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Monolayer-Protected Gold Nanoparticles as a Stationary Phase for Open Tubular Gas ChromatographyGwen M.Gross,†David A.Nelson,‡Jay W.Grate,‡and Robert E.Synovec*,†Center for Process Analytical Chemistry,Department of Chemistry,Box351700,University of Washington, Seattle,Washington98195,and Pacific Northwest National Laboratory,Richland,Washington99352The use of a thin film of monolayer-protected gold nano-particles(MPNs)as a stationary phase for gas chroma-tography(GC)is reported.Deposition of a MPN film was obtained in a2-m,530-µm-i.d.deactivated silica capillary using gravity to force the solution containing the MPN material through the capillary.By SEM analysis,the average film thickness was determined to be60.7nm. The retention behavior for the dodecanethiol MPN column was studied using four compound classes(alkanes,al-cohols,aromatics,ketones),and retention orders were objectively compared to a commercially available column (AT-1,100-nm film thickness).Separation of an eight-component mixture was performed using both isothermal and temperature-programming methods with the dode-canethiol MPN phase and compared to an isothermal separation with the AT-1phase.The AT-1phase separa-tion had an efficiency,N,of6200(k′)0.33)while the dodecanethiol MPN phase separation had an efficiency, N,of5700(k′)0.21)for the same analyte,octane.The reduced plate height,h,for octane was found to be less than1at the optimum linear flow velocity,indicating the MPN column operated near the optimum possible per-formance level.Robustness of the MPN phase is also discussed with consistent performance observed over several months.Overall,MPNs appear promising as a stationary-phase material for GC and as an experimental platform to study their thermodynamic and mass-transfer properties.Nanoparticles and nanoparticle-based materials are attracting great interest for their unique properties and potential for application in diverse areas.1-3Use of nanoparticles for chromato-graphic applications has proven to be advantageous.For example, capillary electrophoresis has found applications of nanoparticles as a pseudostationary phase,enhancing selectivity,as well as a production method for nanoparticles.4-6Monolayer-protected nanoparticles(MPNs)are of particular interest because the surface monolayer stabilizes them relative to aggregation and their properties are influenced by the structure of the monolayer-forming molecules.More recently,the use of high-performance liquid chromatography for the separation of different MPNs based upon nanoparticle size and the chosen monolayer was reported.7 An ordered arrangement of the thiol linkages with uniform, monolayer coverage can be obtained on flat gold surfaces.8 However,with small-diameter MPNs,the curvature of the gold core leads to a larger population of core surface gold atoms,which are defect sites in2D self-assembled monolayers.9This disorder is a desired effect,promoting a noncrystalline interparticle structure,facilitating fast mass transfer via diffusion.Studies of dodecanethiol MPNs have shown the closest core-core spacing is equal to the extended chain length of a single dodecanethiol monolayer,demonstrating the chains interpenetrate to form a disordered packing structure.3Thus,a thin MPN film should function as an efficient stationary phase for gas chromatography (GC).It is envisaged that similar chromatographic efficiencies relative to commercial stationary phases can be realized.Since a wide range of thiol-based organic monolayers is available,ample chemical selectivity could be achieved for different MPN-based GC stationary phases.This report initiates a GC-based platform for thermodynamic and mass-transfer studies of MPNs.With a firm understanding of the chemical selectivity and mass-transfer properties,one could potentially devise MPNs that are tuned for a particular GC application of interest or for other chemical analyzers and sensors as well.The characterization of the MPNs is of considerable interest. In particular,Au-based MPNs have been utilized in a sensor format,10but a greater understanding of their properties as a*Corresponding author.E-mail:Synovec@.