11040-8347/00/$.50 2000 by CRC Press LLC

Critical Reviews in Analytical Chemistry, 00(0):0000 (2000)

Chromatographic Techniques for Petroleum andRelated ProductsBhajendra N. Barman*Equilon Enterprises LLC, Westhollow Technology Center, P.O. Box 1380, Houston, Texas 772511380

Vicente L. Cebolla* and Luis MembradoInstituto de Carboqumica CSIC, Mara de Luna 12, 50015 Zaragoza, SPAIN

KEY WORDS: petroleum, coal products, hydrocarbon type, simulated distillation, gas chromatography; liquidchromatography; supercritical fluid chromatography, supercritical fluid extraction, thin-layer chromatography,size-exclusion chromatography .

TABLE OF CONTENTSI. INTRODUCTION................................................................................................................................

II. GAS CHROMATOGRAPHY.............................................................................................................A. Hydrocarbon Type Analysis by GC ..........................................................................................

1. Capillary Column GC ............................................................................................................2. Multidimensional GC ............................................................................................................3. Hydrocarbon Type Analysis of Coal-Derived Products .......................................................

B. Boiling Range Distribution by GC ............................................................................................1. Simulated Distillation by GC-FID2. High-Temperature Simulated Distillation3. Simulated Distillation by GC-MS

C. Analysis of Wax and Similar Samples by High-Temperature GC ..........................................D. Element-Specific Detectors for Heteroatomic Compounds ......................................................

1. Sulfur Chemiluminescence Detector and Nitrogen Chemiluminescence Detector ............2. Atomic Emission Detector ....................................................................................................

E. Fast GC .......................................................................................................................................F. Pyrolysis GC ...............................................................................................................................

III. OPEN-COLUMN AND MEDIUM PRESSURE LIQUID CHROMATOGRAPHY..................A. Petroleum Distillates ..................................................................................................................

1. Standard Methods ..................................................................................................................2. Separation Schemes ...............................................................................................................

B. Shale Oil and Coal-Derived Products ........................................................................................C. Quantitative Analysis by Open-Column Liquid Chromatography ............................................

IV. HIGH-PERFORMANCE LIQUID CHROMATOGRAPHY .....................................................A. Hydrocarbon Type Determination .............................................................................................

1. Standard HPLC Methods* Corresponding authors.

22. Light and Middle Distillates, and Gas Oils ..........................................................................3. Heavy Distillates and Coal-Derived Products ......................................................................

B. Determination of Target Compounds ........................................................................................1. Standard HPLC Methods.......................................................................................................2. Illustrative Applications ........................................................................................................

a. Polycyclic Aromatic Hydrocarbons .................................................................................b. Heterocyclic Compounds .................................................................................................c. Gasoline and Diesel Additives .........................................................................................d. Lubricant Additives ..........................................................................................................

C. Preparative HPLC.......................................................................................................................D. HPLC Detectors .........................................................................................................................

1. Spectroscopic Detectors ........................................................................................................a. UV-Visible Detector .........................................................................................................b. Fluorescence Detector ......................................................................................................c. Infrared Detector ..............................................................................................................

2. Bulk Property Detectors ........................................................................................................a. Refractive Index Detector ................................................................................................b. Evaporative Light Scattering Detector ............................................................................c. Dielectric Constant Detector ............................................................................................d. Flame-Ionization Detector ...............................................................................................

3. Mass Spectrometric Detector ................................................................................................4. Miscellaneous HPLC Detectors ............................................................................................

a. Element-Specific Detectors ..............................................................................................b. Electrochemical Detector .................................................................................................

V. SUPERCRITICAL FLUID CHROMATOGRAPHY AND EXTRACTION ...............................A. Hydrocarbon Type Analysis ......................................................................................................

1. Light Distillates: Gasoline and Naphtha ...............................................................................2. Middle Distillates: Diesel, Jet Fuel, and Kerosene ..............................................................3. Heavy Distillates: Crude Oil, Gas Oil, and Residuum .........................................................

B. Simulated Distillation by SFC ...................................................................................................C. Analysis of Polycyclic Aromatic Hydrocarbons .......................................................................D. Compound Class Fractionation by Supercritical Fluid Extraction ...........................................

1. Fossil Fuels ............................................................................................................................2. Environmental Samples .........................................................................................................

VI. THIN-LAYER CHROMATOGRAPHY..........................................................................................A. TLC Sorbents, Solvents and Detectors ......................................................................................B. Planar Chromatography ..............................................................................................................C. TLC with Flame-Ionization Detection .......................................................................................

VII. SIZE-EXCLUSION CHROMATOGRAPHY...............................................................................A. SEC Columns, Mobile Phases and Detectors ..........................................................................

1. SEC Columns ........................................................................................................................2. SEC Mobile Phases ...............................................................................................................3. SEC Detectors ........................................................................................................................

B. Molecular Weight Distribution by SEC ....................................................................................C. Preparative SEC..........................................................................................................................D. Anomalous Component Elution in SEC and Other Non-Size Effects .....................................

3E. Typical Coal and Petrochemical Applications of SEC .............................................................1. Paraffin Wax ..........................................................................................................................2. Lubricating Oil ......................................................................................................................3. Crude Oil ...............................................................................................................................4. Coal Liquid ............................................................................................................................5. Heavy Petroleum and Coal Fractions ...................................................................................

VIII. HYPHENATED OR MULTI-TECHNIQUE CHROMATOGRAPHY....................................A. HPLC-GC ...................................................................................................................................B. SFC-GC ......................................................................................................................................C. SFE-Chromatography .................................................................................................................D. Miscellaneous Coupled Systems ................................................................................................

IX. CONCLUSIONS ................................................................................................................................ REFERENCES....................................................................................................................................

ABSTRACT: Recent developments in chromatographic techniques for the separation and quantitative character-ization of petroleum and related products are highlighted. Specifically, scope, applicability, and versatility ofindividual techniques such as gas chromatography, liquid chromatography, supercritical fluid chromatography,thin-layer chromatography, and size- exclusion chromatography are discussed in some detail. In general, analyticalapproaches vary widely depending on the chromatographic technique, instrumentation, minimum detection limit,and sample type. Specific applications include chromatographic separation followed by identification and deter-mination of individual components, measurement of boiling range distribution, and determination of hydrocarbongroup types. With the exception of gaseous samples and light distillates (up to gasoline range materials), theinherent complexity arising from the presence of numerous isomers and compound types, and the finite resolutionafforded by chromatographic methods preclude precise identification and quantitative determination of individualcomponents in fossil fuels and oils. For the middle and heavy distillates, chromatographic techniques have beenapplied mostly for the isolation and determination of compound classes. Several techniques such as open-columnliquid chromatography, medium pressure liquid chromatography, high-performance liquid chromatography, andsupercritical fluid extraction provide separation of compound classes for subsequent characterization by high-resolution chromatography, spectroscopy, or other methods. Hyphenated or multitechnique chromatographicapproaches have also been described. These simplify the characterization of complex samples by incorporatingmultiple separation mechanisms and detection schemes.

I. INTRODUCTION

A feature common to most fossil fuels andoils is that they are complex mixtures of hydro-carbons and related compounds. The hundreds ofindividual compounds constituting petroleum andcoal-based materials vary in molecular structureand size, polarity and functionality or chemicalgroup. The composition as well as the occurrenceof a specific hydrocarbon family in a given prod-uct depends on the type of fuel and its source orprocess history. There have been continuous ef-forts to look for better approaches to characterizethese materials.

Many chromatographic techniques have beenemployed for the characterization of petroleumand related products. In some cases (for an ex-ample, the characterization of polycyclic aromatic

hydrocarbons, PAHs), these techniques are ap-plied to separate and determine individual com-pounds. However, as chemical groups rather thanindividual molecules determine chemical proper-ties of most fossil fuels and oils, hydrocarbontype analysis (HTA) by chromatography, thus far,has been very popular for their chemical charac-terization. With HTA, one can determine the qual-ity of the fuel or oil, evaluate variables for itsconversion processes, elucidate reaction pathwaysand kinetics, and obtain insights into theprocessability of the feed or the quality of thefinal products.

HTA is also known as SARA (saturates, aro-matics, resins, and asphaltenes), and PONA, PI-ANO, or PIONA (paraffins, isoparaffins, olefins,naphthenes, and aromatics), depending on thechemical groups analyzed in each sample. Sepa-

4ration of samples according to the number ofaromatic rings (mono-, di-, tri-, polycyclic aro-matics) may also be considered HTA. Almost allfuels and oils have some polar components, amajority of which are nitrogen, oxygen, and sul-fur compounds. They may exist from ppm topercent levels in various samples. Usually, ana-lytical approaches for hydrocarbon type determi-nation, compound class fractionation, or identifi-cation of individual compounds vary with thesample type, components present, and their con-centrations.

For a variety of applications, fossil fuels arefractionated according to their boiling points bydistillation methods. Chromatographic techniquesare often used to measure boiling point distribu-tions of petroleum or coal-based products. De-pending on boiling point range of the sample,fractionated products fall into three major catego-ries: light distillates, middle distillates, and heavydistillates. All gasoline range materials and naph-thas are considered light distillates. The middledistillates are diesel, jet fuel, and kerosene rangesamples. Heavy distillates can be distillation cutsfrom vacuum gas oils. Most often, methods forheavy distillates can conveniently be applied tovery high boiling materials such as bright stocks,vacuum residua, and asphalts.

