Pesticide Residue Testing – Accurate Standard Preparation

Pesticides are chemical compounds used to kill, repel or control unwanted plants, animals and microorganisms (i.e. those considered to be weeds, pests and diseases). Pesticides such as herbicides, insecticides, fungicides, antimicrobials and disinfectants are used routinely on agricultural products, including food and animal feed, to maximize crop output and protect produce during storage and transportation. Through the consumption of such products, humans and animals are exposed to low levels of pesticides in their diets.

In order to protect consumers, pesticide residues are under strict legislation worldwide. The maximum residue level (MRL) is the highest amount of an individual pesticide residue that is permitted to be present in or on food or animal feed. MRLs vary from region to region across the world and food manufacturers exporting produce to other countries must meet all the individual MRLs in each target region.

The process for analyzing pesticide residues typically involves extracting the residues, clean-up procedures to remove other components, and an analytical procedure to identify and measure the amount of pesticide residue.

The pesticide residues are usually analyzed by the following methods:

• Gas Chromatography – Mass Spectrometry Coupled (GC-MS), especially for volatile compounds in complex samples
• Liquid Chromatography – Mass Spectrometry Coupled (LC-MS), suitable for non-volatile compounds (thermally unstable molecules)
   

Pesticide residue analysis using these techniques enables individual pesticides in a sample to be identified and quantified when compared against a matrix of known pesticides. For best results, the method requires reproducible conditions and the use of high quality reference standards.
 

Why Accurate Preparation of Reference Standards is Crucial

Analysis of pesticide residues is a highly sensitive procedure requiring extremely accurate solutions. Accurate weighing and dispensing of reference standards, as well as dilution with solvents, is therefore critical. Given the inherent errors associated with both pipetting and volumetric glassware, preparing solutions gravimetrically offers an increased level of accuracy.

Pesticide residue analysis requires highly accurate calibration standard solutions to identify and quantify the pesticides present in a sample. This requires preparation and dilution of stock solutions, the accuracy of which can be increased significantly by using gravimetric methods instead of volumetric.

If you need further information or any support on gravimetric standard preparation, our team of experts is here to help. Please do not hesitate to contact us.

The Demands of Standard Preparation

Laboratories performing pesticide residue analyses often test routinely for over 100 different pesticides. Preparation of both stock solutions and working solutions must be done with care, making the process time and labor-intensive. The pesticide reference solutions are often made in bulk and stored for future use. However, the standards must have high stability, as the pesticides can degrade over time, depending on the solvent, length of storage, temperature, light and the presence of contaminants (including traces of water). For low-stability or unstable pesticides, the reference standards must be made fresh each time, adding to the workload of the analyst.

Weighing and Calculations

Reference standards must be accurately weighed and the balance used must meet the accuracy requirements of the analytical process. The purity of the reference standard, in conjunction with the actual dosed weight, must be used to determine the precise concentration of the solution. Therefore, not only is the accuracy of each weighing critical, but also the amount of calculations required to determine the purity of each component adds to the complexity of the analysis.

Traceability of Standards and Data

Traceability is important for safety and audit purposes. Should any problems arise, having traceable results enables the source of the fault to be identified and rectified. Each weighing result, calculation and dilution step must be meticulously documented to ensure traceability. Typically, results are written into a lab journal and then manually entered into a spreadsheet for analysis. Comprehensive labeling of reference stock solutions, working solutions and samples is vital. Such manual documentation procedures have a high risk of human error and place a huge burden on analysts. Laboratories must also provide a report of the analysis to their customers.

A METTLER TOLEDO Excellence level analytical balance with an automated gravimetric liquid dispensing module enables you to prepare highly accurate standard solutions. For a full workflow solution, combine with LabX^® software and benefit from comprehensive data handling and sample tracking functionalities.

