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Analytica
Chimica
Acta
jo u r n 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 / a c a
Coffee-ring
effects
in
laser
desorption/ionization
mass
spectrometry
Jie-Bi
Hu, Yu-Chie
Chen
∗, Pawel
L.
Urban
∗DepartmentofAppliedChemistry,NationalChiaoTungUniversity,1001UniversityRd,Hsinchu,300,Taiwan
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Coffee rings occur during sample preparationforMALDI-MSand LDI-MS.
Theypartlycontribute tochemical heterogeneityofsampledeposits. Coffeeringsmaybe hiddenwithin
samplespots.
Occurrenceof coffeerings permits partialseparationofsample compo-nents.
In somecases,formation of coffee ringscanbesuppressedduring sam-plepreparation.
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Articlehistory:
Received16October2012
Receivedinrevisedform
14December2012
Accepted21December2012
Available online 3 January 2013 Keywords:
Coffee-ringeffect
Matrix-assistedlaserdesorption/ionization
Massspectrometry
Surface-assistedlaserdesorption/ionization
samplepreparation
laserdesorption/ionization
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Thisreportfocusesontheheterogeneousdistributionofsmallmolecules(e.g.metabolites)withindry depositsofsuspensionsandsolutionsofinorganicandorganiccompoundswithimplicationsfor chem-icalanalysisofsmallmoleculesbylaserdesorption/ionization(LDI)massspectrometry(MS).Taking advantageoftheimagingcapabilitiesofamodernmassspectrometer,wehaveinvestigatedthe occur-renceof“coffeerings”inmatrix-assistedlaserdesorption/ionization(MALDI)andsurface-assistedlaser desorption/ionization(SALDI)samplespots.Itisseenthatthe“coffee-ringeffect”inMALDI/SALDI sam-plescanbebothbeneficialanddisadvantageous.Forexample,formationofthecoffeeringsgivesrise toheterogeneousdistributionofanalytesandmatrices,thuscompromisinganalyticalperformanceand reproducibilityofthemassspectrometricanalysis.Ontheotherhand,thecoffee-ringeffectcanalsobe advantageousbecauseitenablespartialseparationofanalytesfromsomeoftheinterferingmolecules presentinthesample.Wereporta“hiddencoffee-ringeffect”whereundercertainconditionsthe sam-ple/matrixdepositappearsrelativelyhomogeneouswheninspectedbyopticalmicroscopy.Eveninsuch cases,hiddencoffeeringscanstillbefoundbyimplementingtheMALDI-MSimagingtechnique.Wehave alsofoundthattosomeextent,thecoffee-ringeffectcanbesuppressedduringSALDIsamplepreparation. © 2013 Elsevier B.V. All rights reserved.
1. Introduction
Matrix-assisted laser desorption/ionization (MALDI) [1] and surface-assistedlaserdesorption/ionization(SALDI)[2]facilitate detectionofawiderangeofanalytes–frombiomoleculesto syn-theticpolymers–withhighsensitivity[3–5];therefore,theyhave
∗ Correspondingauthors.Tel.:+88635731786;fax:+88635723764.
E-mailaddresses:[email protected](Y.-C.Chen),[email protected]
(P.L.Urban).
become importantionizationtechniques usedin mass spectro-metric (MS) analysis during thepast two decades.It is known thatsamplepreparationprotocolsstronglyinfluencethequality ofMALDIandSALDImassspectra.Ithasbeenwidelydiscussed thatthequalityofMALDI-MSresultsishighlydependentonthe qualityofsample depositsonMALDItargets.Numeroussample preparation methodshave beendeveloped in ordertoimprove homogeneity of crystalline sample deposits. Examples include electrospray-aided sample/matrix deposition [6], application of ionic-liquidmatrices[7–9]aswellassolvent-freemethods[10,11]. Nevertheless,oneofthe“classical”andmostwidelyusedsample
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78 J.-B.Huetal./AnalyticaChimicaActa766 (2013) 77–82
Fig.1. Proposeddirectedself-organizationofparticlesandanalytesbycoffee-ring
effectinanevaporatingdropletonahydrophilicsurface.Redarrow:outward
cap-illaryforce;blackparticle:inorganicmatrix;A:analytespecies.Ideogrambasedon
themodeldescribedbyBhardwajetal.[59].(Forinterpretationofthereferencesto
colorinthisfigurelegend,thereaderisreferredtothewebversionofthearticle.)
