High-Efficiency Solvating Gas ChromatographyUsing Packed Capillaries

Yufeng Shen and Milton L. Lee*

Department of Chemistry and Biochemistry, Brigham Young University, Provo, Utah 84602

In this study, column efficiency in packed capillarycolumn solvating gas chromatography (SGC) was inves-tigated. Long (>3 m) fused silica capillaries with an innerdiameter of 250 µm were packed with 10 and 15 µmspherical porous (300Å) octadecyl bonded silica particlesusing a CO2 slurry packing method. A 336 cm × 250µm i.d. fused silica capillary containing 10 µm particlesprovided a total column efficiency of 264 000 plates (k) 0.41), corresponding to a reduced plate height of 1.27,using CO2 as the mobile phase at a column inlet pressureof 260 atm. A minimum plate height of 12.7 µm and amaximum plate number per unit time of 813 plates/swere obtained using packed capillary SGC. Retentionfactors were dependent on the column inlet pressure butindependent of the pressure gradient along the column.Gasoline and diesel samples were separated under SGCconditions, and the results were comparable to thoseobtained using typical open tubular column gas chroma-tography.

Packed column gas chromatography (GC) was extensivelystudied in the past.1-10 In recent years, high-efficiency GC hasbeen primarily associated with open tubular columns. To improvethe total column efficiency when using packed columns, eitherlong columns or small particles are necessary. However, thesecolumns produce a significant column resistance to mobile phaseflow, and a high column inlet pressure is needed to force themobile phase through the column. Therefore, high performancepacked columns are always associated with high column inletpressure. Packed column efficiency can be described by totalcolumn efficiency (N), column efficiency per column length (n),plate height (H), reduced plate height (h), or column efficiencyper unit time (Nt).

Myers and Giddings prepared an extremely long column (4000ft) packed with large particles (50-60 mesh) and obtained a totalcolumn efficiency on the order of 106 with a column inlet pressureof 2500 psi, corresponding to a reduced plate height of 2.2 Thisis the highest plate number and lowest reduced plate height

obtained in packed column GC, and these results confirmed thata high pressure drop and a large mobile phase linear velocitygradient had limited effects on column efficiency. However,because of the use of large particles, a low column efficiency perunit time (less than 80 plates/s) and a large plate height (largerthan 500 µm) required an intolerably long analysis time to obtainthis high efficiency. To improve column efficiency per unit timeand per column length, microparticles must be used.

Myers and Giddings also prepared a 2 m long column packedwith 13 µm particles, and a minimum plate height of 82 µm wasobtained, corresponding to 24 000 plates/column and a reducedplate height of 6.3.3 Corcia et al. packed a 21 cm long columncontaining 20-25 µm particles, and they obtained 17 000 plates/m, corresponding to a plate height of 60 µm and a reduced plateheight of 2.5-3.0.4 This is the lowest reduced plate height valueobtained in packed column GC when microparticles were usedas packing materials. Lu et al. prepared a 10 cm long columncontaining 7 ( 2 µm particles, which produced 45 000 plates/m,corresponding to a reduced plate height of 3.1.5 This is the largestplate number per meter obtained in packed column GC. However,these shorter columns had a limited total column efficiency.

The selection of mobile phase in packed column GC isimportant in order to carry out high-performance separations.Carbon dioxide as a mobile phase in packed capillary GC has thefollowing advantages: (1) higher column efficiency can beobtained using CO2 as the mobile phase in packed column GCthan by using other lighter gases;5 (2) the solvating power of denseCO2 can offset the large retention of solutes in packed columns;(3) the high column inlet pressure can be easily controlled usinga supercritical fluid chromatography (SFC) pump.

It should be mentioned that CO2 is a gas at pressures below72.9 atm. Beyond this point, it becomes a supercritical fluid.Therefore, when using CO2 as the mobile phase with long packedcolumns containing microparticles (which require high columninlet pressures), the mobile phase at the inlet end of the columnis a supercritical fluid, and at the outlet end it is a gas. We havecalled this method “solvating gas chromatography (SGC)”.11

When using other lighter gases such as He and N2 as mobilephases in packed column GC, the term “high-pressure gaschromatography (HPGC)” is probably more suitable because themobile phase behaves as a gas along the whole column length. Aprevious study showed that SGC was more suitable to carry outhigh speed separations than HPGC when packed columns wereused.11

In the past, there have been two limitations in the performanceof packed column GC. First, a reduced plate height as low as 2was never achieved using microparticles as packing material in

(1) Giddings, J. C. Anal. Chem. 1964, 36, 741-744.(2) Myers, M. N.; Giddings, J. C. Anal. Chem. 1965, 37, 1453-1457.(3) Myers, M. N.; Giddings, J. C. Anal. Chem. 1966, 38, 294-297.(4) DiCorcia, A.; Liberti, A.; Samperi, R. J. Chromatogr. 1978, 167, 243-252.(5) Lu, P. C; Zhou, L. M.; Wang, C. H.; Wang, G. G.; Xia, A. Z.; Xu, F. B. J.