Eliminate Major Sources of Error

Gravimetric standard preparation involves weighing not only the solid substance but also the solvent, enabling specific concentrations to be prepared accurately and precisely. Preparing solutions gravimetrically requires only one container – weighing paper, weighing boats, intermediate tare containers and volumetric glassware are no longer needed. The actual weight recorded by the balance is used to calculate the precise amount of solvent required to reach the target concentration. Gravimetric standard and sample preparation thereby eliminates major sources of error in the standard preparation process, reduces the labor requirement, and minimizes variability.

Minimize the Amount of Substances

The required amount of reference standard can be dosed directly into the target container (e.g. chromatography vial). The calculated precise amount of solvent is dispensed automatically into the target container. Small quantities of solutions can be prepared so less material is required, saving costs, reducing waste and minimizing the impact of disposal on the environment.

Automatic Calculations and Documentation

Connecting the automated liquid dispensing system to LabX laboratory software offers a full workflow solution. LabX provides step-by-step SOP instructions on the balance display. Each step must be confirmed by the operator, ensuring SOPs are followed precisely by everyone. All data is handled automatically by LabX and saved in a secure, centralized database to ensure full traceability. When the analyst enters the ID of the reference standard, LabX automatically can retrieve the purity from the reference standard spreadsheet. Based on the purity and the actual amount dosed, the correct amount of solvent is automatically added to the standard to create the desired concentration.

Easy System Integration

Labels can be customized to include user ID, sample ID, actual concentration, production date and expiration date. Connecting a barcode reader allows sample and reference standard IDs to be read in electronically. With the addition of a printer, labels can be printed automatically. Import/export options allow bi-directional transfer of data between existing networks, data archives, LIMS, ERP and other knowledge-management systems. Manual documentation is no longer necessary, eliminating transcription errors entirely.

Watch the video and learn how dispensing liquids gravimetrically in an automated process increases the accuracy of your reference standards. In combination with a barcode reader, a printer and LabX, you can eliminate manual transcription, have all calculations performed automatically, ensure traceability, and dramatically reduce the time and effort required to create accurate reference standard solutions for pesticide residue analysis.

Preparation of pesticide standards is an arduous task in testing laboratories.
Hundreds of different pesticide stock solutions must be prepared regularly.

Automated standard preparation can help to:

• Improve efficiency and reduce substance usage
• Reduce turn-around time to get results faster
• Enhance accuracy by avoiding errors
• Deliver traceable audit-proof data

This application note describes the benefits of using the automated gravimetric liquid dispensing (XPR Automatic Balance) with LabX software compared to traditional manual stock solution preparation in a volumetric flask. 

Before pesticide residue analysis, food samples should be prepared according to their type. Samples may need to be homogenized or processed to a smaller particle size. After homogenization, food samples are weighed and extracted with suitable solvents. Good analytical results always start with a good weighing process; weighing is the first step in virtually any analytical procedure. METTLER TOLEDO Excellence level precision balances deliver fast and accurate results and are perfect for preparing samples for pesticide residue analysis.

Fast and Accurate

The innovative SmartPan™ weighing pan minimizes the effects of air currents on the weighing cell so that you get your results up to twice as fast as with a standard weighing pan. The extraordinary stability provided by SmartPan enables even 1 mg balances to be used without a draft shield in standard conditions.

Ergonomic Weighing

Without the need to keep opening and closing draft shield doors, the number of process steps is dramatically reduced, making processes much faster and more ergonomic. With the optional ErgoStand, the balance terminal can be placed at eye level to avoid excessive bending of the neck during long periods in front of the balance.

Easy Cleaning

The tray underneath the SmartPan catches any spills, and both simply lift off for easy cleaning. With overload protection and resistance to harsh chemicals, these tough balances will provide you with years of reliable service.

Automatic Data Handling

Connect your Excellence level balance to LabX software and benefit further from SOP user guidance on the balance, automatic documentation and calculations, full traceability, data integrity, and centralized control of users, tasks and instruments.