckendorf R. Chromatographia 1997;45:173. [12] Lewis AC, Askey SA, Holden KM, Bartle KD, Pilling MJ. J High Resol Chromatogr 1997;20:109. [13] Schmid B, Andersson JT. Anal Chem 1997;69:3476. [14] Becker G, Colmsjo A. Anal Chim Acta 1998;376:265. [15] Moessner SG, Wise SA. Anal Chem 1999;71:96. [16] Quimby BD, Sullivan JJ. J High Resol Chromatogr 1991;14:110. [17] Ross AB, Junyapoon N, Bartle KD, Jones J, Williams A. J Anal Appl Pyrol 2001;371:58.
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