Chromatogr. 1979, 186, 20-35.(6) Carter, H. V. Nature 1963, 197, 684.(7) Cramers, C. A.; Rijks, J. J. Chromatogr. 1972, 65, 29-37.(8) Huber, J. F. K.; Lauer, H. H.; Poppe, H. J. Chromatogr. 1975, 112, 377-

388.(9) Lauer, H. H.; Poppe, H.; Huber, J. F. K. J. Chromatogr. 1977, 132, 1-16.

(10) Welsch, Th.; Engewald, W.; Poerschmann, J. J. Chromatogr. 1978, 148,143-149. (11) Shen, Y.; Lee, M. L. J. Chromatogr., submitted.

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GC, which is typical of packed column liquid chromatography(LC). Second, the total column efficiency and plate number perunit time were never as high as the values obtained by using opentubular column GC (∼100 000 plates/column, 100-600 plates/s). This has limited the practical use of packed columns for theseparation of complex samples. The aim of this study was toresolve these two limitations by using packed capillary SGC.

EXPERIMENTAL SECTIONMaterials and Instrumentation. Spherical porous (300 Å)

octadecyl bonded silica (ODS) particles having diameters of 10µm were purchased from Phenomenex (Torrance, CA). Sphericalporous (300 Å) ODS particles having diameters of 15 µm werepurchased from YMC (Wilmington, NC). Fused silica capillarytubing was purchased from Polymicro Technologies (Phoenix,AZ). Column connections were made using PEEK tubing andzero dead-volume unions (Valco Instruments, Houston, TX). Thecolumn packing and SGC experiments were carried out using aLee Scientific Model 600 SFC instrument (Dionex, Salt LakeDivision, Salt Lake City, UT). SFC grade CO2 (Scott SpecialtyGases, Plumsteadville, PA) was used for the preparation of packedcapillary columns and as the mobile phase.

Helium (Scott Specialty Gases) was used as a carrier gas inopen tubular column GC and HPGC. Open tubular column GCwas performed using a 22 m × 250 µm i.d. fused silica capillarycoated with 0.25 µm SE-54 stationary phase (Supelco, Bellefonte,PA). An HP-5890A series gas chromatograph (Hewlett-Packard,Avondale, PA) was used for open tabular column GC experiments.HPGC was performed using the same system as used for SGC;however, He was used as carrier gas instead of CO2. Otherchemicals used were purchased from Sigma (St. Louis, MO) andAldrich (Milwaukee, WI).

Preparation of Packed Capillary Columns. A previouslyreported CO2 slurry packing method12 was modified and used toprepare long packed capillary columns containing 10 and 15 µmparticles. One end of the fused silica capillary column wasconnected to a zero dead-volume union using PEEK tubing toposition a steel screen (2 µm pores, Valco) to support the particles,and the other end of the union was opened to the atmosphere.The column was then connected to a steel vessel, in whichmicroparticles were introduced. Using CO2 at room temperatureand 40 atm, the particles were packed into the column to a lengthof approximately 40 cm, and then the column was placed in anultrasonic bath to help settle the particles at the end of the column.Particles were then introduced into another 40 cm length, andthe process was repeated while the pressure was graduallyincreased. At approximately 120 atm, the column was fullypacked, and it was conditioned at room temperature and 180 atmin an ultrasonic bath for 10 min. Finally, the column was leftovernight to allow for slow depressurization.

SGC Experiments. SGC experiments were carried out usinga Lee Scientific Model 600 SFC instrument. A manual liquidinjector (Valco Instruments) with a rotor volume of 0.2 µL wasused for introduction of samples. A tee was connected to theinjector valve by using a 10 cm × 125 µm i.d. steel tube. A 4 cm× 15 µm i.d. fused silica capillary was used for the split line.Separation columns were connected to the tee using PEEK tubing.A 10 cm × 50 µm i.d. fused silica capillary was used to connectthe separation column to the FID. CO2 was used as the mobile

phase in SGC, and its pressure was controlled using the SFCpump. The outlet of the pump was connected to a valve injectorthrough a 50 cm × 1 mm i.d. steel tube, which was preheated inthe chromatographic oven.