To be an ideal chromatographic method forthe determination of individual compounds, HTAor boiling point distribution, the method shouldfulfill the following requirements: it should berugged and reproducible, rapid, adequate for qual-ity control, quantitative, and applicable to thewhole sample without requiring prefractionation.As discussed in this paper, most of these criteriaare met reasonably well by the methods for lightand middle distillates. However, in the case ofheavy materials or samples covering a broad boil-ing point as well as polarity range, currently avail-able chromatographic methods often have somelimitations. Usually, problems arise from inad-equate resolving power, low volatility, low solu-bility of the components in the chromatographicmobile phase, strong adsorption of sample com-ponents on the stationary phase, or low detectorsensitivity. The task is always more challengingwith very complex samples (such as coal liquids)

containing numerous functional groups and hun-dreds of individual compounds.

This overview is a focused discussion on thebasic principle, scope, and applicability of each ofthe major chromatographic techniques used forthe separation and characterization of varieties ofpetroleum and coal products. To this end, moreemphasis is given to major developments of chro-matographic methodologies than to providing acomprehensive collection of references on theirapplications. The applicability of each chromato-graphic technique is discussed along with its majorlimitations. The chromatographic techniques cov-ered are gas chromatography (GC), liquid chro-matography (LC), supercritical fluid chromatog-raphy (SFC), thin-layer chromatography (TLC),and size-exclusion chromatography (SEC).

II. GAS CHROMATOGRAPHY

Gas chromatography has long been vital topetrochemical analysis where its roles range fromon-line monitoring of process streams and char-acterization of feeds, intermediates, and productsin refinery operations, to providing analytical datato meet environmental regulations. The diversityof uses of GC are attributable to modern auto-mated instruments that are equipped with high-performance packed or capillary columns, uni-versal as well as selective detectors, and otheraccessories. Gas chromatographs can be operatedover a wide range of temperatures (typically, from50C to 430C), thus allowing these instrumentsto be suitable for samples ranging from gas tohigh boiling residuum. The use of thermally stablecolumns and the availability of injection portsand detectors that operate at high temperatures(up to 430C) have been helpful for the analysisof many difficult-to-volatilize materials. A list ofmajor GC methods approved by the AmericanSociety for Testing and Materials (ASTM) givenin Table 1, reflects the wide variety of GC appli-cations for petroleum-related materials.

For low-molecular-weight hydrocarbons andheteroatomic molecules having up to 10 or socarbon atoms, capillary column GC provides ex-cellent resolution between major component peakspermitting both identification and quantitation of

5individual components in the sample. However,for heavier samples, because of coelution of nu-merous isomers and interference from isomers ofadjacent carbon numbers, identification of indi-vidual components becomes a difficult task. Ingeneral, GC chromatograms obtained from coalconversion products such as coal-derived oils aremore complex than those from petroleum prod-ucts because they contain a broader variety of

compound families and much higher levels ofpolar compounds, including molecules with het-eroatoms and PAHs.

GC stationary phases used vary from non-polar, such as methylsilicone, to highly polarCarbowax or TCEP (tris-cyanoethoxypropane),depending on the sample components and selec-tivity required for a particular separation.1,2 Flame-ionization detection (FID) is applied extensively

Table 1Major GC Methods Approved by the ASTM

6in the GC analysis of fossil fuel-derived samples.FID and other common GC detectors are listed inTable 2. Basic principles and selectivities of thesedetectors are also highlighted. Major applicationareas of these detectors are given in the last col-umn of Table 2. In many applications, multipledetectors are used in series or parallel configura-tion to take advantage of their selectivity and/orto obtain data from multiple columns. A discus-sion on some element -specific detectors is cov-ered in a separate section to highlight some spe-cial petrochemical analyses.

A. Hydrocarbon Type Analysis by GC

1. Capillary Column GC

High-resolution capillary columns have been usedto separate individual components in gasoline to die-sel range samples.3-6 For the most part, FID has beenused for the quantitative determination of each hydro-carbon component from its peak area using an appro-priate response factor. GC data are then grouped intodifferent hydrocarbon classes by carbon number.3-5

In the past, GC coupled with mass-spectrom-etry (GC-MS) served as the primary technique forthe identification of individual components, par-ticularly in gasoline range samples.7 A list ofabout 400 identified peaks in whole gasolinesamples was provided by Whittemore in 1979.8At present, most commonly retention indices areused to identify components separated by capil-lary column GC-FID. For light petroleum samples,Kovats retention index,9,10 based on the retentiondata of n-alkanes, has been found to be useful toidentify hydrocarbon components irrespective ofminor variations in chromatographic conditionsand column parameters.11,12 A combination ofretention indices and GC-MS has also been ap-plied to identify components in a complex hydro-carbon mixture.13 GC-FTIR and retention indiceswere used to identify components in middle oilfractions from a coal tar.14 As coal-derived liquidscontain high levels of polycyclic aromatic com-pounds (PACs), retention indices based on inter-nal standards naphthalene, phenanthrene,chrysene, and picene were shown to be moreeffective for peak identification than Kovats re-

tention index system. The retention indices ofover 200 PACs were reported.15,16

A complex separation scheme was applied tofractionate a light crude oil to identify and quantitate281 aliphatic, aromatic, and biomarker compounds.17,18The collection points from open-column liquid chro-matography with silica gel were determined using 15n-alkanes and 16 PAHs. The resulting fractions werethen analyzed by GC-FID and GC-MS.

Usually, the column and temperature programprofiles determine how best the individual hydro-carbons can be separated and resolved from eachother by capillary column GC. In practice, manysmall peaks from real-world samples are ignored.However, some coelutions of major componentshave been encountered, and these require closerattention. For an example, a multistep temperatureprogramming was used to resolve some key com-ponents that are difficult to resolve otherwise. 19

The role of GC-MS has been very importantin petrochemical analysis. Besides componentidentification 7,20 and determination (e.g., aro-matics by the ASTM D5769 method) in complexpetrochemical samples, there are GC-MS studiesthat resemble hydrocarbon group type analyses.Capillary column GC-MS methods based onchemical ionization with nitric oxide were ap-plied to both identify and quantify compoundtypes in gasoline 21 and middle distillates.22 Themethod is used to classify hydrocarbon types bytheir carbon number n and hydrogen deficiencyindex z defined by a general formula CnH2n+z,where z can be both positive and negative inte-gers. The distributions of saturates, mono-, di-,and tri-aromatics are obtained by using relativepopulations corresponding to appropriate z num-bers. A GC-MS method has also been demon-strated for the determination of acyclic alkenes ingasoline to light gas oil range samples.23 Themethod involves the detection of an intense(M+43)+ or (M+46)+ adduct ion peak in the ac-etone or acetone-d6 chemical ionization mass spec-tra of alkenes in a petrochemical sample.

2. Multidimensional GC

All multidimensional GC systems describedin the literature utilized two or more columns of

7Tabl

e 2

Com

mon

GC

Dete

ctor

s

8different polarities and selectivities .4,24-27 Althoughseveral multidimensional GC systems were re-ported, only one is commercially available.24,25This automated multicolumn GC system can beoperated under various modes by activating anumber of valves and using several traps andcolumns. This allows separation and quantifica-tion of the hydrocarbon types of light distillates(with final boiling point up to 200C), and theseparation of each type by carbon number. Sev-eral reports have described for PIONA (paraffins,isoparaffins, olefins, naphthenes, and aromatics),PNA, and PONA determinations from light distil-lates, including naphthas, reformates, and alky-lates.6,28-31 In some cases, the multidimensionalGC results were compared with those from othermethods, including capillary column GC 6,29-31and ASTM D1319 fluorescent indicator adsorp-tion (FIA).6,28 The commercial multidimensionalGC system uses columns and traps that include areversible olefin trap, a polar OV-275 column, anon-polar OV-101 column, a 5A molecular sievecolumn and a 13X molecular sieve column.24,28The aromatics are separated from saturates andolefins by the OV-275 column. The olefins areretained in an olefin trap allowing saturates toelute. Isoparaffins plus napthenes are separatedfrom n-paraffins by the 5A molecular sieve col-umn. The 13X molecular sieve column then sepa-rates isoparaffins and naphthenes. The 13X col-umn also allows the elution of componentsaccording to their carbon numbers. The OV-101column separates aromatics according to theirboiling points.

3. Hydrocarbon-Type Analysis of Coal-Derived Products

Separation and determination of chemical fami-lies according to the number of rings have beenachieved for coal-derived products. There havebeen studies on qualitative retention index of elu-tion for polycyclic aromatic compounds, which areusually present in coal samples.15,16,32-35 Conclu-sions from these studies suggest that elution of coalproducts from polymethylsiloxane columns is car-ried out according to the number of rings (whichcan be both aromatics and non-aromatics). In this

manner, an approximate separation by index zonesfor one ring to five rings can be obtained. The useof an average response factor for each zone (calcu-lated using the corresponding PAC standard), aswell as the use of an internal standard (usually, n-decane) allows an approximate quantitation of thesefamilies.32,35 However, these results are subject tolarge uncertainties due to difficulty in delimitingindex zones in real chromatograms and in obtain-ing accurate response factors.

