The fungicide fosetyl-aluminum doesn’t stick around long once it’s been applied in the field. In fact, it will almost certainly be completely hydrolyzed (split by water) into ethanol and phosphonic acid well before the commodity it’s been applied to is harvested. That’s fine for the grower, it’s actually the phosphonic acid that has the fungicidal effect, the point of the fosetyl-aluminum molecule is just way to slowly release it, but what about the poor analyst who has to find a way to measure the residue of a compound that probably won’t last long enough to reach the lab. When faced with a problem like this, regulatory agencies are increasingly requiring that the residue of not only the compound of interest (like fosetyl-Al), but any of the smaller pieces that it may break up into from reaction in the environment (degradants) or by reaction within a plant or animal (metabolites).
…if a grower can apply more than the label rate of fosetyl-aluminum and still have no detectable fosetyl residue it’s problematic from a regulatory standpoint
It is important to remember that MRLs are more about verifying compliance — that the grower used the right amount of pesticide and followed GAPs — than they are about ensuring food safety, so if a grower can apply more than the label rate of fosetyl-aluminum and still have no detectable fosetyl residue it’s problematic from a regulatory standpoint. So instead of defining a fosetyl-Al residue as just the amount of fosetyl detected on the commodity, for instance, the residue of fosetyl-aluminum is defined as the sum of the amount of fosetyl detected to the amount of phosphonic acid (multiplied by 1.33, to adjust for the mass difference between fosetyl and phosphonic acid). The result of this trend is an increase in complexity that anyone involved in the international export of agricultural commodities must be aware of. If sending samples of leafy greens to a lab for glyphosate analysis, for instance, it is critical to know how the lab is defining a glyphosate residue. In the US, a glyphosate residue on leafy greens is determined just by the measured amount of glyphosate. If the greens are going to Australia, however, the glyphosate residue is defined as the sum of not only glyphosate, but the metabolites N-acetyl glyphosate and, aminomethylphophonic acid (AMPA).
The EU definition of fosetyl-aluminum residues as the sum of fosetyl and phosphonic acid (and its salts) has caused in considerable difficulty in the nut industry
Fosetyl-aluminum and propylene oxide represent two more serious examples of the consequences of an increased focus on degradation products. The EU definition of fosetyl-aluminum residues as the sum of fosetyl and phosphonic acid (and its salts) has caused in considerable difficulty in the nut industry, due to the fact that there are many other, non-regulated sources of phosphonic acid, such as fertilizers containing phosphite salts. In addition, an ongoing USDA-DFA research project has shown that certain methods of measuring (LC-MSMS using negative ionization mode) phosphonic acid can get a false positive if the samples contain a high amount of phosphoric acid. A temporary MRL of 75ppm has been issued for tree nuts, but with the previous MRL of 2.0ppm, it was possible to have a high enough level of phosphoric acid in a sample that would give a false positive for phosphonic acid high enough to trigger the MRL for fosetyl-aluminum, which was never applied. Using the proper method of analysis is also of great importance when measuring propylene oxide residues. The loss of methyl bromide as a post-harvest fumigant has led to an increased need to use propylene oxide (PPO) to eliminate undesirable microorganisms. During a sterilization, propylene oxide can react with surface chloride and bromide to form propylene chloro and bromo-hydrin (PCH and PBH, respectively), both of which are compounds of concern to the EU, due to their toxicity. The standard method for determining PPO residues involves blending the commodity in water, this has the effect of not only generating heat, but freeing additional chloride and bromide from the bulk of the commodity, the combination of which causes additional PCH and PBH to form.
With increased public / regulatory concern about chemical residues the detection and quantitation of metabolites and degradation products becomes increasingly critical and with that the need for improved analytical methodologies. In partnership with the USDA-ARS, DFA of California is at the forefront of research to meet these needs and can offer analysis of the previously mentioned compounds.