HPGC Experiments. The same instrument and proceduresas those used for SGC were used to carry out HPGC. Heliumwas used as the carrier gas.

Open Tubular Column GC Experiments. Split injectionwith a split flow of 150 mL/min was used in open tubular columnGC experiments.

RESULTS AND DISCUSSIONSGC and HPGC. In the introduction, we briefly described

the difference between SGC and HPGC. In SGC, both SFC andGC operate in the column when CO2 is used as the mobile phase.This method is appropriately named as a form of GC because theelution of solutes depends on temperature. Solutes must bevaporized before they can be eluted from the column. Thesolvating power of the mobile phase, especially at the column inletend, assists in the elution process. On the other hand, in HPGC,the mobile phase serves only as a carrier, even though highcolumn inlet pressure is imposed. When using lighter gases suchas H2, He, or N2 as mobile phases at high pressures, HPGC results.

In SGC, the solvating power of the mobile phase can decreasethe retention of solutes in the column. Interaction between themobile phase and the stationary phase usually reduces interactionsbetween the solutes and the packing material. However, slowerdiffusion of solutes in supercritical fluids and higher supercriticalfluid viscosities can be disadvantages of SGC. In this study, HPGCand SGC using packed capillary columns were experimentallycompared.

Figure 1 shows HPGC and SGC chromatograms of normal andaromatic hydrocarbons. Great differences were observed. At 130°C and 150 atm inlet pressure, naphthalene and 1-methylnaph-thalene were not eluted in HPGC when He was used as the carriergas. Although other lighter components were eluted, largeretention and poor peak shapes were observed (Figure 1A). Uponincreasing the temperature to 180 °C, all components were eluted;however, low column efficiency and poor peak shapes wereobtained (Figure 1B). When the mobile phase was changed fromHe to CO2, excellent results were obtained (Figure 1C). At 130°C and 150 atm inlet pressure, all components were eluted withexcellent peak shapes. The shortened retention times andimprovement in peak shapes could both be explained by thesolvating power of the mobile phase and the interaction betweenthe mobile phase and the stationary phase, which reduces theinteractions between solutes and the stationary phase. It wasfound that when the same column, split tube, concentration ofsolutes (∼2% total concentration), temperature, and inlet pressurewere used, SGC produced a much higher detector response thanHPGC. Figure 1C was obtained by increasing the attenuation 10times compared to that in Figure 1A, while all other experimentalconditions remained the same. In our experiments, high-efficiencyHPGC could not be obtained when using long packed capillariescontaining microparticles.

Generation of Mobile Phase Flow. At a specific column inletpressure, the resultant mobile phase linear velocity is dependenton the viscosity of the mobile phase. Differing from HPGC, theaverage viscosity of CO2 in the column is higher than high-pressure gases because the mobile phase in SGC is a combinationof gas and supercritical fluid. The higher viscosity of the(12) Malik, A.; Li, W.; Lee, M. L. J. Microcolumn Sep. 1993, 5, 361-367.

2542 Analytical Chemistry, Vol. 69, No. 13, July 1, 1997

supercritical fluid imposes an increased column inlet pressure inSGC to obtain a specific mobile phase linear velocity.

In our experiments, no separation was obtained betweenmethane and ethane when using columns packed with porousODS bonded particles under SGC conditions. Therefore, it wasassumed that methane had no retention in these columns, and itwas used as a marker to measure the dead time. Figure 2 showsthe relationship between column inlet pressure and mobile phaselinear velocity for various length columns packed with 10 and 15µm porous (300 Å) ODS bonded particles under SGC conditions.Longer columns and smaller particles led to less change in mobilephase linear velocity with pressure. When the 10 µm particlepacked column length was reduced 1.35 and 1.87 times from 3.36to 2.50 to 1.80 m, the linear velocity was increased 1.44 and 2.05times, respectively, at any specific column inlet pressure.