B. Boiling Range Distribution by GC

1. Simulated Distillation by GC-FID

In most part, refinery streams are fractionatedaccording to their boiling ranges. Although physi-cal distillation methods (such as ASTM D86,D1160, and D2892) closely resemble actual re-finery operations, they are time-consuming andcost-prohibitive for routine process control. Simu-lated distillation (SimDis) by GC has been veryeffective as an alternative to physical distillation.SimDis is based on the fact that hydrocarbons areeluted from a non-polar column essentially inboiling point order, and that a complete elution ofsample can be achieved as the column tempera-ture is increased through programming. Usually,sample detection is carried out by a FID. Thepeak integration is done in fixed time slices, ratherthan on individual peaks. The time axis is thenconverted to boiling temperature by using a mix-ture of standards with known boiling points. Theusual choices for the standard mixtures are a mix-ture of n-paraffin standards for petroleum prod-ucts, and a mixture of polycyclic aromatic com-pounds in the case of coal liquids. By comparingchromatographic retention times of the sampleand standards, it is possible to calculate the per-centage recovery or process conversion yield atany desired temperature.36 If complete elution ofsample is not possible, the boiling range distribu-tion data are determined by using either internalstandards or external standards.

SimDis methods such as ASTM D3710 andD2887 are intended for gasoline range and lubebase oil samples with final boiling points up to1000oF, respectively, and it is assumed that the

9whole sample elutes from the column and is de-tected. ASTM D5307 was designed to analyzefront ends of crude oils or heavy distillate samplesboiling up to 1000oF using internal standards toestimate the quantitative recovery.

Nonpolar packed columns and capillary col-umns are used for the ASTM approved GC-simu-lated distillation methods. Temperature program-ming is used to achieve separation of samplecomponents with adequate resolution in a reason-able time. Even with packed columns or thick filmcapillary columns, it becomes necessary to usesubambient cooling to resolve all low carbon num-ber alkanes to obtain a more linear calibration curve.

2. High-Temperature SimulatedDistillation

The usefulness of simulated distillation has beenfully realized recently with the development of metalcapillary columns with low-bleed stationary phasesthat withstand very high temperatures. Based onhigh-temperature capillary GC, several variations ofthe ASTM SimDis methods exist. These methodshave made simulated distillation applicable up tocarbon number ~120 or temperature range of about1400F. High-temperature SimDis methods havebeen applied to crude oils and heavy materials suchas high-viscosity-grade base stocks, and atmosphericand vacuum residua.37-43

High-temperature SimDis has some seriouslimitations, such as assignments of the same re-sponse factors for the eluting and noneluting frac-tions, cracking of the heavy components at highoven temperatures, and the degradation of the front-end of the capillary column as nonvolatile compo-nents from the sample accumulate over time.

3. Simulated Distillation by GC-MS

This technique provides information on bothconversion yields and group-type distribution of satu-rates and aromatics. For an example, SimDis by GC-MS on the products obtained by hydroconversion ofa deasphalted vacuum residuum provided relativepopulations of four saturate and four aromatic classesof hydrocarbons along with boiling range distribu-

tions of the products.44 The procedure for spectraldata processing consists of deconvoluting the satu-rates spectrum and aromatics spectrum from the glo-bal data. Cross-contributions from each hydrocarbonfamily to the spectrum of the other are eliminated.After the contribution from each hydrocarbon type tothe total ionization has been calculated, the volumepercentage of the classes of saturates and aromaticsare obtained by normalization.44-46 However, this tech-nique has only been applied to few products (prima-rily petroleum heavy oils and their hydrocrackingproducts), and it requires previous knowledge ofsamples to avoid possible interference.47 The coupledsimulated distillation-MS, where high-temperaturesimulated distillation column serves as an inlet for themass spectrometer, can easily be applied to the char-acterization of base oils and related products.

C. Analysis of Wax and Similar Samplesby High-Temperature GC

The average molecular weights of waxsamples were found to vary between 400 and2000 Da depending on the type of wax: paraffinwax, microcrystalline wax, or polywax polyeth-ylene. Achieving complete elution and resolu-tion of low- to high-molecular-weight wax com-ponents from the GC column has been a greatchallenge. It appears that instability of a GC col-umn at high temperature was a major limitingfactor for the characterization of wax samples byGC in the 1980s.48 Recently, the use of a pro-grammable temperature vaporizer injector for on-column injection, short capillary columns withthermally stable stationary phases, and tempera-ture programming up to 430C or more have beenproviding high-resolution separation of wax com-ponents up to and beyond carbon number 120.48-51

Samples similar to wax such as high-molecu-lar-weight linear alcohols and acids were alsoanalyzed by high-temperature GC.49

D. Element-Specific Detectors forHeteroatomic Compounds

The use of a detector that measures GC efflu-ent in terms of properties that are characteristic of

10

only certain element(s) in the molecule is a pre-ferred way to obtain information about somechemical families. This also allows one to obtainenhanced resolution between component peaksbecause molecules are detected selectively in themidst of others, which remain transparent to thedetector. This is advantageous for identificationand detection of specific types of molecules inmany complex petrochemical and coal samples.Because of their wide (or potentially wide) appli-cability, the following three element-specificdetectors are discussed below.

1. Sulfur Chemiluminescence Detectorand Nitrogen ChemiluminescenceDetector

Ozone-induced SCD,52-54 and NCD55,56 pro-vide enhanced S to C and N to C selectivities,adequate dynamic range, and uniform responsesto different classes of S-compounds (in the caseof SCD) and N-compounds (in the case of NCD).For both detectors, the oxidizable species in theeffluent from the GC column are oxidized. In thecase of NCD, the NO produced combines withozone (O3) to form nitrogen dioxide in the excitedstate (NO2*). The NO2* emits light during its de-cay to the ground state, which is detected by aphotomultiplier tube. In the case of SCD, the SOand O3 produce excited sulfur dioxide (SO2*) thatdecays to the ground state, producing a signal.

GC-NCD was applied for quantitative specia-tion of N-compounds and simultaneous determi-nation of total nitrogen in gasoline and dieselrange process streams. The GC-NCD method wasshown to permit determination of compoundscontributing as little as 100 ppb of nitrogen. Ad-equate resolution between individual componentsof each N-family such as pyridines, pyrroles,anilines, indoles, quinolines, and carbazoles couldalso be obtained.56

GC-SCD has been applied widely for thespeciation of sulfur compounds and determina-tion of total sulfur in a variety of petrochemicalsamples, including natural gases,57 gasolines,58-60diesels,61 and gas oils.60,62 This technique has beenused to analyze groups of organosulfur compoundsin different gas oils and hydrodesulfurized gas

oils with a boiling point range of 150 to 450C,and to identify and quantify individualspecies such as 3-methyl benzothiophene, 4-methyl dibenzothiophene, and 4,6-dimethyldibenzothiophene, which are refractory tohydrodesulfurization.63 However, owing to thecomplexity of the studied products, in this caseSCD was used in a hyphenated chromatographicsystem, which consisted of HPLC-GC-FID-SCD(see later). The speciation of sulfur families, in-cluding thiols + sulfides + thiophenes, ben-zothiophenes, dibenzothiophenes, and ben-zonaphthothiophenes was carried out byhigh-performance liquid chromatography (HPLC).

2. Atomic Emission Detector

AED combines microwave-induced plasmaexcitation with optical emission spectroscopy.64,65In AED, the high temperature produced in a Heor Ar plasma breaks bonds of the GC-elutedmolecules and excites their constituent atoms tohigher electronic states. The excited atoms emitcharacteristic frequencies of light. Intensity ofemission lines at wavelengths characteristic ofthe atoms of interest are then measured.

In the case of petroleum, major elements ofinterest for detection by AED have been C, H, N,O, S, Pb, Fe, Ni, and V. Many petroleum streamsand fractions,66-69 as well as crude oils andresidua,70-73 and shale oils and coal-derived liq-uids72-74 were analyzed by GC-AED. Gasoline-range distillates were analyzed for separation,identification, and quantitation of sulfur com-pounds, oxygen-additives and lead-containingcompounds.66,68,69,75,76 The total amounts of anelement such as sulfur or oxygen in gasoline rangesamples or other materials, including crude oils,can be obtained readily because the element-spe-cific response was found to be largely indepen-dent of the type of compound.66,75,77

Volatile Ni-, V-, and Fe-porphyrins were de-tected in crude oils and residua by GC-AED.70,71This multielement analysis can easily be used tofingerprint crude oils from different sources. Quali-tative analysis of the portion of pyrolyzed coals,which is eluted from a GC-column, has also beenperformed by GC-AED.74 GC-AED was found to

11

be useful to track changes in the sulfur compoundsand other heteroatomic molecules in processes suchas catalytic cracking66 or hydrodesulfurization.63,78

Identification of heteroatomic compounds hasbeen carried out using various means, including theuse of known compounds. GC-MS has been a veryuseful tool for identification of compounds.67 Chemi-cal treatment of the sample for selective oxidationprovided group-type classification of sulfur com-pounds in gasoline range samples.69 For this, mer-captans were selectively oxidized with excess io-dine to disulfides. Hydrogen peroxide was used tooxidize mercaptans and sulfides to yield sulfonicacids and sulfones, respectively. Another exampleof selective chemical treatment (oxidation of sulfurcompounds to sulfones using meta-chloroperbenzoicacid) involves a shale oil sample, which was ana-lyzed for polycyclic aromatic sulfur heterocycles.72

E. Fast GC

There is a growing interest in fast GC, alter-natively called high-speed GC, for rapid analysisof petrochemical samples with adequate resolu-tion. The technique is particularly desirable inrefinery laboratories where there is a high de-mand for high throughput for compositional moni-toring of process streams. The conventional GCanalysis time is 30 min or longer. Typical fastGC analysis time can be from less than 1 min to10 min for a relatively complex sample like gaso-line range naphtha. Another important require-ment for the fast GC technique is that the GC-oven cool-down should be rapid to allow a shortcycle time.

