BIOMEDICAL AND ENVIRONMENTAL MASS SPECTROMETRY, VOL. 15, 517-519 (1988)

S H O R T C O M M U N I C A T I O N S

Analysis of the Polycyclic Aromatic Compounds of Diesel Fuel by Gas Chromatography with Ion-trap Detection

Paul T. Williams, Gordon E. Andrews and Keith D. Bartle Departments of Fuel and Energy and Physical Chemistry, University of Leeds, Leeds LS2 9JT, UK

Paul Bishop and Peter Watkins Finnigan MAT Ltd, Hemel Hempstead, Herts HP2 4TG, UK

Capillary column gas chromatography combined with ion-trap detector mass spectrometry was used in the identifi- cation of 85 constituents of the polycyclic aromatic hydrocarbon fraction of diesel fuel. The main constituents are an extensive range of alkylnaphthalenes; alkylated tetrahydronaphthalenes, benzothiophens, dibenzothiophens biphenyls, fluorenes and phenanthrenes were also identified. Chromatographic retention indices for these com- pounds were determined to calibrate retention data for routine analysis with flame-ionization detection, and to identify positions of alkyl substitution.

INTRODUCTION

The potential health hazard associated with diesel exhaust particulates is derived mainly from the poly- cyclic aromatic compounds (PAC) present in the solvent organic fraction,'.' some of which are known to be mutagenic and/or car~inogenic.~ PAC have been identi- fied and quantified in both diesel fuelL6 and exhaust

and recent work has shown that many of the PAC in diesel particulate are derived mainly from unburned fuel components rather than pyrosynthesis during combustion.

Detailed knowledge of the composition of the aro- matic fraction of diesel fuel is therefore required, and is usually obtained by capillary-column gas chromatog- raphy. Such an analysis is made difficult by the com- plexity of the mixture and by the widely different concentrations of the constituents: the five-ring PAC are present at a level three orders of magnitude less than the two-three ring

Preliminary separation using high-performance liquid chromatography (HPLC) is therefore usually necessary before gas chromatography (GC), but such steps inevit- ably introduce the possibility of losses and increased analysis time, unless coupled LC/GC9 is employed. The ideal analytical procedure for mixtures of such complex- ity as the aromatics of diesel fuel is coupled capillary gas chromatography/mass spectrometry, which com- bines excellent chromatographic resolution with posi- tive identification.

The ion trap.detector (ITD) has many of the advan- tages of traditional mass spectrometry but at lower cost. The ITD utilizes electric fields to hold the ions within the ion storage region. The ITD is then scanned through the mass range, causing the ions to be ejected

0 1988 by John Wiley & Sons Ltd 0887-61 34/88/09051743 %05.00

from this region sequentially, from low to high mass. The ejected ions are detected by a conventional electron multiplier. Thus the characteristic of the ITD is that ionization and mass analysis take place in the same space. This contrasts with a conventional mass spec- trometer which requires a separate ionization source, focusing lenses and analyser, and associated low mech- anical tolerances, with consequent high costs.

In this paper we report the analysis of the aromatic fraction of diesel fuel by combined capillary GC/ITD.

EXPERIMENTAL

Diesel fuel

The fuel was sampled from a stock of 500 gallons of diesel fuel held at Leeds University Diesel Research Test Facility. The fuel was obtained from the Perkins Diesel Engine Company, Peterborough. The fuel was A2 grade with a cetane index of 47.97, density 859 kg rnp3 and hydrogen content 13.05%.

Isolation of the P A C fraction

A PAC fraction was isolated from the fuel as follows. A 10-cm3 aliquot of the fuel was adsorbed on to a 45 cm x 2.2 cm column of 0.125-0.250 mm silica gel pre- viously deactivated by heating at 200°C for 48 h and wetted with n-pentane. Aliphatics were eluted with 1-dm3 n-pentane, or until the fluorescence under ultra- violet light extended throughout the column, and then aromatics were eluted with 1 dm3 benzene. The n- pentane and benzene eluates were evaporated under

Received 9 June 1987 Revised 21 September 1987

518 P. WILLIAMS ET AL.

vacuum and then blown down with nitrogen to 10 cm3 and 5 cm3, respectively, before analysis.

A I , . . 1

Analysis of the PAC fraction

Analysis of the fuel PAC fraction was by two methods: capillary column GC and combined capillary column GC/ITD.

The gas chromatograph was a Carlo Erba Mega Series HRGC 5300 equipped with cold on-column injection and flame ionization detection. The column was a 25-m SE54 cross-linked capillary. The coupled GC/ITD system utilized a Varian capillary gas chro- matograph with splitless injection. The GC column was a 10-m SE54 with a heated transfer line to a Finnigan MAT Model 705B ITD. The ITD used the mass range from 20 to 650 with a scan speed of 0.25 s. The ITD was linked to an IBM PC/XT computer with an NBS/EPA mass spectral library containing 38 752 mass spectra.

RESULTS AND DISCUSSION

Peak identification with ITD detection

The computer software associated with the ITD allows output of the total-ion current chromatogram (Fig. 1) and background subtraction to yield mass spectra which are of individual peaks, often identified by automated search of the mass spectra in the library (Fig. 2).