preparationmethods istheso-called “dried-droplet method”: a matrixsolutionismixedwiththesampledirectlyonthesurface oftheMS target,which isthen followedby theevaporationof thematrixand samplesolvents underambientconditions [12]. Thismethodiseasytoimplement,anditcanreadilybeusedfor thepreparationofsamplescontaininghighamountsofsaltsand othercomponents[13].Forexample,ithasbeenusedinthedirect profilingofcomplexmixtures,includingcrudeextracts,cultures ofbacteria,celllysates,intactviruses,andevenindividual neu-rons [14]. Unfortunately, heterogeneity ofthe resulting sample spotsusuallyleadstopoorshot-to-shotandunsatisfactory sample-to-sample reproducibility of the MS signals. This method also necessitatessearchingfortheso-called“sweet-spots”–microscale locationswithinthesampledepositsthatcontainanelevated con-centrationofanalytemolecules[15].
Theoccurrenceof “sweet spots”isa majordrawback ofthe samplepreparationforMALDI,andithamperstheuseof MALDI-MSforquantitativeanalysis.Inmanycases,onecanalsoobserve ring-likepatternsformedattheperimeterofthesamplespots; theseringscanbeattributedtotheso-called“coffee-ringeffect”
[16].Theringpatternformationisduetotheactionofcapillary flow,whichcarriesthesoluteand/orsuspendednon-volatile parti-clestotheperimeterofthedroplet[17].Particlessuspended in thedrying dropletaccumulateattheperimeterand forma cir-cularringpattern,asopposedtoauniformspot(Fig.1).Infact, thecoffee-ringeffecthasseriousimplicationsformanyconsumer products,suchasinkjetprinting[18],andcoatingpolymers[19], aswellasresearchtechniques,suchasmicroarraytechnology[20], chromatography[21,22],andstructuralanalysisbytransmission electronmicroscopy[23].Therefore,thecoffee-ringeffecthasbeen broadlystudiedbyscientistsinvariousfields.
Onepossibility for improvingthe sample spothomogeneity is usinginorganic powder (micro- and nano-particles), suchas graphiteparticles[2],carbonpowder[24,25],andmetaloxide pow-der[26–29]asmatricesforfacilitatingionizationoftheanalytesof interest.Infact,inorganicmaterial-aidedSALDI-MSanalysishasthe advantagethatsamplepreparationdoesnotrelyonthe crystalliza-tionofanorganicmatrixcompound;itcanbeusedtoconcentrate analytesbased onaffinityinteractions [25,30–36]and generally producesspectrawithlessnoise inthelowm/z range.Todate, variousnanomaterials(e.g.carbonnanotubes[35,36],porous sil-iconandsilicon nanoparticles[37,38],TiO2 [39–41],Fe3O4/TiO2
[33],andgoldnanoparticles[42,43])havebeenproposedas possi-bleassistingmaterialsforSALDI-MS.Nevertheless,Kawasakietal. found that dry deposits of gold nanoparticle suspensions also exhibitformationofthe“coffeerings”followingtheirdeposition
ontargetplates[44].Alternatively,MALDImatrices–covalently immobilizedonasol–gelfilm–havebeenusedtoimprove shot-to-shotreproducibilityandeliminatebackgroundionsinthelow m/zregion[45–48].SegregationofanalytesinMALDIspotswas observedusinghigh-resolutionMSimagingmethods[49,50].