A short ultranarrow bore column with verythin film (e.g., a 6 m x 0.10 to 0.25 mm id x 0.1m GC column) offers fast analysis time. Re-cent advances in GC instrumentation such as aGC inlet for rapid (40 ms) sample introductionwith narrow bandwidth (5 to 10 ms) using cryo-focusing technique,79 and a GCs ability to havehigh inlet pressure, high oven temperature pro-gramming rate (20 to 60C/min),80 high split-ratios to avoid overloading, and fast detectionrates, have made the fast GC possible. A recentarticle on high-speed GC highlights some ofthese developments.81 Usually, TCD, FID, and

MS detector have been used as detectors forfast GC.

Applications of fast GC will grow as instru-ments become available commercially. A few fastGC examples include analysis of permanent gasesand light hydrocarbons in less than 1 min,82 sepa-ration of light hydrocarbons using packed capil-lary column in seconds,83 and analysis of refor-mulated gasolines by GC-MS for benzene andtotal aromatics in about 7 min.84

F. Pyrolysis GC

Pyrolysis GC has been applied to obtain use-ful information on the thermal decompositionbehavior as well as the composition of heavy oilproducts such as asphaltenes, vacuum residua, oilshale, and coal tars. Usually, a pulse heatingpyrolyzer with a platinumstrip sample holder iscoupled with a GC equipped with FID and /orother detectors.85

Typical applications of pyrolysis GC varywidely. Characterization of vacuum residua,asphaltenes, and resins from different crude oilsby pyrolysis GC coupled with FID and FPD re-vealed differences in the programs and genera-tion of different sulfur compounds during pyroly-sis.85 Toluene -insoluble solids from hydrocrackedbitumen products were analyzed by pyrolysis GC-MS. Solids obtained by hydrocracking under mildconditions were found to release alkanes and alk-enes in addition to aromatic compounds, whilethose from high-severity hydrocracking releasemostly aromatic compounds.86 Thermal decom-position behavior and molecular structure ofasphaltenes were determined by pyrolysis GC-MS.87

III. OPEN-COLUMN AND MEDIUMPRESSURE LIQUIDCHROMATOGRAPHY

Open-column liquid chromatography(OCLC), using normal adsorbents such as silicagel or alumina, has been applied extensively forhydrocarbon type separation of fossil fuels andoils.88-91 These OCLC separations are based on

12

well-known retention and elution mechanisms andare carried out using appropriate adsorbents andsolvents or solvent mixtures. In general, interac-tions (arising from polarity differences) betweenadsorbent and sample, solvent and sample, andsolvent and adsorbent govern the sample reten-tion process. The solute displacement is a func-tion of solvent volume when interactions betweensample components and adsorbent and betweensolvent and adsorbent are comparable. With ap-propriate adsorbent and solvent, components thatdiffer in polarity are displaced inversely with theirpolarity. A successive elution with a strongersolvent (with higher polarity) is necessary to elutecomponents that cannot be moved by a low-po-larity eluant.

OCLC-separated fractions are collected andsubjected to gravimetric determination after evapo-ration of solvent. Rotary evaporation under mildvacuum or another form of vacuum aspiration isused to concentrate the collected fractions. OCLC-based methods are often used to isolate sufficientamounts of materials for subsequent characteriza-tion by high-resolution chromatographic methodsor spectroscopic techniques, or for their use asexternal standards.

Normal-phase LC elution has been carriedout in an open-column by a gravitational flowof solvent92-95 or by a force-flow using me-dium pressure (MPLC) to increase separationspeed.96 Usually, saturates are eluted with ann-alkane. The aromatics and polars are elutedwith solvents or solvent mixtures of highereluotropic strengths that include toluene,dichloromethane, acetone, or their mixtureswith other solvents.

Silica gel is, by far, the most widely usedadsorbent. It provides separation of compoundclasses according to polarity. For similar polari-ties, specifically for aromatics, separation of com-pounds is usually carried out according to mo-lecular weight. Alumina has been used whenseparation according to the number of aromaticrings is desired. However, irreversible adsorptiononto the adsorbent is more prevalent with alu-mina than silica gel.97 Complexation packings(including charge-transfer phases and argentationpackings) have also been used for OCLC separa-tion.98

A. Petroleum Distillates

1. Standard Methods

Several OCLC-based ASTM standard meth-ods for hydrocarbon type analysis and compoundclass fractionation of petroleum products havebeen developed. For light distillates, the fluores-cent indicator adsorption (FIA) analysis (ASTMD1319 method) has been used for over 40 years.94This method is based on open-column displace-ment chromatography using silica gel as station-ary phase and isopropanol as eluant. The hydro-carbons are separated into aromatics, olefins, andsaturates (as they are displaced differentially fromthe top of the column). Fluorescent dyes, previ-ously added to silica gel, are also separated selec-tively with the hydrocarbon types and make theboundaries of the aromatic, olefin, and saturatezones visible under ultraviolet light. The volumepercentage of each hydrocarbon type is obtaineddirectly from the length of each zone in the col-umn.

It should be noted that the FIA aromatics alsoinclude some dienes, and compounds containingsulfur, nitrogen, and oxygen, if these are presentin a gasoline range sample. Other factors affect-ing accuracy and reproducibility of the FIA methodcan be summarized as follows:28 nonuniform col-umn internal diameter, indistinct zone boundaries,improper packing of silica gel, incomplete elutionof hydrocarbons by isopropanol, colored compo-nents present in the sample, and excessive zonebroadening due to the presence of C5- and lighterhydrocarbons in the sample.

The ASTM D2549 method has also beenapplied to middle and relatively heavy petroleumdistillates (with b.p. from 232 to 538C), in orderto isolate and recover the aromatic and non-aro-matic fractions.93 In this method, n-pentane isadded to a column containing activated bauxiteand silica gel to elute the nonaromatics. Later,aromatics are eluted using diethyl ether, chloro-form, and ethanol. The nonaromatic fraction is amixture of paraffinic and naphthenic hydrocar-bons if the sample is a straight-run material. How-ever, if the sample is a cracked stock, thenonaromatic fraction may also contain linear,branched, and cyclic olefins. The aromatic frac-

13

tion may contain aromatics, condensed naphthenic-aromatics, aromatic olefins, and compounds con-taining oxygen, nitrogen, and sulfur.

The ASTM D2007 method is used for thedetermination of hydrocarbon types in heavy dis-tillate, residual stocks and extracts.92 This methodinvolves the use of n-pentane to elute saturatesthrough a double column containing clay in theupper section, and clay plus silica gel in the lowersection. During n-pentane elution, polars are lefton clay and aromatics on silica. Subsequently,polars are desorbed from clay using 1:1 (v/v)acetone-toluene mixture. Saturates and polars arethen determined gravimetrically after completeevaporation of solvents. The amount of aromaticsis calculated by difference. As an option, theymay be recovered from silica by Soxhlet extrac-tion using toluene.

The D2007 method has been applied to abroad variety of materials, including petroleum-derived oils with boiling points above 260C andcrude oils. However, its proper application re-quires a previous removal of n-pentane-insolublessuch as asphaltenes. Inaccuracies in the ASTMD2007 data, in most part, originate from over-loading effects causing cross-contamination ofhydrocarbon types, and they have been found tobe very high for high boiling distillates, heavyalkylbenzenes, and highly paraffinic base stocks.99

A group type separation of asphalts was car-ried out in 1969 by Corbett.100 A modified form ofthis original approach was adopted as ASTMD4124 method.101 After asphaltene removal as n-heptane insolubles, maltenes are subjected to se-quential elution with n-heptane, toluene, toluene-methanol (50/50 v/v), and trichloroethylene fromactivated alumina. Thus, asphaltene, saturate,naphthene-aromatic, and polar-aromatic, fractionsare obtained.100

2. Separation Schemes

Many nonstandard separation schemes havebeen applied to the characterization of petroleumdistillates and crude oils. Procedures include one-step or successive102-104 separations using variousstationary phases, including alumina, cation andanion exchange phases, clay, and silica gel, as

well as complexation packings.98 Examples ofsome OCLC separations can be found in a reviewarticle by Altgelt et al.103 Besides group-type sepa-rations, OCLC-based methods have been usedsuccessfully as cleanup steps for the determina-tion and isolation of sulfur, nitrogen, and oxygencompounds in the crude naphtha,105 high end ofcrude oils,102 and other petroleum distillates.104The OCLC methods have also been used forsubfractionation of a family of compounds asexemplified by the separation of n-alkanes andcyclic plus iso-alkanes in a saturate fraction usinga 5A molsieve column.106 A scheme based onselective reactions of n-paraffins with urea and ofisoparaffins with thiourea was applied to separaten-paraffins, isoparaffins, and naphthenes from anonaromatic fraction of a gas oil.107

B. Shale Oil and Coal-Derived Products

Any strong interaction between sample com-ponents and stationary phase (as well as eluant)can introduce bias and high level of uncertaintiesin preparative OCLC.88,108 This problem is morecritical in the case of coal products when polarcompounds are strongly retained in the columnbed and cannot be desorbed easily. For gravimet-ric determination of sample components, removalof typical solvents such as toluene, tetrahydrofu-ran, and pyridine, even under vacuum and tem-perature, are, on occasions, difficult. These, andthe broad chemical variety and complexity of coalproducts have, so far, been responsible for theabsence of any standard OCLC methods in coalchemistry.

