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UTTERWORTH

EINEM NN

0016-2361 95)00108-5

Fuel Vol. 74 No. 10, 1436-1451, 1995p.Copyright 0 199 5 Elsevier Science Ltd

Printed in Great Britain. Allrights reserved0016-2361/95/%10.00+0.00

Temperature programmed retention indices

for g.c. and g.c. m.s. analysis of coal andpetroleum derived liquid fuels

Wei-Chuan Lai and Chunshan SongFuel Science Program, 209 Academic Projects Building, Pennsylvania State University,University Park, PA 16802, USAReceived 12 July 1993; revised 30 November 1994)

Retention indices are very useful in identifying compon ents in liquid fuels by gas chrom atograp hy (g.c.),even when g.c. is coupled with mass spectrom etry (g.c.-m.s.). In this work , temperature-prog ramme dretention indices of over 150 compo unds were determined on an intermediately polar capillary columncoated with 50% phenyl-50% methyl polysiloxane (Rtx-50) and a slightly polar column coated with 5%phenyl-95% methyl polysiloxane (DB-5) at three heating rates (2, 4 and 6 Cmin- from 40 to 310C ).Aliphatic compo unds give nearly constant retention indices at different heating rates. How ever, theretention indices of polycyclic aromatic compo unds exhibit a relatively large temperatu re depende nce. Theuse of a short isotherm al hold (5min) prior to the program med heat-up did not cause any significantdifference in the retention indices. The column polarity can affect the retention indices significantly,depending on the compou nd type. The differences between th e retention indices on the two columns arerelatively small with aliphatic co mpoun ds but becom e larger with polycyclic and polar compo unds. Ingeneral, retention indices and their sensitivity to temperatu re programm ing decrea se with decreasing columnpolarity. Th e usefulness of the temperatu re-program med retention indices was also demon strated in theanalysis of liquid fuels. Combined use of retention indices and mass spectra allows the identification of manymore compounds with higher confidence in petroleum- and coal-derived JP-8 jet fuels. The know ledge onthe effects of tempera ture and column polarity can be applied for selecting appropriate column andtemperatu re program mes for the separation and reliable identification of compou nds in given samples. Inaddition, the present results can be used in combination with a mass spectral library to accomp lish faster andmore reliable compound identification.

(Keywords: retention index; gas chromatography; uel)

Modern gas chromatography-mass spectrometry (g.c.-m.s.) has contributed greatly to the analysis of variousmixtures of organic compounds. However, many com-poun ds in coal- and petroleum-derived fuels are difficultto identify just by mass spectra alone. A commonphenom enon in the g.c.-m.s. of fuels is that two ormore g.c. peaks have very similar or even identical massspectra, altho ugh they have different retention times anddifferent chemical n atures. Examples of such compoundsare the isomers of some cycloalkanes; the isomers ofsubstituted aromatics such as trimethylbenzenes (MW120), methylnaphthalenes (MW 142) and dimethyl-naphthalenes (MW 156); different polyaromatics withsame molecular weigh t but different ring structures, suchas biphenyl and acenaphthene (MW 154) and phenan-threne and anthracene (MW 178); and partially hydro-genated aromatics such as sym-octahydrophenanthreneand sym-octahydroanthracene (MW 186). Thus, usingg.c.-m.s. for compo und identification often requires theassistanc e or confirmation of g.c. retention times. On theother h and, in many cases highly reliable identificationcannot be made by using retention times alone. This is

because co-elution of two or more different compou nds ispossible under given conditions, and whether this occurscannot be determined by g.c. alone for unknown samples.For these reasons, retention times and mass spectra arecomplem entary to each other. The combin ed use ofretention indices and mass spectra can allow theidentifications of individual com pounds in complexmixtures to be made with high confidence.

The present work is concerned with retention indicesfor the tempera ture-program med g.c. and g.c.-m.s.analysis of liquid fuels. This work is a part of an ongo ingresearch programm e at Pennsylva nia State University forinvestigatin g the comp ositional factors that affect thetherma l stability of jet fuels. The sources of futurehydrocarbon jet fuels may include petroleum, coal andother fossil resources1-5. Future high-performa nce air-craft require ad vanced thermally stable jet fuel&.Clarifying the hydrocarbon compo nents in conventiona land alternative jet fuels and identifying the thermallystable and unstable compounds are considered to be keysteps in developing advanced jet fuels with high thermalstability and high density for future high-performance

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GC retention indices of fuels: W.-C. Lai and C. Song

aircraft-15. The need to characterize coal- and petro-leum-derived jet fuels stems from the lack of know ledge oftheir molecular composition and the desire to establishthe relationship b etween their compo sition/structure andthermal stability at high temperatures. Hayes andPitzeri617 and Steward and Pitzer have analysed

several petroleum- and shale-derived JP-4 jet fuels usingcapillary gc. These fuels are deceptively simple inappearance but are actually m ixtures of hundreds ofhydrocarbo ns. They indicated that, even for petroleum-derived jet fuels, the detailed hydrocarbon distribution isfar too complicated to be unravelled by even the mostefficient capillary column 6. The coal-derived fuels aremore complex tha n petroleum-de rived fuels15. There islittle publishe d inform ation on the detailed characteriza -tion of coal-derived jet fuels.

A common practice of qualitative g.c. analysis is the useof retention times. How ever, it is well know n thatretention time depen ds on several factors, e.g. tempera-

ture and flow rate. Thus retention time itself is not an idealparam eter for identification purpose s, especially forcomplex samples such as jet fuels; parameters that areless dependent on these factors are needed. A usefulparam eter is the relative retention time instead of theabsolute retention time. The retention index9,20 system isa measu re of relative retention times referenced to ahomologous series of organic com pounds, and is one ofthe sytc;s$u$, param eters used fios:Tidentification pur-poses . Hayes and Pitzer demonstrated theusefulness of determin ing retention indices for identifyingcompounds in complex mixtures. Although many g.c. andg.c.-m.s. analyses are carried out under isotherm al

conditions, temperature programming26 is known toimprove the separation of complex mixtures such asliquid fuels whose components have widely varyingvapour pressures.

The Kovats retention index relates (interpolates) theretention time of an unknown compound to that ofreference standa rds eluting before and after it underisotherma l conditions. Any homolog ous series of organiccompounds (e.g. n-alkanes) can be used as retention indexreference standards. Each of the standards is assigned anindex I, e.g. Z = 1OOn or a given alkane with a carbonnumber n. The retention index of any unknowncompound is then calculated by logarithmic interpola-tion between the two relevant standards according to thefollowing relationship under isothermal conditions:

z, = 100[

log t, - log t,log &+I - log t,

+ n

where n and n + 1 are the carbon numbers of thebracketing n-hydrocarbons, t, is the absolute retentiontime of the unknown under isothermal conditions andt,and tn+l are the absolute retention times of the alkane s,eluted just prior to and just after the unknown,respectively, under isotherma l conditions. However, incontrast to the logarithmic relationship under isothermalconditions, a quasi-line ar relationship exists for a non-isothermal g.c. analysis w ith a linear temperatureprogramme, as shown by Van den Do01 and Kratz27.

The present work w as aimed at establishing retentionindices for temperatu re-program med g.c. and g.c.-m.s.analysis on two different capillary colum ns at threeprogrammed heating rates for over 150 compounds that

are compo nents of coal- and petroleum-de rived liquidfuels. The effects of heating rate, temperature program meand polarity of the stationary phase o n the retentionindices and the relative elution order were studied. Thetemperature dependence and the column polarity depen-dence of retention indices were examin ed for different

compound classes. The data presented are temperature-programmed retention indices of I-alkenes, n-alkanes,alkylcyclohexanes, alkylbenze nes, polycyclic aroma tics,partially hydrogen ated polycyclic aroma tics an d N-, S-and O-containing compounds. Also presented are theretention indices of many compounds that were not usedin previous studies but are generally found in coal-derivedliquid fuels. The usefulness of using temperature-program med retention indices for g.c. and g.c.-m.s.analysis was demonstrated in the detailed identificationof compo nents in two JP-8 jet fuels, namely a petroleum-derived JP-8P and a coal-derived JP-8C. The resultsreported here are believed to be useful to researchers in

the field of the chroma tographic analysis of liquid fuels.

EXPERIMENTAL

Reagents and uels

Reagent-grade chemicals from Aldrich, Supelco, K&KLaboratories, TCI Ame rica and Fisher Scientific wereused for the determina tion of retention indices. Over 150pure compounds were examined, covering aliphatic,olefinic, aroma tic a nd polycyclic aromatic hydrocarbon sand 0-, N- and S-containing compounds that are relatedto coal- and petroleum-derived liquid fuels. The jet fuel

samples used were two JP-8 fuels supplied by the AirForce Wright Laboratory/Aero Propulsion and PowerDirectorate. The coal-derived fuel, JP-8C , wa s producedby hydrotreating tar liquids produced at the Great PlainsGasification plant1 428. The petroleum-derived JP-8P is aconventional military jet fuel, whose properties areusually selected to be identical with those of comm ercialjet fuel Jet A -129. The JP-8 C fuel is a blend ofhydrotreated and hydrocracked stocks14 . The chromato-graphic fractions of JP-8P an d JP-8C fuels were obtainedusing a neutral alumina column and a series of elutionsolvents. as described elsewhe rei5.