Inthepresentpaper,wediscusstheoccurrenceofthe coffee-ringeffectduringthesamplepreparationforanalysisbyMALDI-MS andSALDI-MS.Wehaveimplementedlaserdesorption/ionization imaging in order to studythe formation of coffee rings in dry depositsofsamplescontainingeitherorganicorinorganic materi-als.Wewillshowthatalthoughthecoffee-ringeffectcangenerally beconsideredasanuisanceinsamplepreparationforMALDIand SALDI,it also enablesrough separation of samplecomponents, whichcanbebeneficial.
2. Materialsandmethods
2.1. Samplepreparation
Thedried-dropletsamplepreparationmethodwasusedinthis study.ForMALDI,9-aminoacridine(9-AA)wasusedasmatrixwhile forSALDI,2-mgraphitepowderandfluorescentplatinum nano-clusterswereusedas SALDI-assistingmaterials.In each case, a 0.2-Lsamplewaspremixedwith0.2Lofthematrixcocktailina microcentrifugetesttube,andtheresultingmixturewasdeposited onvarioustargets:glasssubstratecoatedwithanindiumtinoxide (ITO)layer(ITOthickness:260±20nm;resistivity:∼7cm;light transmission:≥80%),ITOglassslidescoatedwithalayerof polysi-lazane,oronacommercialdisposableAnchorChiptarget(PACII 384/96,CHCAprespottedonconductivepolymer;Bruker Dalton-ics/Eppendorf,Bremen,Germany).Inthecaseofthecommercial disposabletarget,acetoneandisopropanolwereusedtoremove CHCAbeforeexperiments.Subsequently,thesamplesweredried atroomtemperature.
2.2. Synthesisandpurificationoffluorescentplatinum nanoclusters
The fluorescent platinum nanoclusters (PtNCs) were syn-thesized as described previously [51]. Briefly, 150L of an aqueous0.1-MH2PtCl6 solutionwasmixed with15mLof
N,N-dimethylformamide(DMF)pre-heatedto140◦C,andtheresulting mixturewasrefluxedina140◦Coilbathwithvigorousstirringin ambientairfor8h.Then,theas-preparedPtNCswerepurifiedusing prefilledionexchangecolumns(Poly-Prep,AG1-X8resin,200–400 mesh;BioRad,Hercules,CA,USA)toremoveresidualchlorideions andthusdecreasespectralinterferencescausedbythesespecies.
2.3. MALDI/SALDIimaging
AlltheMSmeasurementsandMSimagingwereconductedby meansoftheAutoflexIIISmartbeamMALDI-MSinstrument(Bruker Daltonics, Bremen,Germany), equippedwitha solid-state laser (=355nm),andoperatedinthereflectronmode.DuringMS imag-ing,thelaserbeamwasfocusedtoaspotwithadiameterbetween 60and70m;thedefaultspacing ofthescanraster wassetto 100m;100lasershotswerefiredateachrasterpointwiththe presetfrequencyof100Hz.Themassrangewasnormallysetto 100–1000Daandalltheionsupto100Dawereexcludedinorder topreventsaturationofthemicrochannelplatedetector.Various standardcompoundswerespottedonthetargetsnexttothe sam-plespots,andusedformasscalibrationpriortotheMSimaging scan.
Fig.2. Coffee-ringeffectinSALDI-MS.(A)Opticalandfluorescence(ex=330–380nm;em>420nm)imagesofdryspotscomposedofsub-nanometerfluorescentplatinum
nanoclustersonacommercialdisposableAnchorChiptarget(noorganicmatrixpresent).(B)SALDI-MSspectraofplatinumnanoclustersdepositedonacommercialdisposable
AnchorChiptargetobtainedinthenegative-ionmode.(C)SALDI-MSimageofplatinumnanoclustersacquiredinthenegative-ionmode.(D)Fluorescence(ex=330–380nm;
em>420nm)imageofaspotcontainingAMPandplatinumnanoclusters.(E)SALDI-MSspectrumofaspotcontainingAMPandplatinumnanoclusters(negative-ionmode).