Retention Factor and Mobile Phase Linear Velocity. Sincethe solvating power of the mobile phase in SGC is affected bythe pressure or density of the mobile phase, the mobile phaselinear velocity, generated by imposing a certain column inletpressure, is always associated with the retention factor. Figure 3shows the experimental relationships between retention factor andmobile phase linear velocity for different columns. For each

column, the retention factor rapidly decreased when the mobilephase linear velocity was increased. At a specific linear velocity,the longer column produced a much smaller retention factor, andthe column containing 15 µm particles produced a much largerretention factor than the column containing 10 µm particles. Thiscan be expected because the higher mobile phase linear velocities,longer columns, or smaller particles require higher column inletpressures and result in a decrease in the retention factors ofsolutes.

The decrease in retention factor of solutes with increasingmobile phase linear velocity produces both advantageous anddisadvantageous effects on the chromatographic performance. Adecrease in retention factor can reduce the resolution, but it canalso speed up the elution of solutes and shorten the analysis time.This effect is somewhat similar to that obtained by increasing thetemperature in GC.

In our experiments, it was found that the retention factor wasa direct result of the column inlet pressure and not the pressureor density gradient of the mobile phase along the column. Table

Figure 1. HPGC and SGC chromatograms of test solutes. Condi-tions: 228 cm × 250 µm i.d. fused silica capillary column packedwith 15 µm spherical porous (300 Å) ODS bonded particles, 150 atmcolumn inlet pressure, (A) 130 °C, He carrier gas, (B) 180 °C, Hecarrier gas, and (C) 130 °C, CO2 mobile phase, flame ionizationdetector (FID). Peak identification: (1) benzene, (2) toluene, (3)n-octane, (4) p-xylene, (5) n-nonane, (6) n-decane, (7) butylbenzene,(8) n-undecane, (9) naphthalene, (10) n-dodecane, and (11) 1-me-thylnaphthalene.

Figure 2. Relationship between column inlet pressure and mobilephase linear velocity in SGC. Conditions: 130 °C, methane used asunretained marker, (O) 228 cm × 250 µm i.d. capillary packed with15 µm porous ODS bonded particles, (b) 336 cm × 250 µm i.d. (9)250 cm × 250 µm i.d., and ([) 118 cm × 250 µm i.d. capillariespacked with 10 µm porous ODS bonded particles; other conditionsare the same as in Figure 1.

Figure 3. Relationship between retention factor and mobile phaselinear velocity. Conditions: n-octane as test solute; other conditionsand identification of data points are the same as in Figure 2.

Analytical Chemistry, Vol. 69, No. 13, July 1, 1997 2543

1 clearly shows this phenomenon. Two normal and two aromatichydrocarbons were used to investigate this relationship, usingdifferent length columns packed with 10 µm particles. At anyspecific column inlet pressure, solute retention factors were thesame, regardless of how long the column was or what the pressuregradient was.

In GC, the separation selectivity is determined only by thetemperature and the stationary phase. However, in SGC, it wasfound that the separation selectivity and even the elution orderof solutes were affected by the column inlet pressure or mobilephase linear velocity. Figure 4 shows this phenomenon. At aconstant temperature of 130 °C and increasing the column inletpressure from 140 to 220 atm, the elution order of naphthaleneand dodecane was reversed. This is because the retention factors

of the solutes were influenced by both solvating power andtemperature of the mobile phase. Therefore, in SGC, bothretention mechanisms of GC and SFC are in effect.

Column Efficiency in SGC. Packed columns containingmicroparticles create a significant pressure drop along the column.In SGC, this pressure drop results in a linear velocity gradient ofthe compressible mobile phase (both gas and supercritical fluid)and a retention factor gradient of solutes along the columnbecause the solubility of the mobile phase is dependent on itspressure or density.

Giddings1 theoretically treated the effect of the mobile phaselinear velocity gradient on column efficiency and pointed out thatthe loss of column efficiency could be corrected using a com-pressibility factor (f2). A maximum correction of f2 (Pi . Po) was1.125, which means that the maximum loss of column efficiencydue to the linear velocity gradient was 12.5%. Using a long packedcolumn (4000 ft) and a large column inlet pressure (2500 psi),Myers and Giddings experimentally showed that the minimumreduced plate height could be as low as 2.2 This suggests thatthe mobile phase linear velocity should not be a major factoraffecting the column efficiency when compressible mobile phasesare used.

