AN INTRODUCTION TO OPEN-TUBULAR GAS CHROMATOGRAPHY-­ANALYSIS OF FOSSIL AND SYNTHETIC FUELS

C. M. WHITE ANALYTICAL RESEARCH GROUP PITTSBURGH ENERGY TECHNOLOGY CENTER P.O. BOX 10940 PITTSBURGH, PA 15236 USA

ABSTRACT

High resolution open-tubular gas chromatography will be introduced, and instrumental aspects will be discussed, along with a substantial amount of practical information concerning its applications. Sample introduction systems, detectors, and column technology will be presented. A block diagram of a gas chromatograph depicting the various components will be shown. Several sample introduction systems will be described including split, splitless, and on-column injection. The concept of retention indices will be introduced, as will the use of gas chromatography for measuring thermophysical properties, such as boiling point and heat of vaporization. Lastly, a molecular topological parameter called molecular connectivity will be presented and it will be shown that it can be used to predict the gas chromatographic retention characteristics of aromatic compounds.

1. Introduction

Chromatography is a word coined by its inventor, M. S. Tswett, and is derived from two Greek words, chroma meaning color, and graphien meaning to write. 1-3 All forms of chromatography have two things in common, i. e., they consist of a mobile or moving phase and a stationary phase. The many forms of chromatography differ from one another based upon the state of the mobile phase and stationary phase. In this chapter, we are concerned with gas chromatography (GC), where the mobile phase is a gas, and the stationary phase is either a liquid or a solid. Specifically, we are concerned with open-tubular (frequently called capillary) gas chromatography where the chromatographic column is an open tube or capillary having an immobile stationary phase coated on the inside surface of the tube. Gas chromatography is an analytical technique used to separate mixtures of substances that are volatile and thermally stable at the operating temperature by moving the vapor phase mixture in a carrier gas over a stationary phase in which the components of the mixture dissolve. Some mixture components are more soluble in the stationary phase than others. The compounds that are least soluble in the stationary phase spend less time dissolved in it and elute (emerge) from the column frrst. Those constituents that are more soluble in the stationary phase spend more time dissolved in it and take longer to elute from the end of the column. The result is that what entered the column as a mixture elutes from the column as individual compounds.

107

C. Snape (ed.J, Composition, Geochemistry and Conversion o/Oil Shales, 107-123. © 1995 Kluwer Academic Publishers.

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A block diagram of a gas chromatograph is shown in Figure 1 and consists of (1) a high pressure cylinder of mobile phase commonly called the carrier gas, (2) flow controllers and pressure regulators that reduce the pressure from the cylinder and maintain a smooth delivery of carrier gas to the chromatograph, (3) an injection port where the mixture to be separated and analyzed is introduced to the chromatograph, (4) the chromatographic column that has the stationary phase coated on its inside surface and performs the separation of analytes, (5) the column oven that heats the column at either constant temperature or in a temperature programmed fashion, (6) the detector that detects the compounds when they elute from the column, and (7) the recorder that records the detector signal. The permanent record of the detector signal is called the chromatogram.

A chromatogram consists of a series of peaks arising from a baseline where each peak ideally represents a single compound. The time from sample introduction to the peak maxima is called the retention time and is usually different for each component in the mixture. As described later, the retention times assist in identification of the compounds in the mixture, while ideally, the peak area is directly proportional to the amount of that compound in the mixture. More detailed and complete information concerning various aspects of gas chromatography and open-tubular gas chromatography can be found in texts by Perry,3 Jennings,4 McNair,S Hyver,6 and Klee7.

2. Instrumental And Operational Aspects

2.1 SAMPLE INTRODUCTION (INJECTION)

The sample is injected into the chromatograph using a calibrated syringe. A measured volume of sample is drawn into the syringe and introduced to the gas chromatograph usually by piercing a rubber septum with the needle point, inserting the needle to the base of the syringe barrel, depressing the syringe plunger, and depositing the liquid contents of the syringe into the hot, helium-swept injection port. The syringe is then removed from the septum, allowing the septum to reform a gas tight seal. This is a general description of the mechanics involved in sample injection. Many variations exist depending on the kind of injection hardware mounted in the chromatograph. Some common injection systems used in open-tubular gas chromatography include the following: split injection (vaporizing), splitless injection (vaporizing), and cool on­column (not vaporizing). Other injection techniques are used, but these are the more common ones. Upon injection into a vaporizing injector the liquid sample is converted to a gas via flash vaporization of the solvent andlor volatile sample components in the hot injector, and carried out of the injector by the carrier gas (e.g. helium). Klee has written an excellent book on inlet (injection) systems used in gas chromatography. 7 A much more complete coverage of the topic can be found there and in Hyver's book. 6

