One of the most powerful analytical instruments available to chemists is the HPLC – the high pressure liquid chromatograph. The theory behind HPLC is easy enough to understand. A long, thin tube is filled with an adsorbent of some kind – sometimes silica gel, sometimes long waxy components – and then it is inserted into a high pressure arrangement of tubing. A liquid solvent is then flushed through the system as the pressure is continuously increased until finally it is at the operating pressure. The high pressure allows the solvent to permeate and trickle through the tubing that is packed with the solid material – the “column”, in the parlance of chemistry. A sample of an unknown mixture of compounds – maybe a drop or two of an unknown flask, or the crude reaction product from an organic synthesis – is then injected via syringe at one end of the column. The pressure then pushes this collection of unknown molecules through the column and out the other side, where the presence of organic molecules is noted by a detector and displayed on a computer monitor.
The key to chromatography is that different molecules interact with the solid support of the column in different ways. Some molecules may latch on to the solid support, requiring an extraordinary amount of time to be finally pushed all the way through the column. Some molecules may not interact with the column support at all, and are flushed straight through the column very rapidly. As a result, while the mixture was injected onto the start of the column all at the same time, the individual components of the mixture come out of the other end of the column at different times. This allows a chemist to separate the various components, as each portion can be collected in different flasks. You might have one flask which consists of minutes 1-5, another for minutes 6-10, and another for minutes 10 until the end of the analysis (which normally runs to about 15 minutes). Because the components come out at different times, you have all of component #1 in the first flask, #2 in the second, and so forth.
The most common solvent that is used to flush the compounds out of the column is a chemical called acetonitrile. It’s the most popular choice as it has the perfect set of characteristics (solvent viscosity, UV absorbance, boiling point) to make the HPLC separation the absolutely best that it can be. As the components in an unknown mixture may be almost identical except for one tiny difference, it is important to maximum the chances for optimum separation. Since acetonitrile gives the best possible chance for separations, it’s been the #1 choice of organic solvent for HPLC. However, acetonitrile is not so ideal when you consider that it’s quite toxic. The molecular structure of acetonitrile consists of a carbon connected to a cyanide group. It’s not the most pleasant solvent to use, and although one individual HPLC may not generate a great deal of waste (maybe a liter per day, depending on how many samples are analyzed), many modern pharmaceutical companies have hundreds of HPLC instruments all working in unison. This translates to barrel after barrel of hazardous acetonitrile waste that has to be disposed of in a safe manner. It’s expensive and it contributes to environmental pollution.
I was therefore intrigued by an article I read in the journal Green Chemistry that took the stance of perhaps using alternative solvents. So many advances have been made in recent years that increase the performance of HPLCs, that an argument can be made that acetonitrile may not be as necessary anymore. The use of better column packing, higher pressure, and stricter temperature control during the separation have given chemists are bit of breathing room. It’s usually quite easy to get fantastic separation between various components in an unknown mixture. Therefore, the article argued, maybe it’s time for chemists to switch to a safer, less toxic solvent such as ethanol. It’s a valid argument. Many chemists may not be willing to give up any performance, but the rising costs of waste disposal mean that it may soon become an economical necessity. It will be interesting to see if this call to reason is followed by any of the large pharmaceutical firms, as they are the primary users of HPLC instrumentation and they are the ones who could make the largest impact in this particular area of waste reduction.