†University of Washington.‡Pacific Northwest National Laboratory.(1)Brust,M.;Fink,J.;Bethell,D.;Schiffrin,D.;Kiely,C.J.Chem.Soc.,Chem.Commun.1995,1655-1656.(2)Brust,M.;Bethell,D.;Kiely,C.;Schiffrin,ngmuir1998,14,5425-5429.(3)Templeton,A.;Wuelfing,M.;Murray,R.Acc.Chem.Res.2000,33,27-36.(4)Neiman,B.;Grushka,E.;Ovadia,L.Anal.Chem.2001,73,5220-5227.(5)Sunderhaus,J.;Steinbock,B.;Steinbock,O.Abstr.Pap.Am.Chem.Soc.2002,223,360-PHYS Part2.(6)Viberg,P.;Jornten-Karlsson,M.;Petersson,P.;Spegel,P.;Nilsson,S.Anal.Chem.2002,74,4595-4601.(7)Jimenez,V.L.;Leopold,M.C.;Mazzitelli,C.;Jorgenson,J.W.;Murray,R.W.Anal.Chem.2003,75,199-206.(8)Sabapathy,R.;Bhattacharyya,S.;Leavy,M.;Cleland,W.;Hussey, C.Langmuir1998,14,124-136.(9)Hostetler,M.;Wingate,J.;Zhong,C.;Harris,J.;Vachet,R.;Clark,M.;Londono,J.;Green,S.;Stokes,J.;Wignall,G.;Glish,G.;Porter,M.;Evans, N.;Murray,ngmuir1998,14,17-30.(10)Grate,J.W.;Nelson,D.A.;Skaggs,R.Anal.Chem.2003,75,1868-1879.Anal.Chem.2003,75,4558-45644558Analytical Chemistry,Vol.75,No.17,September1,200310.1021/ac030112j CCC:$25.00©2003American Chemical SocietyPublished on Web08/05/2003sensing medium was desired,serving as impetus for this work. Previous characterization has been done using techniques such as electron microscopies and thermal gravimetric analysis as well as other spectroscopic and thermochemical techniques.9-11The GC platform reported herein can be used to further characterize the gold-based MPNs in a way that is different from the studies done on nanoparticles to ing techniques previously established for materials characterization,the sorptive properties of the nanoparticles as well as their chemical retention behavior can be explored.12-14Gas chromatographic measurements provide a reliable,reproducible,and efficient platform for measuring retention behavior of many compounds on a given MPN phase and,hence,for evaluating chemical selectivity.Reported herein is the development of a stationary phase consisting of dodecanethiol-protected gold nanoparticles and initial characterization of this phase by and for capillary GC.Successful deposition of a thin film of MPNs within a capillary for GC was explored for the first time as well as the methodology for that deposition.Dodecanethiol MPN was coated in a thin film, nominally60nm thick,in a530-µm-i.d.capillary with2-m length.Separations of different compound classes and a test mixture,as well as characterization studies of the nanoparticle stationary phase,were completed.The characterization studies included exploration of the retention behavior of the stationary phase and chromatographic efficiency.An objective comparison between the novel dodecanethiol MPN phase and a commercial AT-1GC stationary phase was completed.Mass-transfer behavior of the dodecanethiol MPN phase was studied by observing the band broadening of several analytes as a function of linear flow velocity. The retention and mass-transfer behavior provided by the dode-canethiol MPN phase will serve as a benchmark for comparison with future MPN-based stationary phases.EXPERIMENTAL SECTIONReagents and Chemicals.All chemicals were reagent grade or a higher grade.Many of the bulk reagents were purchased from Fisher(Fisher Scientific,Fairlawn,NJ).Chromatographic standards from PolyScience(AccuStandard,Inc.,New Haven,CT) were used as analytes for the GC separations with the exception of samples containing benzene and hexane(both Fisher),anisole, octane,decane(all Aldrich,Milwaukee,WI),and chlorobenzene (J.T.Baker,Phillipsburg,NJ)(Table1).Water used in the production of the column was filtered using a NANOpure II filter system(Barnstead/Thermolyne Corp.,Dubuque,IA).The GC carrier gas was hydrogen and was supplied by a hydrogen generator(Whatman Inc.,Kent,U.K.).Nanoparticle Synthesis.The method used for the synthesis of the gold-based MPNs was taken from Wolhtjen and Snow15 but was initially developed by Brust.16The basic formula for the synthesis isA1:1molar ratio of gold to thiol was used,with a dodecanethiol monolayer.Tetraoctylammonium bromide(4.5g)was dissolved in17mL of toluene.A solution consisting of65mL of water with 1.03g of hydrogen tetrachloroaurate was