LC•GC Europe February 20042

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It has been approximately 12 years since theinitial description in chromatographicliterature of the technique now calledcomprehensive two-dimensional (2D) gaschromatography (GC) or GC�GC.1

Subsequently, the number of publicationsin the field has grown rapidly asresearchers have explored GC�GC, itsvariants and related techniques. Theinterest level remains high today, asdemonstrated by the large portion of thispast year’s GC papers and presentationsthat address GC�GC and related areassuch as GC�GC–mass spectrometry (MS).Applications for complex samples anddifficult matrices abound, and theincreased volume of chromatographicinformation provides impetus to revisit orreformulate existing separations solutions.

Although conceptually elegant, thepractical details of the chromatographicmodulation techniques that enablecomprehensive separations can besomewhat complex, and they require alevel of customization that most GC usershad dismissed as belonging to the 1950sand 1960s. The original oscillating thermaldiscontinuity, driven by gears or belts, hasbeen displaced primarily by thermal gasjets, fluidic switches, various valvingarrangements and their combinations, allof which offer significant improvements inreliability and reproducibility, as well asallow a better degree of control of theGC�GC process. But the requirement for

modulation followed by synchronousreconstruction of the detector signal willalways remain, along with requirements fortuning the system properly to avoid artefacts.

These technical hurdles are barriers tothe widespread acceptance of GC�GC,but the technology development andacceptance cycle that is underway is nodifferent than it was for capillary GCcolumns or even temperature-programmedGC ovens. History shows that when givenan application-driven need for a separationtechnique and a clearly defined set ofdesirable outcomes, multiple researchgroups that are actively pursuing thetechnical developments in industry andacademia will eventually find a way to putan initially obscure or difficult newtechnology into general use.

It is impossible to include sufficientreferences in this “GC Connections”column to do justice to the very largenumber of researchers who have madesignificant contributions in the varioussubfields of multidimensional GC.Interested readers should read the excellenttwo-part review of the subject byBertsch,2,3 which is still very relevant today.

Multidimensional GCGC�GC, at its roots, is a multidimensionalseparation technique in which the resolvingpower of two or more different columns isapplied to some or all of the componentsin a sample. Multidimensional GC is not a

new concept — it is almost as old as GCitself. Chromatographers have appliedmultiple columns to difficult separationproblems in various ways. During thebeginning years of GC, separations reliedmore heavily upon a much larger variety ofstationary phases than is now the case;hundreds of phases were used then, incontrast to the 20 or so commonly usedwith modern capillary columns. Packedcolumns dominated the field, but theyrequired more selectivity to overcome theirlackluster efficiencies relative to capillarycolumns. Analysts needed a uniquestationary phase for almost everyseparation class, and they soon turned tomixing column packings coated withdifferent stationary phases as a means tofine-tune difficult separations. The resultingchromatograms exhibited separationbehaviours that were the combination ofseparations on the individual phases; thecontribution of stationary phases could becontrolled by changing the proportions ofeach packing. Strictly speaking, these blendedseparations were not multidimensionalbecause only one column was used.Series-coupled columns: Otherresearchers soon realized that placing twocolumns in series — each with a differentstationary phase — would yield resultsequivalent to mixing the packings, after theeffects of the pressure drops throughoutthe column series were taken into account.Again, the results were proportional to the

Comprehensive Two-Dimensional Gas ChromatographyJohn V. Hinshaw, Serveron Corp., Hillsboro, Oregon, USA.

Comprehensive two-dimensional gas chromatography (GC�GC) hasattracted much attention recently. As the latest newcomer to thecollection of GC techniques, GC�GC delivers more GC information in ashorter time than do other methods; however, the complexity of GC�GCmight be delaying its entrance to the GC application mainstream.

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contribution of the individual packings, butin this situation they were in relation to thelengths of the columns connected in series.By attaching shorter or longer columns inseries, gas chromatographers could alsofine-tune their separations, and thesemanipulations became — and remaintoday — standard fare in dedicated GCsystems such as those found in process-control or on-line monitoring applications.With two columns in series, these systemsproduced the first multidimensionalseparations.Selectivity tuning: Series columnoperation can be easily extended tocapillary columns. In addition, positivecontrol of the pressure at the midpointbetween two columns yields a dynamicallytunable separation system. Conveniently,the contribution of each column to theoverall separation is proportional to thefraction of the total unretained peak timecontributed by either column. Instead ofadjusting column lengths — a time-consuming and costly optimizationprocedure — chromatographers can simplyadjust the linear carrier-gas velocitiesthrough the columns to achieve aselectivity-tuning effect. The polarity rangeof these systems is curtailed by the limitedrange of efficient carrier-gas velocities:too-high or too-low velocities yield reducedoverall efficiencies.

