Column Pore Size- An Underutilized Variable for Optimizing ... · Pore-Size Effects on IP RP HPLC...

Column Pore Size- An Underutilized Variable for Optimizing HPLC Separation

Richard A. Henry, Independent Consultant, 983 Greenbriar Drive, State College, PA [email protected] or [email protected]

Stephanie A. Schuster, Application and Quality Manager, Advanced Materials Technology, Inc., Wilmington, DE 19810 [email protected]

Pittcon 2018 R. A. Henry 1

mailto:[email protected]



2Pittcon 2018 R. A. Henry

• Although the greater surface area of small-pore silica can provide more retention and resolution for small molecules, larger column pores are needed to match larger molecules.

• Full surface area of a 100Å silica is not accessible to molecules such as BSA which is only ca. 1/2 of the average pore diameter.

• For BSA, a better choice would be ca. 400-500Å where the analyte/pore radius ratio is about 1/10.

100Å

300Å

BSA: MW 66 kD, Stokes radius 35.5Å (71Å diameter)

Angiotensin II: MW 1.0 kD, Stokes radius < 10Å

Surface Area and Pore Structure Control Retention

Poor retention and efficiency

Good retention and efficiency

“…entropy plays a significant role (in phase distribution) when one of the phases has a porous structure, providing the mean pore diameter is of the same order of magnitude as the diameter of the partitioning species.”

J. Calvin Giddings

Entropy Effects in Porous Media, pg 31

Unified Separation Science, 1991 John Wiley & Son.

Entropy Effects in Porous Media


• Analytes should be about an order of magnitude smaller than the average particle pore for high performance results!

“In pure SEC, solutes elute with decreasing MW, whereas in LSC (including RP), the reverse elution order is obtained. In mixed mechanisms, LSC can contribute only within pores accessible to the solute. Therefore, total retention cannot be the sum of KSEC and KLSC.”

Klaus K. Unger

Chapter 9, pg 287, Porous Silica (…as a support in column LC), Elsevier Scientific 1979.

Silica as Packing in Size Exclusion Chromatography


• Avoid overlap between SEC and LSC/RP modes for high LC column reproducibility and performance.

Silica as Packing in RP and Other HPLC Modes

“….solutes should encounter at least 10-15 times larger mean pore diameter for fast mass transfer into particles and efficient interaction with stationary phases.”

J. Jack Kirkland

Personal conversation, 2016.


• When analytes become too confined within the pores, column retention, efficiency and peak capacity will be reduced.

Pore Diameter- A New HPLC Resolution Variable

6

Column variables (controlled by suppliers)• Substrate (usually silica)• Bonded phase (ideally, substrates should have no impact)• Pore-diameter (adequately large pores needed to maximize

retention, efficiency and selectivity for larger molecules)

Mobile phase variables (controlled by instruments and users)• Solvent type: ACN, MeOH and Water for RPLC (or HILIC)• Solvent strength (% organic in RP mode)• pH (controls ionic state of substrate, phase and analyte) • Additive type and concentration• Temperature

Sample variables (uncontrolled)• Chemistry• MW (size and shape)

* Presentation by John Dolan, LC Resources, MCF 2009.

** Suppliers should provide full details on column variables.

Pittcon 2018 R. A. Henry

7

Some Mode Overlap Occurs in All Porous Particles

column: Aqueous SEC columns,

30 cm x 7.8 mm I.D.,

5 µm particles

mobile phase: 150 mM phosphate buffer,

pH 7.0

flow rate: 1.0 mL/min

det.: UV at 214 nm

inj.: 10 µL

• Note that molecules 8, 9

with MW ≤1,350 can move

freely within particle pores.

• Molecules 6, 7 with MW

≥10,000 are excluded from

about half of the pore

volume of a 150Å column.

• Molecules 1-5 require

wide-pore columns for

retention modes (RP, etc.)