(F)SALDI-MSimagesofaspotcontainingAMPandplatinumnanoclusters(negative-ionmode).Laserspotdiameter:60–70m;laserrasterspacing:100m.Scalebars:
100m.GrayscaleisusedasarelativemeasureoftheMSsignalintensity.Theredarrowhighlightstheedgeofthespot.(Forinterpretationofthereferencestocolorinthis
figurelegend,thereaderisreferredtothewebversionofthearticle.)
3. Resultsanddiscussion
Inthispresentwork,weinvestigatetheformationofcoffeerings duringsamplepreparationforSALDIandMALDImass spectrome-try.Thestudystartswithtestingtheinfluenceofinorganicmatrices (micro-andnano-particles)ontheemergenceofthecoffee-ring effect,andcontinueswiththeinvestigationof“hiddencoffeerings” inthedepositsofanorganicMALDImatrix.
3.1. CoffeeringsinSALDIspots
Fortheinitialtestswe chosegraphitepowder (particlesize: ∼2m)asamodelSALDI-assistingmaterial[2].Thismaterialis chemicallyinert,conductselectricity,andabsorbsenergydelivered byUVlaser;therefore,ithasbeenwidelyusedinSALDI[2,52,53]. Conventionally,graphitepowderisusedatahighconcentration (≥4mgmL−1),inwhichcasesampledepositsappearhomogeneous, andhighsensitivityisseen[54].Inordertoobservetheappearance ofcoffeeringsduringsamplepreparationforSALDI-MS,weused asuspensionofgraphitepowderatarelativelylowconcentration (2.5mgmL−1).InFigureS1onecanseethatmostgraphite parti-clesaccumulatedintherimofthespotfollowingtheevaporation
ofsolvent.Severallargerparticlesofgraphitealsoaccumulatedin thecentralpartofthespot,whichgaverisetoheterogeneous dis-tributionofthegraphitematrix.Thiseffectcanalsobeobserved inSALDI-MSimageswhichdisplaytheoccurrenceofcarbon clus-terions(C6+atm/z72,C8+atm/z96,andC10+atm/z120)while
scanningthesurfaceofthespotwiththeUVlaser(FigureS1). Inordertoverifyhowparticlesizecontributestothe coffee-ringeffect,wefurthertestedsub-nanometerfluorescentplatinum nanoclusters(FigureS2)asthematrixmaterial.Tostudythe coffee-ringeffectcharacteristicswithindrynanoclusterspots,wetook advantageof various analytical techniques:optical/fluorescence microscopy,SALDIimaging,scanningelectronmicroscopy(SEM), and energy-dispersiveX-rayspectroscopy (EDX). Fig.2A shows opticalandfluorescentmicrographsofring-likepatternsformed byplatinumnanoclusters(∼10mgmL−1,0.2L).Inorderto inves-tigatethedistributionofplatinumnanoclusterswithindryspots, weexecutedSALDI-MSimagingsequences.In bothpositiveand negativemodes,ring-likepatternswererevealed.Inthe negative-ionmode(Fig.2BandC),werecordedMSsignalscorresponding toaPtCl2−ion(m/z266).Inthepositive-ionmode(FigureS3),we
recordedtwounknownsignals(m/z438and454),whichweremost abundantwhentherimofthespotwasscannedwiththeUVlaser.
