Poly(butylene terephthalate) based novel stationary phase (SP), composed of planar aromatic phenyl group together with ester group monomer units, was designed for supercritical fluid chromatography (SFC) use. As expected from its structure, this phase shows planarity recognition of isomeric aromatics and closely similar compounds.
Trang 1jou rn al h om ep a g e : w w w e l s e v i e r c o m / l o c a t e / c h r o m a
DAICEL Corporation, CPI Company, Life Science Development Center, Innovation Park, 1239, Shinzaike, Aboshi-ku, Himeji, Hyogo, 671-1283, Japan
Article history:
Received 26 January 2018
Received in revised form 13 March 2018
Accepted 15 March 2018
Available online 17 March 2018
Keywords:
Supercritical fluid chromatography
Stationary phase
Polymer
Ligand
Poly(butylene terephthalate), PBT
Selector
a b s t r a c t Poly(butyleneterephthalate)basednovelstationaryphase(SP),composedofplanararomaticphenyl grouptogetherwithestergroupmonomerunits,wasdesignedforsupercriticalfluidchromatography (SFC)use.Asexpectedfromitsstructure,thisphaseshowsplanarityrecognitionofisomericaromatics andcloselysimilarcompounds.Interestingly,formostanalytes,theretentionbehaviorofthisSPis sig-nificantlydistinctfromthatofthe2-ethylpyridinebasedSPswhichisamongthemostwell-knownSFC dedicatedphases.Althoughthepoly(butyleneterephthalate)iscoatedonsilicagel,theperformanceof thecolumndidnotchangebyusingextendedrangemodifierssuchasTHF,dichloromethaneorethyl acetateandcolumnrobustnesswasconfirmedbycycledurabilitytesting
©2018TheAuthors.PublishedbyElsevierB.V.ThisisanopenaccessarticleundertheCCBYlicense
(http://creativecommons.org/licenses/by/4.0/)
1 Introduction
Supercriticalfluidchromatography(SFC)isimplementedtoday
asroutinetechniqueinmanylaboratoriesandarisesstrong
sci-entific and practical interest [1 SFC uses pressurized carbon
dioxide(CO2)withmisciblepolarorganicsolvents(modifier)as
amobilephaseandhasbecomeapowerfulseparationtechnique
complementary to conventional highperformance liquid
chro-matography(HPLC)andgaschromatography(GC)[2–10].Inthe
earlydevelopmentsofthetechnique,SFCwasstronglydrivenby
theenantioseparationfieldbenefittingfromthealreadyexisting
chiralstationaryphases(CSPs),particularlyatpreparativescalein
pharmaceuticalindustry[11–18].Recently,SFCexpandedalsoin
theachiralseparationfield,usingachiralphases,butevenapplying
theCSPsaspowerfultoolsinseparationofcloselyrelatedsample
impuritiesormolecules[19,20]
ThemobilephaseinSFChaslowviscosityandhighdiffusivity,
whichmakesitparticularlyadaptedforfastflowanalysis
Further-more,SFCisregardedasanenvironmentallyfriendlyseparation
technique because it uses nontoxic recycled CO2 and the total
amountoforganicsolventsissmallerthaninconventionalHPLC
Thishighthroughput chromatographicperformance, as wellas
∗ Corresponding author.
E-mail address: tr shibata@jp.daicel.com (T Shibata).