1. Thyroglobulin, 670 kDa

2. g-Globulin, 158 kDa

3. Bovine serum albumin dimer, 132 kDa

4. Bovine serum albumin, 66 kDa

5. Ovalbumin, 44 kDa

6. Myoglobin, 17.6 kDa

7. Ribonuclease A, 13.7 kDa

8. Vitamin B12, 1.35 kDa

9. Uracil, 120 Da

Discovery BIO GFC 150



1

4

5

2

3

6

5 6 7 8 9 10 11 12 13

Elution Volume (mL)

Log

(MW

)

8

9

76

150Å

500Å

* Data provided by SupelcoPittcon 2018 R. A. Henry

Size Exclusion Chromatography Calibration Plot

8

Chromatographic Conditions

columns: Aqueous SEC-150, 30 cm x 4.6 mm I.D., 3 µm

mobile phase: 0.2 M potassium phosphate, pH 7.0

flow rate: 0.25 mL/min

pressure: 66 bar

column temp.: 25 °C

detector: UV 215, 280 nm

injection: 0.5 µL

samples: listed below

y = -1.4279x + 8.2024R² = 0.9641

0

1

2

3

4

5

6

7

1.50 2.00 2.50 3.00 3.50 4.00 4.50

log

MW

Elution Volume

1

2,34

56 7

89

10

11

12

V0 VM

Analyte MW (Da) Size (Å) Ksize

1 thyroglobulin 667000 200 0.00

2 SigmaMab 150000 0.06

3 IgG 150000 100 0.05

4 BSA 66400 0.16

5 ovalbumin 45000 0.24

6 myoglobin 17000 40 0.33

7 ribonuclease A 13700 0.38

8 bovine insulin 5700 0.60

9 neurotensin 1700 0.66

10 vitamin B12 1350 20 0.85

11 angiotensin II 1000 0.74

12 uracil 112 1.00

Solute/pore radius 0.1-1.0

Solute/pore radius < 0.1

Data provided by AMT

Mode overlap region shows lower column performance


9

Cylindrical pore model Conical pore model

aar r

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.05 0.1 0.2 0.5 0.75 1

Occupation Partition Coefficient, KOC

KOC = (1- a/r)2

a/r

KOC High Performance

RegionLow

Performance Region

Increasing MW or smaller pore

Effect of Radius Ratio (a/r) on SEC Retention

Pore radius (r) varies within a range for porous particles; an average is usually reported.

Analyte radius (a) increases with MW; a specific ratio (a/r) exists for each sample component (7).

As solutes become larger, they are excluded from pores to create the calibration curve, but lose access to surface area for retention.


Impact of Size Exclusion on Band Broadening

Schure (6) recently described another type of band broadening by estimating reduced diffusion rates inside crowded pores (Dp) relative to free diffusion in mobile phase outside pores (Dm) and plotting it against radius ratio (molecule/average pore).

Klein (7) showed that stationary phase interaction dominates column retention at a radius ratio of 0.1, but diffusion coefficient inside pores has already dropped by almost 50%.

18Pittcon 2018 R. A. Henry

90 Å2.7 µm

160 Å2.7 µm

400 Å3.4 µm

1000 Å2.7 µm

Range of Superficially-Porous HALO® Silica Particles


-0.01

0.04

0.09

0.14

0.19

0.24

-0.01

0.09

0.19

0.29

0.39

0.49

0.59

0.69

10 100 1000 10000

dV

/dlo

g(w

) P

ore

Vo

lum

e

(cm

³/g

·Å)

Pore Width (Å)

dV

/dlo

g(w

) P

ore

Vo

lum

e (

cm

³/g

·Å)

