Optimization of analytical method greenness scores: a case study of amino acid enantioseparations with carbonated aqueous systems

Abstract

Analytical and preparative separation techniques, although perceived as less detrimental compared to industrial manufacturing processes, present a substantial concealed environmental threat. Green analytical chemistry has seen major advances in the past five years in terms of proposing metrics for measuring the greenness of separation methods. One such comprehensive measure is the Analytical Method Greenness Score (AMGS), which was used to benchmark current enantiomeric separation methods in the literature. In this work, we propose pragmatic and mathematical strategies to minimize AMGSs in high-performance liquid chromatographic enantioseparations. A case study of more than 456 chromatograms from the enantiomers of 38 proteo- and non-proteogenic amino acids was generated and assessed. A sustainable method of generating carbonated water-based eluents was introduced, and the H₂CO₃* additive was shown to improve the chromatographic figures of merit (resolution and efficiency) while lowering the AMGS. We show that narrow diameter columns with superficially porous particles reduced the solvent waste 12-fold compared to traditional analyses. The AMGS formula was modified to incorporate the “cycle time” of the chromatograph, which provides a more accurate picture of solvent waste generation in high throughput chemical analyses. Using the principles of mathematical optimization, the AMGS was minimized with respect to flow rate to show that the ideal separation speed differs depending on the solvent composition. The AMGS reached values as low as 1.2 for ultrafast (<15 s) amino acid chiral separations, where 79% was contributed by the cycle time. This AMGS minimization adds a new optimization problem to future method development, with widespread implications for drug development research and other production fields reliant on separation sciences.