Capillary Super Critical Fluid

8/6/2019 Capillary Super Critical Fluid

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the Hildehrand solubility theory (9),although such predictions are first ap-proximations at best. The point ofpractical interest is that while ordi-nary gases can he compressed to pro-duce these effects (9),imilar condi-tions can he met with some otherfluids at pressures and temperatureswell within the limits of ordinaryHPLC technology. Some examples areshown in Table I. Attention will he fo-cused on fluids with low critical tem-peratures, since they will he useful forseparating thermally labile com-pounds.Optimum conditions for capillarysupercritical fluid chromatographywill now he briefly considered. If weintroduce the initial restriction of amaximum allowable pressure drop,P,.,, between the inlet and outlet of acapillary column of radius r , the maxi-mum column length, L is given bythe integrated Poiseuille equation:

where 7 is the mobile phase viscosityand F is the average linear velocity ofthe fluid. The number of theoreticalplates, N, or such a column is:

Since both L and H (plate height) arefunctiuns of i , the maximum efficien-cy, Nma,, an easily he found (3).However, the maximum number ofplates applies here to conditions of in-finite length and zero flow. f, instead,the column is operated under condi-tions of optimum efficiency per unitlength, more realistic efficiencies mayhe calculated.The minimum plate height, H,,,,can he derived from the Colay equa-tion, assuming that resistance to masstransfer in the stationary phase is neg-ligible:Hm,"= 0.577 r (3)

This cundition occurs at:

whereD , is the diffusion coefficientof a solute in the mobile phase. Sub-

stituting equations ( l ) ,3), and (4)into equation (2 ) yields the maximumnumber of plates that can he generatedfrom a column of fixed length:

This equation is similar to that de-rived earlier hy Giddings (3) for thelimiting number of plates used tocompare the relative merits of gas andliquid chromatography. The differ-ence is that equation (5) assumesii =popt,whereas Giddings's expressionholds only for L = m and ii= 0. For agiven pressure drop, operation a t ii =popt ather than L = m and = 0 de-creases the maximum efficiency by afactor of two.While the above considerations areonly valid for a nonretained chromato-graphic peak (capacity ratio, k = O),some interesting observations can hemade. Estimated efficiencies per uni ttime are quite impressive sinceD ,and q are more favorable to a rapidchromatographic separation in a su-percritical fluid than in a liquid mo-bile phase. Whereas literature data onboth these physical quantities for su-percritical fluids are rare, only ap-proximate calculations can he made atthis time. The diffusion coefficient ofn-pentane in carbon dioxide a t 48 atmand 40 "C was estimated to he 1.5 Xcmz s-l (17), hile a viscosityvalue of 3.59 X g cm-1 s--1 at 90atm and 40 "C is available from an-other literature source ( 1 8 ) .Actualdiffusion coefficients may he as muchas one or two orders of magnitudesmaller for large solutes at supercriti-cal pressures, hut no one seems tohave measured such quantities asyet.If we arbitrarily choose a pressuregradient restriction of 1atm, a maxi-mum of 8.1 X lo6 heoretical platescould be generated in 18h for a nonre-taiced component with a 0.24 mm i.d.capillary column (length, 570 m).Through reduction of the column di-ameter to 100pm, 1.4 X 106 theoreti-cal plates could be obtained in just0.54 h (length, 41 m).Practical realization of optimizedcapillary supercritical fluid chroma-tography is dependent on at leastthree crucial criteria:

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The system must he designed sothe entire column and the detectorboth operate at the same high pres-sure:instrumental contributions tohand broadening must he minimal,and* stationary phase films of opti-mum thickness must he permanentlybonded to the capillary wall so the re-sistance to mass transfer in the sta-tionary phase is small, and so thephase cannot he stripped off by thepassing fluid.Development of capillary supercriti-cal fluid chromatography has recentlybecome feasible. Designs of microco-lnmn HPLC equipment (19-22) havemade certain instrumental compo-nents more readily adaptable to capil-lary supercritical fluid chromatogra-phy. Among them, a low-volume loopinjector and a miniaturized spectro-scopic cell are particularly useful. Anumber of fluids are available, such asn-pentane, isopropanol, or carbondioxide, that allow the migration ofrelatively heavy molecules under rea-sonable pressures and temperatures.These mobile phases, combined withthe small-volume requirements of cap-illary techniques, make safety hazardsalmost negligible. Glass capillary col-umns with wall thicknesses similar tothose used in conventional capillarygas chromatography can be easilyused. Advantage can also he taken ofrecently developed bonded phases(22).The capillary supercritical fluidchromatograph operating in our laho-ratory possesses the basic features il-lustra ted in Figure l.A high-pressuresyringe pump operating a t rm m tem-perature delivers, at constant pres-sure, a liquid (n-pentane) or com-pressed gas (carbon dioxide) that isconverted into a supercritical fluid ina preheating coil. The fluid suhse-quently sweeps through a loop valvewhere the sample is introduced direct-ly into the glass capillary column. Thecolumn is heated to temperatures con-sistent with the desired fluid condi-tions in a thermostat. The thermostatalso contains a flow-through cell (ei-ther a UV or spectrofluorimetric de-tector) used to monitor chromato-graphic effluents under high pressure.A length of 50-pm glass capillary afterthe detection cell is used to maintainsystem pressure. Solutes can also hedetected directly in the last section ofthe column. As this comprises the op-tical compartment, no hand spreadingoccurs due to interconnection dead-volume. Increasing the pressure is themost convenient means to influencesolute retention (15-27):herefore,pressure programming will ultimatelyhe used for resolution of complex mix-tures as shown in Figure 1.