“green”aspect,makeSFCveryattractivefornumerousapplications [21–31]
The retention and separation characteristics in SFC mainly dependona combinationbetweenmobilephaseandstationary phase(SP)[6,32–34]Thechemicaldiversityofthecurrently avail-ableSPshasbeensignificantlyextended,benefitingfromthelarge varietyofcommerciallyavailableHPLCSPs(e.g.reversephase, nor-malphase,and/orHILIC)thatcanbealsousedinSFCmode.Besides thistrend,somecolumnmanufacturersandresearchgroupshave originallydevelopedSFCdedicatedstationaryphases.One well-knownSPdesigned specificallyfor achiralSFCseparationis the 2-ethylpyridine(2-EP)bondedsilicaphase.This2-EPSPoffersgood peak shapes,especially forbasic compounds,even withoutany additives[35]
OthernovelachiralSPsdedicatedtoSFChavebeendeveloped [36–40], however,most of them consistedof a low-molecular-weightligand,coatedorcovalentlyattachedontoasolidsupport (e.g.silicagel).Incontrast,onlyfewpolymerictypephaseshave beendescribedsofarforapplicationsintheachiralSFC separa-tionfield.Suchpolymericphasesareexpectedtointeractthrough multipleconcertedmechanismswiththeanalytes[40]
Anotherviewpointiswhatkindofmajorinteractionshouldbe embeddedinaSP.WhileSPswithavarietyofinteractiontypesare neededofcourse,whatarethoserelativelyunmet?Thedesignof newphasesmayhavetostartbydefiningwhicharethe interac-tiontypesneededandcombinedtomakeanefficientSP,butalso whichinteractionmechanismsarerelativelyunmetinthealready
https://doi.org/10.1016/j.chroma.2018.03.032
0021-9673/© 2018 The Authors Published by Elsevier B.V This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ ).
Trang 2Fig 1.Structure of poly(butylene terephthalate) or PBT-based selector of the
col-umn DCpak PBT.
existingSPs.In thislight,thesystematicanalysisofinteractions
inavarietyofcommerciallyavailableSPsunderSFCconditions,as
investigatedbyWestetal.,wouldrepresentaverysuggestive
infor-mation[41–43].Accordingtotheirdiagrams,hydrogen-bonding
andvanderWaalsinteractionsaremoredenselycoveredbyalready
existingphases,whereasdipolarandaromatic()interactionsare
lessrepresented[43]
Basedontheaboveconsideration,whenlookingforinnovative
phases,wefocusedonapolymerwithadipolargroupwith/without
anaromaticgroupinorclosetoitsmainchain.Amongmany
poly-mersevaluated,poly(butyleneterephthalate),wellknownasPBT,
waschosenasanovelselectorconsideringitsmolecular
recogni-tionabilityandpeakefficiency[44].Itsinsolubilityinmostsolvents
andremarkablechemicalstabilitywerealsopositivemerits
sup-portingtheselectorchoiceforcolumncommerciallaunch(Fig.1)
Inthepresentstudy,somefeaturesandapplicationsofthisSP
underSFCconditionsaredescribed
2 Experimental
2.1 Chemicals
ThemodifierusedinthisstudywasJapaneseIndustrialStandard
specialgrademethanol(MeOH)obtainedfromNacalaiTesqueInc
(Kyoto,Japan).Carbondioxideofindustrialgrade(over99.5%)was
purchasedfromTatsumiIndustryCo.,Ltd.(Hyogo,Japan)
o-Terphenyl,m-terphenyl,p-terphenyl,3,4-dihydrocoumarin,
coumarin, 6-methylcoumarin, 7-methylcoumarin, dimethyl
phthalate,dimethylisophthalate, dimethylterephthalate,
triph-enylene, cis-stilbene, trans-stilbene, 2-methylbenzophenone,
3-methylbenzophenone, 4-methylbenzophenone, 2
-hydroxyflavanone, 3-hydroxyflavanone, 4-hydroxyflavanone,
theobromine, phenanthrene, and pyrene were
pur-chased from Tokyo Chemical Industry Co (Tokyo,
Japan) 1,3,5-Tri-tert-butylbenzene, 2-acetylanthracene,
9-acetylanthracene, 2-acetylphenanthrene, 3-acetylphenanthrene,
9-acetylphenanthrene, and paraxanthine were purchased from
Sigma-AldrichCorporation(St.Louis,MO,USA).Naphthacenewas
purchased fromNacalaiTesque Inc 2-Propanol(IPA),chrysene,