80 J.-B.Huetal./AnalyticaChimicaActa766 (2013) 77–82
Fig.3. Coffee-ringeffectinMALDI-MS.(A)Opticalandfluorescence(ex=330–380nm;em>420nm)imagesofasampledepositcontainingvariousmetabolitestandards
co-crystallizedwiththe9-aminoacridinematrix(9mgmL−1,0.2L)preparedinacetonesolvent.(B)MALDI-MSimagescorrespondingtothespeciesrelatedtothe
9-aminoacridinematrix.(C)MALDI-MSimagescorrespondingtothemetabolitestandards.Signalassignment:m/z403,uridinediphosphate;m/z426,adenosinediphosphate;
m/z442,guanosinediphosphate;m/z483,uridinetriphosphate;m/z506,adenosinetriphosphate;m/z522,guanosinetriphosphate;m/z565,uridinediphosphateglucose;
m/z606,uridinediphosphateN-acetylglucosamine.InalltheMALDI-MSimagestheminimum(threshold)intensitywassetto60%(relativetothebasepeak).(D)Signal
intensityprofilescorrespondingtoMALDI-MSimagesin(C).Laserspotdiameter:60–70m;laserrasterspacing:100m.Scalebars:100m.Thegrayscaleisusedasa
relativemeasureoftheMSsignalintensity.
WhenanalyzingthedepositsofplatinumnanoclustersbySEMand SEM-EDX,we confirmedthat mostplatinum nanoclusterswere localizedwithintherimzoneofthespotwhileonlyasmallportion ofplatinumnanoclusterswaspresentnearthecenterofthespot (FigureS4).ThisresultisinagreementwiththereportbyKawasaki etal.ontheuseofnanomaterialsasSALDImatrices [44].Based ontheauthors’experience,thecoffeeringsareespecially promi-nentatrelativelylowparticledensities.Higherparticledensitywill effectivelyreducetheoccurrenceofthecoffeerings.Nonetheless, inmanycasesitisnotpossibletoobtainconcentratedsuspensions ofsomeofthenovelSALDI-assistingmaterials.
Thefollowinganalysisfocusedontheinfluenceofthe coffee-ringeffectonthedistributionoftestanalytes.Weusedadenosine monophosphate (AMP) as a test analyte. As shown in Fig. 2D and F, fluorescence micrographs and SALDI-MS images reveal PtCl2− ions and the ions related to the analyte AMP when
the laser light beam impinged on the rim zone. When AMP and bradykinin samples were deposited on the target with-out adding anymatrix, coffee ringswere observed (Figure S5). Therefore, theoccurrence of thecoffee-ringeffectin SALDI-MS images, as seen in Fig. 2F, can be due to either accumu-lation of nanoclusters at the rim of the spot, accumulation
Fig.4. Theinfluenceofsurfacewettabilityontheemergenceofcoffeerings.Aspotofgraphitepowdersuspension(2.5mgmL−1,0.4L)depositedontohydrophobic(left)
andhydrophilic(right)surface.Thearrowhighlightstheedgeofthespot.Scalebars:100m.
of the analyte molecules, or a combination of these two effects.
The solutes precipitating from homogeneous solutions (e.g. smallmoleculessuchasAMP)arealsoresponsiblefortheformation ofcoffeerings.ThisobservationisrelevanttoSALDI-MSvariants whichusemicro-ornano-particlesasSALDI-assistingmaterials. Therehasbeenconsiderableinterestintheutilizationof nanostruc-turedsurfacesformatrix-freeMS(e.g.Refs.[37,55]);however,since theanalytesthemselvesgiverisetocoffeerings(FigureS5),it can-notbetakenforgrantedthattheimplementationofsuchsurfaces canalwaysprovidehomogeneoussampledeposits.