Figure 1 shows a schematic representationof a selectivity-tuned system. Peaks fromthe first column, usually non-polar, passthrough a midpoint splitter thatcontinuously routes a fraction of thecolumn effluent to the first detector. Thebulk of the effluent passes into the secondcolumn and on to a second detector. Thecarrier-gas velocity in the second column isset by the midpoint pressure, and thevelocity through the first is determined bythe pressure difference between the firstcolumn inlet pressure and the midpointpressure. In practice, a variable restrictor isusually placed between the midpointsplitter and the first detector to balance theflows between the columns.

The two representational chromatogramsin Figure 1 show the effect of a series-columnseparation on relative peak positions for ahypothetical sample. In chromatogram (a),peaks 1 and 2 are coeluted and peaks 4, 5and 6 are poorly resolved after they havepassed through column 1. As the peakspass through column 2, their positions shiftaccording to the strength of theirretentions on the second column. Peak 1 issomewhat strongly retained and it shifts toa later position. Peak 2 is weakly retained

and doesn’t shift much. Peak 3 is lessretained and is eluted relatively earlier.Peak 4 is also eluted earlier, but it interfereswith peak 1 after the two peaks passthrough column 2, as is also the situationwith peaks 6 and 8.

Series columns are the simplestembodiments of multidimensionalchromatography. The effects they produceare limited by carrier-gas velocities andbecause all the solutes transit bothcolumns in a single continuous stream.When working with complex samples,chromatographers might observe thatpeaks that are well separated after passingthe first column might come back togetheror might interfere with other peaks as theypass through the second column. Apeak-delay technique can help control thiseffect by momentarily time-shifting onepeak at the column juncture so thatanother peak, with which the delayed peakwould otherwise interfere, can move alongthe column and be eluted before thedelayed peak.

Series-coupled column systems bythemselves do not yield comprehensiveseparations: the results are a convolutionof the two columns’ separations. The first

separation couples into and stronglyinfluences the second. The number oftheoretical plates and basic resolvingpower are the same as if a single columnwith the length of the two individualcoupled columns had been prepared with astationary phase of equivalent selectivity —as when earlier researchers mixed coatedpackings together in a single column. As Iwill demonstrate in the next section,comprehensive 2D GC means that theseparation power of each column appliesfully to each analyte independently toeliminate the dependencies of the secondcolumn’s separation on the relative positionof peaks from the first column.Heartcutting: In heartcutting systems, oneor several discrete portions of a separationare directed from the first column to thesecond. Because only a few selected peaksenter the second column at a time,interference from other nearby peaks thatprecede or follow the heartcut iseliminated, and the second column’sseparation becomes largely independent ofthe first’s. Figure 2 illustrates the effect ofpassing a limited portion of the column 1effluent onto column 2 for the samehypothetical separation as shown in Figure 1.

Series columns are the simplest embodiments ofmultidimensional chromatography. The effects theyproduce are limited by carrier-gas velocities and because all the solutes transit both columns in a single continuous stream.

tectorB

Figure 1: Selectivity-tuned GC system: Representative relative peak positions (a) at the column 1–column 2 juncture and (b) after transiting the second column.

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In Figure 2, the pneumatic splitter deviceis replaced with a pneumatic switch thatcan direct flow from column 1 to detector Aor to the entrance of column 2. The switchworks by slightly increasing or decreasingthe relative pressures on the two midpointpressure control lines by a few tenths of apound per square inch. A slightly higherpressure on the column 1 side of theswitch will direct column 1 effluentthrough the switch’s middle passage andinto column 2. Alternatively, a slightlyhigher pressure on the column 2 side willreverse the flow through the switch,sending only pure carrier gas into column 2and forcing the effluent from column 1into the detector.

