The separation of several organophosphate pesticides on immobilized polysaccharide chiral stationary phases

Abstract

While not initially a focus or priority, in recent decades, an emphasis has been placed on the activity of individual enantiomers of widely used pesticides. Of particular note are organophosphorus-based pesticides like fenamiphos and profenofos, as examples. This work explores the enantioselective high-performance liquid chromatography (HPLC) separations of seven such organophosphorus pesticides (OP's) on the library of immobilized polysaccharide-based chiral stationary phases (CSPs) with normal phase hexane/alcohol mixtures. Further exploration of the effect of mobile phase strength and temperature on several of the separations was performed using simple factorial design. Equivalent retention of the first eluting enantiomer of several combinations of temperature and mobile phase was compared for peak shape, selectivity, and resolution. Similarly, equivalent selectivity of several combinations of temperature and mobile phase was compared for peak shape, retention of the first eluting enantiomer, and resolution. The results of this study make available several new chiral separations of the OPs included in the work that were not previously documented, including separations on the three most recently commercialized phases, Chiralpak IH, IJ, and IK. Additionally, sufficient understanding was obtained to be able to predict the trade-off of resolution, analysis time, peak sharpness (and thus improve limit-of-detection [LOD]/limit-of-quantification [LOQ]), robustness, and convenience of conditions for further application optimization.

Keywords: chiral HPLC; normal phase HPLC; organo-phosphate pesticides; polysaccharides.

FIGURE 1

Organophosphate pesticides

FIGURE 2

The separation of Crufomate on Chiralpak IA‐3 with 10% EtOH/90% hexane (top), the PDR‐chiral signal for Chiralpak IA‐3 (second from the top); the separation of Crufomate on Chiralpak IB‐N‐3 with 10% EtOH/90% hexane (second from the bottom), the PDR‐chiral signal for Chiralpak IB‐N‐3 (bottom)

FIGURE 3

Example of a two‐factoral 3 × 3 factorial design two factors—temperature and mobile phase strength—each at three levels

FIGURE 4

Effect of varying temperature at 5% IPA/hexane on selectivity and separation of profenofos on Chiralpak IC

FIGURE 5

Vant Hoff plot showing limited effect of temperature on selectivity for Fenamiphos on Chiralpak IA using EtOH/hexane mobile phase. Results were obtained using 5 × 5 factorial design; for better clarity, only three mobile phase levels (7.5% EtOH [blue], 10% EtOH [red], and 12.5% EtOH [green]) are shown

FIGURE 6

Separation of Fenamiphos on Chiralpak IC using 10% EtOH/hexane mobile phase. For this separation temperature has only a slight effect on peak width and retention. Selectivity decreases with increasing temperature

FIGURE 7

Separation of fensulfothion on Chiralpak IH at 15°C, 25°C, and 35°C using 10% EtOH/hexane mobile phase

FIGURE 8

Effect of %EtOH/hexane mobile phase on selectivity for fensulfothion on Chiralpak IA. The effect of temperature on selectivity for fensulfothion on Chiralpak IA is shown in Figure 11D

FIGURE 9

Effect of mobile phase on retention of fensulfothion on Chiralpak IA. Dashed and dotted lines show combinations of mp and temperature that would be expected to give similar retention

FIGURE 10

Combinations of mobile phase (EtOH/hexane) and temperature giving similar retention of fensulfothion on Chiralpak IA as predicted in Figure 9

FIGURE 11

(A) Vant Hoff plot of retention of first enantiomer of fensulfothion on Chiralpak IA. Dashed line indicates mp/temperature combinations that would be expected to give similar retention of 7.35 min (k 3.4). (B) Vant Hoff plot of selectivity of fensulfothion on Chiralpak IA. Note that parallel curves indicate that temperature and mp effect selectivity independently (i.e., no interaction between the factors). (C) Effect of temperature on retention of fensulfothion on Chiralpak IA. Compare with Vant Hoff plot in Figure 11A. Dashed and dotted lines indicate mp/temperature combinations that would be expected to give similar retention of 7.35 and 8.6 min, respectively (t_R‐1 is retention time, in minutes, of first eluting enantiomer). (D) Effect of temperature on selectivity of fensulfothion on Chiralpak IA. Compare with Vant Hoff plot in Figure 11B. Shown are 7.5% EtOH (blue), 10% EtOH (red), and 12.5% EtOH (green)