ABSTRACT

The Analytical Chemistry Laboratory at Pantex Plant utilizes a variety of solvents as mobile phase in High Pressure Liquid Chromatograph (HPLC) instruments to analyze high explosives (HE). A process was developed by two Pantex employees to recycle the solvents in a close-loop system resulting in 88-96% reduction in hazardous solvent waste and savings of . This in-house developed recycling method cost less than $100 to set-up, uses ordinary labware and very simple to operate and still 25-80% more efficient in recycling solvents than commercially available systems costing more than $3,000.

For many years, High Pressure Liquid Chromatography (HPLC) has shown its worth in the separation of sample constituents in complex mixtures of low volatility organic compounds. Indeed, its usefulness has been enhanced by the development of both normal-phase and reverse-phase eluents and columns. Those eluents’ compositions can be tailored to resolve chromatographic peaks. However, the large amounts of solvents consumed during HPLC method development and subsequent use create a significant waste stream volume that requires disposal. Depending upon the analysis, the eluent and dissolved sample constituents may be composed of compounds that are currently classified as hazardous under the Federal Resource Conservation and Recovery Act (RCRA) and thus are expensive to dispose of as waste.

Given the solvent costs associated with HPLC operation, recycling of eluent may be an attractive alternative to disposal. The recycling of the eluent would reduce the cost of solvent purchase and the volume of HPLC eluent waste requiring disposal. With this intent, prototype closed-loop eluent reclamation systems were established for three of the most used HPLCs in our laboratory. Attempts to develop a reclamation process focused on the removal of dissolved sample constituents from the eluent by their adsorption onto activated carbon. Previous experiments performed in our laboratory showed that organic compounds could be removed from a waste stream by using activated carbon.(1) Therefore, an activated-carbon column inserted into a HPLC system may restore the eluent close to its original purity. The recovered eluent could then be reintroduced into the HPLC system, creating a closed-loop process.

An experiment was conducted comparing the analytical precision and accuracy of three different methods of solvent delivery in HPLC systems. The first series of analyses (A) were performed without the reuse of a mobile phase. Another series of analyses (B) were made by direct return of eluent to the eluent reservoir. In the final series of analyses (C), the eluent was passed through an activated-carbon column in a closed-loop configuration.

A second recycling experiment was designed to determine how much eluent could be saved and reused over a one-year period using three of the most used HPLC systems in our laboratory. The first and second HPLC systems were the same as in Experiment #1. In the first of the LDC systems, the normal-phase eluent was a mixture of 49% heptane, 34% n-butyl chloride, and 17% acetonitrile. The second LDC system was equipped with ODS column. This system used a reverse-phase eluent consisting of 55% water and 45% acetonitrile. The third chromatography system was an Axxiom 747 HPLC with Alltech Octadecyl (C18). The reverse-phase eluent composition was the same as the previous instrument.

Various organic explosives were injected into each of the three HPLC systems over a year. Along with HNS and DPE, HMX and RDX, BTF, FEFO, and PETN were assayed in the closed-loop systems.. The carbon clean-up column on each system was packed with Calgon F-300 (2.38 mm x 0.595 mm) activated carbon(2), which functions to adsorb these injected constituents from the eluent.

As shown in Figure 3.1, the flow of eluent through each closed-loop system begins in the eluent reservoir. From the reservoir, the eluent passes through the HPLC pumps, through a solvent mixer. Then, the eluent travels from the HPLC column, through the variable-wavelength UV detector, through an activated carbon column and returns to the eluent reservoir. The filtered eluent is then reintroduced into the HPLC pumps.

4.0 RESULTS AND DISCUSSION

As with any procedure, there are limitations. For example, the closed-loop system is impractical for gradient elutions or for method development where the eluent compositions need to be varied to affect separations. The systems are, however, ideally suited for dedicated HPLC systems performing routine analysis with isocratic eluents.

Cost of eluents calculated are shown in Table 4.2. .

The savings realized due to eluent reuse are shown in Table 4.3 for both normal-phase and reverse-phase eluents.

5.0 CONCLUSION

The data from Experiment #1 show that a high standard of performance for routine HPLC analysis can be maintained when using activated carbon to clean up the eluent. Experiment #2 shows that waste generated by HPLC analysis can be significantly reduced with eluent recycling. In one example, more than 96% of the eluent volume normally used was saved. A cost comparison shows significant (81%) cost savings are also achieved. The closed-loop activated-carbon eluent reuse system meets our laboratory's goals of waste-stream minimization and of HPLC analysis cost reduction while providing high quality data.

6.0 ACKNOWLEDGMENTS

The author wishes to acknowledge C. L. Schaffer and W. T. Quinlin of the Applied Technology Division of Mason & Hanger Company at Pantex Plant. Mr. Schaffer and Mr. Quinlin were responsible for all aspects of development of this waste solvent recycling technique.

7.0 REFERENCES