In chromatogram (a), after peaks 1, 2and 3 go to detector A, the pneumaticswitch is set to send peaks 4, 5 and 6directly to the second column. Next, thepneumatic switch is reset and theremainder of the column 1 effluent goes todetector A. Peaks 4, 5 and 6 don’t appearin chromatogram A (they’re shown as greydotted lines for reference): all of thecolumn effluent goes to column 2 duringthe heartcutting period. Because only thesethree peaks enter column 2, no possibilityof interference from peaks 1 and 8 exists.Trapping: As they transit into the secondcolumn, the heartcut peaks could occupy arelatively large volume of carrier gas andthe switching device could introduce someextra carrier gas or broadening as well.Cold-trapping or otherwise concentratingthe transferred peaks into a smaller volume

after they leave the first column cannarrow the peaks’ profiles at the onset ofthe second separation. This trappingcollapses the localized separation of theselected peaks from each other and resultsin the removal of any contribution from thefirst column to the immediate separation ofthe selected peaks on the second column.

However, the number of heartcuts fromthe first column that can be analysed by asecond column is limited by the analysisand recovery times of the second columnbefore each subsequent heartcut cycle.Peaks that are eluted from the first columnwhile the second column is activelyanalysing a previously sampled heartcut donot benefit from separation in the secondcolumn and are sent to the first detectorinstead. One solution for this problem ofpeaks queuing up and waiting for thesecond column to finish a separation is touse more than one secondary column.Another solution, which is the essence ofcomprehensive chromatography, is to makethe secondary separation faster so thatqueuing peaks don’t accumulate too muchduring each secondary separation pass.

Thus at one dual-column extreme —selectivity tuning and its variants — all thepeaks enter the second column separatedjust as they are eluted from the first. Somemanipulation and delay of the peaks ispossible as they transit the columnjuncture, but these fully coupled systemsproduce a net result that is a convolutionof both separations; peaks that areseparated at the exit of the first column

might recombine as they pass through thesecond column. In the intermediateheartcutting situation, only selectedfractions from the first column enter thesecond column. Those components that dopass through the second column areseparated largely independently of theirbehaviour on the first column, as if theyhad been injected independently of therest of the sample. Heartcutting is veryuseful for directed pinpoint separations ofa few selected peak groups but fails toproduce a complete separation thatcombines all the resolving power of bothcolumns for the entire sample.

Comprehensive SeparationsAs the name implies, comprehensiveseparations apply all the available resolvingpower of both columns to all the peaks ina sample. In GC�GC each peak transitsthe first column, is trapped at the end ofthe first or the beginning of the secondcolumn and is released onto the secondcolumn. The major difference betweenGC�GC and heartcutting is that forGC�GC the trapping time and secondcolumn separation speeds are fast enoughso that many heartcuts can be analysed inthe second column during the course of asingle first-column run. The heartcuts canbe taken rapidly enough that individualpeaks from the first column are slicedacross several sequential secondary columnruns, but going much faster than that hasno advantage.High-speed: Achieving this level ofperformance requires a high-speedseparation for the secondary column andencourages ordinary speeds for the firstcolumn. Faster separations on the firstcolumn can drive the second-column speedrequirement higher than what is practical.Typical secondary column analysis timesused today are on the order of 0.1 min orless with very short (1–2 m) narrow-bore(100–180 mm i.d.) dimensions and linearvelocities several times the optimum.Hydrogen carrier gas is advantageous as itmaintains column efficiency at higher linearvelocities better than helium does.

A secondary column with thosedimensions yields few theoretical plates bythe usual standards for capillary columns,but it doesn’t have to, because it’s onlybeing asked to handle the few peaks at atime that coexist in any one heartcut fromthe first column. This distinction is essentialto understanding GC�GC. Gaschromatographers are not used to thinkingthat short, narrow-bore capillary columnsare practical for anything. Even at linear

Midpointpressurecontrol

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Figure 2: Heartcut GC system: Representative relative peak positions (a) at the column 1–column 2 juncture and (b) for heartcut section after passing alone through the second column.

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velocities well above optimum, however,these columns can deliver 3000–5000theoretical plates, which is sufficient forthe purpose at hand.Modulation: Comprehensive GCseparations can be accomplished manyways. The most prevalent methods in usetoday involve some form of thermalmodulation at the juncture of columns 1and 2. Figure 3(a) illustrates the mainprinciples of GC�GC operation with athermal modulator. The simplestarrangements intermittently blow coolednitrogen or air over a section of thebeginning of column 2. When the coolingflow is turned off, the cooled columnsection rapidly reheats from circulating hotoven air. Some arrangements positivelyheat the cooled section to release thetrapped peaks more rapidly.