A useful OCLC-based method, initially ap-plied to whole coal hydroliquefaction productsderived from the solvent refined coal process, wasdeveloped in 1977 by Farcasiu.88 It was calledsequential elution solvent chromatography (SESC),and the method uses a silica gel column and nineeluants of increasing polarity (from n-hexane topyridine). The first five fractions are assigned tospecific chemical families. From the sixth to ninthfraction, characterization of functionality is impre-cise. These fractions contain multifunctional het-eroatoms and polyfunctional molecules. The tenthfraction is unknown and retained in the column.

14

A method has been described recently for fraction-ation of coal extracts, crude oils, and other fossil fuel-derived oils into compound classes.91 The system usesfive columns (two silica gel, one acidic silica gel, andtwo basic silica gel columns) that provide fiveheterocompound fractions (bases, acids, high-polarity,medium-polarity, and low-polarity nitrogen, sulfur, andoxygen compounds) in addition to conventional satu-rate and aromatic hydrocarbon fractions.

Most of the methods developed later thanFarcasius work are shortened versions of SESCadapted to each particular sample by reducing thenumber of eluants, varying the elution sequence, orusing different stationary phases.32, 109-111 Automatedequipment for the separation of coal products apply-ing medium pressure elution was also described.96Many of these studies involved characterization ofspecific families of compounds. For example, aninitial OCLC separation provided fractions ofhydrotreated shale oils for detailed analyses of nitro-gen compounds by HPLC as well as GC-MS.111Similar separation and identification of nitrogen-containing polycyclic aromatic compounds such ascarbazoles, benz(e)indoles and benz(g)indoles, werealso carried out.110

Most of the coal-derived products are highlyviscous. Preadsorption of viscous samples eitheronto an inert support or onto the whole station-ary phase has been used as a means for sampleintroduction. The elution of materials from sucha preadsorbed stationary phase is known asextrography. It is not a pure chromatographicprocess, and data suggest an extractive mecha-nism for the first eluted fraction (with n-hex-ane), and dominant chromatographic processesfor the more polar fractions.112 Extrography wasapplied to coal hydroliquefaction products andcoal-tar pitches112-114 and crude oil residua.115Although pure separations into families are notobtained as in a chromatographic process,extrography allows a rough fractionation withhigh sample loading.

C. Quantitative Analysis by Open-Column Liquid Chromatography

In an OCLC separation, fractions are collectedon a volume basis, which has previously been

determined for a calibrant sample. This has severelimitations when applied to samples that differ incomposition from the calibrant. It should be notedthat an OCLC analysis (such as hydrocarbon grouptype separation) must be precisely carried out tomonitor compositional changes due to operatingparameters of the conversion processes.

Selection of solvent volume for an elutedfraction is sometimes delicate in preparative liq-uid chromatography because of possible fractionoverlapping. For this reason, there should be astrict control of eluant volumes for each cut if anadequate separation is desired. Subfractions areusually collected, and later off-line characteriza-tion is carried out to check the purity of a fraction.Refractive index (RI) or ultraviolet (UV) detectorcan also be used for continuous monitoring of thefractionation.

TLC-FID99,116 and HPLC-FID117 have beenreported to be useful tools to quantitatively evalu-ate the separation and verify the purity of frac-tions obtained by OCLC methods. Thus cross-contamination in saturate, aromatic and polarfractions from lubricant base stocks obtained byASTM D2007 was ascertained when these frac-tions were analyzed by TLC-FID.99 The TLC-FID method was also used for examining OCLCelution. Both aromatics and polar materials werefound to be present in a saturate fraction whenexcess n-hexane was used to elute saturates froma heavy oil.116

IV. HIGH-PERFORMANCE LIQUIDCHROMATOGRAPHY

HPLC has been applied quite extensively topetroleum distillates, crude oils, and coal-derivedliquids. Its applications include hydrocarbon-typeseparation and determination, as well as separa-tion, identification, and, in some cases, quantita-tive determination of target compounds in manypetrochemical samples.118 The different modes ofHPLC afforded by the use of normal- or reversed-phase columns, isocratic or gradient elution ofsolvents, and various detection systems, and thepossibility of column switching allow an analystto carry out a desired separation with adequateresolution, rapidity, and accuracy.

15

Hydrocarbon type determinations are mostlycarried out by normal-phase HPLC. This is pri-marily due to the solubility and compatibility offossil fuel-derived samples with solvents such assaturated hydrocarbons used in normal-phase elu-tion. Therefore, silica and amino-, cyano-, or diol-bonded silica columns119,120 have been used ex-tensively in petrochemical analyses. A study ofeight different normal-phase HPLC columnsshowed that nitrophenyl- and aminosilane col-umns provide the best selectivity for aromaticcompounds.121

Complexation packings or charge-transferphases98 and argentation chromatographypackings,122 have also been used. Reversed-phasecolumns (with alkyl-bonded silica packing) havebeen found useful for the high-resolution charac-terization of certain compound groups suchas polycyclic aromatic hydrocarbons123 andphenols124,125 in petroleum samples.

Almost all available HPLC detectors havebeen used for detection and quantification of pet-rochemical samples.118 These include spectro-scopic detectors such as UV-visible, fluorescence,diode array (DAD) and infrared (IR), and bulkproperty detectors such as RI, evaporative lightscattering (ELSD), dielectric constant (DCD), andFID. Mass spectrometric detectors have alsobeen used with HPLC (in HPLC-MS). In general,external or internal calibration procedures workwell for the determination of target compoundswith many of the above detectors. However, forhydrocarbon type determinations, problems arisefrom the lack of a detection system that providesuniform response factors for hydrocarbon-types.Moreover, response factors for a given type ofhydrocarbons have been found to vary from fuelto fuel.

Both analytical and preparative HPLC meth-ods have been developed to replace labor-intensiveopen-column liquid chromatography. As will benoted later, the Institute of Petroleum (IP) has stan-dardized several such HPLC methods. Some pre-parative HPLC methods have been developed toseparate target compounds as subfractions. Thesesubfractions are then analyzed by high-resolutionchromatographic method for detailed qualitative orquantitative analyses. They are also subjected tospectroscopic and elemental analyses.

A. Hydrocarbon-Type Determination

1. Standard HPLC Methods

An HPLC standard method for the determina-tion of aromatic hydrocarbon types for sampleswith boiling points ranging from 150 to 400C is IP391. This method has been applied to both gasolineand middle distillate samples and is based on theuse of a polar column (amino- or aminocyano-silica gel) with n-heptane or n-hexane as mobilephase and refractive index detection. Here, afterelution of saturates, and mono- and di-aromatics asdistinct peaks, the column is backflushed to elutePAHs as a single peak. The percent mass of mono-, di-, and polycyclic aromatics are quantified usingan external calibration procedure.

IP 368 is a preparative HPLC method for thedeterminations of saturates and total aromaticsusing silica columns. In this method, hexane isused as the mobile phase. After the elution ofsaturated hydrocarbons from the column, aromat-ics are backflushed.

2. Light and Middle Distillates and GasOils

The role of HPLC as well as other chromato-graphic techniques within the context of the char-acterization of light distillates has been addressedin a general review.126 A typical hydrocarbon typeanalysis depends on the sample analyzed. In therange of gasoline and middle distillates, the hy-drocarbon-types can be saturates (n-paraffins,isoparaffins, and naphthenes), olefins, and aro-matics. Aromatic distributions into mono-, di-,and polycyclic aromatics can also be obtained.The separated hydrocarbons are usually quanti-fied in terms of mass or volume percent of eachcompound class.

In the 1970s, silica gel was widely used asstationary phase for hydrocarbon-type analysisof light and middle distillates and other prod-ucts.127-130 Thus, saturates, olefins, and aromaticsin gasolines (with a boiling point range 60 to215C) were separated on a single column, usinga low-polarity fluorohydrocarbon (Freon FC-78)as the mobile phase, and were detected using an

16

RI detector.127 The low-polarity eluant allowedthe separation of saturates from olefins, saturateseluting first. Aromatics were then backflushedfrom the column. As an extension of this method,Suatoni et al. separated middle distillates (with aboiling range of 190 to 360C) into saturates,monoolefins, diolefins, and aromatics usingtwo silica columns and n-hexane as the mobilephase. 128

In the 1980s, Hayes and Anderson reportedseveral methods for hydrocarbon-type separa-tions.131-136 Separation of saturates, olefins, andaromatics was achieved in 8 min using a 150 mmx 4.6 mm column packed with silica gel coated toa strong cation exchanger (for silver), Freon-123(2,2-dichloro-1,1,1-trifluoroethane) as mobilephase, and DCD.132 Saturates eluted first, fol-lowed by aromatics. Olefins were backflushedfrom the column. For resolution of naphthenesfrom other classes of compounds, Hayes andAnderson136 used a configuration that included anadditional switching valve to the basic systemand a naphthene-selective PONA column (con-taining a microparticulate organic gel phase). Here,Freon-123 was also used as mobile phase to ob-tain adequate separation and compatibility withDCD. PONA (paraffin-olefin-naphthene-aro-matic) type analysis was also achieved with anHPLC-DCD system employing five columns andtwo switching valves.135

