Different responses of E:Z‐isomers of pesticides in liquid chromatography–electrospray ionization–mass spectrometry

Abstract

The mass spectrometric behavior of four pairs of stereoisomers was investigated by liquid chromatography-electrospray ionization-mass spectrometry (LC-ESI-MS). The E- and Z-isomers of the pesticides chlorfenvinphos, dimethomorph, mevinphos and phosphamidon-each with one double bond-were chosen for this study. The MS response of the individual isomers was investigated by infusing the isomers individually into the MS or after the separation of isomer mixtures via high-performance liquid chromatography (HPLC). In the case of dimethomorph, the same MS response was found for the two isomers. In contrast to that, the individual isomers of chlorfenvinphos, mevinphos and phosphamidon showed different MS response both in the single ion monitoring (SIM) mode in single quadrupole MS and multiple reaction monitoring (MRM) mode in tandem MS. The MS response of the isomers partly depends on (1) the declustering potential of the precursor ion in the SIM mode, (2) the selected transition and (3) the collision energy in the MRM mode. Consequently, quantification by summation of the peak areas of the isomers is inaccurate due to over- or underestimating of one of the stereoisomers. Accurate quantitative results can only be achieved when the compound-specific MS parameters are separately determined for each isomer. This can be done by using pure isomers or by the determination of the MS parameters after HPLC separation and the measurement of the actual isomer ratio with an independent technique.

Overestimation of 3α- over 3β-25-Hydroxyvitamin D3 Levels in Serum: A Mechanistic Rationale for the Different Mass Spectral Properties of the Vitamin D Epimers

“…Interestingly, a similar scenario was previously encountered by Mayer-Helm et al for pesticides, where several E / Z isomers gave different responses, which were linked to the individual stereoisomer’s dependence on declustering potential, MS/MS collision energy, and differential affinity to H + , Na + , and NH 4 + .…”

“…Flynn et al also discovered that the mobile phase composition affected overestimation of 3α-25(OH)D 3 , with larger methanol concentrations amplifying the effect. 18 Interestingly, a similar scenario was previously encountered by Mayer-Helm et al 19 for pesticides, where several E/Z isomers gave different responses, which were linked to the individual stereoisomer's dependence on declustering potential, MS/MS collision energy, and differential affinity to H + , Na + , and NH 4 + . As a result of the vitamin D epimers' response factor differences, vitamin D concentration levels of individuals will be measured inaccurately, if no dedicated isotope standards are used for each of the C-3 epimers, 20 even if the epimers are fully separated by liquid chromatography prior to mass spectrometry.…”

High‐throughput UHPLC–MS/MS method for the detection, quantification and identification of fifty‐five anabolic and androgenic steroids in equine plasma

“…Additionally, the responses of all the 17α‐epimers were noticeably lower than those of the relevant 17β‐epimers, boldenone, nandrolone and testosterone. Such differences in the response between stereoisomers such as the 17α‐ and 17β‐epimers have been reported 35–38…”

Identification and characterization of geometrical isomeric photo degradation product of eprosartan using LC-MS and LC-NMR

“…Moreover, observed minor change in J value of H-18 might be due this spatial non-bonding interaction. The supposition of isomerization was verified through correlation of the observed differential MS peak intensities of the drug and the product (Figure 2a and 2b), considering that isomers are known to show different fragment ion intensities [16,17].…”