Download - Understanding Stationary Phases for Reversed-Phase ... · PDF fileUnderstanding Stationary Phases for Reversed-Phase Separations: New Notions for a New Century. Dr. Pat McDonald ...

Patrick D. McDonald*, Bonnie A. Alden,KimVan Tran, Charles H. Phoebe, Jr.,Pamela C. Iraneta, Mark Capparella,Thomas H. Walter, Uwe D. Neue,Barbara K. Grover, John E. O’Gara,

Joseph C. Arsenault, Yuehong Xu, Pamela A. Richards*[email protected]

Waters Corporation, 34 Maple Street, Milford, MA USA 01757Poster: HPLC 2001 Maastricht, 18-19 June 2001

Poster by Patrick D. McDonald, Ph.D., HPLC 2001, Maastricht, 18-19 June 2001. © 2001 Waters Corporation All photos © 2001 by PDMcD. Waters, µBondapak, Symmetry, XTerra, HLPC, HydroLinked Proton Conduit are trademarks of Waters Corporation.

Understanding Stationary Phasesfor Reversed-Phase Separations:New Notions for a New Century

Dr. Pat McDonald

[1980]Barbara Grover

Joe Arsenault

ChemicalProducts

Analysis

Evaluation

Chem R&D

Dr. Tom Walter

SynthesisDr. John O'Gara

Dr. Yuehong Xu

Pam Richards

Pam Iraneta

Mark Capparella

Bonnie Alden

Dr. Chuck Phoebe Dr. Uwe NeueKimVanTran

Applications

Info Center

Carla ClaytonGrace LavalleeMaureeen Allegrezza


Thank You Colleagues! 2

Experiments designed to explore the actual physical reality of the interaction of mobile andstationary phases are fraught with practical limitations that impede drawing significantconclusions from the results. It has been easier for most practitioners to foster the folkloreforged through the citation generations of HPLC literature than to refine the "cartoon-level"view of the chromatographic process to a higher "art".

Hybrid Particle Technology enables creation of chromatographic substrates with interesting,specifically designed chemical modifications that reside not simply at the "accessible" surface,but, rather, throughout the entire backbone of a particle’s molecular make-up.This now permits rational exploration of structure-activity relationships with a view towardunderstanding and explaining observations that previously seemed incongruous.Accessible-surface modifications in the traditional manner add further variables to the studyof the ways in which analyte molecules interact with the mobile and stationary phaseson their "random walk" through the chromatographic bed.

Results of experiments using both bonded-silica phases and new bonded-hybrid particle phases,allied with recent ideas from related disciplines, will be shown to challenge traditionalconcepts of accessible surface, ligand density, silanol interaction, and hydrophobic "collapse".Particular attention will be paid to the role of the structure and physicochemical propertiesof both particle substrate and mobile phase elements as they, together,determine the constitution and function of the "stationary phase".


3Abstract

N

10 20 30 40 50 60 70 80 90 min

Unexpected

Note Peak Shape

Expected

4Poster by Patrick D. McDonald, Ph.D., HPLC 2001, Maastricht, 18-19 June 2001. © 2001 Waters Corporation All photos © 2001 by PDMcD. Waters, µBondapak, Symmetry, XTerra, HLPC, HydroLinked Proton Conduit are trademarks of Waters Corporation.

1980 Curiosity Redux Discovery made by NOTwaiting for dry column

to equilibrate.Same result 20 yrs later:

Column: 3.9 x 150 mm; few pores > 100ÅSol-Gel Silica; fully C18 bondednot endcapped; air-dried > 1 yr

Mobile Phase: 0.5 mL/min70% MeOH [good wetting]

Injections: made every 1.5 min, startingas soon as 1st column volume emerges

Quinoline[weak base]

10 20 30 40 50 60 70 80 min

N


No Loss of EfficiencyColumn: Similar, except dried 18-20 hrs,argon stream, 80-90°; injections 3 mins apart

• Super-dry column• Quinoline peak

shape degradationis NOT due toloss of efficiency.

• Constant tR fornaphthaleneindicatesNO changein phase ratio.

• Analyte-accessiblepores wet quickly.