Reten t ion i ndexStandard mixtures of reagent-grade chemicals were

prepared for the determin ation of retention indices. Thestandard mixtures w ere analysed on a Hewlett-Packard5890 Series II gas chromatograph coupled with an HP5971A mass-selective detector (MSD). The columns usedwere 30m x 0.25mm i.d., 0.25Z~m film thickness, fused-silica capillary columns (intermediately polar Restek Rtx-50 column coated with 50% phenyl-50% methylpolysiloxane; slightly polar J&W DB-5 column coatedwith 5% phenyl-95% methyl polysiloxane). The columntemperature was programmed linearly from 40 to 310Cat three heating rates (2, 4 and 6 Cmin-), unless statedotherwise. Retention index values calculated from threedifferent h eating rates can be used as cross-references toknown materials and aid in identifying unknowncompounds. Other chromatographic operating condi-tions were as follows: detector tem perature, 280C ;injector tempera ture, 280C ; average linear velocity ofcarrier gas (helium) through the column, 33cm s-l;

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Table 1 Retention indices of various compounds determined for intermediately polar (Rtx-50) and slightly polar (DB -5) columns at three heatingrates

CompoundMolecularion m/z)

n-Pentane 72

n-Hexane 86

I-Hexene a4

n-Heptane 100

Cyclohexane 84

I-Heptene 98

Cyclohexene 82

Methylcyclohexane 98

Benzene 78

n-Octane 114

trans- 1,4-Dimethylcyclohexane 112

c&1,3-Dimethylcyclohexane 112

I-Octene 112trns-1,2-Dimethylcyclohexane 112

I-Methylcyclohexene 96

cis-1,4-Dimethylcyclohexane 112

rruns-1,3-Dimethylcyclohexane 112

cis- 1,2-Dimethylcyclohexane 112

Ethylcyclohexane 112

Toluene 92

n-Nonane 128Pyridine 79

1 Nonene 126

n-Propylcyclohexane 126

Ethylbenzene 1061,4-Dimethylbenzene (p-xylene) 106

1,3_Dimethylbenzene (m-xylene) 106

n-Decane 142

1,2-Dimethylbenzene (o-xylene) 106

1 Decene 140

rrans-Octahydro-lH-indene 124

tert-Butylcyclohexane 140

Isopropylbenzene (cumene) 120

cis-Octahydro-1 H-indene 124

n-Propylbenzene 120

n-Butylcyclohexane 140

I-Ethyl-3-methylbenzene 120


1,3,5_Trimethylbenzene (mesitylene) 120

tert-Butylbenzene 134

4,5,6,7_Tetrahydroindan 122


n-Undecane 156

1,2,4-Trimethylbenzene 120

1Undecene 154

set-Butylbenzene 134

truns-Decalin 138

1, I-Bicyclopentyl 138

1,2,3_Trimethylbenzene 120

Phenyl ethyl ether 122

n-Butylbenzene 134

n-Pentylcyclohexane 154

cis-Decalin 138

Indan 118

-

R.Z. on Rtx-50 R.Z. on DB-5

Baseoeak m/z) 2C min- 4C min- 6C min- 2C min- 4C min- 6C min-

43

43

56

43

56

56

67

83

78

43

97

97

5597

81

97

97

55

83

91

43

79

56

83

9191

91

43

91

56

67

56

105

67

91

83

105

105

105

119

79

105

57

105

41105

138

68105

94

9183

67117

500 500 500 500 500 500

600 600 600 600 600 600

606.4 607.0 60 7.7 588.2 587.9 587.9

700 700 700 700 700 700

705.6 706.3 706.0 655.4 656.2 657.6

708.3 708.4 709.5 688.0 689.0 689.4736.1 740.0 739.3 677.1 676.7 678.8748.1 749.5 752.4 718.3 720.0 720.3

765.7 767.4 769.0 654.2 654.8 656.1800 800 800 800 800 800801.8 802.3 803.5 775.6 776.7 777.3801.8 802.3 803.5 772.8 774.7 776.6

808.2 809.2 809.7 788.3 789.3 789.1820.9 822.4 825.0 796.7 798.0 798.4824.1 827.0 829.2 763.9 765.3 766.4828.6 830.5 832.6 804.7 805.3 805.7830.9 832.2 835.4 804.1 805.3 806.2

859.1 862.6 866.7 823.1 824.7 827.3862.3 865.5 867.4 826.9 829.1 830.9870.5 873.6 875.7 758.9 761.3 761.7900 900 900 900 900 900908.9 910.3 913.4 735.6 736.7 739.1909.1 909.5 909.9 888.9 889.5 890.2956.9 960.8 963.4 924.2 927.1 929.1

968.1 971.5 975.2 853.2 856.3 857.2972.3 975.7 979.2 861.7 864.4 866.5974.2 977.9 981.2 861.7 864.4 866.5

1000 1000 1000 1000 1000 10001009.2 1012.5 1015.5 888.0 889.9 891.21010.3 1011.0 1011.3 989.2 989.5 990.71011.6 1015.9 1018.8 947.7 952.0 955.31015.1 1019.3 1022.6 978.1 982.2 985.21028.2 1032.1 1034.7 919.4 922.2 923.6

1053.0 1059.9 1063.2 981.0 985.8 929.51058.0 1063.0 1066.1 946.9 950.5 952.3

1059.5 10 64.2 10 66.5 1026.6 1030.2 1031.6

1068.7 1073.1 1075.7 955.0 958.5 960.8

1069.8 1074.0 1076.6 956.3 960.0 962.4

1074.4 1079.2 1081.6 961.5 964.9 967.1

1097.6 1102.3 1104.5 987.1 990.5 992.0

1097.8 1103.7 1106.9 1008.6 1011.4 1014.8

1098.2 1103.1 1105.3 973.3 977.2 979.7

1100 1100 1100 1100 1100 1100

1106.8 1111.9 1114.6 987.5 991.1 992.8

1110.6 1 111.0 11 11.0 1090.9 1091.4 1091.4

1112.9 1 117.6 11 20.3 1006.4 1008.9 1011.7

1114.7 1122.4 1127 .2 1045.7 1051.8 1055.9

1138.6 1145.9 1151.2 1071.8 1077.6 1080.9

1147.6 1155.0 1158.9 1016.0 1019.9 1022.7

1150.0 1154.1 1156.9 988.8 991.1 992.4

1163.2 1168.6 1171.1 1050.1 1053.7 1055.9

1163.7 1169.4 1172.0 1130.2 1134.7 1136.0

1174.6 1184.4 1190.2 1089.5 1096.1 1100.4

1180.4 1188.7 1193.5 1027.4 1032.4 1035.5

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Table 1 Continued

R.I. on Rtx-50 R.I. on DB-5

CompoundMolecular Baseion (m/z) peak (m/z) 2C min- 4C mini 6C min- 2C min- 4C min- 6C min-

n-Dodecane 170 57 1200 1200 1200

1 Dodecene 168 55 1211.6 1212.4 1211.92-Methylindan 132 117 1212.8 1220.8 1225.8

l-Methylindan 132 117 1222.7 1231.2 1236.5

1,2,4,5_Tetramethylbenzene 134 119 1235.1 1242.4 1247.5

n-Hexylcyclohexane 168 83 1269.0 1274.4 1276.6

1,4-Diisopropylbenzene 162 147 1279.9 1285.1 1288.1

5-Methylindan 132 117 1282.5 1292.7 1298.4

1,2,3,4-Tetramethylbenzene 134 119 1286.4 1296.3 1302.6

2,6-Dimethylphenol 122 107 1297.6 1305.3 1310.7

n-Tridecane 184 57 1300 1300 1300

4-Methylindan 132 117 1301.2 1312.0 1318.8

1 Tridecene 182 55 1311.8 1312.0 1312.4

Tetralin 132 104 1324.1 1336.0 1343.6

1,3,5_Triethylbenzene 162 147 1338.5 1343.6 1346.2

1,2_Dihydronaphthalene 130 130 1345.7 1358.8 1366.2

n-Hexylbenzene 162 91 1370.1 1376.3 1379.9

Bicyclohexyl 166 82 1377.5 1390.4 1397.4

Naphthalene 128 128 1386.8 1401.2 1410.9

2,4,6-Trimethylphenol 136 121 1397.4 1406.7 1412.2

n-Tetradecane 198 57 1400 1400 1400

1-Tetradecene 196 55 1412.5 1412.8 1412.7

Benzothiophene 134 134 1415.7 1432.0 1442.5

2,3-Dihydroindole 119 118 1455.5 1469.2 1479.2

Cyclohexylbenzene 160 104 1475.5 1489.0 1497.7

I-lndanol 134 133 1476.6 1489.0 1496.8

n-Octylcyclohexane 196 83 1480.1 1485.7 1489.6

Quinoline 129 129 1493.6 1510.6 1521.6

n-Pentadecane 212 57 1500 1500 1500

2-Methylnaphthalene 142 142 1501.0 1517.3 1528.6

I-Pentadecene 210 55 1512.7 1513.1 1513.1

1-Methylnaphthalene 142 142 1530.2 1548.7 1559.6

Dodecahydrofluorene 178 97 1556.9 1576.9 1588.7

8-Methylquinoline 143 143 1562.7 1580.1 1591.1

1,2-Dicyclohexylethane 194 83 1568.2 1582.1 1589.2

5,6,7,8-Tetrahydro-3-methylquinc Aine 147 146 1571.5 1586.5 1595.8

n-Octylbenzene 190 92 1580.2 1587.2 1590.6

n-Hexadecane 226 57 1600 1600 1600

1,2,3,4-Tetrahydroquinoline 133 132 1600.0 1617.6 1627.8

2,6-Di-tert-butylphenol 206 191 1603.1 1613.2 1619.7

2-Ethylnaphthalene 156 141 1604.3 1621.6 1632.3

1 Hexadecene 224 55 1613.1 1613.2 1613.1

2,7_Dimethylnaphthalene 156 156 1613.3 1631.8 1641.9


l,l-Biphenyl (diphenyl) 154 154 1615.7 1631.8 1641.9

2-Methylbiphenyl 168 168 1620.2 1634.5 1644.4

I-Ethylnaphthalene 156 141 1621.0 1639.2 1650.0


1,6_Dimethylnaphthalene 156 156 1643.3 1662.5 1675.32-lsopropylnaphthalene 170 155 1660.5 1677.4 1687.9



