In my last post, celebrating the blog’s fifth birthday, I reminisced about the history of chromatography. Over the years, there have been shifts in the analytes of interest to Chemists, as well as the best methods of purification. In this blog, I want to look at Supercritical Fluid Chromatography (SFC) and how the method is currently witnessing a revival due to its many unique features. SFC has taken a back seat over the years and has often been overlooked due to the popularity of HPLC. Read on to find out what happened and why SFC is making a resurgence – Let’s get critical!

I recently rediscovered my old Betamax videotapes in the garage, and the younger members of my family looked suitably perplexed when I told them that this box of dusty old plastic relics was my old movie collection. Even the members of my family who could remember VHS tapes looked puzzled at the narrower odd-looking format. As anyone who bought into Betamax at the time will tell you, the format was superior to VHS, offering a higher resolution. It was aimed at bringing the playback and recording of broadcast-quality media into the home. Sadly though, it never took on for several reasons; the format was more costly to produce at the time, and many movie studios had heavily invested in the VHS format. Over the years, it fell by the wayside. A chromatographic method that also offered a higher resolution than its alternatives almost suffered a similar fate.

Traditional gravity-fed liquid chromatography, performed since the beginning of the 1900s, took hours or even days to complete. Gas Chromatography (GC) became the preferred method as it was more powerful; however, there were issues regarding the thermal stability of the solutes used. Throughout the 60s and 70s, there was an attempt to improve liquid chromatography and get past the pitfalls of gas chromatography. This led to the development of High-Performance Liquid Chromatography (HPLC), which became ubiquitous and is still the method used today for a significant amount of separation and purification needs. Around the same time HPLC took off, however, another promising method had the potential to supersede both HPLC and GC. Chemists had realized the potential of using a Supercritical Fluid as opposed to liquid or gas.

Supercritical fluids were first used as eluents for chromatographic separations in 1962, and the process later became known as Supercritical Fluid Chromatography (SFC). Substances have a specific temperature at which they can no longer remain as a liquid – no matter how much the pressure is increased – known as the critical temperature. There is also a critical pressure above which a substance can no longer remain a gas, no matter how high the temperature is. These pressures and temperatures are indicated as the critical point on the graph below.

Raising the temperature of a gas or compressing a liquid beyond this critical point results in a supercritical fluid. The substance enters a liquid-gas equilibrium possessing three unique qualities. It has increased density (much like a liquid), increased diffusivity (like that of a gas), and a lowered viscosity (on the scale of a gas). An ideal mobile phase must mix well with other particles, transfer the sample through the column and elute it, ideally at speed. Chemists could see the potential for supercritical fluids due to the unique properties of the substance. The solvating power is like conventional organic solvents and higher than that of gases. The diffusion coefficient and viscosity are far higher than in liquids. Chemists knew these properties were very promising; they saw potential in developing methods that relied not only on the composition of the mobile phase but also the temperature and pressure. Many believed that SFC would replace HPLC altogether, much like I thought that Betamax would replace VHS. However, the instruments required to handle supercritical fluids were more complex than those required for other methods. The conditions need to be just right to maintain a supercritical fluid, like Goldilocks’ porridge.

SFC instruments gained some traction in the late 80s, and experimentation results proved the method’s usefulness. Numerous gases were tested, such as He, N₂, CO₂, and NH₃. CO₂ was found to be ideal due to its low critical temperature of 31 °C and relatively low critical pressure of 72.8 atm. It was also non-explosive, non-flammable, less toxic, cheap, and had a low response in detection systems. SFC was also identified as having great promise for the separation of chiral compounds and organometallics. Early instruments, however, struggled to maintain the temperatures and pressures required. They were expensive, not very user-friendly, and plagued with poor reproducibility. This led to SFC falling by the wayside, much like Betamax. It was seen as inefficient and expensive, and HPLC continued to dominate.

The obscure Betamax format was not the end of the story for high-resolution movie playback. Sony persisted with high-definition playback and created Blu-ray, which eventually became the most popular format. Similarly, SFC did not disappear and, more recently, has been making a resurgence. Current instruments are as robust as HPLC instruments, and technological improvements have eliminated the issues that plagued earlier systems. Control of composition, flow, temperature, and pressure is now very accurate. The physical and chemical properties of supercritical fluids are also better understood. Nowadays, SFC systems can perform many of the tasks of HPLC systems and have several advantages. SFC offers shorter separation times, high separation efficiency, and is easy to scale up. It is suitable for the separation of isomers and structurally similar analytes. The ability to adjust temperature and pressure offers a high degree of adaptability, and using CO₂ as a mobile phase is more cost-effective and environmentally friendly than HPLC alternatives.

It almost sounds as if SFC has superseded HPLC in every area; however, there are still areas whereby HPLC may be preferable. The CO₂ used for SFC is very non-polar, meaning it is unable to elute polar compounds. To overcome this, modifiers are used to increase the polarity, the most popular being methanol. Modifiers raise the critical pressure and temperature of the mobile phase; therefore, SFC is not ideal for extremely polar solutes as the amount of modifier needed would require such high temperatures and pressures that the method would become unsafe. As it stands, HPLC can handle certain analytes that SFC cannot and vice versa. Therefore, SFC and HPLC should not be regarded as competitive techniques; they are complementary. However, the huge advantages offered by SFC mean that for most analytes, HLPC can be replaced with the faster and more environmentally friendly solution.

Till next time,