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2.1.1 Split Injection. A split injection system is shown in Figure 2.6 An open-tubular column contains a very small amount of stationary phase, and thus can dissolve only very small amounts of sample, usually submicroliter amounts. Sample amounts this small are obtained by injecting much larger amounts (0.1 to 1.0 ILL) and splitting the carrier gas stream containing the sample so that the major portion of the stream is vented and only a small fraction of the stream reaches the chromatographic column. The split ratio is the ratio of the sum of the flow through the split vent plus the flow through the column to the flow through the column. The injection process is swift, the syringe is removed from the hot injector immediately after depression of the plunger, so that a small slug of sample is delivered to the column in as short a time as possible. Split injection generally results in the narrowest possible peaks. Split injectors vaporize the sample and protect the column from the nonvolatile sample components, which deposit in the injector. The heated injector also causes thermal decomposition of some analytes and discriminates against high boiling sample components as shown in Figure 3, and discussed below. Discrimination is a phenomenon that can occur in vaporizing injectors where the higher boiling sample components are not quantitatively transferred from the syringe to the chromatograph.

2.1.2 Splitless Injection. This injection technique allows much larger sample volumes to be delivered to the chromatographic column (l to 10 ILL). This injection technique is used when the analytes of interest are very dilute. The oven housing the chromatographic column is held about 15· C below the boiling point of the sample solvent so that the solvent condenses and floods the first few coils at the front of the column. During splitless injection, the carrier-gas flow through the split vent is turned off, and the entire contents of the vaporizing injector are swept onto the column. After 20 to 100 seconds the split vent is turned back on and any remaining vapors in the injector are vented. During this entire time the syringe is left in the hot injection port. This would normally result in broad peaks, but because the analytes are refocussed at the front of the column by dissolution into the solvent flooding the front of the column, the peaks are narrow. When the column temperature is increased, the solvent vaporizes and elutes from the column followed by narrow analyte peaks. Splitless injectors vaporize the sample, and protect the column from the nonvolatile sample components, which deposit in the injector. The heated injector also causes thermal decomposition of some analytes and discrimination as in the case of split injection.

2.1.3 CoolOn-Column Injection. In this injection technique, the liquid sample is deposited directly into the capillary column by inserting the syringe needle tip into the end of the column and depressing the plunger. The front of the column may be either cool or cold. The sample is not exposed to high temperature during introduction as in vaporizing injectors. Several designs of this injector exist. This injection technique has many advantages over other sample introduction methods, such as (1) providing the highest accuracy and reproducibility, (2) eliminating thermal decomposition of analytes

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in the hot ~ector, (3) transferring the syringe contents to the column more quantitatively, (4) eliminating discrimination of high boiling sample components, (5) eliminating the rubber septa used for other injection techniques, thus removing a major source of injector and column contamination. The column oven temperature used in on­column injection can be much higher than that used in splitless injection.

An example of the extent of discrimination against high boiling sample components in a mixture of n-alkanes injected using three different injection techniques is shown in Figure 3.8 A mixture of n-alkane standards was prepared so that each was present in nearly identical amounts. Chromatogram A was obtained using split injection where the liquid sample was present in both the syringe needle and the syringe barrel. Upon injection, the syringe plunger was immediately depressed after inserting the needle into the syringe barrel and the syringe removed from the injection port. Chromatogram B was obtained using split injection with the entire sample in the syringe barrel, and the syringe needle was allowed to warm in the heated injection port for several seconds before the plunger was depressed. Chromatogram C was obtained by coolon-column injection. Both types of split injection clearly result in discrimination against higher boiling n-alkanes, while coolon-column injection is relatively discrimination free.

2.2 ANALYTE DETECTION

The most common detector used in open-tubular gas chromatography is the flame ionization detector (FlO). The FlO responds to almost all organic compounds. The FlO responds uniformly to most hydrocarbons on a weight basis. Its response is decreased toward analytes containing oxygen, nitrogen, and other heteroatoms. Element specific detectors, which work well with open-tubular columns include the flame photometric detector (FPD), which is specific for analytes containing either sulfur or phosphorus; the nitrogen-phosphorus detector (NPD); the electron capture detector (ECD) , which responds selectively to halogen containing compounds; and the atomic emission detector, which detects the individual elemental composition of each analyte as it elutes. Spectroscopic detectors including the mass spectrometer, the infrared spectrophotometer and others are also used in combination with open-tubUlar gas chromatography. Operating conditions for commonly used detectors for capillary GC are listed in Table 1.