added to the toluene solution in a three-neck round-bottom flask attached to a nitrogen bubbler system.An additional3mL of water was used to completely transfer the gold-containing solution to the flask. Dodecanethiol(0.50g)was dissolved in2mL of toluene and added to the flask.Sodium borohydride(0.79g)was taken from under vacuum and dissolved in50mL of water while stirring.This solution was then added dropwise to the flask over a5-min period. Once the addition was complete,the flask was capped and allowed to stir for3.5h,after which time the solution was transferred to a separation funnel.The toluene layer was washed twice with150 mL of water.The product was placed under a nitrogen stream overnight prior to purification.Purification was done using350mL of ethanol,allowing the gold MPNs to precipitate from solution.The solution was centrifuged and the solvent decanted.Two additional30-mL ethanol washes were completed in the same way.The product was dissolved in5mL of dichloromethane and transferred to a tared vial.An additional3mL of dichloromethane was used to completely transfer the dodecanethiol MPN product to the tared vial.The solvent was evaporated off under a stream of nitrogen, and a product yield of0.78g was achieved.The purity of the product was tested using thin-layer chromatography(TLC)and Fourier transform infrared spectroscopy(FT-IR)(Vector33, Bruker Optics,Billerica,MA).Of particular interest was the presence of residual thiol or disulfide present in the ing a9:1dichloromethane/methanol solution,the TLC plates were run and then stained with iodine.No additional spots were detected with the iodine stain,indicating that neither residual thiol nor disulfide was present at detectable levels in the final product. The FT-IR spectra(KBr pellet)of the MPN product were consistent with the literature and confirmed the absence of any(11)Warner,M.;Reed,S.;Hutchison,J.Chem.Mater.2000,12,3316-3320.(12)Abraham,M.;Poole,C.;Poole,S.J.Chromatogr.,A1999,842,79-114.(13)Abraham,M.H.;Andonian-Haftvan,J.;Du,C.M.;Diart,V.;Whiting,G.;Grate,J.W.;McGill,R.A.J.Chem.Soc.,Perkin Trans.21995,369-378.(14)Abraham,M.;Ballantine,D.;Callihan,B.J.Chromatogr.,A2000,878,115-124.(15)Wohltjen,H.;Snow,A.W.Anal.Chem.1998,70,2856-2859.(16)Brust,M.;Walker,M.;Bethell,D.;Schiffrin,D.;Whyman,R.J.Chem.Soc.,mun.1994,801-802.HAuCl4‚3H2O+RSH98(C8H17)4NBrNaBH4Au:SRTable1.Twelve Components Chosen for Evaluation ofthe Dodecanethiol Monolayer-Protected GoldNanoparticle Stationary Phase acompound bp(°C)k′(MPN)log L16k′(AT-1)hexane690.01 2.6680.04ethanol780.02 1.4850.01benzene800.06 2.7860.083-pentanone1020.060.121-butanol1180.16 2.6010.10octane1260.21 3.6770.34chlorobenzene1320.65 3.6570.451-pentanol1380.43 3.1060.273-heptanone1480.420.66anisole1540.82 3.8900.803-octanone1680.71 1.59decane174 1.47 4.686 1.99a The retention factors,k′,were determined using methanol as thedead time marker.For comparison,the elution data for two othernonpolar stationary phases are included:log L16and AT-1.The elutionorder for hexadecane as a stationary phase is given by the log L16values.Analytical Chemistry,Vol.75,No.17,September1,20034559significant amount of phase-transfer agent in the final product.17,18However,additional purification steps may be warranted to fullyeliminate the phase-transfer agent.19The final dodecanethiol MPN product was black and stored dry in a sealed vial.Characterization of the nanoparticles was done using thermal gravimetric analysis (TGA;model STA409A,Netzsch,Selb,Germany)and transmission electron microscopy (TEM;model JEM 2010,JEOL,Tokyo,Japan)with TEM software (Digital Micrograph,Gatan Inc.,Pleasanton,CA).Characterization results are presented in Figure ing TGA,it was determined that 75.09%by mass of the nanoparticles was due to the gold core with most of the monolayer loss occurring between 200and 300°C (Figure 1C).While the synthesis mole ratio for gold to thiol was 1:1,the final product mole ratio calculated from the TGA was ∼3:1.The presence of nanoparticles was confirmed by TEM (Figure 1A)using a copper grid airbrushed with a dilute solution (0.2%by mass)of the nanoparticles dissolved in dichloromethane.The TEM image indicates the