I-Isopropylnaphthalene 170 155 1684.5 1701.8 1713.7

n-Decylcyclohexane 224 83 1692.2 1698.0 1702.1


1200 1200 1200

1191.3 1191.3 1191.3

1074.3 1079.5 1082.0

1079.3 1084.8 1087.1

1109.7 1114.1 1115.8

1233.9 1237.7 1239.5

1170.1 1173.4 1174.7

1131.1 1137.1 1139.9

1144.2 1149.9 1153.8

1102.3 1104.6 1105.5

1300 1300 1300

1141.6 1147.7 1151.4

1291.5 1291.3 1292.2

1151.7 1158.5 1162.5

1219.6 1222.3 1224.3

1254.6 1258.4 1260.9

1293.1 1301.5 1307.8

1171.5 1179.7 1183.4

1199.1 1202.5 1204.1

1400 1400 1400

1391.7 1391.4 1392.2

1180.5 1188.3 1192.9

1195.8 1202.2 1206.6

1308.9 1317.7 1322.9

1224.7 1229.9 1232.9

1442.4 1447.5 1449.1

1224.7 1233.0 1237.4

1500 1500 1500

1281.5 1290.5 1296.3

1492.4 1492.5 1492.3

1297.1 1306.8 1313.4

1304.5 1313.9 1319.5

1486.9 1496.3 1501.0

1330.2 1338.6 1343.7

1461.6 1466.5 1468.2

1600 1600 1600

1318.5 1326.8 1332.5

1433.6 1440.1 1443.2

1380.6 1390.6 1396.1

1591.7 1591.6 1592.7

1392.1 1402.2 1409.5

1390.6 1400.9 1407.3

1368.5 1377.3 1381.8

1390.0 1397.6 1402.3

1383.6 1393.8 1400.0

1405.3 1416.5 1422.3

1408.4 1419.6 1427.31443.9 1454.0 1459.1

1424.2 1436.3 1443.2

1428.3 1439.8 1446.4

1451.1 1460.9 1465.9

1650.7 1656.2 1658.4

1439.6 1452.2 1459.1

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Table 1 Continued

MolecularCompound ion (m/z)

n-Heptadecane 240

5,6,7,8-Tetrahydro-I-naphthol 1483-Methylbiphenyl 168

1,8_Dimethylnaphthalene 156

4-Methylbiphenyl 168

Acenaphthene 154

1,2_Diphenylethane (bibenzyl) 182

Dibenzofuran 168

n-Decylbenzene 218

n-Octadecane 254

I-Octadecene 252

3,3-Dimethylbiphenyl 1822-Naphthol 144

4,4-Dimethylbiphenyl 182Fluorene 166

n-Nonadecane 268

2,6-Diisopropylnaphthalene 212

Dibenzyl ether 198

9,10-Dihydroanthracene 180n-Eicosane 282

n-Dodecylbenzene 246

9,10-Dihydrophenanthrene 180

1,2,3,4,5,6,7,8-Octahydroanthracene 186

1,2,3,4,5,6,7,8_0ctahydrophenanthrene 186

1,2,3,4,5,6,7,8-Octahydroacridine 187

1,2,3,4-Tetrahydroanthracne 182

1,2,3,4_Tetrahydrophenanthrene 182n-Henicosane 296

Phenanthrene 178

Anthracene 178

1,2,3,4-Tetrahydrocarbazole 171

n-Docosane 310

I-Phenylnaphthalene 204

n-Tricosane 324

1-Methylanthracene 192

n-Tetracosane 338

Fluoranthene 202

n-Pentacosane 352

Pyrene 202

9,10-Dimethylanthracene 206

n-Hexacosane 366

p-Terphenyl 230

n-Triacontane 422

Chrysene 228

n-Hentriacontane 436

R.I. on Rtx-50 R.I. on DB-5

Baseveak m/z) 2C min- 4C min- 6C min- 2C min- 4C min- t 6C min-

57

120168

156

168

154

91

168

92

57

55

182

144

182166

57

197

92

179

5792

180

186

186

186

182

182

57

178

178

143

57

204

57

192

57

202

57

202

206

57

230

57

228

57

~ I ,

1700

1725.31725.5

1730.1

1735.7

1752.8

1758.7

1783.4

1791.1

1800

1814.4

1837.3

1849.4

1856.61869.3

1900

1925.5

1943.4

1983.3

2000

2005.0

2010.2

-

1700 1700

1743.8 1756.31742.3 1753.7

1752.7 1765.3

1753.4 1766.3

1777.6 1794.2

1776.5 1787.9

1808.2 1825.4

1799.6 1805.5

1800 1800

1814.5 1814.9

1853.9 1865.7

1871.7 1885.6

1875.1 1887.81895.2 1913.9

1900 1900

1943.7 1956.1

1961.4 1974.0

2011.1 2028.7

2000 2000

2014.4 2019.5

2040.3 2059.1

2037.8 2061.9 2086.0

2100 2100 2100

2136.9 2174.0 2196.8

2146.5 2183.0 2207.3

2190.6 2229.6 2250.0

2200 2200 2200

2224.1 2256.1 2276.02300 2300 2300

2400 2400 2400

2494.8 2544.9 2578.2

2500 2500 2500

2580.4 2636.3 2673.1

2582.5 2632.1 2664.6

2600 2600 2600

2627.1 2665.3 2690.0

3000 3000 3000

3036.9 3107.4 3153.2

3100 3100 3100

1700 1700 1700

1440.8 1447.2 1450.91474.5 1483.5 1487.7

1459.7 1472.7 1480.0

1482.8 1492.2 1497.3

1468.0 1481.4 1488.61508.8 1519.2 1524.3

1499.4 1513.0 1520.9

1669.5 1674.8 1677.7

1800 1800 1800

1792.6 1792.1 1793.5

1580.2 1589.0 1594.2

1514.5 1520.5 1523.8

1596.8 1607.6 1614.21565.2 1579.5 1587.9

1900 1900 1900

1716.8 1728.0 1735.51641.4 1650.3 1656.9

1662.1 1676.6 1685.3

2000 2000 2000

1673.5 1689.8 1699.5

1680.2 1694.1 1703.2

1705.5 1721.1 1730.61712.2 1726.5 1736.6

1731.3 1748.7 1760.2

1737.6 1755.2 1767.72100 2100 2100

1756.9 1776.3 1789.2

1767.0 1786.4 1800.01786.6 1800.4 1811.8

2200 2200 2200

1841.7 1858.2 1868.0

2300 2300 2300

1934.8 1959.3 1976.3

2400 2400 24002032.6 2060.1 2076.5

2500 2500 2500

2082.6 2113.4 2132.5

2107.6 2135.5 2152.6

2600 2600 2600

2171.1 2190.9 2204.0

3000 3000 3000

2434.2 2472.3 2494.9

3100 3100 3100

septum purge flow rate, 13 ml min-; and volumetriccolumn flow rate, 0.98 ml min- . The capillary columnssample capacity was also taken into consideration toavoid non-Gaussian peaks (tailing or fronting), whichshift slightly in retention time. Symmetric (Gau ssian)peaks were obtained by injecting an appropriate amount(0.01~1) of standards in the split mode (splitting ratio 66);the amount of each standard compound entering thecolumn was kept in the range 1- 10 ng per component.

The tempera ture-program med retention indices of over150 compounds were determined at three heating ratesusing the equation established by Van den Do01 andKratz27 . They show ed that the retention indices in lineartemperature-programmed g.c. can be calculated by thefollowing quasi-linear relationship:

Iu = 1oo(-=&+n) (2)

1440 Fuel 1995 Volume 74 Number 10

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Table 2 Final elution temperatures and retention indices of I-alkenes on the Rtx-50 column at three heating rates

Heating rate

Compound

2C min- 4C mine1 6C mini

Temperature Temperature Temperature(C) R.I. (C) R.I. Cl R.I.

1 HexeneI-Heptene1 0ctene1 Nonene1 Decene1 Undecene1-Dodecene1-Tridecene1-TetradeceneI-Pentadecene1 Hexadecene1 0ctadecene

43.744.847.151.859.971.083.997.3

110.5123.2135.3157.9

606.4 47.4 607.0 51.1 607.7708.3 49.3 708.4 53.7 709.5808.2 53.4 809.2 59.1 809.7909.1 60.8 909.5 68.1 909.9

1010.3 71.7 1011.0 80.7 1011.31110.6 84.9 1111.0 95.0 1111.01211.6 99.2 1212.4 109.9 1211.91311.8 113.4 1312.0 124.5 1312.41412.5 127.1 1412.8 138.5 1412.71512.7 140.2 1513.1 151.8 1513.11613.1 152.6 1613.2 164.4 1613.11814.4 175.6 1814.5 187.8 1814.9

where the absolute retention times of the compounds aredetermined under conditions of linear temperatureprogramming.

RESULTS AND DISCUSSION

Alkanes, cycloalkanes and aromatics are the three majorclasses of components in liquid fuels and thermallystressed jet fuels . Theoretically, any of the homologousseries of n-alkan es, n-alkylbenzen es and n-alkylcyclohex-anes may be used as the reference standards forinterpolation. Becau se of their comm on occurrence inmost fuel samples, n -alkanes were used as bracketinghydrocarbon s for calculatin g reten tion indices in thiswork. Figure 1 shows that the absolute retention timesof n-alka nes are locally quasi-linear for large n-alkanes(2 n-Cis) with linear temperature programming. Thisimplies that neighbouring n-alkanes are needed to obtainmore accurate retention indices in the lower alkanesregion (< r&is) because of the poorer linearity. O n theother hand, when large neighbouring n-alkanes (2 n-Cis)are not available, retention indices can be estimated byinterpolation between large non-neighbouring n-alkanes,and reasonably accurate results can be obtained, as shownin the following examp le. The retention indices ofnaphthalene calculated by interpolating between non-neighbouring alkanes (n-tridecane and n-pentadecane)are 1388.2, 1403.3 and 1413.4 at 2, 4 and 6 CmiK,respectively; these are comp arable to those calculatedfrom neighbouring n-alkanes: 1386.8, 1401.2 and 1410.9.

-:

2Clmin

.r100

--b

8

4T/min

80 - 6T/min

gF 60

5_2 40

9

d 20

6 12 18 24 30

n-Alkanes (Carbon No.)

