Can SBWC Replace HPLC in Pharmaceutical Analysis?

Subcritical water chromatography (SBWC) is seen as an increasingly enticing prospect to replace high performance liquid chromatography (HPLC) in industrial settings because of its low costs and environmental impact. Yu Yang of East Carolina University, in Greenville, North Carolina, USA, has conducted research into this process for two decades. He recently spoke to LCGC about his work, why companies should consider SBWC, and its role in pharmaceutical analysis.

Q. What is SBWC and what initially led to your interest in this technique?

A:

Theoretically speaking, subcritical water is the liquid water at conditions below water’s critical temperature of 374 °C and critical pressure of 218 atm. But in general, subcritical water is regarded as the water at 100–374 °C and moderate pressures (e.g., ~16 atm for 200 °C) that keeps water in the liquid state at elevated temperatures. In literature, other terminologies such as superheated water, pressurized hot water, and high-temperature water are also used interchangeably to refer to subcritical water (1,2). When subcritical water is employed as the sole mobile phase component, without any organic solvents, for achieving liquid chromatographic (LC) separations, this technique is termed as subcritical water chromatography or SBWC (1,2).   My interest in SBWC evolved from my Ph.D. dissertation research on supercritical fluid technology under the direction of Wolfram Baumann in Germany in the early 1990s. After earning my Ph.D., I moved to the United States and joined Steven Hawthorne’s laboratory at the Energy and Environmental Research Center (EERC) located in Grand Forks, North Dakota, where I started research on subcritical water extraction of environmental samples. During my time at the EERC, Steve’s group and Roger Smith’s group in the UK each published one of the first papers on SBWC (3,4). With Steve’s support, I started subcritical water chromatography research when I joined the Department of Chemistry at East Carolina University in 1997. Since then, my group has worked on both fundamental research and applications of subcritical water chromatography (5–27).  

Q. How does SBWC differ from reversed-phase high performance liquid chromatography (HPLC)?

A:

Subcritical water chromatography is similar to reversed-phase LC in that the stationary phase is less polar than the mobile phase. However, there are differences between SBWC and reversed-phase LC. First and foremost, only water is used as the mobile phase in SBWC while organic solvents such as methanol or acetonitrile are required in reversed-phase LC mobile phases. Because of this difference, SBWC is environmentally benign and economical because of the eliminated costs for purchasing organic solvents and waste disposal. Secondly, an oven or a column heater is required for SBWC while reversed-phase LC does not require column heating. Thirdly, a pressure restrictor or a back pressure regulator is required for SBWC to keep the water mobile phase in the liquid state when the separation temperature exceeds 100 °C.   An ordinary HPLC or reversed-phase LC system can be easily converted into an SBWC system. Nowadays, many HPLC systems sold on the market have a built-in column heater; some of the heating compartments are capable of heating the column up to 150 °C.   
   

Q. In your opinion why is SBWC gaining greater attention?

A:

HPLC is a multi-billion dollar industry and used in almost all analytical laboratories in virtually every industry. However, in strictly economic terms if you can get the same results from a cheaper process then it is worth looking at, especially if it has less environmental impact. The green nature and the low costs of the SBWC technology are appealing to all parties: industry, government agencies, and customers. Companies want to reduce their costs and remove as much risk as possible for their staff, government agencies want industrial companies to reduce their environmental impact, as do customers, and hopefully the savings in solvents and waste disposal would result in lower prices for customers.   

Q. Your most recent research demonstrated the separation and analysis of pharmaceuticals used in “common cold” drugs. How well did SBWC separate the pharmaceutical components? 

A:

Very well, excellent separation and quantification results were achieved using SBWC (15). In this work, pharmaceuticals present in cold drugs – including P&G Vicks formula 44 custom care cough and cold syrup, Bayer Alka-Seltzer plus cold and flu formula capsules, and CVS multi-symptom severe cold relief caplets were successfully separated using SBWC. Both gradient elution and programmed temperatures were used to achieve the best SBWC separation of pharmaceuticals contained in the cold drug samples studied (15).  The recoveries of the active pharmaceutical ingredients (APIs) in the three real-world cold drug samples achieved by SBWC range from 94% to 105% with less than 5% RSD (15).  