nanoparticle core size was polydisperse with diameters ranging from about 1.5to 5nm (Figure 1B)with an average of 3.2nm.Column Preparation.Deactivated silica capillary,530-µm i.d.with a 2-m length,was used for the production of the open tubular MPN column (Supelco,Bellefonte,PA).The capillary was washed with three 50-µL volumes of methylene chloride and with three volumes of water.The methylene chloride washes were evapo-rated from the capillary using a nitrogen stream while the water washes were evaporated from the capillary by baking in an oven at 300°C for 1h.This same treatment was used for a “blank”column (a column without MPN stationary phase to compare with the column with the dodecanethiol MPN phase,i.e.,the MPN column).Sufficient dodecanethiol MPN material was placed in a 100-µL conical insert inside a standard injection vial (Agilent Tech-nologies,Palo Alto,CA),and 50µL of methylene chloride was added.The resulting nanoparticle solution was introduced to prepare the MPN column via capillary action,resulting in a pluglength of ∼4cm.With the column in a vertical position,gravitational force was utilized to move the solution plug and the nanoparticles were deposited in the capillary via evaporation.When the solution plug reached the far end of the capillary,the column was inverted and the process repeated.Additional solution volumes were added until,via visual inspection,a uniform brown color was achieved.To help avoid MPN phase clumping a low-intensity hot air gun (Conair ProStylus 1500,Conair Corp.,Stamford,CT)was used as a low heat source for faster evaporation of the solvent.Several MPN columns were produced with varying shades from light tan to black.The uniform brown for the thin-film MPN column displayed minimal clumping with no observable bare spots and resulted in very reproducible GC separations.Determination of stationary-phase thickness,coverage,and con-firmation of stationary-phase presence were done using SEM as will be described later.SEM Imaging of the MPN Stationary Phase within the Capillary Column.All images were obtained using a LEO 982field emission SEM (LEO Electron Microscopy Ltd.,Cambridge,England).Prior to analysis,four randomly selected pieces of capillary column,three containing MPNs and one blank,were mounted and then coated with a thin platinum layer using a Ladd electronic sputter coating machine (Ladd Research,Williston,VT).Images were obtained in both the normal and backscatter modes.Elemental analysis of the visible stationary phase was performed in the backscattering mode.Chromatographic Instrumentation.All chromatograms were obtained with an Agilent 6890gas chromatograph using a standard FID detector and injector with ChemStation computer control (Agilent Technologies,Palo Alto,CA).Data were imported into Matlab (MathWorks,Natick,MA)for subsequent analysis.Chromatographic Experiments.All chromatograms were obtained with an injection source and FID temperature of 200°C and a hydrogen carrier gas.Samples were introduced by head-space injection.A 2-mm depth of sample containing the compo-nents of interest was placed in the bottom of a standard injection vial,below the injection depth of the syringe into the vial.An injection size of 0.5µL was used with a split injection of 50:1or 1:1as specified.The injection volume and split procedure did not introduce significant chromatographic band broadening.The vial(17)Hostetler,M.J.;Stokes,J.J.;Murray,ngmuir 1996,12,3604-3612.(18)Cai,Q.Y.;Zellers,E.T.Anal.Chem.2002,74,3533-3539.(19)Waters,C.A.;A.J.,M.;Johnson,K.A.;Schiffrin,mun.2003,540-541.Figure 1.Characterization of the synthesized dodecanethiol monolayer protected gold nanoparticles.(A)Transmission electron micrograph of a thin film of the dodecanethiol MPNs.(B)Histogram of the measured core size distribution from the TEM data.