Experimental Strategies Leading to Successful Packed Column Supercritical Fluid Chromatography of Polar Analytes

Transcription

1 Experimental Strategies Leading to Successful Packed Column Supercritical Fluid Chromatography of Polar Analytes Larry T. Taylor Department of Chemistry Virginia Tech Blacksburg, VA and Applied Analytical Inc. Blacksburg, VA (mobile)

2 So what is a supercritical fluid? Assume we start with an ordinary gas. If we pressurize it sufficiently we can condense it to a liquid. For example, water vapor at 100 C condenses to liquid at 1 atm. If we raise the temperature, more pressure is required to condense the vapor to a liquid. Water vapor at 120 C requires 1.96 atm to be condensed. Vapor pressure of H 2 O Temp ( C) Pressure (atm) T > 374 o C no condensation, for CO 2 no condensation T > 31 o C

3 density, g/ml Density Behavior with Pressure Carbon Dioxide 200 C 300 C Pressure, atmospheres 40 C 70 C 100 C 150 C Commercial Instrument Upper Pressure Limit (Typical)

4 Dissolution of TBP-HO 3 in SC-CO CO 2 0 MPa 11.9 MPa 12.1 MPa 12.0 MPa

5 Physical Properties of Supercritical Fluids that Enhance Extraction and Chromatography

6 Additional Features of Supercritical Fluids Compressibility is High (CO 2, 25 0 C) = 7.3 x 10-4 ml/atm (CO 2, 10 0 C) = 4.5 x 10-4 ml/atm (CH 3 C, 25 0 C) = 4.5 x 10-5 ml/atm Turbulent Flow Possible flow through tubing is very often turbulent minimal extra band broadening flow is not turbulent thru packed and capillary SFC columns See: C. Giddings, et al., Turbulent GC, Science, 154, 1966,

7 Zero surface tension of supercritical CO 2 enables matrix penetration Water/Solvent CO 2 co-solvent Hole/trench Sidewall Polymer De-bonding of residue

8 Supercritical Fluids Have Properties Intermediate to Liquids and Gases * SFs have faster rates of diffusion than liquids SFC vs. HPLC greater optimum mobile phase velocity shorter analysis times, re-equilibration is faster SFs have lower viscosity than liquids SFC vs. HPLC longer columns, lower pressure drop higher efficiency, smaller particles, steeper gradients SFs have greater solvating power than gases SFC vs. GC lower operating temperatures analytes may have higher molecular mass *chromatographic implications are in black

9 SFC vs. HPLC Efficiency & Flow Greater Efficiency per Unit Time for SFC SFC HPLC Sample: trans-stilbene Oxide Polysaccharide (5micron) 4.6 mm ID X 25 cm L Plates Flow rate [ml/min] SFC: CO 2 /IPA=90/10, Pressure: 100 bar-outlet, Temp: 25 C HPLC: Hex/IPA=90/10, Temp: 25 C higher optimum flow rate, greater efficiency at that flow rate

10 Efficiency is maintained at high flow rates Separation of ibuprofen as a function of flow rate. A. Kot, P. Sandra, and A. Venema, J. Chromatogr. Sci., 1994, 32,

11 Coupled Chiral Columns Enhance Versatility A. Kot, P. Sandra, and A. Venema, J. Chromatogr. Sci., 32, 439 (1994) Chrialcel OD performs best for basic compounds Chiralpak AD performs for acidic compounds Chiral π donor characteristics Separation of different racemates on a triplet column. Experimental conditions: columns Chiralpak AD (10 µm) + Chiralcel OD (10 µm) + Chrex 3022 ( 5 µm), 3 x 25 cm x 4.6 mm i.e.; flow rate 2 ml/min; temperature 25 C; modifier methanol containing 0.5% triethylamine and 0.5% trifluoroacetic acid, programmed from 4% (5 min) to 30% at 5%/min; pressure 200 bar; detection 220 nm; injected amount ~200 ng each compound.

12 o other fluid has as many desirable properties for SFC as CO 2 ; yet its solvating power is insufficient in many cases. How can we enhance the solvating power of pure CO 2? 1. Increase SF Density which enhances solute fluid interaction. For polar analytes, this increase in solvating power is insufficient. 2. Add a Polar Modifier to the mobile phase: more effective for polar analytes.