6

10 20 30 40 50 60 70 80 min

N

Column: High Purity Synthetic Silica GelC18, endcapped; more surface area in larger pores4.6 x 150 mmdried 18-20 hrs, argon stream, 80-90°

Injections: 3 mins apartMobile Phase: 70% MeOH, 0.7 mL/min


Different Silica, Bonding

Subtle, steady asymmetryincrease for quinoline peak.

R2 = 0.9265

R2 = 0.9482

0.5

1.0

1.5

2.0

0 5 10 15 20 25 30 35

Quinoline

Naphthalene

Injection Number

Asymmetry

No loss of efficiency, constant tR for naphthalene.

How much of the surface in a porous particlecan be accessed by:• bonding reagents? • solvents? • analytes?

What is ligand density in bonded phase?Where are surface silanols? What’s their role?Can modifications to surface or substrate

structure be used to answer our questions?Traditional Cartoons are

misleading, implyingincomplete coverage of

accessible surface& direct analyte–silanol

access.


Challenging Questions

O–SiO–SiOHO–SiO–SiOHO–SiOHO–SiO–SiO–SiOH

O–SiO–Si

O–SiO–SiO–

O–SiO–SiO–

O–SiO–

O–SiO–SiO–SiO–

O–SiO–Si

Mobile PhasepH > 3

Mobile PhasepH < 3

(CH3)2HN+

NH(CH3)2+

Surface

Core

Si

O

O O Si

O

O Si

O

O Si

O

O Si

O

O Si

O

OHOH

OSi

Si

O

O O

O O Si

O

O

Si

O

Si

O

Si

O

O Si

O

O O O O O O O

OH OH OH

SiO O Si O Si O Si O Si O Si OSiO O Si O Si

Bonded Silica Par ticle

8Ink bottles, craters, cylinders, plates are modelswhich lend themselves to simple math, BUT...

REALITY is Geodesic-domed labyrinths –formed withtetrahedral

building blocks.ALL surfaceshave terminal

–Si–OHgroups.


Silica Pore Structure

OHSi

O OO

SiO

OHO

SiO

OHO

Si O

OO

SiO

SiO

OH

SiOH

O

SiO

O

O

Si

SiSi OH

O OSi O

OSi

SiSi

Si

Si

Si

SiHOSi

O

SiO

OSi

O

O

Si OHO O

Si Si

Si

SiOHO

OSi

SiO

HOO

SiO

HO

HO

Si

SiO

SiO

OO

Si

SiOH

O

OSi

OHOO

Si

Si

Si OH

OOSi

OO Si O

OSi

Si

SiSi

SiCH3

CH3

CH3

SiH3C

H3C

CH3


Pore Access Level 1Most accessible

to largesilyl reagents

under favorablereaction

conditions;easiest-to-access

surface reactsfirst &

completely.

Outer surface & surfacein pore spaces > ~100 Åin diameter nearest tooutside of particle

OHSi

O OO

SiO

OHO

SiO

OHO

Si O

OO

SiO

SiO

OH

SiO

O

SiO

O

O

Si

SiSi OH

O OSi O

OSi

SiSi

Si

Si

Si

SiOSi CH3

H3C CH3

Si CH3

H3CCH3

SiO

SiO

OSi

O

O

Si OHO O

Si Si

Si

SiOHO

OSi

SiO

HOO

SiO

O

HO

Si

SiO

SiO

OO

Si

SiH3C

H3CH3C

SiOH

O

OSi

OHOO

Si

Si

Si O

OOSi

OO Si O

OSi

Si

SiSi

SiCH3

CH3

SiCH3

CH3

CH3

SiH3C

H3C


Pore Access Level 2

Accessible tosmaller silylendcapping

reagents&

solventmolecules

Surface in pore spaces > ~50 Ådirectly connected to Level 1 pores.Diffusion distance fromouter surface is still minimal.