Separations according to number of aromaticrings have been of interest for middle and heavydistillates. This has been achieved using eithercharge-transfer columns or normal-phase polarcolumns (such as amino-silica gel). Three groupsof silica-bonded acceptors have been utilizedfor charge-transfer HPLC: nitroaromatics(dinitroanilino-propyl silica, DNAP),137,138tetrachloropthalimido groups,98 and caffeine andrelated compounds.98

Recently, a new method based on HPLC withrefractive index detection was developed andwas demonstrated for the determination of vol-ume percent of saturates, monoaromatics anddiaromatics in diesel fuels.139 HPLC-RI detectionwas also used to obtain total saturates and aro-matics in gas oils140,141 and kerosenes.140 Olefinsand ring number distributions of aromatic hydro-carbons were estimated by detecting them by UV

at 200 nm141 or 210 nm.140 HPLC with both RIand UV detectors was used for the compositionalstudy of gas oils in terms of saturates, mono-,di-, and polycyclic aromatics, and polars.142

3. Heavy Distillates and Coal-DerivedProducts

Hydrogenation, catalytic cracking, lube ex-traction, and coking processes for upgrading ofheavy distillates can be optimized using quantita-tive hydrocarbon-type data obtained by HPLC forfeeds and products. Likewise, HPLC methods havealso been useful for qualitative process monitor-ing or for comparison purposes.

Heavy distillates have been studied by simi-lar HPLC methods developed for light and middledistillates,129,130 although clean separations be-tween hydrocarbon types are more difficult toachieve than in the case of light and middle dis-tillates due to increasing sample complexity.143-146Further, some heavy and polar compounds can beirreversibly adsorbed on the stationary phase pro-ducing column deterioration. Usually, the lattercan be mitigated with appropriate stationary andmobile phases, and, if required, with the use ofbackflushing.

Heavy petroleum distillates are sufficientlysoluble in conventional low-polarity solvents suchas cyclohexane and can be completely eluted bycommon normal-phase HPLC solvents. However,there can be serious problems in the case of petro-leum asphaltenes or coal-derived products due totheir low solubility, even in highly polar solvents.

A single column is often inadequate for sepa-rating the entire range of compound polarities inheavy petroleum distillates,122,138,147 and in coal-products.148 Separations covering a wide polarityrange were often achieved using multiple col-umns, column switching and solventgradients.143,149,150 Grizzle and Sablotny143 devel-oped an automated HPLC system for rapid,semipreparative isolation of hydrocarbon types(saturates, aromatics and polars) as well as aro-matic ring-number fractions from crude oils, bitu-mens, and related materials using two 250 mm X10 mm, 10 m amino-bonded silica gel columnsin series. After the removal of asphaltenes, 30 to

17

60 mg of the sample were injected onto thecolumn and eluted using an n-hexane/dichloromethane gradient. Elution from the col-umn with forward flow yielded saturates and aro-matics or aromatic ring number fractions for col-lection. Elution of the polar fraction wasaccomplished by backflushing after the elution ofthe four-ring aromatics. Separations were moni-tored using RI and UV detectors.

A fully automated HPLC system was alsodeveloped149 to separate each of six hydrocarbontypes (saturates, 1- to 4-ring aromatics, and polars)in a wide variety of heavy distillates (boiling upto 700C). Separation was carried out using two250 mm X 4.6 mm propylaminocyano columnsto separate saturates and monoaromatics, and aDNAP column to separate the aromatic rings.Elution was carried out using a ternary gradientusing n-hexane, dichloromethane, and isopropanol.The quantification of groups was achieved afterdetection using DAD and ELSD. The valve switch-ing and solvent gradient schemes were developedto maximize the resolution of the six separatedgroups.

Multiple columns, column switching, and sol-vent gradients have also been used for carryingout hydrocarbon type separation of coal-derivedproducts.150-152 Thus Padlo et al.150 reported theseparation of coal hydroliquefaction products intosaturates, 1-4 ring aromatics, and polars using amulticolumn system consisting of a 250 mm X4.6 mm aminocyano column, a 300 mm X 4.6mm DNAP-silica column, and a 250 x 4.6 mmdiol-silica column. A gradient elution was carriedout using n-pentane, dichloromethane, and iso-propanol. DAD and ELSD were used for compo-nent detection. Here, the divisions between thearomatic classes were determined based on reten-tion times and UV spectra of standards.

B. Determination of Target Compounds

1. Standard HPLC Methods

Normal-phase columns, reversed-phase col-umns, and combinations of both have been usedin HPLC applications for determining indigenouscompounds, contaminants, or additives in crude

oil and petroleum distillates. Thus, standard meth-ods from the Institute of Petroleum exist for de-termining coumarin in kerosenes (IP 374) and2,4-dimethyl-6-t-butyl phenol antioxidant in lightdistillates (IP 343).

2. Illustrative Applications

a. Polycyclic Aromatic Hydrocarbons

The analysis of any complex sample (such ascrude oil, petroleum distillate, or coal liquid) forPAHs usually involves normal-phase HPLC sepa-ration (for example, on an aminosilica column) toisolate fractions containing isomeric PAHs. Thesefractions are then subjected to reversed-phaseHPLC (usually on C18-silica column) to separateindividual PAH isomers. For quantitative deter-mination of PAHs, appropriate perdeuteratedPAHs are used as internal standards so that eachisomeric fraction contains one internal standard.In reversed-phase HPLC, perdeuterated PAHelutes first, and is usually resolved from the par-ent PAH.153

Individual carcinogenic PAHs (specifically,benzo[a]pyrene), have been determined in al-most all types of distillates. For this, HPLC sepa-ration is carried out (often after a cleanup step),using conventional C18-reversed-phase columns,typical elution with acetonitrile-water, and UV orfluorescence detection. A combination of bothnormal-phase column (for prefractionation) andreversed-phase column such as octadecylsilica (foranalytical separation) was used for determiningbenzo[a]pyrene and other PAHs using fluores-cence detection.154,155

Alkyl biphenyls and naphthalenes were charac-terized by TLC, HPLC, and capillary GC.156 Here, therole of HPLC was to resolve a preparative TLC-separated dinuclear aromatic fraction into distinctsubfractions of alkyl biphenyls and alkyl naphtha-lenes for subsequent characterization by capillary GC.

b. Heterocyclic Compounds

Normal-phase charge-transfer HPLC was usedfor the determination of ppm amounts of azaarenes

18

(aniline, quinoline, pyridine, and isoquinoline) ingasoline, kerosene, and diesel.157 An on-linepreconcentration (on a small precolumnpacked with DNAP-silica or 3-(2,4 dinitro-benzenesulfonamido)-propyl (DNSP) -silica be-fore the HPLC analysis (using either DNAP-silicaor, DNSP-silica column, and dichloromethane assolvent) allowed the detection of pyridine downto 50 ppb by a UV detector. Nitrogen bases(azaarenes) from a crude oil obtained by a selec-tive extraction procedure were separated by re-versed-phase HPLC for their identification byoffline GC-MS.158

Petroleum carbazoles were separated by nor-mal-phase HPLC on silica gel for their subse-quent fractionation based on carbon numberusing a C18- reversed-phase column and acetoni-trile-water mobile phase.159 Each of C1- throughC4 -carbazole subfractions was then fractionatedinto positional isomers by HPLC on silica gelwith hexane/Et3N/isopropanol as the mobile phase.

HPLC-MS was used in the single ion moni-toring mode for rapid (7 min) determination ofdibenzothiophene in coal liquids and crude oils.For this, M+ with m/z 184, and (M-32)+ withm/z 152 were monitored.160

c. Gasoline and Diesel Additives

Reversed-phase HPLC with 5 m octadecylsilica (ODS) column, acetonitrile-water as mobilephase, and RI detector was used to quantitativelydetermine benzene and toluene, and oxygenatessuch as ethanol, methyl-t-butylether (MTBE),ethyl-t-butylether, and t-amyl-methylether in gaso-line.161 After elution of these target compounds,gasoline peaks were backflushed to waste in orderto shorten analysis time to about 10 min. Relativestandard deviations were reported to be 1 to 2%for gasolines containing 0.5 to 10% MTBE.

The color marker Solvent Yellow 124, N-ethyl-N[2-(1-isobutoxyethoxy)ethyl](4-phenylazophenyl)amine was analyzed in diesel fuels on a 5 m C18-reversed-phase column, and determined in thevisible at 420 nm.162 Elution was carried out withacetonitrile-ammonium acetate buffer containingdimethyloctylamine. Other compounds such asphenalenones, their methyl homologues, and re-

lated benzanthrones, were also determined in middledistillates using conventional reversed-phase, col-umn, hexane-chloroform mobile phase and UV-visible detector at 400 nm.163 Cetane improveradditives (amyl-, hexyl-, and octyl-nitrate) weredetermined in diesel fuel by HPLC as an alterna-tive to the ASTM D1389 colorimetric method.164For this, three 10 m-silica gel columns in series,cyclohexane-CCl4 (1:1) as mobile phase, and avariable wavelength infrared detector were used.

d. Lubricant Additives

HPLC has been applied to the analysis oflubricating oil additives such as antioxidants andantiwear agents, ashless dispersants, and viscos-ity improvers.118,165 They have been separatedusing both normal-phase and reversed-phase col-umns and quantified using UV, RI, and ELSD.