The FlO, shown in Figure 4, will be more fully described because it is the detector most commonly used with open tubular columns. The detector is fed a stream of hydrogen and a stream of air or oxygen that are mixed and combusted at the tip of the FlO. The chromatographic column extends to the tip of the FlO and as the analytes elute from the column they enter the flame and are combusted. Two electrodes at opposite ends of the flame measure the current in the flame. The apparent electrical conductivity of a gas, such as helium, is directly related and proportional to the number of ions in it.. When

III

no analytes are eluting from the column, only helium is entering the flame and the number of ions in the flame is constant. This is called the background current. As an analyte elutes from the column, it combusts in the flame creating a large number of ions causing current to flow between the electrodes. This produces a signal that is recorded and is called the chromatogram.

The sensitivity of the FlO depends upon the hydrogen and air flow rates as well as the use of make-up gas (adding extra helium or nitrogen). This is shown graphically in Figure 5. FlO sensitivity is also a function of the ratio of carrier gas (helium) to hydrogen.

The exact position of the capillary column in the FlO can have a dramatic effect on the observed chromatogram. Ideally, the end of the capillary column is placed about 1 mm from the end of the FlO tip to minimize analyte contact with the metal surface of the FlO. As shown in Figure 6, adsorptive analytes that are even slightly polar will adsorb on active surfaces of the FID before they reach the detector.

3. Column Considerations

3.1 COLUMN DIMENSIONS AND MATERIALS OF CONSTRUCTION

Almost all open-tubular columns used today are constructed from fused silica that is coated on the outside with either polyimide or aluminum. Aluminum coated capillaries are used for very high temperature operations (above about 380°C). Less than one dozen stationary phases are in common usage for capillary gas chromatography. Some of them are listed in Table 2 along with their polarity, temperature limits, and some applications. These stationary phases are available on commercial columns in various film thicknesses, column lengths, and diameters. A column coated with a stationary phase from one manufacturer may not display the same retention characteristics as one coated with the same stationary phase available from a different manufacturer. The reasons for this are complex and are due, in part, to batch-to-batch chemical variations in the stationary phase and different column deactivation procedures used by different manufacturers, as shown in Figure 7.6

The column internal diameter directly affects the number of theoretical plates (efficiency) of the column. Commercially available columns have internal diameters ranging from 0.1 to 0.75 mm. Generally, the smaller the internal diameter the more theoretical plates per meter the column will have. However, smaller inside diameter columns contain less stationary phase, and can accommodate smaller amounts of individual analytes before overloading occurs and performance is degraded.

3.2 COLUMN TEMPERATURE

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Most capillary gas chromatographs in use today are capable of operating in either isothermal or temperature programmed modes. They are equipped with very sophisticated ovens and thermostats that allow column temperatures to be controlled to within 0.1 0 C or better. They are also capable of extremely accurate and reproducible temperature programming rates. The retention of analytes is directly related to column temperature. The effect of temperature on retention and the effect of temperature programming on the observed chromatogram is shown in Figure 8.9 As the isothermal operating temperature is increased, retention time of the analytes decreases. When the sample under analysis consists of a wide boiling range of substances, temperature programming is used to elute the highest boiling analytes in a reasonable operating time. Generally, under temperature programmed operation, the slower the temperature programming rate, the better the observed separation.

4. Treatment Of Retention Data: Qualitative Analysis

4.1 RETENTION INDICES

Two pieces of information are obtained with each peak in the chromatogram, the retention time, and the peak: area. Modem gas chromatographs are capable of reproducing the retention times of peaks that elute within the first hour of operation to within a few hundredths of a minute. Retention information is most conveniently expressed as either relative retention or as a retention index. The retention index system proposed by KovatslO is strictly for isothermal operation and has been modified by van den 0001 and Kratz for linear-temperature-programmed operation. 11 In the van den Dool and Kratz method, the retention index of a substance is calculated relative to the retention time of two bracketing standards that are n-alkanes, equation 1.