3.2. CoffeeringsinMALDIspots
Wehavefurtherstudiedheterogeneousdistributionofsmall moleculeswithincrystallinedepositsofanorganicMALDImatrix bymeansofMALDI-MSimaging(Fig.3A).Werecordedsignalsof ionscorrespondingtothe9-aminoacridinematrixfragment(m/z 96 and 193; Fig.3B)and theions correspondingto the small-moleculeanalytespresentinthesample(uridinediphosphate(m/z 403),adenosinediphosphate(m/z426),uridinetriphosphate(m/z 483),adenosinetriphosphate(m/z506),uridinediphosphate glu-cose(m/z565),anduridinediphosphateN-acetylglucosamine(m/z 606);Fig.3C).While someof thesesignalswererelativelyhigh whenthelaserbeamimpingedontherimzoneofthespot,other signalsrepresenteda uniformdistributionwithinthespot.This caneasilybejudgedbasedontheplotsshowingthesignal inten-sityalonganintersectinglinedrawnacrosstheMALDIimageof thespot(Fig.3D).Inparticular,thesignalsofguanosine diphos-phate(m/z442)andguanosinetriphosphate(m/z522)werehigher inthecenterofthespot,whilethesignalsofuridinediphosphate (m/z403),uridinetriphosphate(m/z483),adenosinediphosphate (m/z426),andadenosinetriphosphate(m/z506)wererelatively higher near the droplet periphery. This occurred even though thesample/matrixcrystalappearedhomogeneouswheninspected byopticalmicroscopy.Therefore,weconcludethatseparationof speciesoccurredwithintheMALDIspot.Theapparentseparationof specieswithinthespot(Fig.3CandD)canbeexplainedwithvarious putativecauses,forexample:differentsolubilitiesofanalytes, dif-ferentconcentrationsofmatrixonthetarget,interactionbetween analytemolecules andthematrix, or suppression/enhancement effectsduetotheheterogeneousdistributionofinorganicions.In fact,azonaldistributionofionogenicspecies,whichdonotgive risetosignalsduringanalysisbyMALDI-MS,mayalsocontribute totheapparentseparationofanalytespecieswithinthespots.It shouldbepointedoutthattheobservedeffectisespeciallyclear becausetheintensitythresholdusedintheimagedisplaywasset to60% relativetotheintensityofthebasepeak withinthem/z
rangeused(50–800Da).Thehighthresholdwaschoseninorderto highlightthedifferencesinrelativeintensitiesbetweentheedge andthecenterofMALDIspots.Interestingly,byimplementingthe MALDI imaging technique, a hiddencoffee-ring effectcould be observed(Fig.3BandC), whichis notevidentfrom theoptical images(Fig.3A).Whilethecurrentstudyfocusesontheformation ofringsintheMALDIspotperipheries,itshouldbenotedthat pre-viousstudies(usinghigh-resolutionMALDIimaging)alsoshowed microscalesegregationofanalyteswithinMALDIspots[49,50].
The occurrence of coffee-ringeffect in MALDIraises discus-sionabouttheunderlyingmechanism.Duringtheevaporationof matrix/sampledroplet,severalprocessestakeplaceinparallel.Due totheoutwardcapillaryflow,thecoffee-ringeffectpinsparticleson thecontactline,whichiscounteractedbytheso-called“Marangoni effect”.Thelatterarisesbecausethedifferencesinevaporationrates produceasurfacetensiongradientandinducearadialflowtoward thecenter of the droplet.In fact, undercertain conditions, the Marangonieffectmayreversethecoffee-ringeffect[56].However, inthecaseofMALDIspots(Fig.3),crystallizationoccurs simulta-neouslywithparticletransport(presumablygovernedbythetwo effects).Sequentialpartitioningofdifferentmetabolitestothe pre-cipitatecandictatetheirdistributionwithinthespots.Theanalytes thatpartitiontoprecipitateparticleswillbedriventotheedgeof thespotdespitethepresenceoftheMarangonieffect.Sincethe zonesofdifferentanalytesareformedwithintheMALDIspots,a roughseparationofanalytescanbeachieved.