Figure 1 Retention times of n-alkanes a t three different heating rates(Rtx-50 column, 40-3 10C)

Tabl e 1 presents the retention indices determined usingtwo columns at three heating rates (2, 4 and 6 Cmin-)for 154 compounds generally found in coal- andpetroleum-derived liquid fuels. For calculation of theretention indices shown in Table 1, neighbouring n-alkanes were used as the reference standards (iixed points)for more accurate results. The comp ounds are arranged inincreasing order of retention indices at a heating rate of2C min-i on the Rtx-50 column. The values reported arethe average s of 2-5 rep licates. The retention times werereproducible to within about f0.01 and f0.02min forlow- and high-boiling com pounds, respectively. Ingeneral, the experime ntal error for the retention indicesis of the order of 2 index units (i.u.). Howeve r, it is slightlylarger for those low-boiling comp ounds eluted before n-octane; th is can be rationalized from Equation (2) thatowing to the shorter retention times of these compounds asma ll difference in retention times can cause a larger errorin the retention index calculation.

Tem perat ur e dependence

Several features can be seen fromTab le I regarding thedepende nce of retention indices on the column tempera -ture. First, the retention indices of the compo unds thatare analogous to the reference standards, such asbranched alkanes and 1-alkenes, show a small tempera-

ture dependence. Tabl e 2 giv es the experime ntal results forretention indices and final elution tempera tures for l-alkene s (ranging from 1-h exane to 1-octadecene) at thethree heating rates on the Rtx-50 column. The differencein retention index values at different elution tempera turesis less than fl i.u., which is within the experime ntal error(of the order of 2 i.u.). These data demo nstrate the nearlytempe rature-indepen dent nature of the retention indicesfor these compounds, even for a 30C range (e.g. 157.9-187.8C for 1-octadecene, Table 2).

Second, the other compounds do show a measurabletemperature dependence, as illustrated inFigure 2, whichshow s the retention indices of several representativecompo unds as a function of elution tempe rature. Thenume rical re sults of the linear tempera ture depende nce forcycloalkanes and benzenes are given inTable 3 for bothRtx-50 and DB-5 columns. The second column inTabl e 3denotes the elution temperature range, which indicatesthe lowest (corresponding to 2 Cmin-) and the highest(corresponding to 6C min-) elution temp eratures for


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Table 3 Temperature dependence of retention indices for cycloalkanes and benzenes

Compound

Rtx-50 DB-5

AR.I./AT AR.I./ATTemperature range (C) R Z ange (iu. C-l) Temperature range (C) R Z ange (i.u. C-i)

Cyclohexane 44.7-53.6

Cyclohexene 45.4-55.2Methylcyclohexane 45.6-55.9Benzene 46.0-56.7trans-1,4-Dimethylcyclohexane 46.8-58.6cis-1,3-Dimethylcyclohexane 46.8-58.6trans-1,2-Dimethylcyclohexane 47.7-60.5I-Methylcyclohexene 47.8-60.8cis- 1,4-Dimethylcyclohexane 48.0-61.1trawl ,3-Dimethylcyclohexane 48.1-61.4cis-1,2-Dimethylcyclohexane 49.3-64.1Ethylcyclohexane 49.5-64.1Toluene 49.8-64.8n-Propylcyclohexane 55.5-74.6Ethylbenzene 56.4-76.11 CDimethylbenzene 56.7-76 .51,3-Dimethylbenzene 56.8-76.8

1,2-Dimethylbenzene 59.8-81.3trans-Octahydro-lH-Indene 60.1-81.8tert-Butylcyclohexane 60.4-82.3Isopropylbenzene 61.9-84.0cis-Octahydro-Uf-Indene 64.6-88.1n-Propylbenzene 65.1-88.5n-Butylcyclohexane 65.3-88.6I-Ethyl-3-methylbenzene 66.2-89.9I-Ethyl-4-methylbenzene 66.4-90.01,3,5_Trimethylbenzene 66.9-90.8tert-Butylbenzene 69.4-94.14,5,6,7_Tetrahydroindan 69.4-94.41-Ethyl-2-methylbenzene 69.5-94.21,2,4-Trimethylbenzene 70.5-95.6set-Butylbenzene 71.3-96.4trans-Decalin 71.5-97.41, -Bicyclopentyl 74.6-10 1.01,2,3-Trimethylbenzene 75.7-102.1n-Butylbenzene 77.7-103.9n-Pentylcyclohexane 77.7-104.0cis-Decalin 79.1-106.7Indan 79.9-107.22-Methylindan 84.1-111.91-Methylindan 85.4-113.51,2,4,5_Tetramethylbenzene 87.0-115.1n-Hexylcyclohexane 91.6-119.41,4-Diisopropylbenzene 93.0-121.15-Methylindan 93.4-122.61,2,3,4_Tetramethylbenzene 93.9-123.24-Methylindan 95.9-125.41,3,5_Triethylbenzene 100.8-129.3

n-Hexylbenzene 105.0-134.0Bicyclohexyl 106.0-136.5n-Octylcyclohexane 119.1-148.71,2-Dicyclohexylethane 129.9-161.5n-Octylbenzene 131.4-161.7n-Decylcyclohexane 144.5-175.0n-Decylbenzene 155.4-186.8n-Dodecylbenzene 177.4-209.3

705.6-706.0 0.04

736.1-739.3 0.32748.1-752.4 0.41765.7-769.0 0.31801.8-803.5 0.14801.8-803.5 0.14820.9-825.0 0.32824.1-829.2 0.39828.6-832.6 0.31830.9-835.4 0.34859.1-866.7 0.51862.3-867.4 0.35870.5-875.7 0.35956.9-963.4 0.34968.1-975.2 0.36972.3-979.2 0.35974.2-981.2 0.35

1009.2-1015.5 0.291011.6-1018.8 0.331015.1-1022.6 0.341028.2-1034.7 0.301053.0-1063.2 0.431058.0-1066.1 0.351059.5-1066.5 0.301068.7-1075.7 0.291069.8-1076.6 0.291074.4-1081.6 0.301097.6-l 104.5 0.281097.8-l 106.9 0.361098.2-l 105.3 0.291106.8-l 114.6 0.311112.9-l 120.3 0.291114.7-l 127.2 0.491138.6-1151.2 0.481147.6-l 158.9 0.431163.2-1171.1 0.301163.7-1172.0 0.311174.6-1190.2 0.571180.4-l 193.5 0.481212.8-1225.8 0.471222.7-1236.5 0.491235.1-1247.5 0.441269.0-1276.6 0.281279.9-1288.1 0.291282.5-1298.4 0.541286.4-1302.6 0.551301.2-1318.8 0.601338.5-1346.2 0.27

1370.1-1379.9 0.34 100.2-127.7 1254.6-1260.9 0.231377.5-1397.4 0.65 105.4-134.5 1293.1-1307.8 0.501480.1-1489.6 0.321568.2-1589.2 0.671580.2-1590.6 0.341692.2-1702.1 0.321791.1-1805.5 0.462005.0-2019.5 0.45

44.6-53.1

44.9-53.946.0-56.344.5-53.048.0-60.747.9-60.648.8-62.347.6-59.949.2-63.149.2-63.250.5-65.650.7-66.047.4-59.558.3-78.252.5-69.153.1-70.253.1-70.2

54.9-73.160.7-81.963.9-86.257.8-77.464.2-86.860.6-81.569.5-93.261.5-82.761.6-83.062.1-83.664.8-87.267.2-90.663.4-85.464.8-87.367.0-90.172.0-96.975.3-100.768.2-91.872.5-96.983.1-109.177.6-103.769.6-93.875.6-100.976.3-101.780.3-106.197.3-124.688.6-l 15.083.2-109.7 1131.1-1139.9 0.3385.0-111.8 1144.2-l 153.8 0.3684.6-111.5 1141.6-1151.4 0.3795.4-122.4 1219.6-1224.3 0.17

124.9-153.8 1442.4-1449.1 0.23130.5-160.6 1486.9-1501.0 0.47127.3-156.3 1461.6-1468.2 0.23149.9-179.7 1650.7-1658.4 0.26152.1-182.0 1669.5-1677.7 0.27

655.4-657.6 0.25

677.1-678.8 0.19718.3-720.3 0.19654.2-656.1 0.22775.6-777.3 0.14772.8-776.6 0.30796.7-798.4 0.13763.9-766.4 0.20804.7-805.7 0.07804.1-806.2 0.15823.1-827.3 0.28826.9-830.9 0.26758.9-761.7 0.23924.2-929.1 0.24853.2-857.2 0.24861.7-866.5 0.28861.7-866.5 0.28888.0-891.2 0.18947.7-955.3 0.36978.1-985.2 0.32919.4-923.6 0.21981.0-989.5 0.38946.9-952.3 0.26

1026.6-1031.6 0.21955.0-960.8 0.27956.3-962.4 0.29961.5-967.1 0.26987.1-992.0 0.21

1008.6-1014.8 0.27973.3-979.7 0.29987.5-992.8 0.24

1006.4-1011.7 0.231045.7-1055.9 0.411071.8-1080.9 0.351016.0-1022.7 0.281050.1-1055.9 0.241130.2-l 136.0 0.221089.5-1100.4 0.421027.4-1035.5 0.341074.3-1082.0 0.301079.3-1087.1 0.311109.7-1115.8 0.241233.9-1239.5 0.211170.1-1174.7 0.18

the intermediately polar Rtx-50 column . The thirdcolumn gives the corresponding indices at the lowestand highest elution temperatures. The AR.I./AT ratiosin Table 3 indicate the index change per 1C i.e. theaverage temperature coefficient of retention indices.Columns 5-7 in Table 3 g ive the data on the slightlypolar DB-5 column.