T - T I = 100 R (subs tance) Rn + 100 n

TRn+l - TRn (1)

In equation 1, I is the retention index of the substance of interest, T R( substance) is the retention time of the substance of interest, T Rn is the retention time of the n-alkane that elutes before the substance of interest, T Rn+ 1 is the retention time of the bracketing n­alkane that elutes just after the substance of interest, and n is the number of carbons in the first bracketing n-alkane. Experimentally, a mixture of n-alkanes is added to the sample, injected and chromatographed. The retention indices are calculated by inserting the experimentally determined retention times into equation 1, and solving for l. When using carefully controlled conditions, the calculated retention indices are reproducible within anyone lab to about +/- 0.5 index units, and within about 1.5 index units from one lab to another. The retention indices of hundreds of compounds have been measured and are reported in the literature. 12 By comparing the experimentally determined retention indices from the sample with the measured indices in the literature, compounds

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in the sample can sometimes be tentatively identified.

The technique of using retention indices to tentatively identify unknown compounds in a synthetic gasoline sample is illustrated in Figure 9 and Table 3. 13 The retention indices of the unknown compounds are printed next to the previously published retention indices of known compounds. The comparison between the values is excellent, and allows the analyst to tentatively identify the unknown compounds. When this initial identification is combined with spectral information on the same peaks obtained from combined gas chromatography-mass spectrometry and/or combined gas chromatography­infrared spectrophotometry, then identification of the compounds can be positive.

4.2 MOLECULAR CONNECTIVITY

Sometimes the retention index of a compound of interest may not be in the literature, or a pure sample of the compound may not be available for measurement. The retention index of a compound of known structure can be sometimes be estimated using a concept called molecular connectivity, X, which is a molecular topological index that can be easily calculated for any compound that can be drawn. Molecular connectivity is a description of molecular structure based on a count of skeletal atoms, and weighted by degree of skeletal branching. More detailed information concerning the concept of molecular connectivity can be found in a book by Kier and HaUl4 and a publication by White. IS The first order valence molecular connectivity, lXv, is calculated according to equation 2 .

(2)

Each atom is assigned a value (5), which is the number of bonds to that atom, ipt0ring bonds to hydrogen. Atoms i and j are bonded. An example calculation of Xv for phenanthrene is given in Figure to, and examples of how molecular connectivity can be used to estimate thermophysical properties of planar aromatic hydrocarbons are illustrated. The IXv value correlates directly with the molecule's gas chromatographic retention index and its thermophysical properties. Once IXv is computed, the compound's boiling point, T b' and heat of vaporization, i!:t. Hv ' can be directly calculated using equations 3 and 4.

Tb = 76.21 IXv + 225.7 (3)

i!:t.Hv = 6.6464 IXv + 25.147 (4)

The planar aromatic hydrocarbon's boiling point can be estimated using equation 5, and

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Table 1. Operating CQndiUQnll fQr D~ectQrll CQmmonl~ Used in ClIDill~ Gall ChrQmato&m1'!h~. (AdaPted from a table in referen~e tl.)

Flow Rate (ml/min)

Type Typical Samples Sensitivity Carrier Range + H2 Air

Makeup

FID Hydrocarbons 10-100 pg 20-60 30-40 200-500

TCD General 5-100 ng 15-30 n.a. n.a.

ECD Organohalogenates 0.05-1 pg 30-60 n.a. n.a.

NPD Organonitrogen and 0.1-10 pg 20-40 1-5 70-100 Organophosphorus Compounds

FPD Sulfur Compounds 10-100 pg 20-40 50-70 60-80 (393 nm)

FPD Phosphorus 1-10 pg 20-40 120- 100-(526 nm) Compounds 170 150

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Table 2. Common StationllJ}: Phil~ Coatini:S for Eused-Sili!;;il CapillllJ}: Columns, (Adapted from il tabl~ in r~feRm;~ (l)

Phase with

Composition Polarity Similar Temperature McReynold's Limits Constants

1. 100% Nonpolar OV-l _60°C to 400°C dimethylpolysiloxane SE-30 (Gum)

2. 100% Nonpolar OV-101 O°C to 280°C dimethylpolysiloxane SP-2100 (Fluid)

3. 5% diphenyl95% Nonpolar SE-52 _60°C to 325°C dimethylpolysiloxane OV-23

SE-54

4. 14% cyanopropyl Intermediate OV-1701 _20°C to 280°C phenyl polysiloxane

5. 50% phenyl 50% Intermediate OV-17 60°C to 240°C methyl polysiloxane

6. 50% cyanopropylmethyl Intermediate OV-225 60°C to 240°C 50% phenylmethylpolysiloxane

