Polar pesticides and herbicides are widely used in agriculture because they are affordable and highly effective. One of the most well-known and extensively studied polar compounds is glyphosate, which is used in more than 750 different products across various sectors, including agriculture, forestry, urban, and home use. Although the concentrations of glyphosate residues that persist over time from this and other products are relatively low, concerns have arisen because of the extensive use of this chemical and its potential to endanger animal and human health.
Glyphosate was initially perceived as environmentally safe because it rapidly degrades. However, because glyphosate can negatively impact the survival of healthy plants, food producers have embraced genetically modified crops (GMOs) designed to withstand the chemical’s toxic effects. Unfortunately, the overreliance on glyphosate has spawned the emergence of "super weeds", which have developed resistance to the herbicide, resulting in increased and repeated applications. Consequently, in multiple studies, glyphosate has been detected in soil, crop products, animals that feed on those crop products, as well as in humans and in freshwater. This has raised many concerns relating to its toxicity and possible carcinogenic effects.
As concerns continue to increase, regulatory entities around the world have established targets for minimum acceptable levels of pesticides and herbicides in food, increasing the pressure on food manufacturers to routinely test their products to ensure they comply with evolving regulations.
Even though the development of reliable analytical methods for assessment of these compounds is of utmost importance, creating a successful method design and implementation is far from simple. Glyphosate and other polar compounds constitute an extremely challenging group of molecules to analyze due to their physicochemical properties. However, emerging technologies intended to overcome these challenges are rapidly growing.
Overcoming hurdles to accurate analysis
Polar pesticides and herbicides constitute a category of chemicals characterized by a low “LogKow” value, which reflects the solute concentration ratio between water and octanol and is a widely recognized measure of hydrophobicity. Polar chemicals used in agricultural production can be divided into two primary groups, anionic and cationic. Both types are challenging to analyze and therefore require robust and reliable methods.
There are several common hurdles that food safety analysts encounter. For example, they must use different columns (and systems) to analyze anionic and cationic pesticides in a single sample. This becomes a time-consuming process due to the high number of injections needed for column equilibration and the sample preparation required for derivatization. Thus, the sheer complexity and polarity of the matrices makes the robust determination of analytes difficult to achieve.
Technology advances over the past few years have helped streamline the analysis of anionic and cationic polar chemicals. For instance, the critical first step of the analysis is sample preparation to remove interferences from co-extracts or co-elutes. In recent years, the use of an analytical method called QuEChERS (Quick Easy Cheap Effective Rugged Safe) has become one of the most popular methods for determining pesticides in food matrices. However, one limitation of this method is the recovery of polar compounds, which tends to be very low since the analytes of interest generally stay behind in the aqueous phase.
Another method is the QuPPe (Quick Polar Pesticides) method, which helps to speed up and simplify the analysis of highly polar pesticides and their metabolites in food. The QuPPe method begins with the adjustment of water content, followed by the addition of acidified methanol to extract polar pesticides from food samples. Researchers can improve their results by combining the QuPPe method with liquid chromatography tandem mass spectrometry (LC-MS/MS) to separate and detect the polar pesticides of interest.
Improving separation of polar compounds
LC-MS/MS systems can aid in separating polar compounds. Over the years, reversed-phase chromatography (RPC), hydrophilic interaction liquid chromatography (HILIC) and ion chromatography (IC) have been studied for the separation of polar pesticides. Unfortunately, one limitation of using the RPC approach is that polar compounds do not retain well on a traditional C18 column for reversed-phase chromatography. Often, to overcome this problem, many researchers use a derivatizing agent such as FMOC-Cl to improve retention of these highly polar compounds. However, this could be time consuming and often an inefficient process. HILIC improves peak shape and selectivity but is time consuming and not suitable for use with samples that are only partially soluble in organic solvents. Additionally, while the ion chromatography columns have good retention properties for polar compounds, positive and negative ion analytes usually cannot be analyzed simultaneously. This translates into a higher investment of time and resources to accomplish a dual analysis.
As new technologies continue to emerge, researchers have more options and can now analyze cationic and anionic pesticides on a single column. These columns offer fully porous particle morphology, which allows for high sample loading. They are also designed to provide high retention of polar analytes and to facilitate fast equilibration. They can be used in both positive and negative mode. Early results show effective separation and peak shape for the analytes studied.
As the use of glyphosate continues to increase over the next decade, these technologies will be of critical importance. In the United States, most of the corn and soybean cultivation—totaling more than 150 million acres—now consists of genetically engineered varieties designed to withstand glyphosate. Moreover, many farmers now use the chemical as a desiccant to expedite the drying process of their crops. For these reasons, tools that streamline sample preparation and separation analysis of polar pesticides and herbicides will ease the burdens for researchers tasked with ensuring the safety of the food supply at national and global level.
