HPLC Tech Tip

Troubleshooting the Acetonitrile Shortage

By Rick Lake, Pharmaceutical Marketing Manager

Short-term approaches to reducing acetonitrile use.
Long-term strategies for developing methods with alternate solvents.

The current acetonitrile shortage has many labs scrambling to adjust to limited supplies and increased costs. This preferred organic solvent is a byproduct of plastics manufacturing and the current shortage is caused in part by the economic downturn affecting that industry. Considering the large number of analyses that are validated using acetonitrile-containing mobile phases (it is the leading organic solvent in reversed phase HPLC and HILIC separations), the shortage could have significant impact on basic research, safety testing, and the availability of many consumer goods. Regardless of its duration, the current acetonitrile shortage is an opportunity to review lab practices and explore both short- and long-term responses to the limitation of a critical resource. Here we discuss how solvent consumption can be reduced over the short-term through column scaling and UHPLC, as well as how to develop long-term security by adopting method development strategies based on alternative solvent systems.

Lowering Solvent Use in Existing Methods

As a short-term solution, decreasing the internal diameter of the analytical column is one of the easiest ways to reduce solvent consumption in a developed method. Larger diameter columns require higher flow rates, and thus larger volumes of mobile phase, to reach the desired linear velocity. Typically, a conventional analytical column of 4.0 or 4.6mm internal diameter (ID) is used. By scaling down to a 3.2mm ID column, we can significantly reduce the flow rate and solvent volume needed to reach the same optimal linear velocity, without increasing run time. The new flow rate can be easily determined using Equation 1. For example, if a 4.6mm ID column is being used with a 1.0mL/min. flow rate, the same linear velocity is achieved with a 3.2mm column using a flow rate of 0.48mL/min. This results in a mobile phase reduction of 52%--a considerable solvent cost savings. It also results in less solvent waste generation, another cost savings, and better system performance. Typically, 2-3 fold increases in sensitivity can be expected when injecting the same sample mass on a smaller ID column. The performance of most LC/MS interfaces is enhanced by lower flow rates as well.

Note that while solvent use could actually be reduced 80% by using 2.1mm ID columns, the entire analytical system must be able to accommodate these narrow bore columns. This means that any extra column volume must be minimized (in tubing, connectors, etc.) and a microflow cell must be used in the HPLC detector. This is especially critical when using gradient mobile phase programs because the system dwell volume, or the volume contained between the pumps and the analytical column, becomes a significant factor. Large dwell volumes cannot be swept through quickly enough with low flow rates, making gradients impractical with narrow bore columns. To adjust the effective system dwell volume, simply scale it down by the ratio of the column volumes (Vc2/Vc1). Reducing system dwell volume is generally accomplished by using smaller internal diameter tubing or smaller volume mixing chambers. For more information on lowering system dwell volumes, consult your instrument manufacturer.

For more detail, see our Tech Tip "Reducing Column Internal Diameter"

Strategic Platform Choices

Another way to reduce acetonitrile consumption that can be applied in the short-term is to scale traditional HPLC methods down to UHPLC. UHPLC uses narrow bore columns (typically 2.1mm) packed with smaller silica particles (under 2µm in diameter). While using narrow bore HPLC columns can greatly reduce solvent consumption by decreasing the flow rate as discussed above, this is not the case with UHPLC. Rather, as particle size decreases, the Van Deemter plot changes, resulting in higher optimal linear velocities. Therefore, even though the column ID is smaller in UHPLC, the analyses are typically run at higher linear velocities to get optimal performance and, therefore, the flow rates can reach those used in conventional HPLC. In the case of UHPLC, the solvent reduction occurs not as a result of lower flow rates, but as a result of shortened analysis time. UHPLC, due to greater efficiency and linear velocity, can reduce analysis time by as much as 5-10 fold, reducing the overall volume of solvent required (Figure 1). For more information on the practical application of UHPLC, visit www.restek.com/uhplc.