Analysis of herbicides, insecticides, and fertilizers via 31P benchtop NMR

Due to their roles as crucial agricultural components, pesticides and fertilizers have been the subject of much study.¹ As subclasses of pesticides, herbicides and insecticides have additionally been on the receiving end of much criticism over the years.² These products are controlled by both health and regulatory agencies, and several methods exist for the qualitative and quantitative analysis of their active ingredients, as well as the rest of their commercial formulations.³

While ¹H remains the most widely used nuclide for NMR, heteronuclear NMR offers an elegant alternative for these analyses.⁴ Heteronuclides have much larger chemical shift windows than protons, which minimizes signal overlap and is ideal for peak identification and quantification in mixtures. Organophosphates form an important class of pesticides and fertilizers commonly used by farmer across the world,⁵ and their active ingredients are particularly well-suited for ³¹P NMR analysis. The quantification of phosphorus using ³¹P NMR is straightforward, provided that the internal calibrant used has a known purity,⁶ is soluble and stable in the medium of analysis, and that its signals do not overlap with the others in the sample. For more information about qNMR, please see our previous blog posts on the subject (Part I, Part II, Part III).

One of the most popular herbicides in the world, Roundup®, contains the active ingredient glyphosate, present as the isopropylamine salt. The glyphosate content in Roundup® Super Concentrate is listed as 356 g/L, with the other primary component being water. As such, water-soluble ammonium phosphate monobasic (APM) was used as the internal calibrant to quantify the glyphosate content in this product. The qNMR results of this analysis provided a glyphosate mass of 16.6 mg (average of triplicate runs), matching the theoretical amount of 16.9 mg very closely. One of the ³¹P spectra used for qNMR is presented in Figure 1.

Figure 1. ³¹P (24.3 MHz) NMR spectrum of Roundup® Super Concentrate and ammonium phosphate monobasic in D₂O, used for quantification of glyphosate.

A very common household insecticide, 50% Malathion Concentrate® can also be analyzed and quantified using this same approach. Although this product is formulated as a 50% mixture in petroleum distillates, a single resonance is observed as we are only observing phosphorus-containing species. A stock solution of triphenylphosphine in toluene-d₈ was prepared and a precise amount was added to an accurately weighed solution of the malathion product. An average composition of 48.9% malathion (wt%) was calculated (average of triplicate runs), which matches the composition on the label very closely (50%). One of the ³¹P NMR spectra used for qNMR is presented in Figure 2.

Figure 2. ³¹P (24.3 MHz) NMR spectrum of 50% Malathion Concentrate Insecticide® and triphenylphosphine in toluene-d₈, used for quantification of malathion.

As demonstrated, the use of heteronuclear NMR is a very powerful way of analyzing molecules that might otherwise exhibit complex ¹H spectra, or whose spectra might be overwhelmed by the other components in the mixture, such as water or petroleum distillates. This is particularly applicable to many herbicides, insecticides, and fertilizers as these often contain heteroatoms like phosphorus or fluorine. For a more detailed description of these experiments, as well as some additional studies, please see our application note on this subject. If you have any questions about the analyses presented herein using benchtop NMR, or about how you could incorporate our instruments into your workflow, please don’t hesitate to reach out to us!

References

[1]  Geisseler, D.; Scow, K. M. Soil Biol. Biochem. 2014, 75, 54–63.
[2]  Van Wijngaarden, R. P. A.; Brock, T. C. M.; Van Den Brink, P. J. Ecotoxicology 2005, 14, 355–380.
[3]  Alder, L.; Greulich, K.; Kempe, G.; Vieth, B. Mass Spectrom. Rev. 2006, 25, 838–865.
[4]  Cade-Menun, B. J.; Carter, M. R.; James, D. C.; Liu, C. W. J. Environ. Qual. 2010, 39, 1647–1656.
[5]  Kwong, T. C. Ther. Drug Monit. 2002, 24, 144–149.
[6]  Pauli, G. F.; Chen, S. N.; Simmler, C.; Lankin, D. C.; Gödecke, T.; Jaki, B. U.; Friesen, J. B.; McAlpine, J. B.; Napolitano, J. G. J. Med. Chem. 2014, 57, 9220–9231.