There have been few publications which theoretically analyzedthe effect of retention factor gradient on column efficiency. It isdifficult to treat this theoretically because this effect is alwaysassociated with the mobile phase linear velocity gradient. Duringa study of column efficiency in packed column SFC, this wasexperimentally investigated. Gere et al. showed that, when 3 µmparticles were used in packed column SFC, a reduced plate heightof less than 2 could be obtained.13 Berger et al. connected severalLC columns together and found that the column efficiency wasproportional to the column length.14,15 However, these studieswere based on conventional packed columns. In our presentstudy, packed capillaries were used, and their performances wereinvestigated by determining total plate number (N), plate numberper second (Nt), plate height (H), and reduced plate height (h)under SGC conditions.

Table 2 shows the experimental relationship between N andaverage mobile phase linear velocity (u) for various columns underSGC conditions. Increasing the column inlet pressure and,consequently, the mobile phase linear velocity, the total column

(13) Gere, D. R.; Board, R.; McManigill, D. Anal. Chem. 1982, 54, 736-740.(14) Berger, T. A.; Wilson, W. H. Anal. Chem. 1993, 65, 1451-1455.(15) Berger, T. A.; Blumberg, L. M. Chromatographia 1994, 38, 5-11.

Figure 4. Effect of column inlet pressure on selectivity in SGC.Conditions: 250 cm × 250 µm i.d. fused silica capillary columnpacked with 10 µm porous ODS bonded particles; other conditionsare the same as in Figure 1. Peak identification: (1) naphthaleneand (2) n-dodecane.

Table 1. Relationship between k and Pi in SGCa

180 cm 250 cm 336 cm

Pi (atm) k1b k2

c k3d k4

e k1b k2

c k3d k4

e k1b k2

c k3d k4

e

120 1.88 3.63 6.54 8.95140 1.56 2.86 5.04 6.60 1.57 2.88 5.11 6.67160 1.31 2.26 3.94 5.02 1.29 2.27 3.97 5.10 1.30 2.25 3.94 4.96180 1.09 1.82 3.13 3.90 1.06 1.81 3.15 3.93 1.08 1.79 3.11 3.85200 0.90 1.45 2.49 3.05 0.88 1.44 2.50 3.08 0.90 1.45 2.49 3.02220 0.78 1.24 2.07 2.51 0.77 1.23 2.10 2.53 0.77 1.21 2.04 2.44240 0.68 1.04 1.74 2.08 0.68 1.04 1.76 2.10 0.66 1.00 1.70 2.01260 0.59 0.88 1.47 1.73 0.57 0.89 1.49 1.78 0.57 0.86 1.44 1.71280 0.54 0.77 1.37 1.53 0.53 0.78 1.31 1.53 0.52 0.76 1.26 1.45

a Conditions: various length columns having an inner diameter of 250 µm packed with 10 µm porous (300 Å pores) ODS particles, 130 °C, CO2mobile phase, methane used as unretained marker, FID. b Retention factor of toluene. c Retention factor of n-nonane. d Retention factor ofn-butylbenzene. e Retention factor of n-dodecane.

2544 Analytical Chemistry, Vol. 69, No. 13, July 1, 1997

efficiency was increased for all columns containing 10 µm porousODS bonded particles. All columns produced more than 100 000total plates. A maximum plate number of 264 000 was obtainedby using a 336 cm × 250 µm i.d. column containing 10 µm porousODS particles at a mobile phase linear velocity of 0.90 cm/s,corresponding to a column inlet pressure of 260 atm. This is thehighest plate number reported in packed column chromatographyusing microparticles as packing materials. The column efficiencydecreases with an increase in retention factor (k) when k is lessthan 5, and Figure 5 shows a chromatogram to illustrate thechanges in N with k for the separation of normal and aromatichydrocarbons. Even with a k value greater than 4, more than118 000 plates were obtained using packed capillary SGC. Thesecolumn efficiencies are comparable with those of typical opentubular column GC. Furthermore, excellent peak shapes wereobtained. A 228 cm × 250 µm i.d. column packed with 15 µmporous (300 Å) ODS bonded particles produced a total platenumber of nearly 90 000 in the experimental range of mobile phase

linear velocities studied. However, better column permeabilityallowed this column to carry out fast separations at lower columninlet pressures. Figure 6 shows a fast SGC chromatogram of thesame solutes as shown in Figure 5. At the same temperature (130°C) and column inlet pressure (260 atm), the separation wasfinished within 3 min. Of course, the resolution was reducedcompared with that in Figure 5.

