Supercritical Fluid Chromatography

Colin F. Poole

• Publisher: Elsevier
• Year: 2017
• ISBN:
  9780128093672
• Topic: Ciencias físicas
• Subtopic: Química analítica

Supercritical fluid chromatography (SFC) is a rapidly developing laboratory technique for the separation and identification of compounds in mixtures. Significant improvements in instrumentation have rekindled interest in SFC in recent years and enhanced its standing in the scientific community. Many scientists are familiar with column liquid chromatography and its strengths and weaknesses, but the possibilities brought to the table by SFC are less well-known and are underappreciated.

Supercritical Fluid Chromatography is a thorough and encompassing reference that defines the concept of contemporary practice in SFC and how it should be implemented in laboratory science. Given the changes that have taken place in SFC, this book presents contemporary aspects and applications of the technique and introduces SFC as a natural solution in the larger field of separation science. The focus on state-of-the-art instrumental SFC distinguishes this work as the go-to reference work for those interested in implementing the technique at an advanced level.

• Edited and authored by world-leading chromatography experts
• Provides comprehensive coverage of SFC in a single source
• Extensive referencing facilitates identification of key research developments
• More than 200 figures and tables aid in the retention of key concepts

Chapter 1

Milestones in Supercritical Fluid Chromatography

A Historical View of the Modernization and Development of Supercritical Fluid Chromatography

R. McClain, Merck Research Laboratories, West Point, PA, United States

Abstract

Supercritical fluid chromatography (SFC) has a long history in the field of separation science. Some users have considered this technique to be superior to other forms of chromatography such as gas and liquid chromatography while others have found SFC to be lacking in terms of reproducibility, sensitivity, and general predictability of retention behavior. Numerous historical events and scientific milestones pertaining to SFC have fueled both the positive and negative perceptions of the technique. This chapter will highlight pinnacle milestones that tell the story of SFC and demonstrate how the technique grew from being a controversial topic to a well-understood science.

Keywords

Supercritical fluid chromatography; history; column technology; instrumentation

Outline

1.1 Introduction 2

1.2 The 1960s 2

1.3 The 1970s 4

1.4 The 1980s 6

1.5 The 1990s 9

1.6 The 2000s 13

1.7 The 2010s 15

1.8 Conclusion 20

References 20

1.1 Introduction

The chromatographic technique most widely known as supercritical fluid chromatography (SFC) came into existence in 1962. Since its inception, the technique has been touted for its exceptional performance while simultaneously being demonized for deficiencies experienced during its evolution. SFC milestones encountered over the past 55 years, traverse common themes such as instrument capabilities and limitations, scientific understanding and explanation of the performance parameters and resulting data acquired, and realistic comparison of the techniques performance relative to other separation methodologies utilized at the same point in time. This chapter will highlight a selection of pinnacle milestones that have at times driven more widespread adoption and support of the technique and at times suppressed its acceptance by the scientific community. The milestones will be presented chronologically in hopes of educating the reader about the history of SFC and identifying some of the individuals who are responsible for delivering the capabilities we have today. While key references are included to allow further reading, the bibliography is not meant to serve as an all-inclusive list of references as many of these topics will be covered in greater detail in later chapters.

1.2 The 1960s

In the 1960s gas chromatography (GC) was considered the premier mode of chromatography due to the high efficiency of the technique especially when coupled with the power of flame ionization detection (FID). Analyte volatility as well as thermal stability where two of the limitations of GC that led researchers to pursue other alternatives for separations. Ernst Klesper’s article entitled “High Pressure Gas Chromatography Above Critical Pressure” published in 1962 was the first account of what is known today as SFC and combated the analyte stability limitation of GC [1]. Klesper successfully separated a mixture of Ni etioporphyin II and Ni mesoporphyrin IX dimethylester on a polyethylene glycol stationary phase using dicholorodifluoromethane as a mobile phase at a pressure of 131 bar. Porphyrins were previously found to decompose during traditional GC analysis but x-ray powder patterns of the compounds recovered from Klesper’s high pressure GC (HPGC) analysis showed the compounds remained structurally intact. This experiment demonstrated that operating pressures above the critical point of the mobile phase enhance mobility on a chromatographic column, thus enabling lower operating temperatures compared to traditional GC, and thus SFC was born.