10

OHSi

O OO

SiO

OHO

SiO

OHO

Si O

OO

SiO

SiO

OH

SiO

O

SiO

O

O

Si

SiSi OH

O OSi O

OSi

SiSi

Si

Si

Si

SiOSi CH3

H3C CH3

Si CH3

H3CCH3

SiO

SiO

OSi

O

O

Si OHO O

Si Si

Si

SiOHO

OSi

SiO

HOO

SiO

O

HO

Si

SiO

SiO

OO

Si

SiH3C

H3CH3C

SiOH

O

OSi

OHOO

Si

Si

Si O

OOSi

OO Si O

OSi

Si

SiSi

SiCH3

CH3

SiCH3

CH3

CH3

SiH3C

H3C Porosity deeper

in the labyrinth,

accessible only

to small solvent

molecules;

hydrogen-bonding

molecules are most

favorably attracted,

esp., water.

Infusion of water

into these pores can take minutes.


Least Accessible Level 3

12aPoster by Patrick D. McDonald, Ph.D., HPLC 2001, Maastricht, 18-19 June 2001. © 2001 Waters Corporation All photos © 2001 by PDMcD. Waters, µBondapak, Symmetry, XTerra, HLPC, HydroLinked Proton Conduit are trademarks of Waters Corporation.

Surface CoverageC = % Carbon [from CH analysis]

MWs = Mol. Wt. of silyl functionSC = mole % Carbon in silyl functionMs = Micromoles of Silyl FunctionSA = Surface Area Accessible to Chlorosilane [in sq. meters/gram]

Ms/m2 = C × 106

(SC – C) × MWs × SA

in µmoles/square meter

Ms/m2 = C × 106

(77.4 – C) × 310 × SA

For dimethyloctadecylsilyl:

Ms/m2 = C × 106

(50 – C) × 72 × SA

For trimethylsilyl:

* GE Berendsen, L de Galan, J Liq Chromatogr 1(4) (1978) 403-426; ** A Ulman, Adv Mater 2(12) (1990) 573-582

Accessible Surface Ligand Density May Reach Theoretical Maximum:

Bonding One Silanol Group/1 nm2 = 1.66 µmoles/m2 of coverage.

Density of 4.8 Si–OHs/1 nm2 [estimated from crystal models*]

means maximum coverage is 8.0 µmoles/m2

In Self-Assembled Monolayers of Octadecyltrichlorosilane on silica wafers,

each molecule occupies ~ 0.2 nm2 **.

Does this imply that ~ 5 ligand molecules could bond

to all accessible Si–OH sites in 1 nm2 on a porous silica gel substrate?


12b

Recent Studies in Semiconductor FieldChallenge Myth that High Ligand

Densities on Silica SurfacesCannot Be Achieved:

“Very strong molecule–substrate interactions...result in... formation of chemical bonds...at the interface, molecules try to occupy every available binding site on thesubstrate...in this process they push together molecules that have already adsorbed,thus eliminating free volume....in all SA monolayers the spontaneous adsorptionat the organic material–substrate interface,together with the strong van der Waals attraction amongst the alkyl chains,are the driving force for the formation of highly ordered,and closely packed systems.”*

* A Ulman, Adv Mater 2(12) (1990) 573

High Ligand Density

0

10

30

50

70

90

10 100 1000

Cumulative Percentof Surface Area

Treated Ppt Silicate

BJH Desorption Average Pore Diameter, Å

93% > 50 Å74% > 100 Å50% > 120 Å

0

10

20

30

40

50

60

70

80

90

10 100 1000


Synthetic Silica GelFully RP Bonded Unbonded


84 % > 50Å50 % > 70Å 3 % > 100 Å

170 m2/gavg. 65 Å

87 % > 50Å50 % > 100Å

340 m2/gavg. 91 Å


0

10

30

50

70

90

10 50 100 1000


Synthetic Silica Gel


87% > 50Å50% > 100Å

0

10

30

50

70

90

10 50 100 1000


Si–CH3 Hybrid


99+% > 50 Å70% > 100 Å50% > 120 Å

% Surface Accessible?Distribution of surface area vs pore diameter varies with silica type

& method of preparation. Pore size & diffusion distance limitaccess. Compare % Surface Area in Pores > 100 Å [ ]*.

0

10

30

50

70

90

10 100 1000

Cumulative Percentof Surface Area Sol-Gel Silica


94% > 50 Å3% > 100 Å50% > 83 Å

Avg Pore Diameter isreduced by ~ length ofall-trans silyl function.

Bonding occursin larger pores.Micropores are

blocked.Surface area

cut in half, mostlyin pores >100Å

* All silicas used in these studies were made in our laboratories.