13 Modifiers Increase Solvating Power of CO 2 Modifiers are usually polar organic solvents (most often methanol) added in small quantities to CO 2. T C increases Modifier reduces chromatographic retention and affects selectivity. Caution: With modified fluids liquid-vapor phase separation on the column must be avoided. P = 150 bar T = 50 o C Modifiers are necessary for successful pcsfc of polar analytes.

14 Modifiers Have Multiple Roles in Packed Column SFC Cover active sites on solid support Swell or modify the stationary phase but do not displace adsorbed CO 2 Reduce analyte retention Improve peak symmetry Enhance mobile phase solvating power At maximum total adsorption(co 2 + MeOH): MeOH was 9% of the adsorbed film on ODS and 28% of the adsorbed film on silica total adsorption was ODS(23µmol/m 2 ); Silica (29µmol/m 2 ) Adsorbed film is at least three monolayers thick

15 SOLVET STREGTH VS. ELUTIO STREGTH Solvent strength is a measure of the interaction between solutes (S) and MP Elution strength accounts for S-MP, S-SP, and MP-SP interactions on chromatographic retention The nature of the SP dramatically impacts the effect of the modifier, thus elution strength and solvent strength are not always equivalent. MP composition has a much more profound effect on elution strength than CO 2 density for moderately polar molecules not originally thought to be the case

16 Effect of hexane and methanol on retention of chrysene on a non-polar C 18 column. Such results were interpreted initially to mean that modifiers only deactivated the solid support and did not dramatically change solvent strength. Elution Strength vs. Solvent Strength

17 Effect of hexane and methanol on the elution of chrysene from a polar stationary phase. In this case, a sulfonic acid (ion exchange) column showed dramatic differences between methanol and hexane as modifiers Elution Strength vs. Solvent Strength

18 Modifier Selection is Critical for Successful Chiral pcsfc Flow rate: 2.0 ml/min, Modifier: 10 %, MeOH: α=1.19 EtOH: α=1.11 IPA: α=1.08 AC:α=1.47 Methanol and IPA selectivity normally bracket that of EtOH Selectivity between methanol and IPA may be dramatic, including elution order reversal AC is worth trying.

19 Mobile Phase Additives Enhance pcsfc of Highly Polar Analytes Supelco LC-PC Column without additive with 2.5 mm H 4 OAc with 10 mm H 4 OAc MeO 1. aproxen Me 7. Tramadol Me 2 H Cl + OH CO 2 H 2. Etodolac Et OMe O C H 2 C i-pr OH Et O H 2 + Cl 3. Metronidazole O 2 Me CH 2 CH 2 OH 6. Propanolol HCl CH 2 CH CH 2 O OH 4. Acetaminophen HO HAc 5. Dipyridamole HOCH 2 CH 2 Modifier gradient: 5% hold for 1 minute, increase to 50% at 4%/min, hold at 50/50 HOCH 2 CH 2 CH 2 CH 2 OH CH 2 CH 2 OH

20 Multiple Roles of Additives Suppress solute ionization. Enhance solvating power of the mobile phase. Modify stationary phase. Deactivate solid support. Form ion pairs with ionic analytes Alter enantio-selectivity Most common additives: Bases (diethylamine, isopropylamine) Acids (trifluoroacetic, alkyl sulfonic, citric) Salts (ammonium acetate, sodium alkylsulfonate)

21 Unique Aspects of pcsfc Relative to HPLC FEATURES OF MOBILE PHASE Compressible Varied Amounts of MP Adsorb onto SP ISTRUMETATIO Cooling Device for Pumping System Back Pressure Regulator Pressure Resistant UV Flow Cell

22 Key Components of pcsfc Method Development Today Analyte Solubility- necessary Solid Support - silica Stationary Phase - polar Organic Modifier - yes Additive - yes Temperature - ~30 o C (chiral) o C (achiral) Pressure 150bar