Table 3 shows the relationship between Nt and u for variouscolumns under SGC conditions. Comparable plates per secondof 100-800 as typical of open tubular column GC were obtainedin SGC. For a 180 cm column containing 10 µm particles, theplate number per second increased with increasing the mobilephase linear velocity until the linear velocity reached 1.8 cm/s. Amaximum value of 780 plates/s was obtained for this column. For250 and 336 cm long columns containing 10 µm particles, the platenumber per second increased with increasing mobile phase linearvelocity throughout the experimental range of the linear velocitiesstudied.

Comparing the plate numbers per second obtained using 10µm particle packed columns, it can be seen that, at a specificmobile phase linear velocity, increasing the column length

Table 2. Relationship between Total Plate Number (N) and Mobile Phase Linear Velocity (u, cm s-1) in SGCa

column 1 column 2 column 3 column 4

u N u N u N u N

1.05 68 594 0.86 104 197 0.66 123 860 3.93 88 4851.20 86 562 0.95 143 602 0.72 125 585 4.45 76 4871.36 106 014 1.04 124 895 0.77 190 146 4.85 55 4001.45 112 500 1.11 151 515 0.82 213 368 5.26 43 3501.55 112 503 1.19 155 819 0.86 227 966 5.67 39 3971.67 121 702 1.26 184 817 0.90 264 088 5.94 38 7521.76 118 341 1.33 154 968 0.96 225 008 6.11 34 2451.82 73 426 1.39 173 843 6.38 35 4561.92 70 738

a Conditions: n-octane used as test solute, methane used as unretained marker, CO2 used as mobile phase, 130 °C, FID. Column 1, 180 cm ×250 µm i.d. capillary column packed with 10 µm (300 Å pores) ODS particles; column 2, 250 cm × 250 µm i.d. capillary column packed with 10µm (300 Å pores) ODS particles; column 3, 336 cm × 250 µm i.d. capillary column packed with 10 µm (300 Å pores) ODS particles; and column4, 225 cm × 250 µm i.d. capillary column packed with 15 µm (300 Å pores) ODS particles.

Figure 5. SGC chromatogram of test solutes. Conditions: 336 cm× 250 µm i.d. fused silica capillary column packed with 10 µm porousODS bonded particles, 130 °C, 260 atm column inlet pressure; otherconditions are the same as in Figure 2. Peak identification: (1)benzene, (2) toluene, (3) n-octane, (4) p-xylene, (5) n-nonane, (6)n-decane, (7) n-butylbenzene, (8) n-undecane, (9) n-dodecane, (10)naphthalene, (11) 1-methylnaphthalene.

Figure 6. SGC chromatogram of test solutes. Conditions: 226 cm× 250 µm i.d. fused silica capillary column packed with 15 µm porousODS bonded particles, 130 °C, 260 atm column inlet pressure; otherconditions are the same as in Figure 2. Peak identification: (1)benzene, (2) toluene, (3) n-octane, (4) p-xylene, (5) n-nonane, (6)n-decane, (7) n-butylbenzene, (8) n-undecane, (9) naphthalene, (10)n-dodecane, and (11) 1-methylnaphthalene.

Analytical Chemistry, Vol. 69, No. 13, July 1, 1997 2545

increased the plate number per second. For example, at a linearvelocity of 1.2 cm/s, the 180 cm long column produced ap-proximately 350 plates/s, while the 250 cm column producedapproximately 500 plates/s. However, as shown in Figure 2, theshorter column produced a larger mobile phase linear velocity

when using a specific column inlet pressure, which favors highervalues of plates per second. Therefore, at a specific column inletpressure, a higher plate number per second can be obtained byusing shorter columns. For example, at a column inlet pressureof 220 atm, 732 plates/s were obtained using a 180 cm length

Table 3. Relationship between Plate Number per Second (Nt, plates s-1) and Mobile Phase Linear Velocity (u, cms-1) in SGCa

column 1 column 2 column 3 column 4

u Nt u Nt u Nt u Nt

1.05 187 0.86 180 0.66 131 3.93 7361.20 289 0.95 297 0.72 156 4.45 8131.36 426 1.04 305 0.77 271 4.85 6931.45 525 1.11 421 0.82 340 5.26 6281.55 598 1.19 483 0.86 401 5.67 6311.67 732 1.26 633 0.90 504 5.94 7021.76 780 1.33 579 0.96 464 6.11 6641.82 525 1.39 669 6.38 7381.92 563

a Conditions and identification of columns are the same as in Table 2.