estrone,estradiol,estriol,caffeine,theophylline,tetrahydrofuran
(THF),dichloromethane,andethylacetatewerepurchasedfrom
WakoPureChemicalIndustries(Osaka,Japan).n-Hexane(nHex)
waspurchasedfromKantoChemicalCo.(Tokyo,Japan)
2.2 Instrumentationandchromatographicconditions
DCpakPBTcolumn(initiallylaunchedasDCpakSFC-A),sized
150mm×4.6mm (i.d.), was supplied from DAICELCorporation
(Tokyo,Japan),whichiscomposedofPBT-coated5msilica
par-ticle A 2-ethylpyridine (2-EP) column of 5m particle, sized
150mm×4.6mm(i.d.),waspurchasedfromWatersCorporation
(Milford,MA,USA).TheSFCinstrumentusedinthisstudyis
Nexera-UC supplied by Shimadzu Corporation(Kyoto, Japan) equipped
withaCO2 pump, amodifier pump,a vacuumdegasser,a
col-umnoven,amultiplewavelengthUVdetector,andautomatedback
software(V5.89)wasusedforsystemcontrolanddataacquisition Totalflowratewasfixedat3.0mL/min,columntemperature wassetat40◦C,andtheautomatedbackpressureregulator(ABPR) wassetto15.0MPa,unlessotherwisenoted.Otherconditions,such
asmodifier,sampleconcentration,injectionvolume,anddetection wavelengtharedescribedinthefigures
TheTharSFCinstrumentsuppliedbyWatersCorporationwas usedforSection3.3dealingwiththeorthogonalselectivityofthe twocolumnsandSection3.4dealingwiththemodifiereffect
2.3 Dataanalysis
Relativeretentionfactor(k)wascalculatedwiththeequation below
k= (V/V0)−1, (1) where V is the elution volume of an analyte and V0 is the column void volume V0 was estimated by injecting 1,3,5-tri-tert-butylbenzene as a non-retained marker conducted as an independentanalysisofeachsampleinjection
2.4 Dipolemomentcalculations Dipole moment calculations were conducted using semi-empirical molecular orbital methodwith PM6 implemented in SCIGESSsoftware(version2.3,FujitsuLtd.,Tokyo,Japan)[45]
3 Results and discussion
3.1 Planarityrecognition Basedonitsstructuralfeatures,thenewPBTselectorcomposed
ofnon-polararomaticphenylgrouptogetherwithestergroupunits wasexpectedtointeractwitharomaticcompounds.Inorderto con-firmthispoint,terphenylisomers(1–3)areinvestigated,whichare regardedasmolecularplanarityindicatorsinHPLC[46,47]andSFC [48].Compound1deviatesfromplanarityduetothestrongsteric repulsionoftwophenylringslocatedinortho-position,andthis sterichindrancediminishesfor2and3(inthissequentialorder)
Fig.2showstheSFCchromatogramsof1–3byusingthePBT-based column(Fig.2A),comparedtothe2-EPSP(Fig.2B)underisocratic conditions.Thestrongertheplanaritycharacteroftheanalyte,the longerretentionwasobserved onthenewcolumn Incontrast,
noresolutionbetween2and3wasachievedonthe2-EPcolumn (Fig.2B).Thisplanarityrecognitionmaybeattributedtoplanarand rigidPBTbackbone
3.2 Molecularshaperecognition
Fig.3showsthechromatogramsofcoumarin(5), itsdihydro form(4),andmethylsubstitutedform(6and7).Compound4eluted fasterthan5,probablybecausedihydro4hasless-electronsthan
5,resultinginaweakerinteractionbetweenanalyteandSP.The PBT-derivedselectorcanrecognizetheminordifferenceofmethyl groupposition(6and7),whereasonthe2-EPcolumn,coumarin5
anditsmethylsubstitutedoneselutedalmostatthesametime
Fig.4AandBshowstheSFCchromatogramsofthree plasticiz-ers,dimethylphthalate(8),dimethylisophthalate(9),anddimethyl terephthalate(10)onbothcolumns.ByusingtheDCpakcolumn,
8elutedfirst,followedby9and10(Fig.4A).Dipole momentof
8, 9,and10is2.98,1.60,and0.01,respectively,which is calcu-latedbyusingSCIGESSsoftware.Thus,thesmallerpolarizationof thesample,thelongertheretentiontimetendstobe.Onthe
2-EPcolumn,theelutionorderistotallyinverse(i.e.10elutedfirst,
Trang 3Fig 2.SFC chromatograms of terphenyl isomers on (A) DCpak PBT and (B) 2-EP SPs.
