Enantiomeric Determination of Lactofen Using RP-HPLC–DAD and Chemometrics

A research team from the Universidad Nacional de La Plata (UNLP) and YPF Tecnologia, both in Argentina, developed an analytical method to determine the enantiomeric composition of lactofen, a commonly used herbicide. The study, published in the Journal of Chromatography Open, presents a chemometric approach to overcoming the analytical limitations commonly encountered in the assessment of commercial pesticide formulations (1).

Lactofen is a selective herbicide classified as toxicity class I by the U.S. Environmental Protection Agency. It is primarily applied to broadleaf weeds after they have emerged. Its herbicidal efficacy primarily arises from the (S)-(+)-enantiomer; however, commercial formulations typically contain both the active (S)-form and the less effective or potentially more harmful (R)-form in racemic mixtures. Given that the two enantiomers can have significantly different biological and environmental behaviors, determining their individual concentrations in formulations is essential for quality control and risk assessment.

The team developed a method that combines reversed-phase high performance liquid chromatography (RP-HPLC) with diode array detection (DAD), supported by unfolded partial least squares with residual bilinearization (U-PLS/RBL). This strategy, incorporating second-order calibration, enabled the quantification of enantiomers even in cases of overlapping chromatographic signals and in the presence of matrix interferences.

Conventional chromatographic methods often require extensive separation conditions to distinguish between enantiomers, which can result in long analysis times and high solvent consumption. In contrast, the proposed method significantly reduced the run time to 10 min while minimizing solvent use, without compromising analytical performance. Although this resulted in reduced chromatographic resolution between lactofen enantiomers (R_s = 0.78), the chemometric algorithms applied were capable of accurately deconvoluting the signals.

The study evaluated the method using calibration and validation sets representative of commercial formulation concentrations. The method demonstrated relative prediction errors between 2% and 4% and was found to be both precise and accurate. Figures of merit, including limits of detection (LOD) and quantification (LOQ), were consistent with analytical standards and regulatory requirements.

Subsequently, the method was applied to the analysis of seven commercial lactofen formulations. Results showed a high degree of concordance between measured and labeled concentrations. In some cases, minor discrepancies indicated possible formulation variation or the presence of undeclared enantiomeric impurities. The method proved capable of detecting (R)-lactofen at levels as low as 0.8–1.5% w/v, enabling the identification of impurities in products labeled as containing only the (S)-enantiomer.

The team believes that their study helps to highlight the importance of incorporating enantiomeric analysis in pesticide quality control. Approximately 30% of currently used pesticides are chiral, yet only a small fraction is marketed as pure or enriched enantiomer formulations (2). The European Food Safety Authority has begun to address this issue by recommending the separate assessment of stereoisomers (3). Nevertheless, standardized and efficient analytical techniques are required to support such regulations.

The authors conclude that second-order calibration models can be effectively applied to complex analytical problems involving signal overlap and matrix interference. The use of latent variable-based models, such as U-PLS, facilitates quantification in real-world scenarios where traditional approaches may be inadequate.

References

(1) Díaz Merino, M. E.; Peirano, S. R.; Pellegrino Vidal, R. B.; Padró, J. M.; Castells, C. B. Enantiomeric Purity Determination of Lactofen Formulations Through Chemometric Deconvolution of Partially Overlapped Chromatographic Profiles. J. Chrom. Open 2024, 6, 100144. DOI: 10.1016/j.jcoa.2024.100144

(2) Basheer, A. A. Chemical Chiral Pollution: Impact on the Society and Science and Need of the Regulations in the 21st Century. Chirality 2018, 30, 402–406. DOI: 10.1002/chir.22808

(3) E.F.S. Authority (EFSA), Bura, L.; Friel, A.; Magrans, J. O.; Parra-Morte, J. M.; Szentes, C. Guidance of EFSA on Risk Assessments for Active Substances of Plant Protection Products that have Stereoisomers as Components or Impurities and for Transformation Products of Active Substances that may have Stereoisomers. EFSA J. 2019, 17, e05804. DOI: 10.2903/j.efsa.2019.5804