23 SFC is not HPLC with a Compressible Fluid 1. Works with equipment common to HPLC users 2. Avoids the use of extreme (i.e. high temperature and high pressure) chromatographic conditions 3. RP-SFC and P-SFC are achievable by changing polarity of the SP but keeping the same MP. 4. Retention behavior primarily depends on the nature of the SP opposite of HPLC 5. Reduced column back pressure relative to HPLC results in less frictional heating with pcsfc ( i.e. when particle size is halved, pressure goes up by a factor of four)

24 Current Developments in HPLC Designed to Improve Separation Performance Result in more Drastic Conditions High Temperature to reduce MP viscosity Ultra High Pressure to use smaller particles Column pressure drop: SFC = 20bar; HPLC = 150bar [250 x 4.6 mm, 5µm, 25 o C] Conventional instrument rating: SFC: 6000psi; UHPLC: 20,000psi More narrow HPLC columns required to allow heat to dissipate Injection reproducibility becomes difficult Detector must scan fast enough to collect a minimum of 20 points across the narrow peak Tight temperature control is essential

25 Low Pressure Drop and Short Retention Time sulfamethoxazole H 2 O S H CH 3 O O sulfadimethizole H 3 C O O CH 3 H 2 O S H O sulfaquinoxaline H O S H 2 O 18%MeOH, 2.75mL/min, 65 C, 200 bar outlet; 394 bar inlet, 2µL inj 3x100mm, 1.8µm Zorbax RX-Sil Courtesy of T. Berger 1.0 min sulfamethizole H 3 C S O H S O H 2

26 Fast Separation of Profens and Xanthenes ibuprofen H 3 C CH 3 CH 3 O HO O CH 3 ketoprofen O OH O theophyline H 3 C H O CH 3 O CH 3 caffeine H 3 C O 0.8min 22.5%MeOH,2mL/min 150 bar outlet, 50 C 3x100mm, 1.8µm Zorbax RX-Sil Courtesy of T. Berger theobromine O H CH 3 O CH 3 CH 3

27 Pressure Drop-Mobile Phase Viscosity P = c Lµε T η/d p 2 Carbon dioxide has less than 1/10 th the viscosity of water. When water is mixed with alcohols the viscosity actually increases η, mpa-s H 2 O CO 2 MeOH Mole fraction organic EtOH Carbon dioxide does not strongly interact with alcohols and pressure drop across a column is a linear function of modifier concentration. One should always try to keep the % modifier low to get the maximum speed out of SFC 350 Pressure inlet pressure Pure MeOH P= 200 bar Pure CO2 P= 9 bar Courtesy of T. Berger outlet pressure % Modifier

28 C. Brunelli, Y. Zhao, M-H. Brown, P.Sandra, J. Chromatogr. A, 1185, 2008, The effect of a pressure differential along the column on retention, efficiency, and selectivity is minimal in pcsfc.

29 Chiral Supercritical Fluid Chromatography

30 Why SFC for Chiral Analysis? Lower mobile phase viscosity allows faster flow rates Lower viscosity allows use of small particles Lower viscosity allows coupling of columns in series Faster diffusion gives higher efficiency separations which means faster method development CO 2 is miscible with MeOH and AC; whereas hexane is not

31 Polysaccharide Columns 1. Most commonly used stationary phases. Available as coated and bonded phases. Cellulose and Amylose versions available with most separations accomplished on four (AD, AS, OD, OJ) of them. H-series is better (5µm). Carbamate derivatives. 3. Changing modifier usually gives a large change in selectivity if SFC but not LC

32 Trans Stilbene Oxide (TSO) 2PrOH is superior to MeOH and EtOH but MeOH has low viscosity and boiling point 2,000 1,950 1,900 1,850 1,800 1,750 1,700 1,650 1,600 1,550 1,500 1,450 1,400 1,350 1,300 1,250 1,200 1,150 1,100 1,050 1, SPW 0.20 STH Sample approx.1mg.ml-1 in MeOH 2uL injection 5 micron 250mm x 4.6mm 230nm 35C, 5 ml.min-1 10% MeOH Standard column OD-H Resolution 10.4 Selectivity 1.56 AD-H Resolution 13.0 Selectivity 1.65 OD-H AD-H Different Stationary Phases

