Preface
OUTLINE
0.1 Scope of the Book
Supercritical fluid chromatography (SFC) is more than 50?years old. Chapter 1 entitled "Historical Development of SFC" recaps over a much greater time-frame of the discovery of supercritical fluids and their development as a medium for chromatographic separation of both volatile and nonvolatile analytes. A real interest in SFC using either packed or open tubular columns began in the early 1980s when the first commercial preparative SFC instrument became available [1]. This development led to growing interest in the separation of stereoisomers which started with the pioneering work of Frenchman Marcel Caude and his research group in 1985 [2]. Thus, a wide variety of chiral separations were reported and applied near the turn of the century employing both analytical and preparative packed column (pcSFC) technology. SFC with open tubular columns (otSFC) also peaked in the 1980s but fizzled during the following decade. Interest in pcSFC is currently higher than ever before. For example, the technique is capable of generating peak efficiencies approaching those observed in gas chromatography (GC). On the other hand, pcSFC separations can achieve much higher efficiencies per unit time than in high performance liquid chromatography (HPLC). pcSFC has embraced a critical mass of separation scientists and technicians in terms of the number of workers in the field worldwide. Hundreds of supercritical fluid chromatographs currently are in use. Furthermore, pcSFC is (i) detector and environmentally friendly, (ii) interfaceable with sample preparation, (iii) relatively economical in cost, and (iv) is a superior purification tool. Chapters 3 and 4 provide discussion of these critical developments that earlier had been referred to as dense gas chromatography [3]. Related work in the field currently uses both supercritical and subcritical mobile phase conditions to perform separations as well as purifications.
During the past 20?years, pcSFC has created a bonafide niche for itself as the go-to workhorse in chiral separations. Chapter 6 discusses in detail this topic. It has afforded many advantages for rapid separation of enantiomers over HPLC due to its greater separation efficiency per unit time. The advantages of pcSFC over HPLC which are also discussed later in the book, however, are practical but not fundamental. The greatest difference between pcSFC and pcHPLC is just simply the need to hold the outlet pressure above ambient in separations in order to prevent expansion (i.e. boiling) of the mobile phase fluid.
Enantiomeric separations are more compatible with ambient SFC than with high temperature HPLC because chiral selectivity usually favors decreasing temperature wherein the risk of analyte racemization is minimized. On the other hand, the risk of analyte thermal decomposition as in the GC of cannabis - related components is lessened. Furthermore, the straightforward search (primarily by trial and error) for a highly selective chiral stationary phase is a key step in the development of chiral pcSFC separations that address industrial applications. In this regard, a number of screening strategies that incorporate a wealth of stationary phases are discussed in the book that take advantage of short columns, small particles, high flow rates, and fast gradients.
Upon scale-up of analytical chromatography to preparative supercritical fluid separations as discussed in Chapter 8, the resulting decrease in solvent usage and waste generation relative to preparative scale HPLC is strikingly dramatic. SFC product can be routinely recovered at higher concentration relative to HPLC which greatly reduces the amount of mobile phase that must be evaporated during product isolation. Higher SFC flow rates contribute to higher productivity. The faster SFC process makes the separation cycle time significantly shorter such that it becomes practical as well as feasible to make purification runs by "stacking" small injections in short time windows without compromising throughput. Table 0.1 lists additional advantages of supercritical fluid chromatography.
Table 0.1 Advantages of supercritical fluid chromatography.
- High diffusivity/low viscosity yield greater resolution per unit time.
- Longer packed columns afford greater number of theoretical plates
- Low temperature reduces risk of analyte isomerization
- Scale-up of separation and isolation of fractions are facilitated
pcSFC (as most analytical techniques) has had a tortuous development history, but it appears that analytical and preparative scale chiral SFC are currently on the firmest foundation ever experienced with vendors that are strongly committed to advancing the technology. Extensive, new developments in achiral SFC and a much broader spectrum of applications outside the pharmaceutical area are already happening. Unlike reversed phase HPLC, the identification of the correct column chemistry is critical for the successful application of achiral pcSFC. Very different selectivity can be achieved depending on the column chemistry. Basic, neutral, and acidic compounds are well eluted on most columns that indicates the suitability of pcSFC for a broad range of chemical functionalities. The number of "SFC" columns for achiral purifications has also grown rapidly in the past three years. Activity in (i) agricultural and clinical research, (ii) environmental remediation, (iii) food and polymer science, (iv) petrochemicals, and (v) biological chemistry immediately come to mind. Additional Chapters 9-12 have been introduced into the book since writing began that reflect numerous additional applications of pcSFC such as pharmaceuticals, petroleum, food, personal care products, and cannabis. Additional advantages of SFC are listed in Table 0.2.
0.2 Background for the Book
While there have been numerous books published concerning SFC as both monographs and edited volumes, there appear to be only two texts that have had teaching as a major emphasis. One, published in 1990, was edited by Milton L. Lee (Brigham Young University) and Karin E. Markides (Uppsala University, Sweden) and written by a committee of peers is entitled "Analytical Supercritical Fluid Chromatography and Extraction" [4]. For chromatographic discussion, this book focused almost entirely on wall coated open tubular capillary column SFC (otSFC), which is not widely performed today having been replaced almost 100% by packed column SFC (pcSFC).
In the early days, otSFC and pcSFC coevolved and vigorously competed with each other as described in Chapter 1. otSFC lost ground and eventually faded away, mainly as a result of poor chromatographic reproducibility issues in terms of flow rate, gradient delivery, pressure programming, and sample injection. The early systems were costly and not user friendly, which resulted in the technique being marginalized as too expensive and inefficient. While otSFC was capable of outstanding feats such as the separation of nonvolatile polymeric mixtures and isomeric polyaromatic hydrocarbons, most workers in the field would agree nowadays that the approaches used in otSFC are among the worst parameters to test with pcSFC.
Table 0.2 Additional advantages using pcSFC.
- No pre-derivatization to achieve solubility and/or volatility
- Shorter cycle time with gradual gradient elution
- Faster separation facilitated by higher fluid diffusivity
- Reduced column diameter/particle size via lower fluid viscosity
- Less extreme chromatography conditions
- Routine normal phase chromatography
Another book entitled "Packed Column SFC," published by the Royal Society of Chemistry and authored by Terry A. Berger [5] was published in 1995. Given that over 20?years have elapsed since the publication of Berger's book, the book presented here today provides ample references that reflect the current state-of-the-art as understood today. We have written our book that incorporates a more pedagogical style with the explicit intention of providing a sound education in pcSFC. Relatively new users of SFC in the early days were largely forced to rely on concepts developed for either HPLC (in the case of packed columns) or GC (in the case of open tubular columns), which were often inappropriate or misleading when applied to both otSFC and pcSFC. Our book addresses these deficiencies.
In this regard, a detailed discussion of current SFC instrumentation as it relates to greater robustness, better reproducibility, and enhanced analytical sensitivity is a focus of the book (Chapter 3). Originally, SFC was thought to be solely for low molecular weight, nonpolar compounds. Today, we know that SFC spans a much larger polarity and molecular mass range. Even though modern pcSFC books may be more adequately described as either "Carbon Dioxide-Based HPLC" (as Terry Burger once suggested) or "Separations Facilitated by Carbon Dioxide" (as suggested by Fiona Geiser) than "Packed Column Supercritical Fluid Chromatography," a change in...