The Sie and Rijinders research group from the Koninklijke/Shell Laboratorium in Amsterdam was also very active in HPGC at this time but favored the use of carbon dioxide (CO₂) as a mobile phase due to its lower critical pressure (73 atm). Their research focused on evaluating the effects of pressure on partition coefficients (K) for numerous solutes analyzed on squalane and glycerol columns [2]. This groundbreaking work revealed the favorable effect of decreasing retention factors (k) with increasing mobile phase pressure as seen in Fig. 1.1. This finding was a pinnacle as previously a practitioner of gas chromatographer could decrease retention factors only through increasing temperature, which was an issue not only with respect to analyte stability but also for the upper operating temperature limit for GC columns and instruments. The stage was set to evaluate SFC as a possible avenue to extend the upper molecular weight limits of the existing GC technique.

Figure 1.1 Separations of n-paraffins on a squalane column at 40°C with carbon dioxide as a carrier gas at differrent pressures and comparable linear mobile phase velocities. Column: 1-m length of 3-mm-i.d. tube, filled with 25% w/w squalane on Sil–O–Cel 100/120 mesh support. Sample size: approximately 15 μL. Source: Figure reproduced from Sie S, Beersum W, Rijnders G. Sep Sci 1966;1:459–90 with permission.

Two years after the group from Amsterdam hypothesized the possibility of handling higher molecular weight compounds via HPGC, Giddings and coworkers at the University of Utah provided experimental data confirming this hypothesis to be true. The Giddings’ group performed solubility studies where compounds such as silicone gum rubber possessing a molecular weight of 400,000 was solvated in carbon dioxide at 1200 atm and 40°C [3]. They also performed separations of high molecular weight biomolecules such as β- and α-carotene with carbon dioxide at 500 atm pressure. These analytes were previously incapable of being analyzed by traditional GC techniques due to inadequate thermal stability. The ability to solubilize material through the use of a supercritical fluid (note: the name of the paper containing dense-gas chromatography) was explained as a result of enhanced intermolecular interactions provided by the compression of the gas to a liquid-like density.

Through a comprehensive study of enhanced migration of over 80 large molecules in compressed gases, Giddings’ group was able to prepare a table attempting to correlate the eluotropic strength of compressed gases to the known eluotropic strength of traditional liquids [4]. Ironically, the proposed assignment of carbon dioxide with a liquid-like density to possess eluotropic strength similar to isopropyl alcohol could be the first widely accepted, but major misconception about SFC. Experts later disproved the over-estimated solvent strength of supercritical carbon dioxide and suggested it could have delayed wider adoption of the technique [5,6]. Today it is common knowledge that the solvent strength of supercritical carbon dioxide is closer to that of a small hydrocarbon such as pentane or hexane, not an alcohol such as isopropanol [6,7].

Within its first several years of practice, SFC proved capable of separating thermally unstable compounds not conducive to existing GC conditions as well as extending the operating range to higher molecular weight compounds previously inseparable by traditional GC methods. These two attributes allowed SFC to compete with GC, which at this time was still the dominant, high-performance separation technique. The study of the elution strength of the mobile phase in SFC prompted Giddings’ group to postulate that pressure programming during a chromatographic analysis would tune the solvating power of the mobile phase, resulting in elution of the compounds as they become soluble [8]. The ability to perform such a pressure gradient in SFC would allow this emerging technique to compete with liquid chromatography (LC) which commonly utilized a binary mobile phase and composition gradient. This capability would be realized in less than 1 year from Giddings original work, further testament of the attention and effort being dedicated to SFC in its infancy.