33 SFC Screening Auto Sampler UV Detector CO 2 Pump (-15º) Back Pressure Regulator Modifier Pump Modifier CO 2 Waste

34 Parallel SFC/MS System Equipped with a Multiplex Ion Source Interface (recent study) Chirality, 20, 2008, Takeda San Diego, Inc. Laskar, Zeng, Xu, and Kassel J. Chromatogr., 1169, 2007, Automated method development 24 unique modifier/column combinations in as little as 30 minutes

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36 Optimization is next

37 ormal Phase HPLC Largely Disappeared When Reversed Phase HPLC Emerged WHY? Uncertain Traces of Water in the Mobile Phase»Plus Very Slow Re-equilibration Times Poor Gradient Performance Large Volumes of Flammable, Toxic Waste Limited Range of Solvent Strength pcsfc is the better way to perform normal phase HPLC

38 Observations for Achiral pcsfc SFC requires polar stationary phases to separate most polar solutes. More variety than with RPHPLC Column screening is less productive than with chiral chromatography on-polar stationary phases have a limited role in SFC. Some columns require more modifier than others due to stronger interaction between the solutes and the SP. Adsorption (i.e. hydrogen-bonding) and partitioning most influence retention.

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40 Achiral pcsfc can be more than ormal Phase HPLC 1. Reversed phase SFC 2. ormal phase SFC 3. Ion pair SFC 4. Ion exchange SFC 5. HILIC SFC

41 Ion Pair Supercritical Fluid Chromatography amine salts with sulfonate additives sulfonate salts with amine additive salts

42 Quaternary Amine Salt via Ion-Pair SFC (CH 2 ) 15 CH 3 + Cl 30% 2.5 mm sodium ethanesulfonate 30% 2.5 mm sodium 1-heptanesulfonate Cetylpyridinium chloride 30% 2.5 mm sodium 1-decanesulfonate 40 C, 120 bar UV (215 nm) Deltabond C o elution w/o additive 30% 2.5 mm ethanesulfonic acid 30% 2.5 mm ammonium acetate

43 HILIC Supercritical Fluid Chromatography CO 2 + modifier + additive + H 2 O

44 HILIC A technique useful for separation of polar or ionized solutes which may have limited retention in RP-HPLC A hydrophilic stationary phase + mostly hydrophobic organic mobile phase (i.e. CH 3 C) that contains an appreciable amount of water A thin film of dense water is thought to act as part of the stationary phase

45 thymine, uracil, adenine, cytosine salt + water appears to help

46 HILIC-pcSFC

47 Unprotected Peptide Mix on C18 with 0.2% HTFA-Methanol Intensity Minutes

48 Unprotected peptides on C18 with 90:10 Methanol:Water with 0.2% HTFA Intensity Minutes

49 Application of SFC to Polypeptides 1. Ion Pair 2. Ion Pair + HILIC

50 Polytides - Protected Separation of Linear and End Capped Dodecapeptides with Identical Molecular Mass Different Amino Acid Sequence Ac-Gly-Phe-Leu-Gly-Leu-Ala-Leu-Gly-Gly-Leu-Lys-Lys-H 2 Ac-Gly-Gly-Leu-Gly-Leu-Ala-Leu-Gly-Phe-Leu-Lys-Lys-H 2 phenylalanine and glycine exchange Molecular Mass = Da Ac-Gly-Val-Leu-Gly-Leu-Ala-Leu-Gly-Gly-Leu-Lys-Lys-H 2 Ac-Gly-Gly-Leu-Gly-Leu-Ala-Leu-GlyVal-Leu-Lys-Lys-H 2 valine and glycine exchange Molecular Mass = Da

51 3.2x10 6 pcsfc/esi-ms 2.4x10 6 D = HA-PYR E = 4EP F = AMIO G = 4EP 547-F 551-F Intensity 1.6x x G 551-G 547-E 551-E 547-D 551-D Minutes overlaid chromatograms of peptide pair with MM = 1166

52 1.2x x A 551-A A = 0.2% TFA-MeOH B = 0.2% IPAm-MeOH C = 10 mm H 4 OAc-MeOH Intensity 6.0x x B 547-B 547-C 551-C Minutes 1.40x10 6 comparison of 3 additives on HA-Pyridine 541-A 1.05x A Intensity 7.00x x B 541-B 541-C 544-C Minutes