C. Preparative HPLC

The separation mechanisms, solvents, columntypes, and typical run times of preparative orsemipreparative HPLC are similar to those ofanalytical HPLC of fossil fuel-derived samples.However, these techniques do differ, primarily insample loading, column size, particle size of thecolumn packing, and flow rate. Preparative HPLCcolumns are much larger in size to allow largersample loading (of few tenths of a gram or more)in each run. The preparative column packingmaterials may have larger particles than those inan analytical column. While a flow-rate of about2 mL/min is common for analytical separations,those for semipreparative or preparative HPLCcan be 20 mL/min or higher.

Preparative separation has often been carriedout for gravimetric determination of hydrocarbontypes after removal of solvent(s) from collectedfractions. This has also been useful for obtainingsample fractions for the characterization of indi-vidual compounds or functionalities. For the lat-ter, fractions are usually subjected to off-line oron-line analysis by high-resolution chromato-graphic and spectroscopic methods. For complexpetroleum- or coal-derived samples, cleanup of

19

the original sample is often desired prior to pre-parative separation.

Examples of preparative HPLC separationsare given in Table 3. Although these cover abroad range of materials, we note that they repre-sent only a small fraction of studies reported inthe literature. As noted in Table 3, almost allanalytical separations of complex petrochemicalsamples involve some sort of preparative separa-tion by OCLC, HPLC, or other means.

D. HPLC Detectors

The following provides highlights of differ-ent HPLC detectors. The discussion addressesadvantages, limitations, and applications of eachdetector.

1. Spectroscopic Detectors

a. UV-Visible Detector

UV-Vis detectors are suitable for aromaticsand polars, but they are unable to detect saturatesunder the conditions used in conventional HPLCdetection. Further, responses of compounds arestrongly dependent on their chemical structures.The UV response at specific wavelength varieswith the numbers of rings as well as with theisomeric compounds having the same number ofrings.

In general, detection and quantitation of indi-vidual sample components require sample prepa-ration and cleanup. There have been attempts touse the UV-Vis detector for the determination oftarget components in complex petrochemicalsamples without an elaborate sample preparationand cleanup.148 HPLC with a diode-array detec-tor was used for the detection of PAHs and alky-lated PAHs in coal liquids. Simpler chromato-grams were obtained (as maxiplots) to view allcomponents eluting at maximum absorbancesacross the UV range.

Recently, an HPLC system has been devel-oped to quantify hydrocarbon types and aroma-ticity distributions in heavy oils using com-bined DAD and ELSD.149 The DAD response

factors for aromaticity have been based on av-erage values determined from the 204 to 430-nm spectra of over 80 model compounds. There-fore, absorbance of each peak has beenintegrated over this wavelength range. Aroma-ticity results were validated with 13C-NMR. Aquantitative hydrocarbon-type determinationwas carried out using a procedure for wide-range calibration of ELSD. The use of averageresponse factors for aromatics is based on aprevious work by Lafleur et al.,180 who found anarrow distribution of normalized response fac-tors for 16 polycyclic aromatic hydrocarbonswhen a UV absorption bandwidth between 200and 400 nm was utilized.

For the detection of metal porphyrins, UVdetection was applied using wavelengths between400 nm and 410 nm.181-184 The wavelengths of553 nm and 573 nm were also used as -bandsfor Ni(II) and VO(II) petroporphyrins, respec-tively.181

A UV detector at 260 nm was used to detectand determine cobalt, copper, iron and vana-dium as their bis (acetylpivalylmethane) ethyl-enediamine complexes. There was excellentagreement between the HPLC data and thoseobtained by atomic absorption spectrometry formetals.185

b. Fluorescence Detector

Fluorescence detectors were mostly used forthe determination of PAHs.154,157 For an example,PAHs in coal-tar pitch binder were determinedusing HPLC-fluorescence detector, and the re-sults were compared with those obtained byHPLC-particle beam MS detector.186

c. Infrared Detector

IR detector is useful for selective monitor-ing of functional groups. Infrared detectionwith variable wavelength was used for quanti-fying alkyl nitrates in diesel fuels (using theNO2 stretch absorption band) in order to over-come the problem of UV absorbing compo-nents in the sample.164

20

Table

3

Typi

cal P

repa

rativ

e HP

LC S

epar

atio

ns

21

22

2. Bulk Property Detectors

a. Refractive Index Detector

RI detectors have been used quite extensivelyfor hydrocarbon-type determinations. Althoughan RI detector is considered universal and it givesa lower variation in responses than a UV detectorfor aromatics and polars, it presents several majordisadvantages. An RI detector is incompatiblewith elution gradients often necessary in normal-phase HPLC of petrochemicals. Although accu-rate determination of aromatics is possible usingexternal calibration as in the IP 391 method, theestimation of saturates is problematic because ofthe large variation in refractive indices for differ-ent saturates and the small difference between therefractive indices of saturates and that of the mobilephase such as n-hexane. If the determination ofexperimental response factor for a hydrocarbongroup is necessary, this is done by a preparativeseparation and collection of the hydrocarbon typefollowed by analytical separation and RI detec-tion of the collected fraction.

A new approach for the hydrocarbon-typedetermination involves the use of HPLC experi-ments with RI in two different mobile phases ofvarying refractive indices (such as hexane andnonane). The method is based on the premise thatthere is a linear relationship between measuredrefractive index and detector response (in termsof peak area per unit volume of the analyte).139The novelty of this approach lies in the fact thatunlike any conventional approach (such as the IP391 method), no external calibration is needed.

b. Evaporative Light Scattering Detector

ELSD offers practical advantages over RI withrespect to more rapid stabilization, higher linearinterval,187 and better sensitivity.187,188 An ELSDalso allows the use of gradient elution.149,150 ELSDis suitable for the detection of individual compo-nents boiling above ~315C. Thus, many small,volatile PAHs such as methylnaphthalenes(~240C), acenaphthene (b.p. 279C ), and fluo-rene (b.p. 298C ) present in coal and petroleumproducts are not detected even under mild work-

ing conditions. However, recent results189 obtainedusing modern ELSD technology demonstrate thata variety of small-sized PAHs (from 3 to 6 rings)can be determined quantitatively using adequateconditions (such as evaporation temperature: 30C,and mobile phase: tetrahydrofuran) without sig-nificant loss due to volatilization. ELSD workingparameters (nebulization parameters, evaporationtemperature, and chromatographic conditions) canalso be optimized to obtain, in some cases, nearlyuniform response for different compounds. How-ever, this may not be valid for all applicationsbecause ELSD responses were found to dependin part on other factors, including solute densi-ties.189-192

Usually, ELSD provides a nonlinear relation-ship between mass and signal. The signal can belinearized using a power-law model such as c = msb , where c and s are mass and signal, and m andb are proportionality constant and power-law ex-ponent, respectively.

ELSD has been applied to evaluate theheaviest part of heavy coal and petroleum prod-ucts,193-197 although many of these applicationsinvolved size-exclusion chromatography. Hydro-carbon-type determinations147 and simulated dis-tillation150 have been carried out using ELSD ei-ther with isocratic elution or with eluant gradients,and as a single detector or in combination withother detectors.

c. Dielectric Constant Detector

DCDs have been used for hydrocarbon-typedetermination.117,133-136 For instance, DCD wasemployed for hydrocarbon type analysis of lightdistillates134 and lubricant oils.117 It was foundthat the use of an eluant with a relatively highdielectric constant (such as halogen containingsolvents) provides uniform detector response.134

d. Flame-Ionization Detector

HPLC with FID is highly desired for theanalysis of fossil fuels because of the nearlyuniversal response and supposedly similar re-sponses from different classes of compounds.

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However, its applications with HPLC havebeen limited because of the lack of effectiveinterface to remove the solvent and to trans-fer the sample to the detection system. Arotating disk FID was coupled to HPLC toanalyze samples boiling above 340C to ob-tain hydrocarbon type data.198 Attempts weremade for coupling FID to HPLC using mov-ing wire and belt interfaces.199 This presentedtechnical problems, primarily from variationof responses due to a nonuniform depositionof the effluent. Another design that uses athermospray (TSP) interface,117 was appliedfor the determination of total aromatic con-tents in lube base stocks.

HPLC-FID with coupled aminocyanosilaneand cyano columns, and mixtures of benzene inhexane, and tetrahydrofuran in MTBE as mobilephases, was used to obtain saturates, aromatic andpolars in coal liquids.200 The data provided trendsin the product compositions with the changes inliquefaction temperature and coal rank.

3. Mass Spectrometric Detector

MS detectors, using different HPLC-MS in-terfaces, have been used, although their applica-tions appear to be limited to a more in-depthcharacterization of separated fractions and to theidentification of individual compounds. HPLC-MS has also been applied for hydrocarbon-typeanalysis,201-204 providing some qualitative infor-mation concerning the fractions separated byHPLC. For example, it facilitates the differentia-tion of alkylaromatics from naphthenoaromatics,or between aromatic hydrocarbons and thiophenes.Relative abundances of different groups can alsobe obtained.