1.3 The 1970s

Jentoft and Gouw at Chevron continued the advancement of SFC thorough incorporation of a pressure programmer into an LC set-up, utilizing high-pressure nitrogen as the pressure source, enabling the execution of an SFC pressure gradient. A 900 average molecular weight polystyrene sample was separated using n-octane bonded to Poracil C as the stationary phase with n-pentane containing 5% methanol as the mobile phase. The pressure programmer allowed a 6 psi/minute pressure gradient to be performed over 60 minutes, starting with an initial pressure of 650 psi and concluding with a final pressure of 1000 psi [9]. The polystyrene sample was resolved into 32 oligomers over the course of a 60 minute analysis, Fig. 1.2. Several of the individual peaks were captured post UV detection and chromatographed a second time to ensure they were not artifacts, very similar to Klesper’s confirmational analysis after the inaugural SFC separation [1]. The ability to deliver pressure ramps to control elution provided a “new” mechanism to increase peak capacity, while operating at linear velocities up to an order of magnitude greater than those in LC and at the time SFC was suddenly being compared and competing with not only GC, but also LC.

Figure 1.2 Chromatogram of a 900 average molecular weight polystyrene sample. Source: Reproduced from Jentoft R, Gouw T. J Chromatogr Sci 1970;8:138–42 with permission.

The effects of pressure and temperature on chromatographic performance were studied by Novotny at the University of Houston. Various analytes such as durene, naphthalene, biphenyl, and chrysene were separated on packed columns of different particle size using n-pentane as a mobile phase. Changes in retention factors and plate heights (HETP) were monitored while changing pressure and temperature. The prediction of chromatographic performance by changing temperature was found to be more difficult than pressure. Novotny’s findings, at that time, suggested that an increase in the column pressure drop resulted in a decrease in chromatographic performance [10]. This led to numerous inaccurate conclusions regarding the performance of packed column SFC, the effects of pressure drop on column performance, and the separation speed. The chromatography community, as a result of these conclusions transitioned the development of SFC from using the packed column format to embracing open tubular columns. This transition, primarily centered around column type, would be questioned frequently over the next decade.

1.4 The 1980s

Capillary SFC originated through the work of Milos Novotny and Milton Lee who believed the pressure drop experienced in packed column SFC had “disastrous consequences as far as the column efficiency goes” [11]. These groups sought to design an SFC system that would maintain constant pressure across the entire column length and detector as well as use a stationary phase of minimal thickness to reduce resistance to mass transfer. They used a 58 m long×0.2 mm i.d. glass capillary with a phenylmethyl polysiloxane coating as the stationary phase ...

Table of contents

• Cover image
• Title page
• Table of Contents
• Copyright
• List of Contributors
• Handbooks in Separation Science Series
• Chapter 1. Milestones in Supercritical Fluid Chromatography: A Historical View of the Modernization and Development of Supercritical Fluid Chromatography
• Chapter 2. Theory of Supercritical Fluid Chromatography
• Chapter 3. Practical Approaches to Column Selection for Supercritical Fluid Chromatography
• Chapter 4. Column Characterization
• Chapter 5. Method Development in Supercritical Fluid Chromatography
• Chapter 6. Application of Multiple Column Supercritical Fluid Chromatography
• Chapter 7. Evolution of Instrumentation for Analytical Scale Supercritical Fluid Chromatography
• Chapter 8. Hyphenated Detectors: Mass Spectrometry
• Chapter 9. Theories for Preparative SFC
• Chapter 10. Practical Aspects and Applications of Preparative Supercritical Fluid Chromatography
• Chapter 11. Validation of Supercritical Fluid Chromatography Methods
• Chapter 12. Separation of Stereoisomers
• Chapter 13. Supercritical Fluid Chromatography of Petroleum Products
• Chapter 14. Separation of Lipids
• Chapter 15. Separation of Natural Products
• Chapter 16. Pharmaceutical Applications
• Chapter 17. Applications to Food Analysis
• Chapter 18. Physicochemical Property Measurements Using SFC Instrumentation
• Index