(C)Thermal gravimetric analysis plot indicating the loss of the dodecanethiol monolayer from the gold core.4560Analytical Chemistry,Vol.75,No.17,September 1,2003was not heated,so the amount of each component present in the injected sample was determined by component volatility at rooming samples prepared in this manner did result in sample-to-sample concentration variation for each component. However,this sample preparation and injection procedure was satisfactory for chromatographic evaluation of retention time, efficiency,N,and reduced plate height,h,for the components of interest.Due to the relatively short length and wide diameter of the MPN column,a short restrictor column(100µm i.d.,0.5m in length)consisting of a methyl-deactivated silica capillary followed the MPN column to avoid FID blowout(Supelco).The oven temperature was50°C unless otherwise noted.Additional reten-tion due to the restrictor column was found to be insignificant.The MPN stationary phase was compared to a commercial nonpolar AT-1stationary phase(Alltech Associates,Inc.,Deerfield, IL).All parameters were identical for the comparison except stationary-phase thickness.The AT-1phase thickness of0.10µm was as near as possible to the MPN phase thickness,nominally 0.060µm.The commercial column dimensions and chromato-graphic conditions were the same as those used for the MPN column separation:530-µm i.d.and2-m length,temperature of 50°C,and linear flow velocity of22cm/s.RESULTS AND DISCUSSIONUsing SEM analysis,the presence of the gold-containing dodecanethiol MPN stationary phase within the capillary GC column was confirmed.Stationary-phase thickness was measured at random spots along the column.Visually,the majority of the surface area of the inside wall was a uniform brown(∼97%),while a minor fraction of the surface area contained small black clumps (∼3%).Both uniform and nonuniform areas were imaged using the SEM.For a typical location on the capillary wall appearing uniform,the stationary-phase thickness was60.1nm(Figure2). There was very little variation((2nm)around the capillary circumference in these predominating uniform regions.For a rare area of nonuniformity,the average thickness was55.7nm,ranging from12.7to115.2nm.No bare areas of silica capillary were observed during any of the SEM measurements.The average film thickness for the full length of column was60.7nm.The retention characteristics of the dodecanethiol MPN stationary phase were studied using four different compound classes(Figure3).The analytes studied had a boiling point range from69to174°C(Table1).It should be noted that the analytes are not retained strictly upon the basis of boiling point.3-Octanone has a higher boiling point(168°C),but the less polar anisole, with a boiling point of154°C,is more highly retained.Retention order is based upon boiling point only within a homologous series of compounds.This can be seen within the three homologous series of alkanes,ketones,and alcohols.Additional support for the separation due to the MPN stationary phase was obtained using the“blank”column.This experiment addressed the pos-sibility of additional retention if exposed silica areas were present within the MPN column.Each of the four compound classes (Table1)were analyzed with the blank column under identical conditions as the MPN column.The bare silica capillary did not contribute significantly to the net analyte retention.This experi-ment confirmed the separations achieved with the MPN column were not due to bare silica areas,consistent with the SEM results.An expanded study of the retention behavior for the alkane homologous series was also done(C10-C16).The log of the retention factor(k′)was plotted against log L,16the Oswald solubility coefficient(gas-hexadecane partition coefficient at25°C).For straight-chain alkanes,log of k′is directly related to the log L16(Figure4).14The slope of the line is the l value of the stationary phase,used to describe the stationary phase in the solvation parameter model.12,14,20The l term is a measure of the hydrophobicity of the stationary phase and is an indicator of the ability of a stationary phase to separate members of a homologous series.12The solvation model is temperature dependent and so a change in the slope of the line was seen for the three temperatures examined(100,75,50°C).The k′values at25°C were extrapolated from the other three temperature series and plotted against the log L16values.The linearity of the data indicates the dodecanethiol MPN stationary phase is well correlated