In the same fashion, Table 4 presents the numericalresults of the linear temperature dependen ce for aroma ticsand NSO (N-, S- and O-containing) compounds.Considering the data on the Rtx-50 column as an

example, it can be seen that, in general, alkylatedcyclohexanes and benzenes displayed aA R . I . I AT ofabout 0.33 f 0.05 i.u. C- whereas benzenes w ith multi-substitution (such as tetramethyl) and long side-chains(decyl and dodecyl) exhibit slightly higher values(- 0.46 i.u. C-), Com pared with alkylated cyclohex-anes and benzenes, the retention indices for compoundswith a two-ring structure exhibit a higher temperaturedependence. For example, indan, l- and 2-methylindan,bicyclopentyl and trans-deca lin all have values of about0.49 i.u. C-l; 4- and 5-methylindan, cis-decalin an d


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Table 4 Tempe rature dependence of retention indices for aromatics and N-, S- and O-containing compounds

Rtx-50 DB-5

AR.I./AT AR.I.fATCompound Temperature range (C) R Z ange (i.u. C-l) Temperature range (C) R I ange (i.u. C-l)

Tetralin

1,2-DihydronaphthaleneNaphthalene2-Methylnaphthalene1 MethylnaphthaleneDodecahydrofluorene2-Ethylnaphthalene2,7_Dimethylnaphthalene2,6-Dimethylnaphthalene1, -Biphenyl2-MethylbiphenylI-Ethylnaphthalene1,7_Dimethylnaphthalene1,3_Dimethylnaphthalene1,6_Dimethylnaphthalene2-Isopropylnaphthalene2,3_Dimethylnaphthalene1,4_Dimethylnaphthalenel,S-Dimethylnaphthalene1-Isopropylnaphthalene1,2_Dimethylnaphthalene3-Methylbiphenyl1,8_Dimethylnaphthalene4-MethylbiphenylAcenaphthene1,2-Diphenylethane (bibenzyl)3,3-Dimethylbiphenyl4,4-DimethylbiphenylFluorene2,6-Diisopropylnaphthalene9,10-Dihydroanthracene9,10-Dihydrophenanthrene1,2,3,4,5,6,7,8-Octahydro-anthracene1,2,3,4,5,6,7,8-Octahydro-phenanthrene1,2,3,4,-Tetrahydroanthracene1,2,3,4_TetrahydrophenanthrenePhenanthreneAnthracene1 PhenylnaphthaleneI-MethylanthraceneFluoranthenePyrene9,10-Dimethylanthracenep-TerphenylChrysenePhenyl ethyl ether

2,6_Dimethylphenol2,4,6_TrimethylphenolBenzothiophene2,3-Dihydroindole1 1ndanolQuinoline8-Methylquinoline5,6,7,8-Tetrahydro-3-methylquinoline1,2,3,4_Tetrahydroquinoline2,6-Di-tert-butylphenol5,6,7,8-Tetrahydro-I-naphtholDibenzofuran2-NaphtholDibenzyl ether1,2,3,4,5,6,7,8-Qctahydroacridine1,2,3,4-Tetrahydrocarbazole

98.9-128.9

101.8-132.1107.2-138.3121.8-153.8125.3-157.7128.6-161.4134.3-166.7135.3-167.9135.6-168.1135.6-167.9136.1-168.2136.2-168.8138.3-171.2138.7-171.6138.8-171.8140.8-173.3141.4-174.6142.2-175.6142.9-176.4143.6-176.3144.5-178.1148.2-180.9148.7-182.2149.3-182.3151.2-185.5151.8-184.8160.3-193.3163.4-195.7163.7-198.5169.5-202.8175.2-210.2177.9-213.2

189.9-226.4190.8-227.4197.8-233.6

220.6-259.0 2494.8-2578.2 2.17227.3-266.4 2580.4-2673.1 2.37227.5-265.8 2582.5-2664.6 2.14230.9-267.8 2627.1-2690.0 1.71259.9-300.2 3036.9-3153.2 2.88

76.0-101.8 1150.0-1156.9 0.27

95.4-124.3 1297.6-1310.7 0.45108.6-138.5 1397.4-1412.2 0.50110.9-142.5 1415.7-1442.5 0.85116.0-147.3 1455.5-1479.2 0.76118.7-149.7 1476.6-1496.8 0.65120.9-152.9 1493.6-1521.6 0.88129.3-161.7 1562.7-1591.1 0.87

130.3-162.3 1571.5-1595.8 0.76 110.4-139.5133.8-166.2 1600.0-1627.8 0.86 108.8-137.9134.2-165.2 1603.1-1619.7 0.53 123.8-153.0148.2-181.2 1725.3-1756.3 0.94 124.7-154.0154.6-188.9 1783.4-1825.4 1.22 132.1-163.1161.6-195.5 1849.4-1885.6 1.07 133.9-163.4171.3-204.7 1943.4-1974.0 0.92 148.9-179.6

: 180.5-215.9 2037.8-2086.0 1.36 156.9-188.7194.8-231.2 2190.6-2250.0 1.63 164.9-197.1

1324.1-1343.6 0.65

1345.7-1366.2 0.681386.8-1410.9 0.771501.0-1528.6 0.871530.2-1559.6 0.911556.9-1588.7 0.971604.3-1632.3 0.861613.3-1641.9 0.881615.5-1643.9 0.871615.7-1641.9 0.811620.2-1644.4 0.761621.0-1650.0 0.891638.6-1670.2 0.961642.2-1673.2 0.941643.3-1675.3 0.971660.5-1687.9 0.841665.9-1698.5 0.981672.6-1707.4 1.041678.1-1714.2 1.081684.5-1713.7 0.891692.4-1728.6 1.081725.5-1753.7 0.861730.1-1765.3 1.051735.7-1766.3 0.931752.8-1794.2 1.211758.7-1787.9 0.891837.3-1865.7 0.861856.6-1887.8 0.941869.3-1913.9 1.281925.5-1956.1 0.921983.3-2028.7 1.302010.2-2059.1 1.39

2136.9-2196.8 1.642146.5-2207.3 1.662224.1-2276.0 1.45

86.0-113.1 1151.7-1162.5 0.40

88.8-116.3103.8-132.9106.0-135.3

1171.5-1183.4 0.431281.5-1296.3 0.511297.1-1313.4 0.56

117.0-146.7 1380.6-1396.1 0.52118.5-148.5 1392.1-1409.5 0.58118.3-148.2 1390.6-1407.3 0.56115.4-144.8 1368.5-1381.8 0.45118.3-147.6 1390.0-1402.3 0.42117.4-147.3 1383.6-1400.0 0.55

120.2-150.2 1405.3-1422.3 0.57120.6-150.9 1408.4-1427.3 0.62125.1-155.1 1443.9-1459.1 0.51

122.6-153.0123.1-153.4126.0-156.0124.6-155.1128.9-158.9127.1-157.8130.0-160.1128.1-159.0133.2-163.5141.8-172.1143.8-174.5140.0-171.3157.4-188.6151.2-182.9152.5-184.6

1424.2-1443.2 0.631428.3-1446.4 0.601451.1-1465.9 0.491439.6-1459.1 0.641474.5-1487.7 0.441459.7-1480.0 0.661482.8-1497.3 0.481468.0-1488.6 0.671508.8-1524.3 0.511580.2-1594.2 0.461596.8-1614.2 0.571565.2-1587.9 0.721716.8-1735.5 0.601662.1-1685.3 0.731673.5-1699.5 0.81

153.3-185.0 1680.2-1703.2 0.73

156.1-188.1 1705.5-1730.6 0.79158.9-191.4 1731.3-1760.2 0.89159.6-192.2 1737.6-1767.7 0.93161.7-194.6 1756.9-1789.2 0.98162.8-195.8 1767.0-1800.0 1.00170.7-203.1 1841.7-1868.0 0.81180.2-214.2 1934.8-1976.3 1.22189.7-224.1 2032.6-2076.5 1.28194.4-229.4 2082.6-2132.5 1.43196.7-231.2 2107.6-2152.6 1.30202.4-236.0 2171.1-2204.0 0.98224.6-260.9 2434.2-2494.9 1.67

65.0-87.2 988.8-992.4 0.16

79.2-104.5 1102.3-1105.5 0.1392.6-l 19.4 1199.1-1204.1 0.1990.0-117.8 1180.5-l 192.9 0.4592.1-119.8 1195.8-1206.6 0.3996.1-123.6 1224.7-1232.9 0.3096.1-124.3 1224.7-1237.4 0.45

107.0-136.1 1304.5-1319.5 0.51

1330.2-1343.7 0.471318.5-1332.5 0.481433.6-1443.2 0.331440.8-1450.9 0.351499.4-1520.9 0.691514.5-1523.8 0.311641.4-1656.9 0.501712.2-1736.6 0.771786.6-1811.8 0.79

bicyclohexyls show an even higher retention index-temperature dependence (- 0.60 i.u. C-l). Multi-ring

alkylated (Ct-Cs) naphthalenes and six biphenyls inTabl e 4 show an average A R . I . / AT of 0.94 and 0.86 i.u.

aromatics exhibit the largest temperature dependence of C-, respectively, with a standa rd deviation of aboutthe retention index among the studied compounds. The 160.08 i.u. C-l. The values for three-ring aroma tics in


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G C r e t e n t io n i n d i c e s o f f u e l s : W . C . L a i a n d C . S o n g

Table 5 Retention times and retention indices on DB-5 column measured at a heating rate of 4 Cmin-with and without a n isothermal initial hold(IH) period

Retention time(min)

Retention index

Compound No IH IH=5min Difference No IH IH=5 min Difference

Benzene 2.21Cyclohexane 2.221-Octene 3.88cis-1,2-Dimethylcyclohexane 4.65Ethylcyclohexane 4.76Ethylbenzene 5.43n-Propylcyclohexane 1.39I-Ethyl-3-methylbenzene 8.414,5,6,7_Tetrahydroindan 10.17n-Butylcyclohexane 10.85truns-Decalin 11.63n-Butylbenzene 11.705-Methylindan 14.741,2,3,4-Tetramethylbenzene 15.212,3-Dihydroindole 17.142,4,6_Trimethylphenol 17.151,3,5-Triethylbenzene 17.86lIndano 18.13n-Hexylcyclohexane 18.411-Tridecene 20.33Bicyclohexyl 20.69I-Methylnaphthalene 20.871,2,3,4-Tetrahydroquinoline 21.551, I-Biphenyl 23.262-Ethylnaphthalene 23.711-Ethylnaphthalene 23.822,6_Dimethylnaphthalene 24.065,6,7,8-Tetrahydronaphthalenol 25.55n-Octylcyclohexane 25.561,2_Dimethylnaphthalene 25.712Isopropylnaphthalene 25.11n-Octylbenzene 26.171,8-Dimethylnaphthalene 26.37Acenaphthene 26.654-Methylbiphenyl 27.001,2-Dicyclohexylethane 27.13Dibenzofuran 21.652-Naphthol 21.88Fluorene 29.703,3-Dimethylbiphenyl 29.991-Hexadecene 30.07Dibenzyl ether 31.79n-Decylcyclohexane 31.96n-Decylbenzene 32.509,10-Dihydroanthracene 32.559,10-Dihydrophenanthrene 32.92