Modifier, MeOH (isocratic conditions, 3%); temperature, 40◦C; ABPR, 15 MPa; flow
rate, 3.0 mL/min; UV detection, 254 nm.
followedby9and8,seeFig.4B).Therelationshipbetweendipole
momentsandretentionfactorsbyusingtwocolumnsaredisplayed
inSupplementalMaterial(TableS1)
Estron(11),estradiol(12),andestriol(13)arenaturalestrogenic
hormones,whichhavealmostsameskeletonwithdifferentnumber
ofhydroxylgroups.Compound11hasonehydroxylgroup,12has
two,and13hasthree.InteractingwiththePBTselector,polar13
elutedfirst,followedby12and11(Fig.4C).Incontrast,byusing
2-EP,less-polar11elutedfirst,thenfollowedby12and13(Fig.4D)
ThisresultindicatesthatDCpakPBTcanstronglyretainlesspolar
samples,whereasitwillshowlessretentionformorepolarsamples
(oppositetothe2-EPobservations).Theresultsdisplayedinthis
sectionsuggeststhehighorthogonalityofthePBTandthe2-EP
derivedSPs
Inordertodiscussthecharacteristicmolecularshape
recogni-tionbehaviorofthenewcolumn,wethenanalyzednaphthacene
(14),chrysene(15),andtriphenylene(16)underisocratic
condi-tions.TheseC2orC3symmetricpolycyclicaromatichydrocarbons
(PAHs)havethesamenumberofaromaticringsand-electrons
butdifferentmolecularshape,whichareoftenusedasmolecular
shaperecognitionindicators(Fig.5).Wiseetal.proposed
length-to-breadth(L/B)ratiofordescribingtwodimensionalaspectratio
of suchPAHs[49] The smallerL/B ratioindicatesthe disk-like
molecule.Indeed,L/Bratioof14, 15,and16is1.89,1.72,and1.12,
respectively.OnDCpakPBT,disk-like16elutedfirst,followedby
15,and14withalargeaspectratioelutedlastly(Fig.5A).Contrary
tothis,byusing2-EP,14withalargeaspectratioelutedfirst,
fol-lowedby15,and16elutedlastly(Fig.5B).Theretentionfactor(k)of
thesePAHsonthetwocolumnsissummarizedinTableS2in
Sup-plementalMaterial.Theirselectivitytrendsareorthogonalagain.It
isclearlyseenthatthePBTselectortendstoretainthelinearPAH
Fig 3.SFC chromatograms of coumarin derivatives on (A) DCpak PBT and (B) 2-EP SPs Modifier, MeOH (isocratic conditions, 2%); temperature, 40◦C; ABPR, 15 MPa; flow rate, 3.0 mL/min; UV detection, 220 nm.