with the nonpolar n-alkane series used(R2of∼0.99for all temperature series)and behaves as expected by the model.The increased slope(l value) indicated the stationary phase separates n-alkanes better with a decrease in temperature.Lower temperatures are not always conducive to GC analysis,however,so it is useful to know that extrapolation of retention values can be done for the MPN stationary phase for use in future studies using the solvation model.From the original12analytes(Table1),8analytes were selected to demonstrate a more complex mixture separation.Each of the four compound classes were represented in the mixture. An acceptable isothermal separation was obtained with the dodecanethiol MPN column in a little over30s as shown in Figure 5A,comparing reasonably with the commercial AT-1column separation in ing temperature programming,the same mixture was separated with the MPN column in just over 25s with improved resolution(Figure5B).The isothermal separation was done at50°C,far below the boiling point of the higher boiling analytes in the mixture.It is not surprising that an improvement in the separation was observed for the later-eluting(20)Du,C.M.Thesis.The Application of Physiochemical Desciptors to theCharacterisation of Liquid and Solid Phases.University College,London, 1995.Figure2.SEM image of the dodecanethiol MPN stationary phasewithin the capillary.Area A is the silica capillary.Area B is the gold-based dodecanethiol MPN stationary phase,60.1nm thick at thising a random piece of column,this image was obtainedusing an end-on cross-sectional view.Analytical Chemistry,Vol.75,No.17,September1,20034561peaks (Figure 5B),even for the short temperature program (40-80°C at 70°C/min).Band broadening and separation efficiency were examined for the dodecanethiol MPN column.The reduced plate height,h ,was plotted versus the linear flow velocity,u ,for two aromatic analytes,chlorobenzene (k ′)0.65)and anisole (k ′)0.82).The van Deemter plots obtained for the two analytes were satisfactory (Figures 6and 7).21The reduced plate height,h ,is the experi-mentally determined plate height,H ,divided by the inside diameter of the ing reduced plate height plots,future comparisons between different systems with varying column diameters will be straightforward.The minimum reduced plateheight,h min ,is obtained at the optimum linear flow velocity.A h min less than or equal to 1is indicative of a high-performance open tubular GC system.22The theoretically obtainable h min for open tubular chromatography under ideal conditions is 0.8.23,24Thus,the h min values of 0.90(chlorobenzene)and 0.88(anisole)obtained from Figures 6and 7,respectively,are indicative of an efficient open tubular system.The h min near the optimum and the relatively shallow slope of h at high linear flow velocity both indicate the MPN column performance was consistent with the thin-film stationary phase being used (∼60nm).The efficiency of the MPN column separations was compared to commercial AT-1column separations.To compare the novel nanoparticle stationary phase with a commercial stationary phase it was decided that the two columns should be as dimensionally identical as possible.The AT-1phase was chosen since it is a nonpolar phase based on 100%poly(dimethylsiloxane).The AT-1phase thickness was 100nm compared to 60nm for the MPN column.Both columns were 2m long with 530-µm internal diameters.The slight difference in film thickness was not anticipated to hamper the ing the same experi-(21)Giddings,J.C.Unified Separation Science ;Wiley:New York,1991.(22)Grant,D.Capillary Gas Chromatography ;John Wiley &Sons:New York,1996.(23)Knox,J.J.Chromatogr.Sci.1980,18,453-461.