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Pesticide Residue Testing – Accurate Standard Preparation

Preparation of Standards for Analysis of Pesticide Residue Levels

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Pesticide Residue Testing - Standard Preparation


Pesticide Residue Testing Workflow

1.    Prepare Calibration Standard Solution


1.1 Prepare Stock Standard Solution

Individual stock standard solutions typically are prepared to a concentration of 1 mg/mL. The required amount of pure pesticide reference standard (typically 10 – 50 mg) should be weighed out using a 4- or 5-decimal place balance or scale, depending on the method. Transfer to an appropriate volumetric flask, dissolve in a suitable solvent, and make up to the required volume to achieve the desired concentration. Label the flask with all relevant information. Liquid reference standards can be dispensed, by weight, directly into the solvent. It is also possible to purchase ready-made stock standard solutions of pesticide reference standards, if they are certified by the manufacturer or by an independent source.


1.2  Prepare Intermediate / Working Standard Solution

Working standard solutions typically have very low concentrations, in the range of 0.005 – 0.01 µg/mL. To achieve such low concentrations, it may be necessary to create intermediate solutions and dilute further to achieve the required concentration. For a multi-residue method, several pesticide standards are mixed. Combine appropriate quantities of the individual stock standard solutions to create the concentrations required for comparison against the sample under investigation.


2.    Prepare an Extract of the Sample

Preparation of samples for pesticide residue analysis is a complex procedure and is dependent upon the sample type, the pesticides under investigation and risk of cross-contamination. In a complex sample matrix, it is necessary to remove unwanted interferences in order to focus on the pesticide content. Common extraction preparation techniques for pesticide residue analysis are QuEChERS (Quick, Easy, Cheap, Effective, Rugged and Safe) and Accelerated Solvent Extraction (ASE).


3.    Measurement by GC-MS / LC-MS

Samples are typically analyzed using gas chromatography (GC) or high performance liquid chromatography (HPLC) in conjunction with mass spectrometry (MS) for identification and quantification of the pesticides present. The extracted samples are introduced into the analyzer in accordance with the manufacturer's instructions.


4.    Data Evaluation and Storage

The results from the sample are compared to those from the working standard solution to identify and quantify the pesticides in the sample. This is typically done automatically by the analyzer, delivering results in mg/kg. To comply with pesticide limit regulations, the amount of each residue must not exceed the respective MRL of the target market. Results must be documented fully to ensure traceability.



Pesticide Residue Analysis Know-How

Over 1000 pesticides are currently registered worldwide. Pesticides are heavily regulated. However, regulations and MRLs (Maximum Residue Levels) differ from region to region across the world. In the US, the Environmental Protection Agency (EPA) is responsible for setting MRLs (also referred to as 'tolerances'). In the EU, the European Commission (EC) defines MRLs. Each individual MRL is the upper allowable limit for the specified pesticide on or in a food product which is deemed safe for consumer consumption. Such levels are set following rigorous risk assessments.

Analytical Guidance Documents for Pesticide Residue Testing

In the US and EU, all laboratories carrying out official testing of pesticide residues must be accredited to ISO/IEC 17025, which sets out the general requirements for the competence of testing and calibration laboratories. Guidance documents also place a focus on the preparation and management of pesticide standards, stock solutions and working solutions (calibration solutions). In the preparation of stock standard solutions, it is specified that not less than 10 mg of the reference standard should be weighed out using a 5 decimal place balance (EU guidance document SANTE/11813/2017).

Errors in the Standard Solution Preparation Process

The two largest sources of errors in the laboratory come from standard solution processing and human operations. With the amount of laboratory time spent processing standard solutions estimated to be greater than 60%, measures that can reduce the incidence of such errors can significantly improve overall accuracy and laboratory productivity. Traditional standard solution preparation involves the use of volumetric glassware. While instrumentation and software have provided dramatic improvements in analysis and data processing, the manufacturing process for volumetric flasks with accuracies similar to those in use today has remained unchanged for decades. The traditional volumetric method is time consuming and prone to error at many points in the process.

Volumetric standard solution preparation is highly manual, subjective and variable.

Volumetric standard solution preparation is highly manual, subjective, and variable.

Making the Switch to Gravimetric Standard Solution Preparation

It is universally accepted that a gravimetric measurement is intrinsically more accurate that a volumetric measurement. In fact, pipettes and volumetric measuring equipment are calibrated using gravimetric methods. In a fully gravimetric system, the user defines the concentration required and the target amount of solution. The system software calculates the target amount of solid to dispense and then, according to the actual amount of solid dispensed, delivers the appropriate amount of diluent to prepare an extremely accurate and precise concentration. In a liquid dispensing system, the software calculates the required amount of diluent according to the actual amount of solid dosed manually; the result is still a highly accurate concentration. Implementing a gravimetric sample preparation system reduces laboratory errors and Out-of-Specification (OOS) incidents by up to 50 percent, while simultaneously increasing laboratory efficiency and reducing waste.

Gravimetric standard solution preparation eliminates subjectivity and variability.

Gravimetric standard solution preparation eliminates subjectivity and variability.


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FAQ – Frequently Asked Questions on Accurate Standard Preparation for Pesticide Residue Testing

What's the best way to keep track of all my standards, solvents and samples?

It's crucial to ensure that all standards, solvents and samples are correctly labelled. Printing labels is more efficient than writing labels by hand and avoids transcription errors. Using barcodes on labels speeds up the process even more and further reduces the risk of human error.