Pore-Sizes of Halo Silica for Different Molecules

Small drugs

Small peptides

Large peptides

Small proteins

Large proteins

Pittcon 2018 R. A. Henry12

160Å90Å

Molecule Diameter

Small drugs 5-20 Å

Small peptides 20-30 Å

Large peptides 30-50 Å

Small proteins 50-100 Å

Large proteins 100-200 Å

400Å1000Å

90 Å160 Å400 Å1000 Å

* Data provided by AMT

13

Effect of Pore-Size on “Small Analyte” Efficiency

HALO C18, 90 Å pores

HALO ES-C18, 160 Å pores

High efficiency is critical to high resolution; both particles show high peak capacity for small molecules.This is an indication of minimal crowding within the pores

Pittcon 2018 R. A. Henry Data provided by AMT

14

HALO Particles

160 Å

90 Å

Bovine insulin: MW 5733

Pore-Crowding Reduces Efficiency of Larger Solutes

High efficiency is the key to high resolution and peak capacity; a larger pore particle is needed for high efficiency with bovine insulin.

Insulin has a diameter of about 50Å. Even peptides of 1000 MW may show lower performance. Pore crowding is not limited to peptides.

Pittcon 2018 R. A. Henry * Data provided by AMT

0 1 2 3 4 5 6 7 8 9 10

Ab

sorb

ance

(m

AU

)

Time (min.)

0.0625

1. Ribonuclease A (13,700 g/mol)

2. Bovine Insulin (5733 g/mol)

3. Human Insulin (5808 g/mol)

4. Cytochrome c (12,400 g/mol)

5. Lysozyme (14,300 g/mol)0.2330 0.1784

0.24270.0582

0.0783 0.0752

0.0825

0.04250.0408

90 Å pores

2.7 µm HALO

160 Å pores

2.7 µm HALO

Fig.3 from J. Pharm. Anal. 2013;3(5):303–312

Columns: 100 mm x 4.6 mm HALO C18 (90 Å pores) and 100 mm x 4.6 mm HALO ES-C18 (160 Å pores); mobile

phase: A: water/0.1% trifluoroacetic acid; B: acetonitrile/0.1% trifluoroacetic acid; gradient: 25–42% B in 10 min; flow

rate: 1.5 mL/min; temperature: 30 °C; detection: 215 nm; Peak widths in minutes above each peak.

Halo 90 Å: Pores Too Small for Large Peptides


0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 18.0

W50% = 0.306 min

W50% = 0.099 min

SILU Lite SigmaMAb on Halo 160Å and 400Å


SPP Prototype 160 Å C4

2.7 µm; 90 m2/g

SPP 400 Å C4

3.4 µm; 15 m2/g

Time (min.)

• 160Å pore size column is much too small for SigmaMAb (147 kDa)

• Larger pore 400Å column is better, showing similar retention for 6-fold lower surface area and narrower peaks with higher resolution.

• Only about 20% of the pore volume is accessible to mAb on smaller pore column.

Columns: 150 mm x 2.1 mm; mobile phase: A: water/0.1% difluoroacetic acid; B: acetonitrile/0.1% difluoroacetic acid; gradient: 27-37% B in 20 min; flow rate: 0.4 mL/min; temperature: 80 °C; injection volume: 2 µL; detection: 280 nm

* Data provided by AMT16

-1

0

1

2

3

4

5

6

7

8

9

-1

0

1

2

3

4

5

6

7

8

9

6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0 10.5

Time (min.)

Ab

sorb

an

ce (

mA

U)

Ab

sorb

an

ce (

mA

U)

2.7 µm HALO 160 Å

ES-C18

Prototype 2.7 µm HALO 400 Å

ES-C18

Heavy chain subunits @ ~220 kDa

Large Protein on 160Å and 400 Å C18

Myosin cellular protein >200 kDa

Pittcon 2018 R. A. Henry 17* Data provided by AMT

Adapted from Fig. 3 in Journal of Chromatography A, 1440 (2016) 135–144.

Pore-Size Effects on IP RP HPLC of DNA Digest

• Same DNA restriction digest analyzed on columns with different pore openings.

• Restriction digest base pairs too large for 80Å pore size column.

• Increased retention and sharper peaks observed on 150Å pore size column.