withlargeaspectratio,and2-EPtendstoretaindisk-likePAHwith smallaspectratio
3.3 Orthogonalityinvestigations
Asdescribedinprevioussections,theseparationbehaviorof twoinvestigatedcolumnswassignificantlydistinct.Therefore,they wereexpectedtodisplaycomplementaryselectivity,i.e orthogo-nalselectivity.Toobtainadeepinsightintoorthogonalityaspects,
wethencomparedtheirretentionfactorsmeasuredunderisocratic conditions byusingcommercially availableneutral and slightly basic23 samples.Thesetest compoundsare classedintoseven differentisomericorcloselysimilarsamplefamilies.Fig.6shows thedoublelogarithmicplotsofkobtainedbytwocolumns.The detailofsamplesandretentionfactorsaresummarizedinTable S3inSupplementalMaterial.Asexpected,theplotswerewell dis-persed.Indeed,theirPearson’scorrelationcoefficient(R2)was0.62, indicatingthereisnotstrongcorrelationbetweenthem
3.4 Modifiereffect ConsideringthatthePBTselectorwascoatedonsilicagel,one mayfear“columndamage”ofselectorbyusingextendedsolvent choices,suchasTHF,dichloromethane(CH2Cl2),orethylacetate (EtOAc) We then examined the stability of retention by using regioselectiveacetylated anthracene(17, 18)and phenanthrene
(19–21)asanalytes
Trang 4Fig 4.(A, B) SFC chromatograms of plasticizers (phthalates) and (C, D) estrogenic hormones on (A, C) DCpak PBT and (B, D) 2-EP SPs (A, B) Modifier, MeOH (isocratic conditions, 1%); temperature, 40◦C; ABPR, 15 MPa; flow rate, 3.0 mL/min; UV detection, 230 nm (C, D) Modifier, MeOH (isocratic conditions, 30%); temperature, 40◦C; ABPR,
15 MPa; flow rate, 3.0 mL/min; UV detection, 210 nm.
Fig 5. SFC Chromatograms of C 2 or C 3 symmetric polycyclic aromatic hydrocarbons (PAHs) on (A) DCpak PBT and (B) 2-EP SPs Modifier, MeOH (isocratic conditions, 25%); temperature, 40 ◦ C; ABPR, 15 MPa; flow rate, 3.0 mL/min; UV detection, 254 nm.
Fig.7AshowstheSFCchromatogramoffiveanalytesmixture
with10%ofMeOH.Althoughsomepeakswereoverlapped,these
peakselutedwithoutheavypeaktailing.Afteranalysis,the
mod-ifier wasgraduallychanged by gradientprogram The program startedafter1minholdwith5%ofMeOH,lineargradientramped
upto30%ofMeOHover20min,followedby39minholdat30%
Trang 5Fig 6. Double logarithmic plots of retention factor (k) obtained with DCpak PBT
and 2-EP SPs Filled circles (䊉), 2-methylbenzophenone, 3-methylbenzophenone,
and 4-methylbenzophenone; open circles (), cis-stilbene and trans-stilbene;
filled squares (䊏), o-terphenyl, m-terphenyl, p-terphenyl, and triphenylene; filled
triangles (), 2-acetylanthracene, 9-acetylanthracene, 2-acetylphenanthrene,
3-acetylphenanthrene, and 9-acetylphenanthrene; open diamonds (♦), caffeine,
theo-phylline, theobromine, and paraxanthine; filled diamonds (), 2 -hydroxyflavanone,
3-hydroxyflavanone, and 4-hydroxyflavanone; open triangles (), hydrocortisone
and prednisolone Modifier, MeOH (isocratic conditions, 5%); temperature, 40◦C;
ABPR, 15 MPa; flow rate, 4.0 mL/min.