Figure 7. Open tubular column GC, packed capillary SGC, and packed capillary HPGC chromatograms of a gasoline sample. Conditions:temperature programming from 40 °C (4 min) to 300 °C at 2.5 °C min-1. (A) 22 m × 250 µm i.d. fused silica capillary column coated with 0.25µm SE-54 stationary phase, He carrier gas. (B) 180 cm × 250 µm i.d. fused silica capillary column packed with 10 µm porous ODS bondedparticles, CO2 mobile phase, pressure program from 110 to 150 atm at 0.5 atm min-1. (C) He carrier gas, 150 atm column inlet pressure. Otherconditions are the same as in Figure 1.

2546 Analytical Chemistry, Vol. 69, No. 13, July 1, 1997

column, while only 340 plates/s were obtained using 336 cmcolumn.

It was found that, by increasing the particle size from 10 to 15µm, the plate number per second was improved under SGC

Figure 8. Open tubular column GC and packed capillary SGC chromatograms of a diesel sample. Conditions: Temperature program from 60°C to 300 °C at 2.5 °C min-1. (A) 22 m × 250 µm i.d. fused silica capillary column coated with 0.25 µm SE-54 stationary phase, He carrier gas.(B) 226 cm × 250 µm i.d. fused silica capillary column packed with 15 µm porous ODS bonded particles, CO2 mobile phase, column inletpressure program from 160 to 200 atm at 0.5 atm min-1. Other conditions are the same as in Figure 1.

Table 4. Relationship between Plate Height (H, µm) and Mobile Phase Linear Velocity (u, cm s-1) in SGCa

column 1 column 2 column 3 column 4

u H u H u H u H

1.05 26.2 0.86 24.0 0.66 27.0 3.93 25.71.20 20.8 0.95 17.1 0.72 22.8 4.45 29.81.36 17.0 1.04 20.0 0.77 17.7 4.85 41.11.45 16.0 1.11 16.5 0.82 15.7 5.26 52.61.55 16.0 1.19 14.0 0.86 14.8 5.67 57.81.67 14.8 1.26 13.5 0.90 12.7 5.94 58.81.76 15.2 1.33 16.1 0.96 14.9 6.11 66.51.82 22.0 1.39 14.4 6.38 64.01.92 25.4

a Conditions and identification of columns are the same as in Table 2.

Analytical Chemistry, Vol. 69, No. 13, July 1, 1997 2547

conditions. For a 228 cm column packed with 15 µm ODS bondedparticles, values of 700-800 plates/s were obtained at high mobilephase linear velocities, and a maximum value of 813 plates/s wasobtained using this column.

Although relatively high values of plates per second wereobtained using columns packed with porous (300 Å) particles,such columns have not produced more than 1000 plates/s. Usingnonporous particles (3 µm nonporous ODS bonded particles) aspacking materials, the value of Nt was increased to 1200 plates/s.11

Table 4 shows the relationship between H and u for variouscolumns under SGC conditions. All three 10 µm particle packedcolumns produced plate heights of less than 15 µm. A minimumvalue of 12.7 µm was obtained by using the 336 cm column. Thisis the lowest value reported in packed column GC. Higher plateheights (e.g, 25.7 µm) were obtained using a 15 µm particlepacked column than from a column packed with 10 µm particles.Therefore, reducing the particle size favors smaller plate heightsin SGC.

Table 5 shows the relationship between h and u for variouscolumns under SGC conditions. Minimum reduced plate heightsof 1.48, 1.35, and 1.27 were obtained for 180, 250, and 336 cmcolumns containing 10 µm particles. Since all columns containing10 µm particles produced minimum reduced plate heights of lessthan 1.5, this suggests that the CO2 slurry packing method issuitable to prepare repeatable and highly efficient capillaries forSGC. The minimum reduced plate height of 1.27 obtained usinga 336 cm × 250 µm i.d. column containing 10 µm porous ODSparticles with a mobile phase linear velocity of 0.90 cm/s is thelowest reduced plate height reported in packed column chroma-tography for a column inner diameter to particle diameter ratio>10.

Theoretical considerations pointed out that a well-packedcolumn can produce a minimum reduced plate height of 2, andthis value decreases when the ratio of column inner diameter toparticle size is less than 8.16 Very narrow bore (25-50 µm i.d)packed capillaries have been prepared, and a reduced plate heightof ca. ∼0.8 has been obtained in LC when this ratio approached8.17,18 However, in our experiments, a minimum reduced plateheight of 1.27 was obtained, even though the ratio of column innerdiameter to particle diameter was 25.

