Ensuring the Purity of Acetonitrile During Quality Control Testing

Acetonitrile is a critical solvent for a number of important technologies. Its low viscosity and low chemical reactivity make it a popular choice as a mobile phase for liquid chromatography. It also plays an important role as the key solvent used in the manufacture of DNA oligonucleotides from monomers. Industrially, it is used as a solvent in manufacturing pharmaceuticals. In addition, acetonitrile is used in military lithium battery manufacturing because of its relatively high dielectric constant and ability to dissolve electrolytes. For similar reasons it is widely used in cyclic voltammetry associated with the manufacture of ultracapacitors. In all of these applications, the purity and consistency of the reagent is critical.

Industrial grades of acetonitrile are produced as a by-product of acrylonitrile manufacturing. The initial by-product material is highly contaminated and often contains only about 60% acetonitrile. The most common impurities are nitriles, aromatics, oxyzole, carbonyls, heterocycles, water, and surfactants and plasticizers picked up during handling. High-purity acetonitrile grades are produced from these industrial by-product streams and contain residual levels of these impurities. The impurities found in commercial acetonitrile grades may cause problems in sensitive products or processes by causing undesirable side reactions, by binding to separation media or complex biomolecules, or by interfering with detectors.

Bulk manufacturers , such as Purification Technologies, Inc. (PTI) (Chester, CT), perform high-volume purification and repackaging of acetonitrile, as well as methanol, and other ultrahigh-purity products. Solvents are sold in bulk tank wagons up to 25,000 L and isotankers up to 20,000 L in volume. Customers include laboratory suppliers, large-scale oligonucleotide synthesis companies, lithium battery manufacturers, and specialty polymer companies.

The purity and consistency of acetonitrile in bulk tanker quantities must be of a quality that ensures optimum performance, which PTI achieves using a patented process that produces ultrahigh-purity acetonitrile from a variety of foreign and domestic feedstocks. Because quality varies significantly among different suppliers, the company must rigorously test incoming raw materials to ensure they meet its quality standards. To do this, the company employs liquid chromatography during critical stages of the production process.

Improving quality control

Because acetonitrile is used for trace-level analyses in thousands of different applications, impure acetonitrile can negatively influence the results of laboratory tests. Manufacturers could suffer the loss of entire lots of product if the solvents employed in their production contained unanticipated chemical compounds.

To ensure the continued supply of acetonitrile of the highest possible quality to customers and differentiate itself from competitors, PTI developed very sensitive LC methods. To test for impurities in its acetonitrile solvent, the company developed and utilized an HPLC gradient test, which required 2 hr per sample, with many samples required weekly. If LC identified a contaminant, the method had to be run twice in order to properly purge and reequilibrate the LC system.

Figure 1 - Prior to purification, a UPLC method is used to profile certain acetonitrile impurities in the feedstock. The chromatogram shows a 3-D plot from an isocratic UPLC analysis of common impurities in commercial acetonitrile.

Figure 2 - The chromatograms show the sensitivity of the UPLC gradient test and the low impurity levels of purified acetonitrile. The chromatogram is an overlay of a finished product chromatogram (black) vs a 10-ppb toluene standard (blue).

In 2005, Jerry Richard, PTI President, heard about the Waters® (Milford, MA) then-year-old ACQUITY UPLC® system. Utilizing sub-2-µm pressure tolerant particles, high-pressure fluidic modules, and high-sensitivity detectors, UPLC® increases speed, resolution, and sensitivity compared to traditional HPLC.

Jerry and his technical director decided to investigate. With samples of their acetonitrile in hand, they visited Waters applications laboratories for a demonstration. They found trace-level impurities that had not been detected by their HPLC system. PTI subsequently installed the ACQUITY UPLC® technology, incorporating the system into their QC testing process. 

PTI’s QC testing involves taking samples at various stages of the purification process, so as not to interrupt the process, to ensure specific impurity specifications are being met along the way (see Figure 1) . Using ACS methodology, the gradient method, involving a mix of acetonitrile/water and gradually ramping the percentage of acetonitrile/water to 98%/2%, is designed to preconcentrate any impurities on the head of the column before eluting them off the column and into the PDA detector. The analyst then looks for an absence of specific peaks, which indicates the level of purity of the acetonitrile. PTI is now more certain of results because UPLC can detect impurities at a lower level (1–10 ppb) than traditional HPLC (10–100 ppb) (see Figure 2). 

UPLC has had a significant impact on the company’s operations. It drives PTI’s purification process by allowing users to react to situations much more quickly. This ensures that the company’s refined product meets the client’s purity requirements.

Conclusion

After installing the ACQUITY UPLC system, PTI noted a number of additional benefits to its Quality Control laboratory , which included:

  • Greater throughput—The 2-hr HPLC gradient QC test is now completed in 24 min, an 80% reduction
  • Significant improvement in sensitivity—A gradient test with a 1-ppb toluene standard is undetectable by HPLC but registers approximately 5 mAU with UPLC
  • Risk reduction—Better product QC data are available faster, allowing PTI to recognize problems sooner (12 vs 100 min), which makes it easier to minimize or eliminate the occurrence of problems in the production process
  • Improved productivity—The LC analyst is able to complete sample testing more quickly, and thus can focus efforts in other areas of the laboratory or process, such as analyzing every acetonitrile container of 20–200 L rather than just a sample of containers
  • More effective utilization of capital investment—The UPLC system handles the workload that previously required two HPLC instrument systems
  • Method development—Faster run times allow for more comprehensive evaluation of method validation parameters
  • Decreased laboratory waste.

Mr. Eaton is Marketing Manager, Chemical Analysis & Informatics, Waters Corp., 34 Maple St., Milford, MA 01757, U.S.A.; tel.: 508-482-2950; fax: 508-482-2674; e-mail: [email protected].