14a

0 40 min

Vo

Retention Times Stable:77 Injections, 20 hrs

Stopped Flow > 10 minRestart Flow: Retention Lost

100% aqueous mobile phase


Wetting Pores‡

‡T Walter, et al, Poster, HPLC’97, [Waters Applications Library #980947, http://www.waters.com/pdfs/TWHPLC97.pdf]

Water on C18

γ = 72.8 dynes/cmθ = 110.6°*Pc ~ + 2200 psi

Methanol on C18

γ = 22 dynes/cmθ = 39.9°*Pc ~ –1500 psi

Pc = – cos θ4γd

d = capillary or pore diameterγ = suface tensionθ = contact anglePc = capillary pressure

θ > 90°cos θ is –

θ~110°d 67Å

θ~40°

θ < 90°cos θ is +

d 67Å

Observation: Explanation: Young-Laplace Theory

* B Janczuk, T Bialopiotrowicz, W Wojcik, Colloids Surfaces 36 (1989) 391-403

Organic-water

mixturesrequire Pc

between theseextremes.Column

Length is acritical

variable inwetting.

Column Void Volume1 1.2 1.4

0

20

60

100

1 1.2 1.4 mL

% Dewetting

2.0 µmol/m2 C18R2 = 0.825 N = 9

y = 263 + -188x

3.2 µmol/m2 C18R2 = 0.984 N = 4

y = 421 + -340x

28 17 0

t0

Decrease in Vo

correlates with% Dewetted

These data suggest that:as stationary phase dewets,mobile phase is extruded from pores.Hydrophobic Collapse is a MYTH!

Vo Decreases

14b

C18 C8 0

20

40

60

80

100% Dewetting after Stopping Flow

100% Aqueous95% Aqueous90% Aqueous

Mobile Phases:

RP[n=12]

RP[n=8]

embedded polar groupin Carbamate RP Phases

–O–Si–(CH2)3–O–C–NH–CnH2n+1

O

CH3

CH3


Polar Group Effect

‡T Walter, et al, Poster, HPLC’97, [Waters Applications Library #980947, http://www.waters.com/pdfs/TWHPLC97.pdf]

Less dewetting with anembedded polar group,even with:• high ligand density• small pore size [65Å]

*

* U Neue, C Niederländer, J Petersen,U.S. Patent #5,374,755 (1994)

Proton Transfer AnalogyBR-StateProton Path

ProtonPickup

http://www.worthpublishers.com/lehninger3d/index.html

15aPoster by Patrick D. McDonald, Ph.D., HPLC 2001, Maastricht, 18-19 June 2001. © 2001 Waters Corporation All photos © 2001 by PDMcD. Waters, µBondapak, Symmetry, XTerra, HLPC, HydroLinked Proton Conduit are trademarks of Waters Corporation.

PROTON PUMP:

Protons move

rapidly through

hydrophobic

regions of

bacteriorhodopsin

via a

proton path or

conduit

formed by

hydrogen-bonded

moieties in protein

structure & water

molecules.Proton Release


A Proton Pump

http://www.worthpublishers.com/lehninger3d/index.html

15b


Protons in Water Wire

12

34

56H3O+

12

34

56

H5O2+

12

34

56

12

34

56

H3O+

12

34

56

H3O+

12

34

56

H5O2+

12

34

56

H3O+

Time: 0 fsecsTime: 30 fsecs



Time: 280 femtoseconds RR Sadeghi & H-P ChengJ Chem Phys 111(5) [1999] 2086-2094

Proton Transfer is one of the fastest processes in solution.Protons hop back & forth through “Water Wires”, by means of

rapid H-bond length fluctuations.


12

34

56HA

B

12

34

56A-

BH+

12

34

56A-

B

12

34

56A-

H3O+ B

Time: 0 fsecs

Time: 940 fsecsRR Sadeghi & H-P Cheng

J Chem Phys 111(5) [1999] 2086-2094

• Relative O–H

orientation important

• Momentum is crucial

for movement in a

given direction

• Sensitive to temperature

• Time scale is rapid!