1,2,3,4,5,6,7,8-Octahydroanthracene 33.061,2,3,4,5,6,7,8-Octahydroacridine 33.912,6-Diisopropylnaphthalene 34.011,2,3,4-Tetrahydroanthracene 34.59Phenanthrene 35.361-Octadecene 35.801,2,3,4-Tetrahydrocarbazole 36.03I-Phenylnaphthalene 31.54Fluoranthene 42.59Pyrene 43.859,10-Dimethylanthracene 44.36Debenzyl sulfide 44.62p-Terphenyl 45.64

Chrysene 51.64

2.34 0.132.35 0.134.82 0.946.09 1.446.25 1.491.36 1.93

10.36 2.9711.80 3.3914.07 3.9014.88 4.0315.77 4.1415.92 4.2219.30 4.5619.82 4.6121.89 4.1521.93 4.1822.68 4.8222.92 4.1923.24 4.8325.26 4.9325.60 4.9125.75 4.8826.43 4.8828.17 4.9128.66 4.9528.78 4.9629.02 4.96

654.8656.2189.3824.7829.1856.3927.1958.5

1011.41030.21051.81053.71137.11149.91202.21202.51222.31229.91237.71291.31301.51306.81326.81317.31390.61393.81400.9

653.8 -0.9654.9 -1.2788.4 -0.9824.4 -0.2828.4 -0.8855.5 -0.8926.4 -0.7958.6 0.2

1009.9 -1.51028.9 -1.31049.8 -2.01053.3 -0.51135.1 -2.01148.2 -1.61200.5 -1.71201.6 -0.91222.0 -0.41228.5 -1.41237.1 -0.61291.9 0.51301.2 -0.31305.5 -1.31325.1 -1.71375.4 -1.91389.6 -1.01393.1 -0.11400.0 -0.9

30.51 4.96 1447.2 1446.1 -1.130.54 4.98 1447.5 1447.1 -0.530.67 4.96 1452.2 1451.1 -1.130.75 4.98 1454.0 1453.6 -0.531.16 4.99 1466.5 1466.3 -0.231.35 4.98 1472.7 1472.1 -0.531.62 4.91 1481.4 1480.5 -0.932.00 5.00 1492.2 1492.3 0.032.09 4.96 1496.3 1495.0 -1.232.62 4.91 1513.0 1512.0 -1.032.87 4.99 1520.5 1520.1 -0.334.68 4.98 1579.5 1578.9 -0.634.98 4.99 1589.0 1588.6 -0.335.08 5.01 1591.6 1591.9 0.336.71 4.98 1650.3 1649.7 -0.136.95 4.99 1656.2 1655.9 -0.331.49 4.99 1674.8 1674.5 -0.331.54 4.99 1676.6 1676.2 -0.337.90 4.98 1689.3 1688.6 -0.7

38.06 5.00 1694.1 1694.1

38.96 4.99 1726.5 1726.239.01 5.00 1728.0 1728.0

39.59 5.00 1748.7 1748.740.36 5.00 1776.3 1776.340.80 5.00 1792.1 1792.141.02 4.99 1800.4 1800.042.54 5.00 1858.2 1858.241.59 5.00 2060.1 2060.148.85 5.00 2113.4 2113.449.35 4.99 2135.5 2135.149.60 4.98 2146.8 2145.950.64 5.00 2190.9 2190.9

56.63 4.99 2412.3 2471.8

0.0

-0.40.0

0.00.00.0

-0.40.00.00.0

-0.4-0.9

0.0

-0.5

general range from 1.20 to 1.65 i.u. C-l. Four-ring C-l. The A R . I . / AT ta ios in Tabl e 3 are useful in thataromatics all display a significant temperature depen- they may be easily used to estimate the retention indices ofdence, judging from the large values of AR.Z./AT (> 2.0 these compounds over the studied temperature range byi.u. C-l); chrysene shows a value as large as 2.88 i.u. interpolation. On the other hand, the large temperature

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R.I. Rtx-50) - R.I. DB-5)

1850,

9 - Acenaphthene

g 1700 b._ - Decylcyclohexane

- Biphenyl

-+- 2.Ethylnaphthalene

- I -Methylquinol ine

120 130 140 150 160 170 180 190 200

Elution Temperature (C)

2700

2600

cv 2500 5

- p-Terphenyl

9 + Pyrene

* 9,10-Dimethylanthracene

- - F luoranthene

* I-Phenylnaphthalene

- Anthmene

- Phenanthrene

180 200 220 240 260

Elution Temperature (C)

280

Figure 2 Retention index versus final elution temperature (Rtx-50column, 40-310C)

c

J

trans-Me

O-Me

cis-

+&Me

Figure 3 Retention index difference between Rtx-50 and DB-5 columns(ApolR.I.) for 20 representative compounds

dependen ce of the retention indices of multi-ringaromatics implies that using the AR.Z./AT ratios inTabl e 4 to estimate their retention indices over the studiedtemperatu re range by interpolation will be less accurate.The NSO compounds in general show larger retentionindices and exhibit a slightly greater temperaturedependence (A R.Z./AT) than the corresponding hydro-carbons.

trans-

al

trans-

aYMe

15100 2o:oo 25:00 30100

Retention Time (min)

Figure 4 G.c.-m.s. total ion chromatogram of unfractionated JP-SC jet fuel (Rtx-50 column, heated from 40 to 310C at 4 Cmin-, split)


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Table 6 Hydrocarbons in coal-derived JP-8C jet fuel identified bymatching retention indices on Rtx-50 column at two heating rates

Compounds

RI. at 4C min- RI. at 6C min-

Std JP-8C Std JP-8C

Cyclohexane

MethylcyclohexaneBenzenetrans-1,4-Dimethylcyclohexanecis-1,3-Dimethylcyclohexanetrans- 1,2-Dimethylcyclohexanecis- 1,4-Dimethylcyclohexanetrans-1,3-DimethylcyclohexaneEthylcyclohexaneToluenen-PropylcyclohexaneEthylbenzene1 CDimethylbenzene1,3_Dimethylbenzene1,2-Dimethylbenzenetrans-Octahydro-1 H-indeneIsopropylbenzenecis-Octahydro- 1H-indenen-Propylbenzenen-Butylcyclohexane1 Ethyl-4-methylbenzene1,3,5Trimethylbenzene1-Ethyl-2-methylbenzene1,2,4-Trimethylbenzenetrans-Decalin1,2,3-Trimethylbenzenen-Butylbenzenen-Pentylcyclohexanecis-DecalinIndan2-MethylindanI-Methylindan1,2,4$Tetramethylbenzenen-HexylcyclohexaneS-Methylindan1,2,3,4_Tetramethylbenzene4-MethylindanTetralinn-HexylbenzeneBicyclohexylNaphthalenen-OctylcyclohexaneCyclohexylbenzene2-MethylnaphthaleneI-Methylnaphthalene1,2-Dicyclohexylethanen-Octylbenzene2,7_Dimethylnaphthalene

l,l-Biphenyl2,6_Dimethylnaphthalene1,3-Dimethylnaphthalene1,6_Dimethylnaphthalenen-Decylcyclohexane3-Methylbiphenyl

706.3

749.5767.4802.3802.3822.4830.5832.2865.5873.6960.8971.5975.7977.9

1012.51015.91032.11059.91063.0 1063.31064.2 1064.51074.01079.21103.11111.91122.41155.01168.61169.41184.41188.71220.81231.2 1230.91242.4 1242.51274.41292.71296.31312.01336.01376.31390.41401.21485.71489.01517.31548.71582.11587.21631.8

1631.81633.11661.81662.51698.01742.3

705.4

750.0767.4802.4b802.4822.9831.2832.9865.3873.5960.9971.6975.5977.4

1012.31016.01032.41060.2

1074.41079.31103.1 1105.3 1105.71112.2 1114.6 1115.11122.41155.01168.31168.81183.91188.41219.8

1127.2 1127.81158.9 1159.21171.1 1171.81172.0 1172.21190.2 1190.61193.5 1193.91225.8 1225.51236.5 1236.21247.5 1247.31276.6 1277.41298.4 1298.4

1274.21292.41296.3 1302.6 1302.61312.0 1318.8 1318.9

1343.6 1343.31379.9 1380.31397.4 1397.91410.9 1410.8

1335.91376.11389.51401.21485.91489.01517.41548.61582.01587.51631.2b

1489.6 1489.61497.7 1497.31528.6 1528.01559.6 1559.71589.2 1589.61590.6 1591.01641.9 1642.2b

1641.9 1642.2b1643.9 1644.21631.2b1632.51662.0 1673.2 1673.41662.7 1675.3 1674.91698.0 1702.1 1702.11742.9 1753.7 1753.7

706.0 706.0

752.4 751.8769.0 768.7803.5 803.5803.5 803.5b825.0 824.6832.6 832.4835.4 835.2867.4 866.9875.7 875.4963.4 963.5975.2 975.5979.2 978.5981.2 980.5

1015.5 1015.61018.8 1019.41034.7 1035.01063.2 1063.71066.1 1066.21066.5 1067.11076.6 1076.81081.6 1082.3

a Excluding the n-alkanes that are also present in JP-8C jet fuel.b Identified with the assistance of mass spectra.