(24)Novotny,M.Anal.Chem.1988,60,A500.Figure 3.Chromatographic separations using the dodecanethiol MPN column of four different compound classes:(A)alkanes,(B)aromatics,(C)alcohols,and (D)ketones.The extra peak eluting before 3-pentanone in (D)is an impurity peak.The separations were obtained using a 2-m,530-µm-i.d.column with dodecanethiol MPN phase nominally 60nm thick,at 50°C with a 0.5-µL headspace injection using a 50:1split on the inlet,operated under constant pressure conditions at 25psi (∼22cm/s).Figure 4.The log of the retention factor (k ′)versus log L 16,the Oswald solubility coefficient (gas -hexadecane partition coefficient at 25°C),for n -alkanes C10-C16.The k ′data were experimentally obtained at 50,75,and 100°C.The k ′values at 25°C were extrapolated from the other three temperature data sets and plotted against the log L 16values.4562Analytical Chemistry,Vol.75,No.17,September 1,2003mental parameters as initially used for the MPN column charac-terization,the AT-1column was used to separate an alkane mixture (hexane,octane,decane)and the eight-component mixture previously described (Figure 5).For octane as the representative analyte,the AT-1column had an efficiency,N ,of 6200(k ′)0.33)while the MPN column had an efficiency of 5700(k ′)0.21),indicating the dodecanethiol MPN column at its optimum is operating essentially the same as the commercial AT-1stationary phase.There were slight retention order differences obtained with the two phases,however,as indicated by the eight-component separations (Figure 5A and C).Note that the relative peak heights for each analyte in the two separations vary slightly due to the injection procedure employed.The AT-1column separation occurred in the same time frame as the MPN column separation.However,two of the components not resolved for the AT-1column separation,benzene (k ′)0.08)and 1-butanol (k ′)0.10),were resolved by the MPN column.In addition,the analyte peaks are distributed differently in the two chromatograms,indicating there is some difference in the chemical retention behavior despite both stationary phases being primarily nonpolar.The retention differ-ences are summarized in Table 1and compared to the log L 16values that indicate the elution order when hexadecane is applied as the stationary phase.Retention order variation is observed when all three stationary phases are compared.The variation in retention order for the AT-1and MPN phases is not surprising,since it was not anticipated that they be identical.However,a more in-depth analysis of the intermolecular interactions that contribute to the observed retention with the dodecanethiol MPN phase is warranted.Of particular interest in the MPN stationary-phase development is not only how it compares thermodynamically and kinetically with commercially available stationary phases but also the robust-ness of the phase.Little to no degradation in the MPN-based separations was observed even after a large number of injections,∼650sample runs to date.The hexane,octane,and decane separation was used as a “benchmark”for the MPN column performance at least once every 4weeks.The benchmark runs indicated little to no degradation occurred over time.It is known from thermal gravimetric analysis of the MPNs that,at temper-atures greater than ∼175°C,the monolayer begins to rapidly strip off the gold cluster (Figure 1C).Since we desired to conclude all initial characterization studies on the same MPN column,this temperature was used as the upper limit for separationmethodFigure 5.Eight-component mixture (subset of Table 1)separations with the dodecanethiol MPN column:(A)isothermal at 50°C and (B)temperature programmed,40-80°C at 70°C/min.The boiling point range was 78-174°C with the following elution order:ethanol,benzene,1-butanol,3-heptanone,chlorobenzene,3-octanone,anisole,and decane.(C)Isothermal separation at 50°C on the AT-1commercial column.Elution order:ethanol,benzene,1-butanol,chlorobenzene,3-heptanone,anisole,3-octanone,and decane.All other parameters were the same as those in Figure3.Figure 6.A van Deemter plot for chlorobenzene (k ′)0.65)on the dodecanethiol MPN column.Reduced plate height,h ,versus linear flow velocity,u ,is plotted.The standard deviation error bars were calculated from sets of five runs.Column parameters: 1.7m,530µm i.d.with MPN phase,operated at 50°C.Sample was introduced via a 200°C inlet under constant-pressure conditions with a sample size of 0.5µL with a 1:1split on the injectedvapor.Figure 7.A van Deemter plot for anisole (k ′)0.82)on the dodecanethiol MPN column.All parameters are the same as those stated in Figure 6.Analytical Chemistry,Vol.75,No.17,September 1,20034563。