Q. And how does this process compare to the separation, which would be achieved with the more traditional approach of HPLC?

A:

The results achieved by SBWC for the active pharmaceutical ingredients reported in our paper are comparable to those achieved by traditional reversed-phase LC or HPLC. The results in our earlier work on the separation of niacinamide contained in skincare creams revealed that not only did SBWC achieve comparable quantitative results to those obtained by HPLC, but also SBWC yielded better separation efficiency (16).  

Q. Do you think SBWC is suitable for other areas of pharmaceutical analysis?

A:

SBWC is suitable for polar and moderately polar APIs that are labile at elevated temperatures. However, SBWC separation of drug products such as impurities and degradants would be much more challenging because the identity of such compounds are normally unknown and subcritical water may be too weak to elute them. Therefore, at this stage I do not recommend SBWC for drug products other than polar and moderately polar APIs.    

Q. Are there any limitations to SBWC?

A:

Like any other techniques, SBWC has its limitations. Since high temperature is required in SBWC, the main concern is the potential degradation of the stationary phases and analytes. The good news is that several recently developed more thermally stable packing materials can stand high temperatures for a certain prolonged period of time as summarized in our review articles (1,2,28–32). However, more durable, efficient, and thermally stable stationary phases are the key for further SBWC developments.   To ensure the reliability and accuracy of SBWC analysis, analyte degradation during the SBWC process needs to be evaluated before the SBWC method can be recommended.    Although the polarity of water is decreased by increasing the temperature, high-temperature water is still too polar for nonpolar solutes. Therefore, SBWC is limited to separation of only polar and moderately polar solutes.  

Q. You mention that SBWC has great potential for industrial applications, especially in replacing existing HPLC methods, do you feel companies will be increasingly turning to SBWC in the future as a result of its benefits?

A:

I have no doubt that more companies will take a good look at SBWC and embrace this green chromatography technique. Early in my career when I presented SBWC research findings at conferences, the audience did not normally pay much attention. Water as the sole mobile phase component for HPLC? That sounds like a joke. Water is way too polar to get the job done. As more chromatographers got to know this emerging technique, the perception gradually changed. Editors started inviting me to write reviews on SBWC. Since 2003, I have published five invited review articles that partially covered SBWC (28–32). I have also published two lengthy and thorough invited reviews on SBWC since 2007 (1,2). All of these made a difference as evidenced by the Procter & Gamble funding of my SBWC research (16–18). Other companies consulting the SBWC technique have also contacted me. I will not be surprised if one day SBWC is adopted by industry as one of the routine separation and analysis techniques for certain classes of analytes. Reviews involving SBWC by other authors can also be found in the literature (33–38).  
   

Q. What are you currently working on?

A:

My laboratory at East Carolina University runs comprehensive research on subcritical water separation technology. Current programmes include the industrial application of subcritical water chromatography, development of new durable stationary phases for SBWC, and subcritical water extraction of medicinal herbs. To ensure that the subcritical water chromatography and extraction methods developed are feasible and reliable, we also conduct fundamental research such as organic solubility and stability under subcritical water conditions. The outcomes of this fundamental research will further direct and facilitate the application of subcritical water separation technology and I'm sure only add to the appeal of SBWC.  

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Dr. Yu “Frank” Yang

 is a professor in the Department of Chemistry at East Carolina University, located in Greenville, North Carolina, USA. He received his Ph.D. in analytical chemistry from Johannes Gutenberg University of Mainz, Germany, in 1993 and joined the Department of Chemistry at East Carolina University in 1997. Dr. Yang's principal areas of interest and expertise include green chemistry, environmental chemistry, subcritical water chromatography and extraction, organic solubility and stability in subcritical water, and pharmaceutical analysis. Honours include the University Five-Year Achievement for Excellence in Research Award and the UNC Board of Governors Distinguished Professor for Teaching Award.