Third, an importa nt consequen ce of the observeddifferences in the tempera ture depende nces A R . I . / AT )among the compounds is that compounds which co-eluted at one temperature may be separated at some othertemperature(s). This means that co-elution problems thatoccur when using a given heating rate may be solved byusing a different hea ting rate. For examp le, 4-m ethylin-dan and 1 tridecene co-elute on Rtx-50 at a heating rate of4Cmin-, but they can be resolved by decreasing orincreasing the heating rate (to 2 or 6C min-). Similarly,1,2_dimethylnaphthalene and 2-isopropylnaphthalene

co-elute on DB-5 at a heating rate of 6 Cmin-, butthey can be resolved by decreasin g the heating rate to2C min-. 1-Alkenes co-elute (or almost co-elute) withsome compounds at a heating rate of 2C min-, but theycan be easily separated from the respective co-elutingcompounds at 6C mint, e.g. 1-decene and 1,2-dimethyl-

benzene; 1 undecene and see-butylbenzene; 1 dodeceneand 2-methylindan; and 1-hexadecene and 2,7-dimethyl-naphthalene. Some other examples, which can be found inTable 1 , include 2-methylbiphenyl and l-ethylnaphtha-lene; 1,2_dimethylnaphthalene and n-decylcyclohexane;acenaphthene and 1 Zdiphenylethane; 9,1 O-dihydroan-thracene and dodecylbenzene; and pyrene and 9,10-dimethylanthracene.

Although the retention indices in Table 1 weremeasured using linear temperature programming with-out an isotherma l initial hold period, the results may alsobe applied to g.c. analysis with a short initial hold time.Tabl e 5 presents the retention times and retention indices

measured with a 5 min initial isothermal hold at 40C andalso those without an initial hold for 60 representative(out of 154 in Tabk 1 ) compounds. The data werecollected using a heating rate of 4 Cmin- on the DB-5column. The retention indices with a 5 min hold time werealso approximated by Equation (2). It was found thatadding a 5 min holding time at low temperature (40C)has only a small effect on retention indices, altho ugh theretention times may differ by as much as 5 min. It wa sapparent from Table 5 that adding an initial hold timeonly serves to delay the elution of heavier compoundswhich have retention indices > 1400 by as much time asadded, e.g. 5 min in this study. For the 154 compounds

studied, the retention index was lowered, on average, byabout 1 i.u. when a 5min of hold time wa s added at theinitial temperatu re. The retention index decremen t forheavier compo unds is in general negligib le. Therefore, thetemperature-programmed retention indices inTable 1may also be applied with minor adjustment to analyseswhich incorporate a short initial hold time.

Col um n po la r i t y dependence

Retention indices exhibit a significant dependence oncolumn polarity, as shown inTable 1. There are severalcharacteristics to be pointed out. First,Tabl e 1 shows thatthe retention indices of all the comp ounds studieddecrease a s the column p olarity decreases (from Rtx-50to DB-5 ). Similarly to the temperatu re depende nce, thedecrement in the retention indices depends on thestructure of the comp ounds. For examp le,Figure 3displays the retention index difference betwee n the Rtx-50 and DB-5 columns (An,,tR.I.) for 20 representativecomp ounds. The retention index difference ranges from assmall as 20 to over 600. Compared with other com pounds,1-alkenes, which are analogous to the reference standards(n-alkan es in this work), display the smallest polaritydependence. A ,,,R.Z. is about 20 f 2 for I-alkenesranging from 1-hexene to 1-octadecene. Alkylcyclohex-anes (from methylcyclohexane to n-decylcyclohexane)also show a small polarity dependence; A,,tR.I. is about36 f 6. On the other hand , the retention indices ofalkylbenzenes, aromatics and N-, S- and O-containingcompounds are highly dependent on the column polarity.Multi-ring aromatics exhibit the largest polarity depen-dence of the retention index amon g the studiedcompounds. It is worth noting that the more polar

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O

n-C1

10.00 15 00 20 00 25 00 30 00


Figure 5 G.c.-m.s. total ion chromatogram of unfractionated JP-8P jet fuel (Rtx-50 column, heated from 40 to 310C at 4C min-, split)

column (Rtx-50) retains more polar compounds longerthan less polar isomer compounds and results in largerA,,rR.Z. This can be demonstrated by 2,6-dimethyl-naphthalene and 1,8-dimethylnaphthalene; the latter isrelatively m ore polar and displays a larger A,,rR.Z . thanthe former. Overa ll, the behaviou r of the polaritydependence of retention indices can be summarized bythe facts that the retention of polar com pound s decreaseswhereas that of non-polar compounds such as n-alkanesincreases wh en the polarity of the stationary phasedecreases.

Second, a change in compound elution order ondifferent colum ns c an be observed because differentcomp ounds display different degrees of retention indexchang es w ith column polarity. For example, the effects ofcolumn polarity on the elution order of comp ounds arevery evident by comparing 1-alkenes and n-alkanes. A I-alkene, which eluted after the n-alkane with the samecarbon number on the Rtx-50 column eluted before the n-alkane on the DB-5 column. More examples of changes inelution order can be found inTable 1. Based on theknowledge on the change in compound elution order,compound identification can be improved by usingcolum ns of different polarity. Ano ther application of theknow ledge of the retention indices is that they canfacilitate the choice of column stationary phase toreduce or even eliminate co-elution within complexsamples. For example, n-propylbenzene and n-butyl-cyclohexane co-elute on the Rtx-50 column but areresolved on the DB-5 column at any of the heating ratesused (2, 4 and 6 Cmin-). On the other ha nd, 2,4,6-trimethylpheno l and 2,3-dihydroindole co-elute on theDB-5 but are resolved on the Rtx-50 column.

Third, for the two capillary colum ns studied, in generalthe values of AR .Z./AT are larger for the Rtx-50 tha n forthe DB-5 column, i.e. almost all the compounds studieddisplay a higher temperatu re sensitivity on the more polarcolumn (Rtx-50) than on the less polar column (DB-5), ascan be seen from Tabl es 3 and 4.

Charac t e r i za t i on o f JP -8 j et f uel s

Retention indices determined in this work can be useddirectly in identifying comp ounds in complex m ixturessuch as jet fuels. In many cases, the unknown compoundsmay be identified by calculatin g their retention indicesand comparing them with those of standards shown inTable I , if the match is within f2 i.u. for a given heatingrate. The choice of 2 i.u. is based on and close to theexperime ntal error for the retention indices. Coal-derivedJP-SC fuel was analysed by g.c. under the same flowconditions as those for the standard compounds, and twoheating rates (4 and 6 C min-) were used for the purposeof cross reference. The capillary colum ns sa mple capacitywas again taken into consideration and symmetric g.c.peaks were obtained. Figure 4 presents the g.c.-m.s. totalion chroma togram (TIC) of coal-derived JP-8C fuel onthe Rtx-50 column , together with the structures of majorcomponents. Besides the n-alkane components, morethan 50 representative compounds present in JP-8C wereidentified by matching retention indices, as shown inTable 6. The first column in Table 6 denotes therepresentative comp ounds identified. The second andthird columns display the retention indices (at 4C min-)of known standa rd compounds (denoted Std) and thoseof the JP-8C , respectively. Similarly, the data obtaine d at6C min- are given in the last two columns. The reported


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16

18/

13I 23

29

35,I

16. n-Nomme n-C9)17. hanr-l-ElhyI-Z-melhylc~k~~18. cis-I-Elhyl-Q-meulykyclohenanc19. n-hpykyclolwane20. 2.6~Dimelhylactane21. 3-Elhyl-3-mahylhepane22. I-Elhyl-3-melJlylbeJlzene23. 4-Melbylmmane24. 2-Methylwane25.26.

1,3.5-Trimethylbenzenc3-Melhyhhmane

27.28.

1.2.4-Trimethylbennen-Decane nclo)

29.30.

2,bDimelhylnonanen-Butylcye hexane

37

39I

3;

::

::5.

;:

t :10.

1:.13:14.IS.

n-Hepcane n-CT)Melhylcyclohexane2-MahylheptwleEMdYUleptane

1,3- + rrans-I,4-Dimethylcyclohexan eIrons-I 2-Dimelhylcyek heaanen-oclane (n-es)cis-1,4- + rr~s-1.3-Dime~y lcyclo~x~2,bthnethylheplaneElhylcyebbexane1,1.3-Trime~hylcyclohexane2-Mahylouane1.4- + 1.3~Dimethyhwene3-Muhylaaanecis-I-Ethyl-3-meUlylcycloexanc

54

31.32.

3.7~Dimethylnonanetronr-M

33.34.

5-Me yl cane

35.4-Meahykkane

36.L-MeAhyldsane

37.3-Mahykkcanen-Undecane nC11)

38.39.

2,bDimelhykkcane

40.3.7-Dimelhykkane

4 I.n-Pentylcyclohexane

42.S-Methylundecaw

43.4-Methylwdecane2-Methyhdecane

44. 3-Mehhdecawn-Dodeime n-Q)2.6-Dimethylu&canen-Hexykycloheaane6-MethyldodeaneS-Methykhkcane4-Methyldodwne2-Methylddcane

t$X Z$ndecanen-Tridecane nJ&)n-HcpcykyclohexanebMethylWecane5-MeIhylWecarte4-M&ylWecsne2-Me4hyloidecane3-Me4hyl~

2,6,10-Trime hylen-Tetradecancn-C ~4)n-OetykyebhexaneS-Methyltetradecane4-MethylteW2.6.10-Trimelhyleidanen-Pentadecane n-C]s)n-Ncnykckhexanen-Hex&sane (n-f&j)n-Hcptadecanc t W

67

--.-r--v---~---l--I - 1-1-T---7---1--7-- ,--.00 10.00 15.00 20.00 1-1 --LG---- 30.00


Figure 6 G.c.-m.s. total ion chromatogram of fraction1 (saturate fraction, n-pentane eluate) of JP-8P jet fuel (DB-5 column, heated from 40 to310C at 4C min-, split)

data are the mea n retention indices of three replicates.The deviations betweenR.I.std and R.I.JP_8C in Table 6are less than 1 i.u., which is within the experimental errorfor the retention indices (of the order of2 i.u.). We alsoanalysed the petroleum-derived JP-8P fuel in a similarfashion. Figure 5 show s the g.c.-m.s. TIC of petroleum-derived JP-8P fuel on the Rtx-50 column, together withthe structures of major com ponents.

