Pesticide Prudence
What does a chemist do to ensure your food is safe from pesticides? By Elizabeth Suk-Hang Lam
Pesticides kill pests. Pesticides are poison. So, pesticides are bad, aren’t they? It turns out that pesticides do play an important role in food production. They are the guards of our crops and are particularly important in countries that face food shortages. According to the World Health Organization, pesticides only bring harm to people when they are above a certain safe level of exposure. If people were in contact with large quantities of pesticide, they may have acute poisoning or long-term health effects, such as cancer. Therefore, food safety is directly linked to the dosage of pesticides exposed. So, all we need is to find ways to quantify the amount of pesticides in food. Sounds simple? Nah. There are indeed more than 1000 pesticides used around the world. In fact, pesticides are not one pure substance. Pesticides are all chemicals that kill or harm pests. Pesticides can be classified into six groups according to the types of pests which they kill. Insecticides kill insects by poisoning them. Herbicides kill plants that compete with nutrients or pose harm to crops. Rodenticides are used to kill rodents such as rats and mice. Bactericides kill bacteria that harm crops. Fungicides kill fungi that parasites to crops. Larvicides kill larvae such as mosquitoes. With such a diversity in the pesticides’ world, it is a complex issue to detect each and every one of them - each pesticide has different properties and toxicological effects. Imagine one needs to perform 1000 experiments to detect each pesticide in an apple. It is simply not practical in reality! Therefore, testing laboratories usually analyse pesticides in a bunch. Chemists name the process of detecting several pesticides in one experiment as multi-residues analysis. One of the common ways to perform the analysis is combing a sample preparation procedure called QuEChERS and mass spectrometry.
sample, we need to make an apple puree. This is called homogenization of sample, which can maximize the surface area for efficient extraction of any containing pesticides in the sample. After weighing a known amount of the apple sample into a test tube, we add an organic solvent. It is nothing magical but is carefully selected to ensure it can extract most of the pesticides from the sample. Then just like cooking, we add a certain amount of salt to the sample. It is not for enhancing flavour, however, it is for the absorption of water in the sample. This forces the pesticides in the sample to enter the organic solvent phase. This whole process is called the extraction. Now, pesticides in the sample should already be present in the organic solvent. But this extract does not just contain pesticides, it also contains other compounds from the food sample such as fat and wax. These co-extracted substances cause interference to instrumental analysis and therefore, we need a sorbent to remove them. The sorbent is another combination of salts which could remove fats, colourings and acids that could interfere with instrumental analysis. After this clean-up procedure, the extract is finally ready for spectrometry analysis. Just like we call people by names, we identify pesticides by their masses (or weights). The instruments used are often liquid chromatography-mass spectrometry (LC-MS) or gas chromatography-mass spectrometry (GC-MS). Chromatography provides a running track for pesticides in liquid for LC-MS or in gas for GC-MS. Each pesticide completes its race at different times due to their different properties. Mass spectrometry then breaks down the pesticide into fragments. By identifying the fragments at different times and quantifying their intensities, we can calculate the level of each pesticide in the apple sample.
QuEChERS is the acronym for Quick, Easy, Cheap, Effective, Rugged, and Safe method. In practice, a testing laboratory needs to be able to test tens or hundreds of food samples quickly, easily, economically, effectively while ensuring the method is rugged and safe to use.
This analysis method is powerful as it can detect multiple pesticides in a single run or experiment. However, due to the existence of a large variety of pesticides in the world, there are still many challenges to pesticides detection. For example, pesticides that are highly soluble and those that are completely insoluble in water requires different extraction and instrumental methods. There is still extensive research work required for chemists to understand and unveil the different levels of pesticides in our daily food and drinks.
Imagine we are going to test the pesticide level in an apple. To ensure we have a representative portion of the
So every time you enjoy a delicious meal, say thanks to chemists for ensuring our food safety!