8 Top Tips For Pesticide Screening
How To Guide May 29, 2018 | By Zoe Griffiths.
Pesticides are utilized all over the world to maximize food crop production and yield in response to the huge consumption demands of an ever-increasing human population. However, the potential for pesticides to remain in both consumer products and the environment is of significant concern to governments due to their potentially harmful effects on human and environmental health. Consequently, legislations and regulations introduced and enforced to protect both consumers and the environment means an increasing demand for pesticide screening on a variety of products, including foods, surface water and baby milk, among others. Gas chromatography tandem mass spectrometry (GC-MS/MS) and liquid chromatography tandem mass spectrometry (LC-MS/MS) have quickly become the techniques of choice for pesticide screening due to their ability to identify and quantify a diverse set of pesticides with high sensitivity. This list provides a few tips and tricks to help you best utilize these techniques for pesticide screening.
The procedure used to collect a sample to be screened is one of the most critical steps for successful pesticide screening. Proper sampling ensures that results are fully representative of the types and quantities of pesticides present in the product of interest. If point-sampling methodology is used, biases will be introduced into results as pesticide presence and quantification will vary substantially by location. Using a statistical-based method to ensure sampling across any spatial variation is imperative to avoid biases. It is also of high importance to test samples quickly after collection to avoid pesticide loss as a result of degradation and volatility. If it is not possible to test samples promptly, samples should be frozen prior to screening in order to slow down the degradation process.
Sample comminution, by cutting, grinding or mixing, is imperative to achieve sample homogenization with respect to analyte distribution. This should be carried out in sub-ambient or cryogenic conditions. This is because many reactions causing pesticide degradation occur at room temperature, including hydrolysis, oxidation, enzymatic activity and chemical complexation. Sample freezing followed by homogenization by blending with dry ice to form a powder is an effective method to achieve these conditions.
The QuEChERS-based approach (Quick, Easy, Cheap, Effective, Rugged, and Safe) is the most frequently used technique for sample processing. The two-step process includes both the extraction and clean-up phases. There are a number of variations to the method, and the best variation to use will depend on the properties of both the matrix and the pesticide compounds of interest. As a general rule, for base-sensitive compounds, it is best to use an extraction product with sodium acetate. For non-base sensitive compounds, it is recommended to use an extraction product with sodium chloride or sodium citrate.
Clean-up sorbent selection should be based on the matrix type. The table below provides a rough guide to sorbent requirements for different, common matrices.
Common QuEChERS issues and resolutions
There are a number of common issues arising from the use of the QuEChERS method for sample processing which can easily be fixed by slight adjustments to the methodology to achieve optimization. Loss of planar pesticides can be resolved by using a product with less graphitized carbon black (GCB); loss of acidic compounds can be overcome by using a product containing magnesium sulfate and C18; and loss of compounds during subsequent analysis is easily fixed by adding an analyte protectant, such as sorbitol or toluene. Problems also arise when the pH of the final extracted sample for analysis is not between pH 4-6. Although the QuEChERS technique does some buffering, it is important to neutralize any acidic or alkaline samples prior to extraction.
Choosing an analytical instrument
A number of analytical instruments can be used to screen for pesticides. GC-MS/MS is the most frequently used method for pesticide screening due to its speed, sensitivity and ability to detect numerous pesticides in a single run. However, it has limited application to more volatile molecules and requires larger sample sizes. LC-MS/MS is being utilized more and more often for pesticide screening due to its ability to perform high sensitivity quantification of hundreds of target pesticides in a single run. However, the method requires extensive compound-dependent optimization and so can only be utilized for targeted compounds. Coupling high resolution accurate mass (HRAM) with ultra-high pressure LC can address these issues allowing the additional screening of untargeted compounds. It is now common to use both GC-MS/MS and LC-MS/MS in tandem due to capabilities in detecting different pesticide compounds. Utilizing both techniques therefore both reduces the risk of missing a pesticide compound that is present in the matrix. Ion chromatography mass spectrometry (IC-MS/MS) is also used in cases where pesticides are difficult to detect by GC-MS/MS or LC-MS/MS due to high polarity, low volatility or thermal instability.
GC-MS/MS optimization tips
Commonly, GC-MS/MS problems tend to originate at sample injection. Whether you are using the splitless or programmed temperature vaporization (PTV) technique, careful optimization is required for both to achieve optimal results. Splitless sample injection is less commonly used for pesticide screening due to its limited sensitivity. However, it can be extremely useful for overcoming matrix issues. The precise control of the PTV technique means it is extremely useful for thermally labile and non-volatile pesticides. GC liner selection is also an important point for optimization due to the polarity of the acetonitrile solvent used in QuEChERS extraction. The liner volume must be sufficient to hold the sample in gaseous form, made tricky by the high expansion volume of acetonitrile. Dedicated selection reaction monitoring (SRM) method development is also recommended to reduce the time and effort required to add new compounds into the methodology.
LC-MS/MS optimization tips
A full scan approach, using high resolution accurate mass (HRAM), coupled to ultra-high-pressure LC (UHPLC) is ideal for rapid and sensitive screening and detection of targeted and non-targeted LC amenable pesticide residues. Column choice is critical to LC optimization. Conventional C18 stationary phases are typically used for pesticide monitoring, however the selectivity and retention are poor for more polar species. In contrast, Ultra Aqueous C18 columns are ideal for multi-pesticide residue monitoring methods. Triple-quadrupole MS is the gold standard choice due to its high compatibility with QuEChERS extraction and its high sensitivity. A drawback of this selection is its time-consuming optimization. However, the use of a comprehensive database of selected reaction monitoring transitions can help to avoid time-consuming manual optimization.
The use of all analytical instruments discussed (GC-MS/MS, LC-MS/MS and IC-MS/MS), especially when used in tandem, produces large volumes of raw data, meaning that automated solutions are highly advised for data interpretation. Such software are also useful in the optimization phases of methods, alerting analytical failures and report creation. Software packages are available that are tailored specifically to pesticide identification and are even able to produce reports based on your data output for you.