The g.c.-m.s. analysis of the whole jet fuel also revealsthat many compounds are present in trace amounts orco-eluted with other compounds, making them difficultto identify from analy sing the whole fuels. Anothe rapproach used in this work is to separate the jet fuelsinto chemically similar compound classes by liquidcolumn chromatographic separation, which also servesto concentrate the compounds present in whole fuelsin very low concentrations and to eliminate (or reduce)the co-elution of different comp ounds from the capillaryg.c. colum n, followed by g.c.-m.s. analysis of thechroma tographic fractions. Th e fuels were separatedinto several fractions using a neutral alumina gelcolumn and a series of elution solvents, as describedelsewhere15. The identifications of compounds in JP-8P

fraction 1 (n-pentan e eluate), fraction 2 (5% benzene-pentane eluate) and fraction 3 (benzene eluate) aredescribed as follows.

Figure 6 shows the TIC and the detailed identificationresults for JP-8P fraction 1 (saturates). All the identifica-tions in Figure 6 were made by using retention indices andfurther confirmed by using m.s. The combined use ofretention indices and mass spectra allows compoundidentification to be performed with higher confidence. TheJP-8P saturated fraction con sists primarily of open-cha incompounds. In their mass spectra, straight-chain alkanesshow weak molecular ions but typical an d relativelystrong [C,H2,+# fragment ions, and specific compoundscan be accurately identified using g.c.-m.s. Man ybranched alkanes exhibit very weak molecular ions, andin some cases such ions disappear from their m ass spectra.It should be noted that H ayes and Pitzer167 published theretention indices of some branched alkanes a nd alkylatedbenzenes. Their data were also used to help identifyseveral more peaks. It can be seen that the dominantconstituents in fraction 1 of JP-8P are long-chain alkaneswith carbon number ranging from C7 to C17, with mostfalling between C9 and C 14. Straight-chain (normal)

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3

6lli514 71

,,-~-~--T-l-T-TT-T-r I I 7 1 I I 1 1 1 I e, I I , , 1 , 8 16.00 8.00 10.00 12.00 14.00 16.00 18.00 20.00 22.00 24.00

:.3:4.

::I.8.9.10.II.12.13.14.15.16.17.18.

:;21:22.

Elh)4bW?J1.4- + 1.3~Dinwhylbenzenel,2-Dimelhylbmzulen-Plopylbawne1-Elhyi-3-nwhybenzenel-Elhvwnle&v1.3,skrinwh ibelwleI-Elhvl-2-m benzene&l1.2~TrimeItlyibenzene1,2.3-Trimelhylbenzene

wanl.3-DiclhylbenzeneI-Methyl-3-prupylbenzeneI-Mc&yl4pmpylLxnzenen-Butylbenzene1.3~DinlcIhylJ-eIhyllxn7A1.2-w 1.4.DiithylknzcneI-Mchyl-2-propylben~ne1,4-DinWhyl-2dhylbe.~1.3~Dimethyl4-cthylben7~nc2-Mahylhlan1.2-Dimethyl-4elhylbenzene

28

G C r e t en t i o n i n d i c e s o f f u e l s : W. C . L a i a n d C . S o n g

5::fi:27.

:t30.31.32.

33.34.35.36.37.38.39.40.41.42.43.44.45.46.47.48.49.50.

I-MechylilldWll.3-Dimethyl-2-e1hyIbenzeneI-M&ylbltyll?eIWlel.2-Dimethyl-3-e1hyl~I 2,4.5-Te~nnn&ylbenzenel,2.3,5-TeoamethylbenzeneJ-MuhylbuIylbenxneS-Mahylinden4-Melhylindan1.2,3.4-Tebamexhylbenzenc

II-FWllyltCN.eneTtinI-Methyl- -isobutylbenzcncNaphM2-Me&ylIebaiiiCyclepen~ylbcnzene + I-Mcthyltebalin5-Fdhylindann-Hexylbenzene6-Melhylleldin1.3~DimelhylbulylbenzcncPentamelhylbenzene5-MetiyllelmlinCyhhexylbcnzcneI-Ethyltebalinn-HepylbauencI-Frcpyltetralin2-lsopowlnaphthalencn-Oclylbenxnc

42


Figure 7 G.c.-m.s. total ion chromatogram of fraction 2 (monoarom atic fraction, 5% benzene-pentane eluate) of JP-8P jet fuel (DB-5 column,heated from 40 to 310C at 4C min-, split)

1

I -1rT I I I I1 ~ 1 16.00 18.00 20 3 22.00 24.00 26.00 28.00 30.00 32.00 34.00

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Figure 8 G.c.-m.s. total ion chromatogram of fraction 3 (aromatic fraction, benzene eluate) of JP-SP jet fuel (DB-5 column, heated from 4 0 to 310Cat 4C min-, split)

Fuel 1995 Volume 74 Number IO 1449

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alkanes are predominant; many branched alkanes are alsopresent, and the positions of the side-cha in are alsoindicated for them in Figure 6.

Figures 7 and 8 show the detailed g.c.-m.s. results forJP-8P fractions 2 and 3, respectively. Fraction 2 iscompo sed of mono aromatics and is free of saturate s.

Alkylbenz enes are the major com ponents in this fraction,and minor components include tetralins and indans,whose concentrations are much lower than those found incoal-derived jet fuel. The analysis sho ws that multi-substituted alkylbenzen es (instead of long-cha in n-alkylbenzenes) are the major compounds. Fraction 3from JP-8P represents a concentrated diarom atics frac-tion, and consists of mainly naphthalene and alkyl-naphth alenes. The identifications of ethylnap hthalenesand dimethylnaphthalenes were based on our ownretention indices and g.c.-m.s. results; however, tri-methylnaphthalenes were identified by comparing thecalculated retention indices or relative retention times in

this study with those reported in the literature for similarcapillary columns30>31.The above results indicate that the coal-derived JP-8C

fuel is significantly different from the petroleum -based JP-8P in composition. The saturate fraction constitutes 78%of JP-8C and 8.5% of JP-8P, a nd the aromatic fractionsmake up 20% of JP-8C and 12% of JP-8P15. The majorcompounds in JP-8P saturates are straight-chain andbranched alkanes ranging from C7 to Ci7. However, theJP-8C satura tes c onsist mainly of monocyclic, bicyclicand tricyclic alkanes, together with some long-chainalkanes. The aromatics in JP-8P are dominated byalkylbenzenes and alkylnaphthalenes, whereas the JP-8C

aromatics are rich in hydroaromatic compounds such astetralin, alkyltetralins, ind ans, cyclohexylbenzene andsome partially hydrogenated three-ring compo unds,together w ith some alkylbenzenes and alkylnaphtha-lenes. These results show that jet fuels derived fromdifferent sources may have distinctly different molecularcompositions.

CONCLUSIONS

We have determined temperature-programmed retentionindices of over 150 pure com pounds using two capillarycolum ns with different stationary phase polarities a t threeheating rates . The retention characteristics of varioushydrocarbons and N-, S- and O-containing compoundson the different column s w ere establishe d. Alipha ticcomp ounds give nearly constant retention indices atdifferent hea ting rate s, but the retention indices ofpolycyclic aromatic comp ounds exhibit a relatively largetemperature dependence. The use of a short isothermalhold (5 min) prior to the programmed heating does notcause any significant difference in the retention indices.Their dependences on the heating rates and temperatureprogramme and on stationary phase polarity have beendemo nstrated. The column polarity can affect theretention b ehaviour significantly, depend ing on thecompo und type. There also exist relationsh ips betweenthe tempera ture depende nce (or column polarity depen -dence) of retention indices and the compo und type.Almost all the compounds studied display highertemperatu re sensitivity on the more polar column (Rtx-50) than on the less polar column (DB-5), i.e. the values of


AR.Z./AT are larger for the Rtx-50 than for the DB-5column . Retention indices and their sensitivity to thetemperature programme decrease with decreasing columnpolarity.

The temperature-programmed retention indices arevery useful for g.c. and g.c.-m.s. analyses of coal- and

petroleum-derived liquid fuels, as has been demonstratedin identifying comp onents in two JP-8 jet fuels and theirliquid chroma tographic fractions. Over 100 fuel compo-nents were identified in detail. The results revealed thatthe coal-derived JP-8C fuel is significantly different incomposition from the petroleum-based JP-8P.

The knowledge of the effects of temperatu re andcolumn polarity on the retention behaviour of variouscompounds can be applied to improving compoundidentification by using different tempera ture program -ming rates or using columns of different polarity. Anothe rapplication of the present results lie s in the selection of anappropriate column coating phase and temperature

program me for more efficient analysis of given samp lesand for elimina ting or reducing the co-elution of certaincomp ounds. In addition, the present resu lts for retentionindices can be used to build a comp uter-assisted library ofretention indices, which in combination with a massspectral library can lead to faster and more reliableautom atic peak identification through computer librarysearching.

ACKNOWLEDGEMENTS

We are very grateful to Professor H. H. Schobert for hisencouragement, support and kind review. This work was

jointly supported by the US Depa rtment of Energy,Pittsburgh Energy Technology Center an d the Air ForceWright Laboratory/Aero Propulsion and Power Directo-rate, Wright-Patterson AFB. We thank D r D. M. Starchand M r W. E. Harrison, III, of WL and D r S. Rogers ofPETC for providing technical support and jet fuelsamples, and D r P. G. Hatcher for his instrumentalsupport to the maintenance of analytical equipmen ts. Oneof the authors (C.S.) thanks Dr J. Shiea for helpfuldiscussio ns on the g.c.-m.s. analsysis of branch edalkanes.

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