METTLER TOLEDO balances have different connection ports to make it quick and easy to add both a printer and a barcode reader. LabX laboratory software can greatly simplify sample and data handling processes. Barcodes from standards, solvents and user IDs can be read-in via barcodes. LabX prints barcoded sample labels automatically. The sample IDs are read in via the barcode and the weight result is automatically stored securely with the ID and all relevant metadata. Samples are fully traceable. LabX can transfer the data to other LIMS/ERP systems as required.

I work with many different pesticide reference standards and different batches sometimes have different purities. How can I make sure the correct purity is used when calculating the concentration of my stock solutions?

Typically, when a company takes delivery of their reference standards, all relevant data, including purity, is saved in a database of the standards. LabX uses the relevant data from the database; this can be done automatically by scanning the barcode on the container of the standard. When the balance has the automated liquid dispensing system installed, LabX uses the purity from the database and the actual weight of standard dosed to dispense the precise amount of solvent automatically to create the desired concentration. In this way, it is not critical to weigh the standard exactly as LabX adjusts the amount of solvent accordingly, and the concentration of the resulting solution will be as desired.

We are testing tobacco for pesticide residues. Are there any recommendations regarding weighing standards and samples for this application?

There are several guidelines on general procedure. The European Union document SANTE/11813/2017 [Ref. 12.1] sets out best practices for the general quality control procedures for pesticide residue analysis in foodstuffs. For tobacco, the Cooperation Centre for Scientific Research Relative to Tobacco (CORESTA) Guide No. 5 provides a technical guideline for Pesticide Residues Analysis on Tobacco & Tobacco Products. Regarding weighing and preparing stock standards, standards should be prepared with not less than 10 mg ‘pure’ standard material using a 5 decimal place analytical balance.

We are a small/medium sized testing lab carrying out pesticide residue testing and must comply with ISO/IEC 17025 to ensure our continued accreditation. How can you help ensure our weighing processes meet the regulations?

ISO/IEC 17025 is the quality standard which declares a lab is technically competent to carry out analyses and provide accurate results. METTLER TOLEDO can support you to meet the requirements of the regulations in several different ways:

GWP® Recommendation

Based on METTLER TOLEDO's globally accepted Good Weighing Practice™, this free service helps you make sure you have the right balance to meet the needs of your weighing processes.

GWP® Verification

Through a thorough risk-based analysis of your applications, we can provide you with an optimum balance calibration and routine testing schedule, including which tests to carry out when and the recommended test weights. With the optimized testing schedule, you can avoid over-testing your balances and potentially make considerable time and cost savings.

Excellence Balances

METTLER TOLEDO's Excellence Level balances have built-in quality assurance functionalities so you can be sure that the results generated are always valid. Intelligent functions, such as StatusLight™, LevelControl and GWP Approved, help ensure right-first-time results by actively monitoring that all relevant conditions for correct weighing are met.

Automated Powder and Liquid Dispensing

Automated dispensing with Excellence level analytical balances enables you to achieve a level of dispensing accuracy unmatched in a manual process. Dilution with automated dispensing avoids the need for pipetting and eliminates the errors associated with pipetting processes. The increased level of accuracy, reduces the balance's minimum weight and enables you to use less of each substance. Considerable materials savings can therefore also be made.

LabX Laboratory Software

LabX provides step-by-step instructions on the balance display and ensures all analysts follow SOPs. All data and calculations are handled automatically by LabX – transcription and calculation errors are eliminated entirely. Data is saved securely in a centralized database, providing full traceability and audit-proof documentation.

Which balance do I need to use to get accurate standard solutions?

Every measurement on ANY balance is subject to uncertainty. Understanding uncertainty is key to ensuring accurate weighing results. It is not the readability that determines the accuracy of a weighing instrument, but rather its repeatability. When weighing out very small quantities, the balance's minimum net sample weight capability is also of critical importance. The minimum weight value is determined by a service technician at the balance's place of operation and is unique to every balance and its working environment. Weight values below the minimum weight value cannot be trusted to be accurate. According to the criticality of the application, a safety factor is usually applied to the minimum weight value to ensure that weighing always take place in the safe weighing range of the balance. The lower boundary of the safe weighing range is determined by using the minimum weight and the required process accuracy.

METTLER TOLEDO's global weighing standard GWP® helps you choose the right balance to meet your application requirements. Ask your local representative for a free GWP® Recommendation to choose the right balance for your application and to discover if your existing balance is fit for purpose.

GWP Recommendation

I have to prepare many different standards on a daily basis. It's time-consuming and takes a lot of effort. How can I be more efficient without compromising accuracy?

By making the switch to gravimetric sample preparation you can actually increase the accuracy of your solutions. The amount of solvent required is calculated according to the actual amount of substance dosed to deliver a highly accurate concentration. Gravimetric sample preparation is also considerably faster than volumetric methods; one testing lab reduced their bi-monthly preparation of stock solutions from 2 weeks to 2½ days. Smaller quantities can be made so you use significantly less substance and solvent, saving costs, reducing waste, and minimizing the impact of disposal on the environment. Dilution with automated dispensing avoids the need for pipetting and eliminates the errors associated with pipetting processes.

If using volumetric methods, consider attaching a printer and barcode reader to your balance. Using barcodes to identify your reference standards, solvents and samples saves time and avoids errors. Printing sample labels and reports saves time and avoids transcription errors.

Preparing reference standards and dilution samples for pesticide residue testing generates so much data and paperwork, and it all has to be transferred to the database in our LIMS/ERP system. How can we do this in a faster and more efficient way?

LabX laboratory software can handle all of your pesticide residue analysis data and calculations for you automatically. Data is saved securely in a centralized database and can be transferred to your internal data management system as required.