53 Polytides Un-Protected Separation of Linear and Un-Protected Dodecapeptides with Identical Molecular Mass Different Amino Acid Sequence Gly-Phe-Leu-Gly-Leu-Ala-Leu-Gly-Gly-Leu-Lys-Lys Gly-Gly-Leu-Gly-Leu-Ala-Leu-Gly-Phe-Leu-Lys-Lys phenylalanine and glycine exchange Molecular Mass = Da Gly-Val-Leu-Gly-Leu-Ala-Leu-Gly-Gly-Leu-Lys-Lys Gly-Gly-Leu-Gly-Leu-Ala-Leu-Gly-Val-Leu-Lys-Lys valine and glycine exchange Molecular Mass = Da

54 Elution of a Single Un-Protected Peptide MeOH w/ 0.2% TFA Single Peptide Intensity :10MeOH:Water w/ 0.2% TFA Minutes GFLGLALGGLKK 1.00x x10 5 Extracted: Single Peptide Thar Method Station II Empower Software Princeton 2-EP 250x4.6mm, 5µm Linear gradient 5-50% modifier Flow Rate: 2.0mL/min Temp: 40C BP:100 bar Waters ZQ Intensity 5.00x x Minutes GFLGLALGGLKK

55 isomeric uncapped 12mer peptides light scattering detection silica column (150 x 4.6mm, d p =5µm) 2mL/min., MeOH gradient: 5% for 1min to 50% at 6%/min, hold for 5min. then back to 5%, Equilibrate 5min

56 isomeric uncapped 12mer peptides light scattering detection

57 3/7/2011 Aurora SFC Systems 57 SFC SPAS WIDE RAGE OF POLARITY

58 Why are we interested in supercritical fluids for chromatography? Green Chemistry Economical Fast Turn-Around Detection Flexibility Scaleable The primary fluid is CO 2, and it is either supercritical or near-critical in most practical analytical applications

59 Review of Key Points Part 1 1. Advantage of Packed Column SFC is More than Green Chemistry 2. Keys to SFC are Polar Stationary Phases and Polar Mobile Phase Additives. 3. SFC Enjoys a Wider Range of Stationary Phases than RP-HPLC 4. SFC is as powerful a separating technique as reversed phase HPLC with orthogonal selectivity 5. SFC is fast and easy using similar hardware to HPLC 6. SFC is more than a chiral technique

60 Review of Key Points Part 2 SFC can be 3-5 times faster than HPLC for the same result without the expense of high temperature, sub2µm particles, and extremely high pressure. ormal phase chromatography is best accomplished today by pcsfc. With dramatic improvements in sensitivity, SFC is likely to find a new place in trace analysis and validated methods. SFC can deliver 3-5 times the speed with 1/4th the pressure drop of UHPLC using standard 400bar systems.

61 Separation Strategies Importance of Parameters in Adjusting Performance (most important to least important) Retention Selectivity Efficiency Most important percent modifier temperature flow rate pressure pressure pressure temperature percent modifier temperature Least important flow rate flow rate percent modifier

62 Acknowledgment Mehdi Ashraf-Khorassani Present

63 Conclusions Sulfonate additives enhance the elution of amine salts and vice versa from conventional bonded phases. Ethylpyridine stationary phase is probably protonated in methanol-modified CO 2 thus promoting the elution of cationic amine salts without the need for additives. Anionic analytes are irreversibly retained. Separation of polypeptides is enhanced when charged. Pyridine phases separate phospholipids by polarity completely and in part by hydrophobicity; while cyano and diol phases separate by only polarity

64 Conclusions Polypeptides were successfully eluted from Ethylpyridine column with, and sometimes without, mobile phase additives. The strength of the acidic additives was critical to achieve good peak shapes. We speculate that protonation of the pyridine functional groups on the stationary phase and of the amine groups of the peptides helped the elution of peptides due to repulsion between the stationary phase and the analytes. When salts were used as additives, tailing instead of fronting peak shapes indicated more interaction among the analytes, the additive, and the stationary phase.