I I \

1 I I I

30 40 50 60

Figure 1. Ion trap detector total ion current chromatogram of polycyclic aromatic compound fraction of diesel fuel. Inset: expanded portion of chromatogram. Peak identifications: (1 ) methyldibenzothiophene; (2) C,-biphenyl; (3) 3- methylphenanthrene; (4) 2-methylphenanthrene; (5) 9- or 4- methylphenanthrene; (6) 1 -methylphenanthrene.

Figure 2. (a) Ion trap detector mass spectrum after background subtraction of a single peak in chromatogram of polycyclic aro- matic compound fraction of diesel fuel. (b) Mass spectrum of 1.2- dimethylnaphthalene matched to spectrum (a) by automated library search.

Compounds identified in the aromatic fraction of diesel fuel from ITD mass spectra are listed in Table 1. The complex mixture of diesel fuel aromatics containing

Table 1. Polycyclic aromatic compounds identified in the am- matic fraction of diesel fuel

Compound

Naphthalene Methyltetrahydronaphthalene 2-Methylnaphthalene 1 -Methylnaphthalene Dimethyltetrahydronaphthalene Dimethyltetrahydronaphthalene Dimethyltetrahydronaphthalene Dimethyltetrahydronaphthalene Dimethyltetrahydronaphthalene Biphenyl 2-Ethylnaphthalene C,- Benzothiophen 2,6- or 2.7-Dimethylnaphthalene 1.7-Dimethylnaphthalene 1,3- or 1,6-Dimethylnaphthalene Methylbiphenyl 1,4- or 2.3-Dimethylnaphthalene 1.5- Dimethylnaphthalene Trimethyltetrahydronaphthalene 1.2- Dimethylnaphthalene C,-Biphenyl Trimethyltetrahydronaphthalene Methylbiphenyl C,-Naphthalenes

2.3.6-Trimethylnaphthalene C,-Naphthalene C,-Biphenyl Fluorene C,-Naphthalenes C,-Biphenyls

9- Methylfluorene C,-Naphthalenes

2- Methylfluorene 1 - Methylfluorene C,-Biphenyl C,-Naphthalene

Measured retention

index

200.0 21 4.4 220.2 223.2 224.6 229.0 229.6 231.5 232.2 236.4 239.0 237.7 240.5 242.8 243.4 245.2 246.3 246.8 247.7 248.5 250.5 251.2 254.0 255.7, 256.3, 256.9, 257.8, 258.8, 259.8, 260.9, 261.9, 262.6, 264.0 265.0 266.3 269.0 269.7 270.0, 270.4 271.4. 272.0, 273.0 273.5 275.2, 276.2, 276.7, 279.5, 280.4, 280.9, 283.7, 286.1

288.0

289.5

287.5

288.9

Literature retention

index

200.0

220.2 223.0

236.4 238.6

240.3

243.3

246.0 246.9

242.8

248.5

265.1

269.7

273.8

288.4 289.2

ANALYSIS OF DIESEL FUEL 519

Table 1. (cont)

Compound

C,-Biphenyls

C,-Naphthalene Dibenzothiophene C,-Naphthalene Phenanthrene C,-Biphenyl C,-Fluorene C,-Biphenyl C,-Fluorene C,-Biphenyl Methyldi benzothiophen C,- Biphenyl 3- Methylphenanthrene 2- Methylphenanthrene 9- or 4-Methylphenanthrene 1 - Methylphenanthrene MW 208

C,- Dibenzothiophen C,- Phenanthrenes

C,- Phenanthrenes

Measured retention

index

290.2, 291.4, 292.1, 292.9 294.8 295.8 297.6 300.0 304.9

307.4

31 0.7 31 2.1 31 6.4

31 9.7 322.7 323.6

329.4 335.1

340.5, 343.1

305.4. 306.8

308.4

31 8.0

326.6, 328.7.

335.9, 338.1,

354.1. 356.8

Literature retention

index

296.0

300.0

31 9.2 31 9.9

323.6 322.8

preliminary fractionation. Both major and minor con- stituents may be identified: these are naphthalene, ben- zothiophen, dibenzothiophen, biphenyl, fluorene and phenanthrene and their alkylated derivatives containing up to five substituent carbon atoms. The extensive range of alkylnaphthalenes (C,-C,) is especially note- worthy; at least five Cz naphthalenes, 14 C, naphtha- lenes and nine C, naphthalenes are present. The mass spectra also show the presence of tetrahydro- naphthalenes and their C1-C3 derivatives.

These results are particularly valuable in extending and complementing identification methods based on gas chromatographic retention indices. GC with ITD detec- tion provides a calibration for retention data so that routine analysis may be made with the aid of flame ion- ization detection. Retention indices based on the Lee system" (naphthalene = 200.00, phenanthrene = 300.00, chrysene = 400.00) for 85 constituents of the aromatic fraction of diesel fuel are listed in Table 1. Comparison with retention indices of standards allows the positions of substitution of some of the alkylated derivatives to be determined.

Acknowledgement

compounds with between two and five 2WX'natic rings in We thank the Science and Engineering Research Council for support widely differing concentrations were identified without of this work through grants (GR/C/59208 and GR/C/91444).

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