of MeOH Afterthe gradient rinsingprogram wasfinished, the sameacetylated PAHmixturewasinjectedunderisocratic con-dition(CO2/MeOH=90/10).Fig.7BshowsitsSFCchromatogram with10%ofMeOH,andretentiontimeandpeaksymmetrywere essentiallyunchanged.Then,therinsingmodifierwaschangedto THF/MeOH=5/1,thesamegradientcyclewasrepeated,and the sameacetylatedPAH mixturewasevaluated.Itshouldbe men-tionedthatnounidentifiedpeakwasobservedduringthisrinsing process.Fig.7 shows theSFCchromatogramof thesame mix-tureunder10%ofMeOH.Again,unidentifiedpeaksdidnotappear duringthegradientprogram,andthechromatogramwasalmost sameasoriginalone.Thisgradientrinsingprogramwasrepeatedby usingCH2Cl2/MeOH=5/1asmodifier,andthesamesamplemixture wasanalyzed,whose chromatogramwasalmostsame(Fig.7D) Finally,thisprocedurewasrunoverbyEtOAc/EtOH=5/1,andthe samesamplemixturewasanalyzedwithnochangeintheresulting chromatogram(Fig.7E)
Accordingtotheseexperiments, nosignofdegradation was observedduringthegradientprocessorthechromatographic test-ing,whichsuggestthatnocolumndamagewasobservedbypassing throughtheextendedrangemodifiersequence.ThePBTselector doesnotdissolveinmanyorganicsolvents,whichenabledusto usealmostallorganicsolventsthoughitwascoatedonsilicagel
Fig 7.SFC chromatograms of acetylated anthracene(17, 18)and phenanthrene(19–21)on DCpak PBT SP by passing through various modifiers Chromatographic conditions
◦
Trang 6Fig 8. (A) Cycle dependent SFC chromatograms and (B) cycle versus retention factors of PAHs on DCpak PBT column Modifier; MeOH (isocratic conditions, 25%); temperature,
40◦C; ABPR, 15 MPa; total flow rate, 3 mL/min; UV detection, 230 nm; sample concentration, 0.2 mg/mL in nHex/THF = 9/1; injection volume,1 L.
3.5 Cycledurabilitytest
Toconfirmthecycledurability,threePAHs,phenanthrene(22),
pyrene(23),andtriphenylene(16)wereusedasanalytes,under
isocraticconditions.Thesameexperimentwasrunover80cycles
Thistestwasnotperformedwiththeaimofcoveringmethod
vali-dationparameters,buttoconfirmthelackofsolubilityofthecoated
selector.Anyselectorlossunderoperatingconditionswouldlead
tochangeswiththeappliedprotocol
Fig.8A showstheSFC chromatogramofthefirstinjectionof
threePAHsonthecolumn,wherethethreepeakswerewell
sep-arated.Thechromatogramsafter20,40,60,and80cyclesarealso
showninFig.8A,andtheretentiontimeofthesesamplesnever
changeduntilthe80cycles
Fig.8Bshowscycleversustheirretentionfactors(k)and
con-firmsthecolumnstabilityincycledurabilitytestattributedbythe
macromolecularligand
4 Conclusions
AnovelPBTbasedstationaryphasewasdesignedandconfirmed
asversatiletoolforSFCuse.ThisSPshowscharacteristicplanarity
recognitionofisomericPAHsandstructurallyrelatedanalytessuch
ascoumarinderivatives,phthalateplasticizers,andestrogenic
hor-mones For mostcompounds, theretention behavior of this SP wasfoundtobesignificantlydistinctfromthatofthe2-EPbased
SP,whichindicatetheorthogonalretentionrelationshipbetween them.Itsstability ofthePBT selectortowardsanextended sol-ventrangeandthecycledurabilityofthecolumnintheoperating conditionswerealsoconfirmed
Theseresultsdemonstratedthatsyntheticpolymersmightbe promisingcandidatesas selectorsfor achiralseparationsin SFC mode Indeed, this column was used as SFC column screening formethoddevelopmentinthepharmaceuticalindustry(seeFig 10.8inRef.[1]).Furtherinvestigationsdealingwithapplications
ofthisnewSPindifferentchromatographicmodesarecurrently
inprogress.Thedesignandscreeningofothersyntheticpolymer basedligandsarealsointhescopeofourresearchteam
Acknowledgments
Theauthors wish tothank Dr.Pilar Franco and Tong Zhang
inChiralTechnologiesEuropeS.A.S.forvaluablediscussions.The authorsalsoappreciateDr.MasashiIwayamainDAICEL Corpora-tionfordipolemomentcalculations
Thisresearchdidnotreceiveanyspecificgrantfromfunding agenciesinthepublic,commercial,ornot-for-profitsectors
Trang 7Appendix A Supplementary data
Supplementarydataassociatedwiththisarticlecanbefound,
intheonlineversion,athttps://doi.org/10.1016/j.chroma.2018.03
032
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