• Sharpest peaks and highest retention observed on 400Å pore size column.

Adalimumab: Too Large for 300Å Columns


0.2 mL/min each

Retention time, min

mA

U, 2

15

nm

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Acquity® UPLC® Protein 300Å BEH C4, 1.7 µm

2450 psi (initial)

-0.02

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.0 2.0 4.0 6.0 8.0 10.0 12.0

Halo 400Å C4, 3.4 µm

1230 psi (initial)

* Data provided by Supelco

Humira® Monoclonal Antibody

HALO 1000Å ES-C18, 2.7µm251 bar

HALO 1000Å C4, 2.7µm227 bar

FPP 300Å C18, 3µm154 bar

FPP 300Å C4, 1.7µm406 bar

2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 min

0

25000

50000

75000

100000

125000

150000

175000

uV

2.1 x 150 mm, A = water/0.1% TFA, B = ACN/0.1% TFA, 34-42% B in 16 min, 0.4 mL/min, 60 °C, 2 µl @ 2mg/mL in 0.1% TFA, 280 nm

Effect of Pore Size on RP Separation of Trastuzumab

Data provided by AMT

Conclusions

• Column retention and efficiency can both be lost for larger sample components when size exclusion mode significantly overlaps with retention modes; partial exclusion of large molecules interferes with stationary phase access and normal diffusion processes.

• Guidelines have been proposed that solutes should be smaller than ca. 10% of the column exclusion limit for optimum performance (radius ratio of 0.1). For example, if exclusion limit is ca. 150,000 Da, largest solute size to avoid significant peak retention and efficiency loss might be estimated at 15,000 Da.

• Preliminary sample screening by SEC can save valuable time by identifying solutes that might be subject to excessive pore exclusion and require evaluation with larger pore columns.

• If data on solute sizes and average column pore size is not available to select optimum radius ratios in advance, screening with larger pore columns is recommended for unknown samples.

Pittcon 2018 R. A. Henry21

References


1. J. C. Giddings, Unified Separation Science, Chapter 2, Entropy Effects in Porous Media, Wiley (1991).

2. J. C. Giddings, Dynamics of Chromatography: Principles and Theory, CRC Press, Boca Raton, FL (2002).

3. K. Unger, Porous Silica, Chapter 9 Size-Exclusion, Elsevier (1979).4. F. Eisenbeis and S. Ehlerding, Kontakte 1/78 (1978) 22-29.5. G. Barka and P. Hoffman, J. Chromatogr., 389 (1987) 273-278.6. B. M. Wagner, S. A. Schuster, B. E. Boyes, T. J. Shields, W. L. Miles,

M. J. Haynes, R. E. Moran, J. J. Kirkland and M. R. Schure, J. Chromatogr. A, 1489 (2017) 75-85.

7. J. Klein and K. Treichel, Chromatographia, 10 (1977) 604-610.8. High Performance Liquid Chromatography, J. H. Knox, ed.

Edinburgh University Press (1978).9. L.R. Snyder, J.J. Kirkland, J.W. Dolan, Introduction to Modern Liquid

Chromatography, 3rd ed., John Wiley & Sons, Hoboken, NJ, 2010.

Acknowledgements


Thanks to Stephanie Schuster, Joe DeStefano and Jack Kirkland of Advanced Materials Technology for data and helpful discussions; Mark Schure also contributed helpful information.

Thanks also to William Betz, Hillel Brandes, Stacy Squillario and Roy Eksteen of Supelco Division of MilliporeSigma who also provided data.

www.advanced-materials-tech.comwww.mac-mod.com

Halo and Fused-Core are registered trademarks of Advanced Materials Technology. Acquity BEH is a registered trademark of Waters, Inc.

http://www.advanced-materials-tech.com/

http://www.mac-mod.com/

Column Pore Size- An Underutilized Variable for Optimizing ... · Pore-Size Effects on IP RP HPLC...

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