– sub-picosecond

Base Creates Momentum 17

HydroLinked Proton Conduit™

* RR Sadeghi & H-P Cheng, J Chem Phys 111(5) [1999] 2086-2094** LF Scatena, MG Brown & GL Richmond, Science 292(4 May) [2001] 908-912

HLPC ConceptPoster by Patrick D. McDonald, Ph.D., HPLC 2001, Maastricht, 18-19 June 2001. © 2001 Waters Corporation All photos © 2001 by PDMcD. Waters, µBondapak, Symmetry, XTerra, HLPC, HydroLinked Proton Conduit are trademarks of Waters Corporation.

Two Physical Conditions Requiredfor Water Wire Formation*:

1. Weak Interaction betweenwater molecules & surroundings

2. Single strand of water moleculesresponsible for conduction of protons

18

Vibrational studies show that dipolar interactions betweenweakly hydrogen-bonded water molecules and hydrophobic surfaces

cause strong orientation effects in the interfacial region. This has“important implications” for a molecular-level understanding of “some

of the most important technological and biological processes.”**

OHSi

O OO

SiO

OHO

SiO

OHO

Si O

OO

SiO

SiO

OH

SiO

O

SiO

O

O

Si

SiSi OH

O OSi O

OSi

SiSi

Si

Si

Si

SiOSi CH3

H3C CH3

Si CH3

H3CCH3

SiO

SiO

OSi

O

O

Si OHO O

Si Si

Si

SiOHO

OSi

SiO

HOO

SiO

O

HO

Si

SiO

SiO

OO

Si

SiH3C

H3CH3C

SiOH

O

OSi

OHOO

Si

Si

Si O

OOSi

OO Si O

OSi

Si

SiSi

SiCH3

CH3

CH3

SiCH3

CH3

CH3

SiH3C

H3C

CH3


MeOH-Water Wire 19Not only does the

organic mobile phase componentfacilitate ‘wetting’ the pores.

Protic solvent moleculessuch as methanol can formlinks in the proton conduit.

OHSi

O OO

SiO

CH3O

SiO

OHO

Si O

OO

SiOH3C

SiO

O

SiCH3

O

SiO

O

O

Si

SiH3C

Si CH3

O OSi O

OSi

SiH3C

SiSi

SiCH3

Si

SiO

Si CH3H3C CH3

SiH3CO

SiO

OSi

O

O

Si OHO O

Si SiCH3

Si

SiOH3C

OSi

SiO

HOO

SiO

O

H3C

Si CH3

SiO

SiO

OO

Si

SiH3C

H3CH3C

SiOH

O

OSi

CH3OO

Si

SiH3C

Si O

OOSi

OO Si O

OSi

Si

SiH3C Si CH3Si CH3

O

NH

SiCH3

CH3

CH3

SiH3C

H3C

CH3

O O

NHO

SiCH3H3C CH3


RP Hybrid* PhaseThroughout

backbone,one third of

–Si–OH groupsare replaced

with –Si–CH3.Hydrophobicity

is increasedin micropores

as well asin mesopores

& macropores.

Polar embeddedcarbamate group**

* Patent Allowed** U Neue, C Niederländer, J Petersen, U.S. Patent #5,374,755 (1994)

21

~Si–OH ~Si–O-0

k20

40

60

3 pH5 7 9 11

Si–CH3

HybridSilica GelPolymer

Sol-GelSilica

Silica GelPolymer

Amitriptyline pKa ~ 9.4 [water] [8.5 [20% ACN] ]NH(CH3)2+

N(CH3)2


Compare SilicasRetention of strongly

basic analyte reaches amaximum at same pH

for two differentkinds of silica.Hybrid silica is

clearly different:hydrophobicity

within pores maintainsk at a higher level at

higher pH. Acidity ofsilanols is different -

smaller population ofSi–OH functions may

reduce opportunity forH-bonding between

adjacent silanols.