7.50% trifluoropropyl Intermediate OV-21O 45°C to 240°C 50% dimethylpolysiloxane

8. polyethylene glycol- Polar OV-351 60°C to 240'C TPA modified SP-lOOO

9. polyethylene glycol Polar Carbowax 20M 60°C to 220°C

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TABLE 3. ComJlQunds Identified in th~ LiQuid CQnd~nsat~ and Their APll[Qxima~ CQn~entration in the Liqyid CQndensate, The LiQuid CQndensat~ is APllrQximat~l~ ~7 W~igbt 19 Qf the TQtal Produ~t,

Method of Identificati.on Estimated

Peak # Com.pound Hame GC-MS FTIR Measured Known Weight

R.I:. R.I. Percent

6 Ch1oroethane 423.96 424.09 0.003

9 !.-Pantene 483.67 483.39 0.004

10 2-Chloropropane x x 490.72 491..40 0.012

11 2-Methyl-1-butene x 493.79 493.58 0.009

13 (B)-2-Pentene 505.09 504.91 0.027

14 (Z)-2-Pentene 510.30 510.35 0.014

17 2"'Chloro-2-methylpropane x 530.66 530.04 0.146

18 cyc1open'tane x x 554.24 554.13 0.022

23 2-Ch1orobutane x 598.87 598.38 0.030

25 (B) -3-Hexane 601.03 601.42 0.014

26 ( Z ) -3-Rexene 602.28 602.19 0 .. 037

30 Methylcyclopentane 620.67 620.81 0.296

33 2-Chloro-2-lI.ethylbutane 647.89 647.88 0.91.1

34 Cyc10hexane x 651.09 651.14 0.016

35 2-Methyl.hexane x 661.02 661.15 0.165

38 cycl.ohexene 667.76 667.80 0.021

39 3-Hethy~hexane 670.56 670.60 0.162

54 Hethylbenzene x 748.65 749.00 2.452

55 2-Methylbeptane x 763.50 763.03 0.078

56 4-Methylheptane x x 764.38 764.57 0.044

63 Ethylbenzene x x 844.50 844.74 1.473

6. l,3-Di.et.hy1benzene x 853.85 853.38 8.789

65 l,4-Dia.thy1benzene 854.85 854.75 4.847

67 l,2-Di •• thy1benzene 875.88 875.89 4.041

72 1,3,5-Triaethylbenzene x 954 .. 28 954 .. 69 0.701

74 1,2,4-Tri •• thylbenzene x 980.80 978.78 45.343

77 1,2,3-Trimetbylbenzene 1004.87 1005.04 0.382

90 1,2,4,5-Tetraaethylbenzene x 1102.07 1101.84 5.381

91 1,2,3,S-Tetraaethylbenzene x 1104.97 1104.26 0.717

94 1,2,3,4-Tetrahydro 1140.47 1140.85 0.120 naphthalene

95 HaEhthalene x 1160.26 1158.51 0.150

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Bonte Enlarged eros Section

Fig. 1 Block diagram of a gas chromatograph. Reprinted with permission from reference 1.

SePtum--___ ~~3

Carrier Gas -=:=;;~:;t';:1 Needle Valve

Vaporization Chamber Spll Ratio

"- Needle Valve Capillruy Column

Fig. 2 Drawing of a split injector. Reprinted with permission from reference 6.

118

i 10

12 , i 114 15

,

-

!

I

-

16

18

injection 'In needle'

20

inject 'hot

I ion needle'

1

'" JJ on-column inJection

Fig. 3 Discrimination against n-alkanes as a function of injection technique. Reprinted with permission from reference 8.

AIr Inlet --___ -"

caf~~'o;~I~~n (1-2 mm from Top of Jet)

FID Detector Assembly

119

Fig. 4 Schematic diagram of a flame ionization detector. Reprinted with permission from reference 6.

I i n-Heptane

20

> .§ \·5

II ~ i'---

] / J.O

0·5

o 10 20 30 40 50 60

Hydrogen flow ml min

Fig. 5 Sensitivity change of a flame ionization detector toward heptane as a function of H2 flow rate. Reprinted with permission from reference 5.