While cooling is applied, peaks elutedfrom column 1 are trapped in place at thebeginning of column 2. When heated, thepeaks are released and begin to move againin a narrow band to start the secondaryseparation. However, during the heatingstage, any new peak material exiting column 1will not be trapped and will enter column 2along with the already trapped material.This situation creates some undesirableoverlapping and smearing of peak bandsbetween adjacent heartcut sections.

A second thermal modulator solves thisproblem, as Figure 3(a) shows. Themodulators are heated and cooled out ofphase with each other. Peaks are trappedat the first modulator when it is cooleddown. The second modulator is also cooleddown and then the first modulator isheated. The trapped peaks move to thecooled second modulator zone along withany material that leaks through while thefirst modulator is hot. When the firstmodulator is cooled off again, the twotrapping zones are effectively isolated fromeach other. The second modulator isheated, and the peaks trapped within arereleased into the secondary column.Because the first modulator is still cooled,any new material coming from column 1 istrapped inside and does not enter thesecond column until the next secondaryanalysis is ready to start. This schemeeffectively isolates the two columns fromeach other for the purposes of GC�GC.

Thus, a series of rapidly repeatinginjections are performed effectively ontothe secondary column with slices trappedoff the first column. What does theresulting chromatogram look like? Figure 3(b)shows a representation of a raw GC�GCchromatogram section that is 2.5 min long.

I have used the same set of eight samplepeaks to represent the separation fromcolumn 1 as in the previous series-coupledand heartcut examples. For GC�GC, thechromatogram from the secondary columnappears with discrete groups or bands ofpeaks. Each secondary column run isrepresented by the 0.1-min-long bands alongthe baseline.

Peaks 1 and 2, which are coeluted fromcolumn 1, are separated in the sixth secondarycolumn run. Peaks 4, 5 and 6, which werepoorly resolved on column 1, are alsoseparated, but the separation occurs in the16th secondary column run. Significantly,none of these peaks interfere with eachother as they did with the series-coupledsystem. Peak 1, which is coeluted withpeak 4 on the secondary column, does notinterfere in GC�GC because it passesthrough the secondary column 10 cyclesbefore peak 4 enters the column; peaks 6and 8 behave similarly. It is this result —that the separation of the peaks on the firstcolumn is maintained — that gives GC�GCits tremendous resolving power.Data reduction: Additional relationshipsbetween peaks in GC�GC exist, but theydo not become apparent until some data

manipulation is performed. The mostcommon data transformation is theconstruction of a 2D representation inwhich one axis represents the separationon the first column and the other axisrepresents the secondary columnseparation. A contour plot using elevationlines or colour coding represents the signalintensity. A third dimension can be addedoptionally by making the z-axisproportional to the signal intensity.

To generate the reconstructed 2D plot,chromatographers use the timing of thethermal modulators to locate the start ofeach secondary column run in the raw datastream. The raw data stream is sliced intomultiple subchromatograms that representeach of the secondary runs. As Figure 3(c)shows, these slices can be assembled side byside to produce a two- or three-dimensional(3D) map of the overall separation. The mapis very useful because it shows relationshipsbetween groups of peaks in the primaryand the secondary column separations.Both types of plot are shown in Figure 4.

Example — Light Cycle OilGC�GC has been applied to hundreds ofcomplex and challenging samples. One

Column 1 (non-polar) Column 2(polar)

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Figure 3: GC�GC system: (a) Schematic diagram with dual thermal modulators, (b) representation of a raw GC�GC chromatogram showing positions of individual slices, and (c) representation of a 3D chromatogram reconstructed from the raw chromatogram.

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LC•GC Europe February 20046

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area in which GC�GC excels is theelucidation of petroleum samples. Oils andother related materials of natural origincontain homologous series of hydrocarboncompounds: normal paraffins, isoparaffinsand cycloparaffins; unsaturates; and one-,two- and multiple-ring aromatic compounds.A non-polar column provides separationsby order of increasing carbon number forsuch samples. Usually, the normal paraffinsdominate the mixture and produce arecognizably spaced series of peaks from thebeginning to the end of a one-dimensionalchromatogram.