It is should be mentioned that the data in Tables 4 and 5 cannotbe used to quantitatively analyze the sources of peak broadeningusing van Deemter or Knox equations. The reason for this isthat every data point has a different retention factor under SGCconditions.

Separations of Complex Samples Using Packed CapillarySGC. The separation of authentic, complex samples can be achallenge to any chromatographic technique. In GC, long opentubular columns are primarily used for such problems. However,the low sample capacities of open tubular columns are a disad-vantage. Packed columns, including packed capillary columns,can provide much larger sample capacities than open tubularcolumns and would be desirable for many applications if they couldprovide similar resolution as open tubular columns.

In chromatography, the separation can be performed usingtemperature, pressure (or density), and mobile phase compositionprogramming. Temperature programming is primarily used inGC, pressure (or density) programming is widely used in SFC,and composition programming is mostly used in LC. In SGC, allof these methods can be used. A composition gradient can beused to adjust the solute retention factors. However, gascomposition gradients are experimentally difficult to generateusing readily available chromatographic instrumentation. Cur-rently, temperature and pressure (or density) programming canbe easily accomplished. Packed capillaries are more effective thanlarger, conventional packed columns for temperature program-ming because of their better heat transfer characteristics.

For typical open tubular column GC (∼20 m columns), thecolumn efficiency is usually about 100 000 plates/column. Thiscolumn efficiency is similar to efficiencies produced using 2 mpacked capillary columns containing 10 and 15 µm particles. Inthis study, a commercially available 22 m × 250 µm i.d. fusedsilica capillary column coated with 0.25 µm SE-54 stationary phase(Supelco) was used to carry out separations of gasoline and dieselsamples, and the results were compared with those obtained byusing packed capillary SGC.

In these experiments, it was found that, when using the sametemperature program, a longer analysis time was needed for SGCat a specific column inlet pressure. By increasing the columninlet pressure, the analysis time was decreased; however, theseparation of lighter components in the samples became worse.In this study, pressure programming was used together withtemperature programming. Figure 7 shows separations of agasoline sample using open tubular column GC (22 m column)and packed capillary SGC (1.8 m column). In approximately the

(16) Knox, J. H.; Parcher, J. F. Anal. Chem. 1969, 41, 1599-1606.(17) Kennedy, R. T.; Jorgenson, J. W. Anal. Chem. 1989, 61, 1128-1135.(18) Cole, L. J.; Schultz, N. M.; Kennedy, R. T. J. Microcolumn Sep. 1993, 5,

433-439.

Table 5. Relationship between Reduced Plate Height (h) and Mobile Phase Linear Velocity (u, cm s-1) in SGCa

column 1 column 2 column 3 column 4

u h u h u h u h

1.05 2.62 0.86 2.40 0.66 2.70 3.93 1.701.20 2.08 0.95 1.71 0.72 2.28 4.45 1.971.36 1.70 1.04 2.00 0.77 1.77 4.85 2.741.45 1.60 1.11 1.65 0.82 1.57 5.26 3.511.55 1.60 1.19 1.40 0.86 1.48 5.67 3.861.67 1.48 1.26 1.35 0.90 1.27 5.94 3.921.76 1.52 1.33 1.61 0.96 1.49 6.11 4.441.82 2.20 1.39 1.44 6.38 4.291.92 2.54

a Conditions and identification of columns are the same as in Table 2.

2548 Analytical Chemistry, Vol. 69, No. 13, July 1, 1997

same analysis time, packed capillary SGC provided better resolu-tion of sample components, especially for the lighter componentsin the sample. This resulted from either the higher retention onthe packed capillary or the different selectivities of the stationaryphases. Figure 8 shows separations of a diesel fuel sample usingopen tubular column GC and packed capillary SGC. Largerparticles (15 µm) resulted in lower packed column efficiencycompared to the open tubular column; however, smaller particlesresulted in increased retention of the higher molecular weightcomponents in the sample and longer analysis time. Further workis under way to optimize the conditions for both speed andefficiency.

Since separations similar to those possible with open tubularcolumn GC can be obtained using SGC, this technology has

potential in practical use for the separation of complex samples.A previous study showed that SGC could provide high columnefficiency per unit time (1200 plates/s) when using small particles,and was suitable for high speed separations.11 The study of specialstationary phases, including inert and selective packing materials,is important for further developments in SGC.

Received for review January 2, 1997. Accepted March 31,1997.X

AC970011J

X Abstract published in Advance ACS Abstracts, May 15, 1997.

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