26 Oct 2020
Achiral SFC Columns
What is SFC?
Supercritical fluid chromatography (SFC) is an intermediate chromatographic technique, with properties lying somewhere between gas and liquid chromatography. Utilising more extreme conditions of temperature and pressure, it maintains the mobile phase as a supercritical fluid, landing it between the two states.
In general selectivity terms, SFC can be regarded as a derivative of normal phase chromatography with the added advantage of low viscosity (and high diffusivity) of supercritical fluids, resulting in high column efficiency. Packed SFC columns are of the same type as those currently used in normal phase chromatography including a host of bonded phase column types.
Supercritical fluid chromatography (SFC) is widely recognized as a preferred technique for preparative chiral applications. However, this does not mean that the technique cannot be used for achiral molecules. The use of compressed CO2 as the primary mobile phase provides many benefits. It is:
- Readily available,
- Relatively inexpensive
These benefits generally lead to SFCs designation as a "green" technology. As previously mentioned, the viscosities of compressed CO2 and mixtures with polar modifiers are much lower than those of aqueous mixtures. This allows for much shorter chromatographic run times, which are approximately one-third to one-fifth as long as typical HPLC runs. These advantages are clearly applicable to a broad range of separations, beyond just chiral separations.
What columns are available for achiral SFC?
SFC columns (as well as many other types of liquid chromatographic columns) consist of a stainless-steel tube filled with the stationary phase. The stationary phase may be bare silica or silica coated with a bonded phase. SFC columns use end fittings that allow them to connect to the rest of the chromatographic system and are identical to those used in HPLC columns. The stationary phase is retained at each end of the tube by a sieve or frit. SFC fittings and accessories are generally made of stainless steel also.
The surface quality of the tube inside wall is important as it affects the flow behaviour through the column. Uneven, rough, or irregular surfaces will adversely affect the separation of analytes flowing through the column.
SFC capillary columns are of similar design to their GC counterparts, with the stationary phase being chemically immobilized onto the inner wall of the capillary, which is typically made from silica coated with a polyimide to give it strength and flexibility. These columns have very narrow external diameters; therefore, chemical means (rendering ceramic frits) are preferred to external fittings to immobilize the column bed.
There are five specific achiral SFC stationary phases:
- 2-ethyl pyridine,
- 4-ethyl pyridine,
These stationary phases offer different selectivity characteristics in SFC analyses of acidic, basic, and neutral compounds. The Celeris family of achiral stationary phases has been specifically designed for SFC separations, delivering high capacity, broad selectivity, excellent peak shapes, and reproducible performance over long column lifetimes. SFC provides a greener alternative to HPLC separation of complex samples containing a range of different functional groups and polarities. SFC provides better resolution, faster separation, and higher sample throughput compared to HPLC. However, these achiral columns are optimized to work with both the high pressures of SFC and HPLC systems.
1 – 2-ethyl pyridine
Ethyl pyridine phases are the workhorse phases for preparative and process scale SFC purifications. Regis Celeris 2EP (2-ethyl pyridine) media is designed to be highly reproducible based on tight product specifications and low metal content. Celeris 2EP is especially well-suited for separation of acidic compounds and exhibits broad selectivity towards a variety of other compound types. Celeris 2EP SFC column delivers high-performance separations comparable to competing ethyl pyridine phases at a much lower price.
2 – 4-ethyl pyridine
The Celeris 4EP (4-ethyl pyridine) phase offers alternate selectivity to the 2EP phase. Celeris 4EP SFC columns deliver high-performance separations comparable to other ethyl pyridine phases at a much lower price. Celeris 4EP media is designed to be highly reproducible based on tight product specifications and low metal content.
3 – propyl-amine
Celeris Amino columns are packed with a traditional propyl-amine phase bonded to 100Å silica. This offers a high degree of polar selectivity over traditional silica or Ethyl Pyridine phases, allowing a high degree of retention for polar amine compounds.
4 – polyethylenimine
Celeris polyethylenimine (PEI) columns are packed with a modified polymer, which includes several tertiary amine groups in the skeleton of the phase. The phase has nearly three times more amino groups than traditional Amino phases. This provides unique selectivity and allows for faster elution to give high-quality separations in shorter times.
5 – arginine
Celeris Arginine columns are packed with a silica surface modified with the amino acid arginine and exhibit both acidic and basic functionality. The phase has a strong affinity to hydrophilic compounds and offers a mixed-mode type of selectivity compared to other SFC phases.
The above columns provide a range of selectivity for achiral compounds using SFC. These columns can also provide alternative selectivity by using them in HILIC HPLC. With multiple chemistries, the throughput of the screening platform will improve, achieving excellent separation of various mixture of compounds, including acidic, basic and polar neutral molecules. Additionally, modern SFC instruments permit rapid column screening of different nature while altering (if required) the mobile phase. These systems are especially suited to column selection under supercritical conditions and will usually contain automated switching valves.