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Flgure 1. Block diagram showing the components of a capillary supercritical fluidchromatographWe wish to report some preliminaryresults on capillary supercritical fluidchromatography, using n-pentane asthe mobile phase a t 210 "C and 32atm. The glass capillary columns usedwere fabricated with phenylmethyl

polysiloxane film bonded to the capil-lary wall similar to he procedure ofBlomberg and Wh nm an (22).As acompromise with currently availablesampling conditions, 0.2 and 0.3 mmi.d. glass capillary columns were used.Th e plate height vs. linear velocitycurves were measured for the fluo-rimetrically detected pyrene peak (A,= 335 nm; A = 425 nm, 10-nmbandwidth). Figure 2 shows the curvesobtained for the two column diame-ters. While pyrene is only slightly re-tained in the phase system used, theplate height values are larger thantheory predicts. Reasons for this dis-crepancy are not currently obvious;

while it would he useful to have moreaccurate values for the diffusion coef-ficient, D,, under the conditions used,we believe that the current design ofthe injection device is the largest con-tributor to the plate height increase.Efforts are being made at present toeliminate such problems. The directinsertion of the capillary tube into thespectral measurement cell reducessimilar difficulties at t he outlet.A model mixture of polycyclic aro-matic hydrocarbons is shown in Figure3. A 58 m X 0.2 mm i.d. glass capillarycolumn was used to separate this mix-ture under isobaric conditions withn-pentane as the carrier fluid. Whilethis result is only a preliminary one[capillary gas chromatography canachieve better separation of such com-pounds (23)],capillary supercriticalfluid chromatography clearly meritsfurther exploration.

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lure 2. Plot of HETP vs. linear velocity for pyrene on a 0.30 mm 1.d. (blue) and1.20 mm i.d. (green) column.The mb il e phase was Rpentane at 210 "C and 32nThe high efficiency demonstratedove with capillary supercritical fluidromatography is a goodreason forvsuing this research. Other facts ofiportance also appear. For separa-Ins where both selectivity and highlumn efficiency are needed (e.g., res-ition of certain isomeric sub-mces), it may be of advantage thate interaction of solute and mobilew e molecules ean he sensitively ad-ited by pressure. In addition, whileadient elution techniques are needed"program" solute retention in liq-

uid chromatography, pressure pro-gramming in supercritical fluid chro-matography can he used to meet simi-lar goals.As the former programmingtechnique is incompatible with certaindetection systems, fewer problems areanticipated with supercritical fluidsunder increasing pressure. For exam-ple, one can envisiona complex mix-ture of nonvolatile substances in su-percritical carbon disulfide being sep-arated hy pressure programming,while the resolved components are in-troduced into a high pressure cell ofan infrared detector.Some attention has already beengiven to the coupling of supercriticalfluid chromatography with maas spec-trometry through a je t molecular sepa-rator (24) . While the technologicalproblems that exist here are formi-dable, they appear no more seriousthan with the now extensively studiedLCIMS.As research in capillary supercriti-cal fluid chromatography proceeds,different f luids and pressure regionscan he explored to achieve chromato-graphic migration of various nonvola-tile and unstable molecules. As point-ed out hy Giddings and co-workers(9), he Hildebrand solubility parame-ters can be employed to predict ap-proximate suitability of mobile phaseafor various molecular separations. Asour capability of performing precisemeasurements in supercritical fluidchromatography gradually improves,meaningful measurements of solut*solvent interactions in the vicinity ofthe critical point and beyond will a bbecome available.References

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Time (min)5 3 0 4 5 1 'Dum3. isobaric run of several PAHandards in a 0.20 mm 1.d. X 58mlpillary column. Conditions were as njure 2. The standards were: 1) an-racene. 2) pyrene, 3)benzo[k]fluo-nthene, 4)benzo[e]pyrene, 5) lbenz-tbclanthracene*6, benzo[ghilpe~le, and 7 )caonene 92-32,(1 ) Golay,M. . E.A w l . Chem. 1957,29,