OHSi

O OO

SiO

CH3O

SiO

OHO

Si O

OO

SiOH3C

SiO

O

SiCH3

O

SiO

O

O

Si

SiH3C

Si CH3

O OSi O

OSi

SiH3C

SiSi

SiCH3

Si

SiO

Si CH3H3C CH3

SiH3CO

SiO

OSi

O

O

Si OHO O

Si SiCH3

Si

SiOH3C

OSi

SiO

HOO

SiO

O

H3C

Si CH3

SiO

SiO

OO

Si

SiH3C

H3CH3C

SiOH

O

OSi

CH3OO

Si

SiH3C

Si O

OOSi

OO Si O

OSi

Si

SiH3C Si CH3Si CH3

O

NH

SiCH3

CH3

CH3

SiH3C

H3C

CH3

O O

NHO

SiCH3H3C CH3

Protons hopwithinwater layer,instead ofmovingtowardbasicanalyte.Peakshapeimproves.


RP Ligand Disrupts Wire

Hybrid ParticleInhibits Wire

FormationHydrophobicitybuilt into hybridparticles’ pores

lowers attraction forwater molecules.

Water wiresless likely to form.

22


New Notions - Summary• Pore surface area accessibility is key to effective

surface modification & RP HPLC performance.• Ligand density may reach maximum in accessible

pores – total surface area calculation is misleading.• Pore dewetting, not hydrophobic collapse,

causes retention loss.• Silanols, even in micropores, may affect analytes

via water wires.• Water wires may be disrupted by embedded polar

groups, inhibited by hybrid particle pores.• Hybrid particle technology holds promise for

stationary phase performance improvements.

24aWaters Authors•Observations on the Wetting of HPLC Packings, T Walter, P Iraneta, M Capparella, Poster #P-202/A, HPLC’97, Birmingham

(1997) 19 pp [http://www.waters.com/pdfs/TWHPLC97.pdf or Search Waters Applications Library for 980947]•Dependence of cyano bonded phase hydrolytic stability on ligand structure and solution pH, JE O’Gara, BA Alden, CA

Gendreau, PC Iraneta, TH Walter, J Chromatogr A 893(2) (2000) 245-251•Systematic Study of Chromatographic Behavior vs Alkyl Chain Length for HPLC Bonded Phases Containing an Embedded

Carbamate Group, JE O’Gara, DP Walsh, BA Alden, P Casellini, TH Walter, Anal Chem 71(15) (1999) 2992-2997•Improving Our Understanding of Reversed-Phase Separations for the 21st Century, PD McDonald et al., Lecture #154,

ISC 2000, London, 3 Oct 2000Proton Transfer & Protein Pumps, Hydrogen Bonding in Water•The Dynamics of Proton Transfer in a Water Chain, RR Sadeghi, H-P Cheng, J Chem Phys 111(5) (1999) 2086-2094•Lehninger Principles of Biochemistry, 3rd ed, http://www.worthpublishers.com/lenhinger3d/iindex.html•Comment on the mechanism of proton-coupled electron transfer reactions, S-I Cho, S Shin, J Molecular Structure 499 (2000)

1-12•Time-Resolved Dynamics of Proton Transfer in Proteinous Systems, M Gutman, E Nachliel, Annu Rev Phys Chem 48 (1997)

329-356•Biophysical aspects of intra-protein proton transfer, S Brandsburg-Zabary, O Fried, Y Marantz, E Nachliel, M Gutman, Biochim

Biophys Acta 1458 (2000) 120-134•Ab initio analysis of proton transfer dynamics in (H2O)3H

+, PL Geissler, C Dellago, D Chandler, J Hutter, M Parrinello, ChemPhys Lett 321 (2000) 225-230

•Interactions of Hydration Water and Biological Membranes Studied by Neutron Scattering, J Fitter, RE Lechner, NA Dencher, JPhys Chem B 103 (1999) 8036-8050

•A direct-dynamics study of proton transfer through water bridges in guanine and 7-azaindole, Z Smedarchina, W Siebrand, AFernández-Ramos, L Gorb, J Leszczynski, J Chem Phys 112(2) (2000) 566-573

•Oxygen and Proton Pathways in Cytochrome c Oxidase, I Hofacker, K Schulten, PROTEINS: Structure, Function, & Genetics30 (1998) 100-107

•The dynamics of hydrogen bonds and proton transfer in zeolites - joint vistas from solid-state NMR and quantum chemistry, HKoller, G Engelhardt, RA van Santen, Topics in Catalysis 9 (1999) 163-180