Column Inser1ed through Jet Column Terminates at Jet

l·oetanol 22~6.~~ ~==c.<.!!:"""':"

3,.·OMP

dleyelohtlylanlnt !=---- mnophtllyl,n,

dleyelohtlylanlnt letnlphthyl,n,

l·dodte.nol

n-C'5

Column: Fused SlIIel, SP·2100., (CiVbowax* 20M DeactlYifed) 20 m)( 0 20 mm

Fig. 6 The relationship between the column terminus in the FID and the quality of the observed chromatogram. Reprinted with permission from reference 6.

120

,. I

1, Tetradecane 2, Pentadecane 3. n-Oetanol 4, Hexadecana S. Butyric Acid 6. Dlcyclohexylamine 7. Heptadecane

--"-

1. 2. l. 4.

2. l. ._ 7 . ..

, . s .•

~. '--"-'--'--__ JlJ'--.JL-J '---

Fig. 7 Chromatograms of a test mixture on four commercially available columns all coated with cross-linked Carbowax 20M. Reprinted with permission from reference 6.

I.Jr-------------~

i 1 I 'oolhe,mal ,

i .lA· ,3 }\ • I:~;":Y ~ 'Q~ "'WC~ J~.

o 1020 30

TIME. min

Fig. 8 The effect of different isothermal operating temperatures in the observed chromatograms is displayed in a and b. Chromatogram c shows the effect of temperature programming. Reprinted with permission from reference 9.

i

CHLOROMETHANE/ZEOLITE LIQUID CONDENSATE

10

90

,\i -/ 130

20 TIME (MIN)

100

140 TIME (MIN)

110

i 150

121

80

120

160

Fig. 9 High resolution gas chromatogram of a synthetic gasoline obtained using a 100m x O.25mm open-tubular column coated with a 0.5 I'm film of dimethylpolysiloxane. The column was temperature programmed from 30 0 C to 2200 C at 10 C per minute. Some numbered chromatographic peaks are identified in Table 3. Reprinted with permission from reference 13.

122

(3) k

(3)

(3)

(3)

d (3)

IX - ~ I + /T + /T + /T + IT V - 4·3 V 3.3 V 3.3 V N V M

abc d e

+~ +4 +~3~4+~+~41.4 k L m n 0

+~ 4~4 = 4.815 P

Fig. 10 An example calculation of a first order molecular connectivity for phenanthrene. Reprinted with permission from reference 15.

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its retention index can be estimated, with a standard error of estimate of 8.8 index units, using equation 6. Thus, the thermophysical properties of an aromatic compound can be accurately estimated using these techniques.

Tb = 1.0672 1+ 282.82

I = 69.69 lXv - 41.93

(5)

(6)

Information concerning the fundamental physical, chemical, and thermodynamic properties of fossil fuels and individual components present in fossil fuels is needed to properly design and operate fossil fuel processing plants. Since many of these values have not been measured, the estimation techniques illustrated above are particularly useful for engineers designing fossil fuel processing equipment.

5. References

1. V. Heines, Chern. Tech., 1971, 1,281. 2. M. S. Tswett, J. Chern. Educ., 1967,44, 238. 3. J. A. Perry, Introduction to Analytical Gas Chromatography, Marcel Dekker

(1981). 4. W. Jennings, Gas Chromatography With Glass Capillary Columns, Academic

Press (1978). 5. H. M. McNair and E. J. Bonelli, Basic Gas Chromatography, Varian Instrument

Division (1969). 6. High Resolution Gas Chromatography, K. J. Hyver, ed., Hewlett Packard

(1989). 7. M. S. Klee, GC Inlets-An Introduction, Hewlett Packard (1990). 8. K. Grob, and G. Grob, J. High Resolut. Chrornatogr., 1979, 2, 109. 9. H. W. Habgood and W. E. Harris, Anal. Chern., 1960,32,450. 10. E. Kovats, Helv. Chirn. Acta, 1958,41, 1915. 11. H. van den Dool and P. D. Kratz, J. Chromatogr., 1963, 11,463. 12. C. M. White, J. Hackett, R. R. Anderson, S. Kail, P. S. Spock, J. High

Resolut. Chrornatogr., 1992, 15, 105. 13. C. M. White, L. J. Douglas, J. P. Hackett, R. R. Anderson, Energy & Fuels,

1992,6,76. 14. L. B. Kier and L. H. Hall, Molecular Connectivity in Chemistry and Drug

Design, Academic Press (1976). 15. C. M. White, J. Chern En2 Data, 1986, 31, 198.