Figure 4(a) shows the raw data from aGC�GC analysis of a sample of light cycleoil. The hydrocarbons stand out as large,regularly spaced peaks. Superimposed overthe normal hydrocarbon series are the otherseries of isoparaffins, unsaturates andaromatic compounds. With many thousandsof compounds present, a single-columnchromatogram suffers from peak overlap,often to the point of completely mergingthe peaks into one or more lumps. Whenthese samples are run on polar columns,the peak groups are sorted by degree ofunsaturation and branching more than bythe number of carbons, but the result issimilar for complex samples — a largedegree of overlap occurs.

GC�GC, with its independent-but-combined non-polar and polar separations,can resolve an astonishing number ofsubstances. Even so, the total number ofpeaks in complex petroleum samplesusually exceeds even this enhancedresolving power. Figure 4(b) illustrates a 2Dcontour plot constructed from the rawdata in Figure 4(a) for the light cycle oilexample. The colour of the plot representsthe signal intensity. In particular, note thedistinct grouping of the various functional-group homologous series. The horizontalgroup of peaks running from left to rightclosest to the x-axis are the paraffins, andthe groupings higher on the plot arevarious types of aromatic compounds.Figure 4(c) shows the same data in a 3Drepresentation, as viewed from the right-hand side of Figure 4(b).GC�GC–MS: This example clearlydemonstrates the potential of GC�GC togenerate a wealth of additionalinformation about a complex sample. Buteven so, it is obvious that much moreinformation is present. One way to add yetanother dimension to the information is totack a mass spectrometer onto the end ofa GC�GC system. In doing so,chromatographers can make additionaldemands of the MS system for scan speed

4.0

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Figure 4: GC�GC analysis of light cycle oil: (a) raw GC�GC data; (b) a reconstructed 2D chromatogram plot; and (c) a reconstructed 3D chromatogram. Column 1: 30 m � 0.32 mm, 0.25 µm df RTX-5; Column 2: 1 m � 0.1 mm, 0.1 µm df BPX-50; oven temperature programme: 50 °C (1 min isothermal) to 300 °C at 3 °C/min; modulationcycle time: 4 s; injection mode: 0.4 µL, split ratio 200:1; flame ionization detector sampling rate: 200 Hz; instrument: Trace GC Ultra in 2D GC configuration with dual-stage carbon dioxide modulator, split–splitless injector and fast flame ionizationdetector; data system: HyperChrom SW for qualitative and quantitative analysis ofbidimensional GC�GC separation patterns. Peak groupings: 1 � paraffins; 2 � mono-aromatics; 3 � naphthenic mono-aromatics; 4 � di-aromatics; 5 � naphthenic di-aromatics; 6 � tri-aromatics. (Courtesy of Thermo Electron Corp.,Waltham, Massachusetts, USA.)

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and data storage beyond conventional oreven so-called high-speed GC systems thatrequire rethinking of how such systems aredesigned. The amount of data generatedby a GC�GC–MS system is truly large, andreducing those data to meaningful resultspresents yet another challenge for analysts.

ConclusionThe power of GC�GC lies in its ability topull the polar-column information into thesecond-dimension axis while retaining thefirst-dimension non-polar columninformation. The results are striking in theirvisual appearance — patterns formed bythe various compound groups seem to leapoff the page. Clearly, GC�GC offerstremendous potential for the furtherelucidation of thousands of GC sampletypes that will occupy researchers for along time to come. But this technology isnot one that is easily accessible to averagegas chromatographers. A successfulGC�GC system or accessory involves bothhardware and software developments thatmust be well integrated, which essentiallyrequires the resources of an instrumentcompany along with academic participation.Thus, this newcomer must follow theclassic path of earlier major developmentsin chromatographic instrumentation.

References1. Z. Liu and J.B. Phillips, J. Chromatogr. Sci., 29,

227 (1991).2. W. Bertsch, J. High Resolut. Chromatogr., 22,

647–665 (1999).3. W. Bertsch, J. High Resolut. Chromatogr.,

23(3), 167–181 (2000).

“GC Connections” editor John V.Hinshaw is senior staff engineer atServeron Corp., Hillsboro, Oregon, USA,and a member of the Editorial AdvisoryBoard of LC•GC Europe.

Direct correspondence about this columnto “GC Connections,” LC•GC Europe,Advanstar House, Park West, SealandRoad, Chester CH1 4RN, UK, e-mail: dhills@advanstar.com

For an on-going discussion of GC issueswith John Hinshaw and otherchromatographers, visit theChromatography Forum discussion groupat http://www.chromforum.com

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