HPLC-MS hydrocarbon-type analysis of arelatively heavy petroleum distillate was carriedout using a thermal desorption-based moving-belt interface, which allows conventional high-and low-voltage electron impact (EI) as well aschemical ionization (CI).201 However, this tech-nique was unable to analyze highly nonvolatilespecies, which either do not vaporize or undergothermal decomposition. TSP was reported to beuseful for nonvolatile, thermally labile mol-

ecules.203 This is essentially a CI technique inwhich the solvent vapor acts as a reagent gas.For a heavy petroleum sample (boiling above565C), TSP selectively ionizes aromatic hydro-carbons by protonation, leaving saturated hydro-carbons largely unionized. HPLC-field ioniza-tion MS was used to carry out molecularspeciation of saturated hydrocarbons in lubri-cant base stocks and to quantify naphthenes,normal paraffins, and isoparaffins.205 A DNAPsilica column was used for the separation ofsaturates from aromatics and polars.

HPLC-MS using an atmospheric pressure chemi-cal ionization interface (APCI) was used to character-ize vanadyl porphyrins fraction of a shale oil. Morethan 50 porphyrin components were observed.206

4. Miscellaneous HPLC Detectors

a. Element-Specific Detectors

Graphite furnace atomic absorption detec-tor183,207,208 and inductively-coupled plasma atomicemission detector (ICP-AES)167,209 were coupledwith HPLC for speciation of metal-containingspecies such as metal porphyrins.183,207,208 Nor-mally, petroporphyrins are extracted and purifiedby Soxhlet or other means, including OPLC, priorto HPLC separation.183 HPLC-ICP-AES with adirect injection nebulizer was used for simulta-neous element speciation of process streams, shaleoil, solvent refined coal, and crude oil.209

b. Electrochemical Detector

Electrochemical detection (by CeIV oxidationof PAHs to quinones) have been used for deter-mining PAHs in different distillates.210 This wasalso used for the detection of electroactive com-pounds such as phenols.124,125

V. SUPERCRITICAL FLUIDCHROMATOGRAPHY AND EXTRACTION

As an analytical technique, SFC is comple-mentary to both GC and HPLC. SFC can be

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applied to materials that are too heavy to meetvolatility and thermal stability requirements forGC. Above the critical point, density and sol-vating power of a gas approach those of a liq-uid, but viscosity and diffusivity remain similarto a gas. The combination of both gas-like andliquid-like properties of a supercritical fluidenables the elution of high-molecular-weightmaterials and faster solute transport within thecolumn. This also results in shorter analysistime for SFC than that for HPLC. Althoughseparating power of GC, in most cases, is farsuperior to SFC, the resolution achievable bySFC is comparable to or better than that byHPLC.

Commercial SFC instruments offer flexibil-ity for optimization of separation conditions.These are equipped with hardware and soft-ware that allow automated sample injection,and programming of temperature, fluid pres-sure or density, flow-rate, and composition ofmobile phase. Packed (2.0 to 4.6 mm i.d.),microbore (100 to 1000 m i.d.), and capillarycolumns (< 100 m i.d.) are used in SFC.211,212Many standard normal-phase and reversed-phase HPLC columns have been found suitablefor SFC. Small-diameter fused-silica columnshave been used extensively. Virtually all GCand LC detectors have been interfaced withSFC. These include flame-ionization detector ,UV/Vis detector, fluorescence detector, mass-spectrometry detector, infrared detector, nuclearmagnetic resonance detector, thermionic detec-tor, flame-photometric detector, sulfur chemi-luminescence detector, and atomic emissiondetector.213-215

SFC has been applied extensively to fourmajor areas: hydrocarbon-type analysis, simu-lated distillation, analysis of polycyclic aro-matic hydrocarbons, and compound class frac-tionation using supercritical fluid extraction(SFE). The reasons for the inclusion of SFE aregiven in subsection D. In a majority of petro-chemical applications of SFC, supercritical CO2has been used as the mobile phase. Thesupercritical fluid state of CO2 can be main-tained easily because its critical temperatureand pressure are 31.3C and 72.8 atm, respec-tively.

A. Hydrocarbon Type Analysis

1. Light Distillates:Gasoline andNaphtha

Light distillates may contain olefins in addi-tion to saturates and aromatics. Aromatics areusually limited to one- and two-ring compounds.In 1984, Norris and Rawdon216 demonstrated thathydrocarbon-type analysis can be carried out bysupercritical fluid chromatography using CO2 asthe mobile phase, standard (25 cm X 0.46 cm)HPLC columns, and a flame-ionization detector.Silica and silver nitrate-impregnated silica col-umns in series were used to achieve sequentialelution of saturates, olefins, and aromatics presentin petroleum liquids boiling below 350C. Theanalysis time was less than 5 min with supercriticalCO2 flow-rate of 3 mL/min at 35C and 210 atm.The results for gasoline, kerosene, diesel, shaleoil, and coal-derived liquid samples were reported.For validation, SFC data were compared withthose obtained by the fluorescent indicator ad-sorption method.94 The SFC and FIA hydrocar-bon-type data were found to be in reasonableagreement.

Other early developments in the SFC sepa-ration of paraffins, olefins, and aromatics in-clude the use of a single microbore silica col-umn, SF6 as the mobile phase, and pressure andtemperature programming.127,217 Similar hydro-carbon type separation was achieved when amicrobore (25 cm X 0.1 cm) silica column anda silver ion loaded cation-exchange silica col-umn were used with a mixed mobile phase of 10% CO2 in SF6.218 The silica column was used toretain aromatics, while saturates and olefins wereseparated in the silver-loaded column. After theelution of saturates from the silver-loaded col-umn, both olefins and aromatics were backflushedfrom the silver loaded silica and silica column,respectively. In other studies involving columnswitching and backflushing, neat CO2 was usedas the mobile phase.219-221 In one of these stud-ies,221 four silica columns in series and a silverion cation-exchange column were used to obtainsequential elution of paraffins, naphthenes, aro-matics, and olefins present in gasoline and jetfuel samples.

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Currently, few laboratories are applying SFCfor the routine hydrocarbon type analysis of gaso-line range samples. This is usually carried out byGC methods. For the most part, capillary GC isused to obtain much superior resolution of indi-vidual compounds for their identification as wellas hydrocarbon type determination.3,12 Multidi-mensional GC utilizing a number of columns andtraps (having high selectivity for individual grouptype) provides hydrocarbon type data as well ascarbon number distributions.24,28,222

2. Middle Distillates: Diesel, Jet Fuel,and Kerosene

The ASTM 5186 method has been in use forthe determination of monoaromatics and polycy-clic aromatic hydrocarbons in diesel and jet fuelsby SFC coupled with FID.223 According to thismethod, relatively mild conditions are required toachieve the separation of aromatics fromnonaromatics (saturates plus olefins). Typical op-erating conditions with packed silica columns areas follows: oven temperature 30 to 40C, CO2pressure 115 to 200 atm and flow-rate 20 to 40mL/min (of the decompressed fluid). Besidesround robin testing by many laboratories, thevalidation efforts for this method include exami-nation of the effects of temperature and pressure,evaluation of relative response factors of differ-ent group types, and comparison of SFC-FID datawith those obtained by FIA and nuclear magneticresonance spectrometry (NMR).224-226

An incomplete resolution between saturatesand one-ring aromatics, and tailing peaks associ-ated with incomplete separation of one-ring aro-matics from polycyclic aromatic hydrocarbons,contribute uncertainties to the quantitative dataobtained by SFC. Optimization studies show thata low oven temperature (

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Separation of nonaromatics and aromatichydrocarbons in gas oil was carried out with andwithout backflushing of aromatics.235 Thebackflush time was determined graphically byplotting the percent peak areas of aromatics andnonaromatics against backflush time. The threegas oils characterized by SFC had aromatic con-tents in the range of 25 to 75%. Results for nor-mal-elution SFC, SFC with backflushing of aro-matics, and HPLC were found to be consistentwith each other. However, SFC-FID withbackflushing provided sharper peaks for both non-aromatics and aromatics.

B. Simulated Distillation by SFC

Simulated distillation by SFC-FID was carriedout to cover a broader boiling point range than canbe achieved by the widely used standard GC-FIDsimulated distillation method ASTM D2887.238 TheASTM D2887 method accommodates boiling pointdistributions up to 538C. In this respect, SFC-FID is an alternative to high-temperature simulateddistillation that allows the elution of high boilingpetroleum fractions and asphalt with atmosphericequivalent boiling points up to 800C.37,40,41,239-241Column temperature of 150C or less is required toachieve measurements of boiling range distribu-tions by SFC-FID.242-244 As relatively mild condi-tions are used, the SFC-FID method is advanta-geous over the GC-FID approach where a columntemperature of 430oC or higher is required to elutehigh boiling petroleum components. Such high tem-peratures are believed to be detrimental to the col-umn life, and, to some extent, to the thermal integ-rity of the samples.

Capillary columns,240,242-245 short packedmicrobore columns,246 and packed capillary col-umns with alkyl bonded silica phase 247 were usedfor simulated distillation by SFC-FID. The col-umn temperatures can be within the range of 70to 150C depending on the sample. Usually, lin-ear pressure programming, such as 100 to 415atm at 3.5 atm/min, was found to be adequate toachieve good resolution between adjacent homo-logues.247 Under these conditions, the elution andseparation of hydrocarbons with carbon numbersfrom C20 to C130 can be achieved.

As in simulated distillation by GC-FID, aseries of normal paraffins and/or polywax sampleare used to obtain a boiling point vs. retentiontime calibration curve. This calibration curve isthen used to determine boiling p