References 1

24b•How water takes shape at hydrophobic surfaces, GL Richmond, C&EN, Sept 11 (2000) 29•Autoionization in Liquid Water, PL Geissler, C Dellago, D Chandler, J Hutter, M Parrinello, Science 291 (16 Mar 2001) 2121-

2124•An Introduction to Hydrogen Bonding, GA Jeffrey, Oxford University Press, New York (1997) pp 119-123•The Structure and Properties of Water, D Eisenberg, W Kauzmann, Oxford University Press, New York (1969) pp 225-227

Monolayers, Surfaces, Pores•Self-Assembled Monolayers of Alkyltrichlorosilanes: Building Blocks for Future Organic Materials, A Ulman, Adv Mater 2(12)

(1990) 573-582•Self-Assembled Mono- and Multilayers of Terminally Functionalized Organosilyl Compounds on Silicon Substrates, S Heid, F

Effenberger, Langmuir 12 (1996) 2118-2120•Formation of Uniform Aminosilane Thin Layers: An Imine Formation to Measure Relative Surface Density of the Amine Group,

JH Moon, JW Shin, SY Kim, JW Park, Langmuir 12 (1996) 4621-4624•Pore-Resolved NMR Porosimetry, RS Drago, DC Ferris, DS Burns, J Am Chem Soc 117 (1995) 6914-6920•The Surface Tension Components of Aqueous Alcohol Solutions, B Janczuk, T Bialopiotrowicz, W Wojcik, Colloids Surfaces

36 (1989) 391-403

Reversed-Phase HPLC•Solvophobic Interactions in Liquid Chromatography with Nonpolar Stationary Phases, C Horváth, W Melander, I Molnár, J

Chromatogr 125 (1976) 129-156•The Molecular Mechanism of Retention in Reversed-Phase Liquid Chromatography, JG Dorsey, KA Dill, Chem Rev 89 (1989)

331-346•Retention in reversed-phase chromatography: partition or adsorption?, A Vailaya, C Horváth, J Chromatogr A 829 (1998) 1-27•Revisionist look at solvophobic driving forces in reversed-phase liquid chromatography, PW Carr, J Li, AJ Dallas, DI Eikens, LC

Tan, J Chromatogr A 656 (1993) 113-133


References 2

24cSilica & Bonded Phases•Porous Silica, KK Unger, J Chrom Library, 16 Elsevier (1979) 359 pp•Synthesis of spherical porous silicas in the micron and submicron size range: challenges and opportunities for miniaturized

high-resolution chromatographic and electrokinetic separations, KK Unger, D Kumar, M Grün, G Büchel, S Lüdtke Th Adam,K Schumacher, S Renker, J Chromatogr A 892 (2000) 47-55

•A Geometrical Model for Chemically Bonded TMS and PDS Phases, GE Berendsen, L de Galan, J Liq Chromatogr 1(4) (1978)403-426

•Preparation and Chromatographic Properties of Some Chemically Bonded Phases for Reversed-Phase Liquid Chromatography,GE Berendsen, L de Galan, J Liq Chromatogr 1(5) (1978) 561-586

•Preparation of Various Bonded Phases for HPLC Using Monochlorosilanes, GE Berendsen, KA Pikaart, L de Galan, J LiqChromatogr 3(10) (1980) 1437-1464

•Determination of Bonded Phase Thickness in Liquid Chromatography by Small Angle Neutron Scattering, LC Sander, CJGlinka, SA Wise, Anal Chem 62(10) (1990) 1099-1101

•Geometry of Chemically Modified Silica, I Rustamov, T Farcas, F Ahmed, F Chan, R LoBrutto, HM McNair, YV Kazakevich, JChromatogr A [HPLC 2000 Proceedings Volume] in press

•Interpretation of the Excess Adsorption Isotherms of Organic Eluent Comoponents on the Surface of Reversed-PhaseAdsorbents. Effect on the Analyte Retention, YV Kazakevich, R LoBrutto, F Chan, T Patel, J Chromatogr A [HPLC 2000Proceedings Volume] in press

•Chromatographic silanol activity test procedures: the quest for a universal test, SD Rogers, JG Dorsey, J Chromatogr A 892(2000) 57-65

Other key references cited in footnotes on individual pages.


References 3

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