Capillary electrophoresis quantification of fluoride residues following sulfuryl fluoride treatment of grain.

Even with the implementation of integrated pest management (IPM) programs over the last 50 years, postharvest fumigation is still an essential tool for disinfesting durable goods, such as cereal grains. Raw product must be fumigated for disinfestation of “production” insect pests within hours or days after harvest, prior to storage. Once in storage, fumigations are conducted for disinfestation of “stored product” pests whenever the need arises. Like other pesticide labels, those for fumigants are parameterized to address applicator, worker, bystander, and environmental health concerns. Fumigants labels specify a minimum and/or maximum allowed applied dose, exposure, duration, temperature, or combinations thereof.

Enforcing label-permitted use of fumigants through maximum residue levels (MRLs) can be complicated, due in part to their high volatility. By the time a regulatory body collects a sample, there may be no residue of the parent fumigant, even if applied above maximum label allowance. In some cases, where the fumigant degradation results in the formation of an inorganic salt, this problem is addressed by defining the residue as the sum of fumigant and the inorganic salt; however, this can present a whole new set of challenges. Fluoride residues resulting from the postharvest treatment with sulfuryl fluoride (SF) are an excellent example.

Sulfuryl fluoride (SF) is commonly used for the postharvest control of insect pests in low moisture “durable” commodities such as cereal grain, tree nuts, dried fruit, and cocoa) (U.S. EPA, 2016a); Eisenbrandt and Hotchkiss (2010). With a vapor pressure of 12,087 mmHg and a K_ow of 0.14 at 20 °C, an applied dose of sulfuryl fluoride (typically 50 – 128 g/m³) typically reaches equilibrium between the enclosure headspace and the commodity surface within an hour of application. Sorbed SF is subject to heterogeneously-catalyzed hydrolysis to yield fluoride and fluorosulfate ions; while further hydrolysis of fluorosulfate to sulfate and an additional fluoride is possible, this happens at a much slower relative rate(Chen et al., 2024a, Chen et al., 2024b); Eisenbrandt and Hotchkiss (2010); Nie et al. (2014). After the end of the fumigation treatment, which in the US can’t exceed a Ct exposure of 1500 g h/m³, the enclosure headspace is vented until the chamber SF concentration is below the re-entry level of 1 ppmv. Treated commodity can then be moved into storage or through channels of trade (U.S. EPA, 2016a).

A residue of fluoride found in food can be attributed to a number of sources, both biogenic and anthropogenic, besides SF exposure Ahmad et al. (2022); (Anastassiadou et al., 2021); Donohue and Duke (2010); Margherita et al. (2021). A natural environmental contribution comes from mineral forms of fluoride found in the soil and water. Another contribution could come from other agrochemicals such as cryolite (Na₃AlF₆). Fluoride exposure is a subject of recent regulatory interest to both the US Enivronmental Protection Agency (U.S. EPA, 2010); Donohue and Duke (2010) and EFSA (Anastassiadou et al., 2021). While fluoride in drinking water (and in beverages consisting mostly of water) is the main source of dietary exposure to fluoride Donohue and Duke (2010), residues from foodstuffs have the potential to increase dietary exposure past recommended amounts (U.S. EPA, 2010). Accordingly, understanding the relative contribution of natural “background” versus anthropogenic, and particularly, agrochemical-derived levels of fluoride in various food stuffs is of interest. EFSA has recently requested such data for a variety of SF treated foodstuffs, including grain (Anastassiadou et al., 2021).

Fluoride can be difficult to quantitate in foodstuffs; the use of common methods of analysis such as liquid chromatography – mass spectrometry (LC-MS) or inductively coupled plasma – mass spectrometry (ICP-MS) are not practical due, respectively, to low molecular weight and high ionization potential. Ion selective electrodes (ISEs) seem to be the most frequently utilized method for fluoride analysis in food Lopez and Navia (2009); (EU Reference Lab for Single Residues, 2023); Rocha et al. (2013); Singer and Ophaug (1986), with can afford less matrix interference than ion chromatography (IC). F-ISE measurements are time consuming, however, have low sensitivity, especially when a high level of dilution is required to reduce matrix effect, and can only be used for the analysis of one analyte at a time. Capillary electrophoresis Guimarães et al. (2009); Jaramillo et al. (2021); Pobozy and Trojanowicz (2021); Travassos Lemos et al. (2015) (CE) offers an alternate mechanism of separation that is automated, sensitive, and flexible while generating minimal waste. Coupling CE with contactless conductivity detection (C4D) allows for enhanced sensitivity, compared to previous work using indirect-UV detection. Another benefit of CE analysis, compared to ISE, is that it can be expanded to cover more than just a single residue so that other anionic compounds of interest can be quantified simultaneously. Anionic compounds like chlorate, perchlorate, bromide, sulfate, nitrate and bromate are all degradants of sanitizers, fertilizers, pesticides and other regulated agrochemicals Leri et al. (2024); Muñoz-Arango et al. (2023); Rice (1983); Thermo Fisher Scientific and Scientific (2016); Shanmugavel et al. (2020); Singh et al. (2022); Wang and Zhang (2019) but, like fluoride, they also occur naturally.

Reproducible, sensitive, and efficient methods of analysis are needed to diagnose anthropogenic residues in foodstuffs. Understanding the relative contribution of SF treatment towards fluoride levels, and how that varies as a function of treatment and geographic region is critical to establishment of MRLs, dietary inputs, and associated allowances. For the first time, a CE-C4D method was applied toward this end, affording a direct comparison of effectiveness relative to F-ISE, at least for cereal grains (i.e., IR4 crop group 15-22). Cereal grains were chosen as representatives of commodity group 5 (high starch, low water and fat) from European Union guidelines for pesticide residue analysis European Commission (2021) due to their high world-wide consumption. The comparison between CE-C4D and F-ISE, as a benchmark method already accepted by regulators, is a critical step towards the acceptance of the use of CE-C4D in a regulatory environment. The CE-C4D method was then extended to include other anions (bromide, chloride, sulfate, nitrate, perchlorate, chlorate, oxalate and bromate) that occur in the residue definition of agrochemicals, and whose detection is likewise confounded by “background” environmental levels. While SF can also be used on groups 3 (high sugar, low water: i.e. – dried fruit and honey) and 4a (high fat, low water: i.e. tree nuts and oil seeds) commodities, additional work is required to remove interfering compounds (likely organic acids and fatty acids, respectively) that occur in high levels.