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(2) Giddings, J. C. Dynamics ofChroma-tography; Marcel Dekker: New York,1QfiS(3)Giddings, J.C. Anal. Chem. 1964.36.1890-92.(4)Tsuda, T.; Nakagawa,G. J . Chroma-togr. 1980,199, 49-58.(5)Knox,J. H.; Gilbert. M.T.J. Chroma-tog?. 1979.186. 405-18.(6) Klesper, E.;Corwin, A. H.;Turner, D.A. J . Ore. Chem. 1962,27,7WI.(7) Myers. M. N.; Giddings, J. C. Separa-t ion Sci. 1966. I, 761-76,(8) McLaren.L.;Myers. M. N.;Giddings,J . C . Srimee 1968,159, 197-9.(9)Giddings. J. C.;Myers, M. N.; MeLar-en, I..: Keller, R . A. Science 1968. 162,67-73.(10) Sie, S.T.: van Beersum, W.; Rijnders,G. W. . SeparationSei. 1966, . 459-w l(IyYSie, S. T.; Rijnders,G. W. . Separa-

(12)Sie, S.T.; ijnders. G. W. A. Separa-(13)Sie. S.T.; leumen.J. P.A,; Rijnders.

tion Sei. 1967.2, 729-53.t ion Sei. 1967,2, 55-77.

G. W. A. In Gas Chroma raphy1968; Harbourn, C. L.A.%d.; Instituteof Petroleum: London, 1969: p 235.(14)Klesper. E.Angew. Ckem . n t . Ed .Engl. 1978.17.738.16.(15)Novotny, M.; Rertseh,W.; Zlatkis, A.J . Ckromatogr. 1971,61, 17-28.(16) Gouw.T. H.; Jentoft, R. E.J . Chro-matogr. 1972.68, 303-23.(17) Sie. S.T.:Riinders.G. W. A. And.Chim.Aeto 1967.3R,31-44.Phy.vieo 1964.30, 161-81.

(18) Kestin. J.; Whitelaw,J. H.;Zien,T. F.119) Ishii.D.: sai. K.: Hibi. K.: onoku-.chi, T.; Nagaya.M. .Ckromatogr. 1977,144, 157-68.togr. 1979,186.521-8.( 2 0 ) Hirata Y.;Novotny, M.J . Chromo-(21) Scott. R.P.W.: Kucera. P. . Chro-motogr. 1979.169.51-72.matogr. 1979.168.81-8.Anal. Ckem. 1976.44 156672.Anal. Chem. 1978.50, 1703-5.

(22)Blomberg. L.;Wannman,T. . Chro-(23) Lee, M. L.; Novotny. M.; Bartle, K. D.(24)Randall, L.G.; Wshrhsftig, A. L.

P.A. Peaden S. H. Springs fon

Paul A. Peaden received his BS inchemistry at Brigham Young Univer-si ty , and ispres ently working on hisPhD in analytical chemistry. His re-search topic is the development of l iq-uid Chromatographic and supercriti-cal fluid chromatographic techniquesfor th e analys is of high molecularweight polycyclic aromatic com-pounds.

Milos No votny is currently professorof chemistry at Indiana Universi ty . Anative of Czechoslovakia, he receivedhis undergraduate edu cation inchem istry and a doctoral degree inbiochemistry at the University ofBrno. Nouotnys research interes ts in-clude capillary gas chrom atograp hy,HPLC, and GC-MS ,as well as bio-medical and enviro nmenta l applica-tions of these methods.Jo hn C . Fjeldsted received a B A i nchemistry at Brigham Young Univer-sity , and is also working toward hisPhD degree in analytical chemistry.His research is concerned w ith t he de-velopmen t of ins trum entatio n and se-lective detection system s applicableto high resolution GC and sup ercriti-cal fluid chromatography.Milton L. Lee received his B A at theUniversity of Uta h. and earned aPh D in analytical chemistry at Indi-ana Universi ty in 1975. He spent oneyear at M IT performing post-doctoralresearch before accepting his pr esen tposition on he faculty at BrighamYoun g U niversity. His research inter-ests includ e the su rface chemistry ofglass and silica as it appl ies to GC,GC -MS . LC , and supercritical fluidchromatography,and th e applicationof these techniques to th e analysis ofcomplex mixtures in environmental ,coal, and coal-derived samples.Stephen R. prin gsta n receiued hisBS degree fr om Virginia PolytechnicInsti tute and State Universi ty ,Blacksburg, Va., in th e field of chem-istry. He is cur rently a doctoral de-gree student a t Indiana Universi ty .His the sis research is concerned withth e fundam ental principles, instru-mentation, and applications of su-percritical fluid chromatography.

414A ANALYTICAL CHEMISTRY, VOL. 53. NO. 3. MARCH 1981

Capillary Super Critical Fluid

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