Pesticide Residue Analysis Sample Preparation

pH s.r.l., a contract laboratory in Italy specializing in the fields of industrial hygiene, safety on work sites, alimentary auto-regulation, environment, quality and training, employs many different sample preparation and analysis instruments, which include some RETSCH mills, such as one high-speed Ultra Centrifugal Mill ZM 200, one GRINDOMIX GM 300, and four GRINDOMIX GM 200 knife mills.

pH Lab performs food and feed analysis for several key Italian food manufacturers. Preparing food samples for neutral-to-analysis is critical, particularly for detecting pesticides and mycotoxins. This type of analysis requires a thoroughly homogenized sample as well as prevention of heating up of the sample in order to preserve the volatile components.

Advantages of RETSCH Mills

Knife Mill GRINDOMIX GM 200

Figure 1. Knife Mill GRINDOMIX GM 200

pH Lab continuously strives to optimize the sample preparation process. Time saving is a key problem for the lab as it handles on an average 15 to 20 samples daily. The lab employs the Knife Mill GRINDOMIX GM 200 for the size reduction and homogenization of smaller sample volumes and employs the GM 300 for larger sample volumes to obtain better results as it can process a sample volume of up to 4,500 ml.

Knife Mill GRINDOMIX GM 300

Figure 2. Knife Mill GRINDOMIX GM 300

The GM 300 allows the lab to process as high as eight bananas at once, devoid of pre-cutting the sample. The use of the patented gravity lid dynamically decreases the grinding chamber volume, thus yielding a perfectly homogenized result.

Role of RETSCH Tools in QuEChERS Method

Once the sample is prepared, the pH Lab then mixes the QuEChERS reactive solution into the sample to perform pesticide contamination analysis. Based on the sample properties, the lab adds dry ice to the sample at a ratio of 2:1 in order to optimize the breaking behavior. It uses the stainless steel container for this purpose.

Michelangelo Anastassiades of Chemical and Veterinary Investigation Office CVUA in Stuttgart, Germany, developed the so-called QuEChERS (quick, easy, cheap, effective, rugged and safe) technique to prepare samples for performing more-efficient pesticide residue analysis. The method comprises three steps, namely Homogenization, Extraction and Analysis.

Since some pesticides are volatile in nature, it is mandatory to avoid the sample to heat up during the homogenization process. Hence, it is preferred to cool the sample in order to optimize the breaking behavior, which, in turn, helps to achieve a better homogeneity and higher fineness. The patented gravity lid of the GM 300 can be utilized for this purpose to grind samples mixed with dry ice. Another option is the RETSCH CryoMill, which allows continuous cooling of the sample with liquid nitrogen.

Once the homogenization of the sample is done, then extraction of 10 g of the food sample is carried out with 10 ml acetonitrile, followed by drying and testing of the organic phase for pesticide residues with chromatographic analysis. The sample extraction is carried out in the presence of a salt mixture in order to shun ghost peaks in the chromatograms. The use of a professional sample divider such as RETSCH's PT 100 can yield this mixture. This rotating divider provides reproducible mixing of the salts, thus enabling each fraction to have identical composition.

The pesticides are transferred into the organic phase from the sample by agitating the mixture with acetonitrile and salt for 1 to 3 min. A laboratory mill like RETSCH's Mixer Mill MM 400 can be used for agitating the mixture.

The Mixer Mill MM 400

Figure 3. The Mixer Mill MM 400

The sample is moved through a 50 mL Falcon tube at a frequency of up to 30 Hz in order to ensure thorough mixing of the sample, which facilitates the subsequent extraction.

The new adapter for the MM 400 allows to use up to 8 Falcon tubes (50 mL)

Figure 4. The new adapter for the MM 400 allows to use up to 8 Falcon tubes (50 mL)

Conclusion

Mr. Manuele Mecacci of pH Lab is satisfied with the advantages of the GM 300. Mecacci informed that the lab is convinced with the performance of the new RETSCH GM 300 featuring the gravity lid. Obtaining a high degree of homogenization was not achievable with previous equipment. Besides fulfilling this requirement, the GM 300 facilitates the lab to achieve time saving by processing most samples within seconds.

This information has been sourced, reviewed and adapted from materials provided by RETSCH GmbH.

For more information on this source, please visit RETSCH GmbH.

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