3.3. Anattempttosuppresscoffee-ringeffectinLDI-MS
Severalmethodstosuppresstheoccurrenceofthecoffee-ring effecthavebeendescribedin literature.Theyincludemodifying the composition of particle suspension [57–59], adjusting sub-stratetemperature[56,60],oralteringsurfacewettability[61–63]. Forexample,Mugeleandco-workerspointedoutthatusing elec-trowettingtechnologycaninhibitcoffee-ringeffectandenhance theintensitiesofMALDI-MSsignals[63].Hereweproposeanother simplemethod,whichiscompatiblewiththedried-dropletsample preparationinSALDI.Itinvolvestheuseofahydrophobicsubstrate. ThesampletargetwasanITOglassslidecoatedwitha hydropho-biclayerofpolysilazane.Whendepositedonsuchaslide,graphite powderdidnotpintothehydrophobicsurfaceandthe suspen-sion dropletcontracted during theevaporationof solvent. This wasfollowedbyformationofadryresiduedeposit(Fig.4).The deposithadasmalldiameter(∼750m)comparedtothedroplet size(∼1400m)anddidnotexhibitformationofacoffeering.We furtherappliedthismethodtosamplepreparationinMALDI, how-ever,inthiscase,thesuppressionofcoffeeringswasnotaseffective asin thecaseofgraphite powder(datanotshown).Overall,as
82 J.-B.Huetal./AnalyticaChimicaActa766 (2013) 77–82
notedelsewhere[61,62],thecoffee-ringeffectcanbesuppressed bychangingsurfacewettability,andundercertainconditions,this strategymaybeapplicabletosamplepreparationinSALDI-MS.
ItshouldbenotedthattheformationofaMALDIsampledeposit isamuchmorecomplexprocessthandepositionofasuspension of particles (asin the case of SALDI). Organic matrices precip-itatedue toa lowered solubilityupon mixingwith thesample solventandcontinuous evaporationofsolvents. Theprecipitate mayinitiallyundergorearrangementaccordingtothecoffee-ring effectand theMarangonieffect.However,thesuspension parti-clesarepromptlypinnedtothesurface,whichisnotonlydueto precipitationandevaporationofsolventbutalsoduetothe for-mationofcrystallinejunctionsconnectingindividualcrystalsand theresultingimmobilizationonthesurface.Theseprocessesare accompaniedbyadsorption,occlusion,andco-precipitationwith analytesandothercomponentsofthesamples,allofwhichare selectiveandcontributetotheheterogeneousdistributionof chem-icalspecieswithintheresultingdeposits.
4. Conclusions
In the present work, distributions of analytes (phosphate metabolites) and matrices in MALDI and SALDI sample spots havebeenstudiedusingoptical/fluorescencemicroscopy,andMS imaging.Inbothcases,wehave observedtheoccurrenceofthe coffee-ringeffect.Wearguethatthecoffee-ringeffectplaystwo concurrentroles,whichhaveadvantagesanddisadvantages.The formationofthecoffeeringsispartlyresponsibleforheterogeneous distributionoftheanalytesandthematrix,whichcompromises analyticalperformanceandreproducibility.Ontheotherhand,it enablesaroughseparationofthesample/matrixcomponents. Cof-feeringswereobservedevenintheabsenceofinorganicororganic matrices.Wealsoreportedonahiddencoffee-ringeffectinMALDI whereundercertainconditionsthesample/matrixdepositappears homogeneous.Nevertheless,thecoffeeringswerestillfoundby MALDI-MSimaging executedusingdenselaserrasterscans.We foundthattosomeextent,formationofcoffeeringsinSALDIcould besuppressedbymodifyingthesurfacewettability.
Acknowledgments
Wethank MsWei-Ru Cioufor her assistancewithSEM and EDX, and Dr Jonathan Fang for correcting our manuscript. We acknowledgetheNationalScienceCouncilofTaiwanforthe finan-cialsupport.
AppendixA. Supplementarydata
Supplementarydataassociatedwiththisarticlecanbefound,in theonlineversion,athttp://dx.doi.org/10.1016/j.aca.2012.12.044.
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