03/09

03/09

Actual Topic

Harvest Time · QuEChERS Time

“Quick, Easy, Cheap, Effective, Rugged and Safe“ – those are the demands on sample preparation in modern food analysis. The QuEChERS method, introduced by Anastassiades et al. 2003 [1], and the subsequent development of ready-to-use mixes meet these demands. QuEChERS became the method of choice in sample preparation for the analysis of pesticides in fruit and vegetables.

During harvest time the analysis of pesticides from fruit and vegetables is of major importance for public and private laboratories.

On the one hand, the worldwide turnout of several 100,000 tons of various pesticides supports an adequate supply with wholesome food, because it allows an effective control of many pests. On the other hand these substances pollute air, soil, and water and also affect human health. A comprehensive control of pesticide residues in food with multi-methods is needed to protect people effectively. While the basic determination methods GC and HPLC allow a high sample throughput with determinations up to 100 analytes, conventional sample preparation methods, especially the classical liquid/liquid extraction, are often demanding in terms of labour, time and cost.

Since its introduction QuEChERS evolved into the method of choice for an economical and environmentally compatible sample preparation for GC-MS and LC-MS. The advantages in comparison with classical clean-up methods are:

high sample throughput due to a quick and easy few-step procedure
low need of laboratory glasses, bench space and equipment
low consumption of solvents without use of chlorinated solvents
broad range of pesticides can be determined
rugged method with high and safe recovery rates

The QuEChERS method is basically performed with only a few easy steps in a centrifuge tube, as the following figure shows.

Following the standard protocol, pesticides from fruit or vegetable samples – often containing high amounts of water – are extracted with acetonitrile. In comparison with the also applicable acetone and ethyl acetate, acetonitrile extracts less of lipophilic ingredients of the matrix, like lipids and waxes. Furthermore, when using acetonitrile, water residues can be better removed from the sample with magnesium sulphate. Addition of sodium chloride controls the polarity of the extraction solvent and influences the fraction of matrix ingredients, which are also extracted. On the contrary, too high amounts of salt reduce the solubility of very polar pesticides. Thus the salt concentration has to be adjusted to the matrix and the analyte to be determined.
By subsequent addition of an SPE adsorbent with primary and secondary amine functions (PSA), such as CHROMABOND® Diamino, to the supernatant after centrifugation, dissolved matrix compounds are adsorbed. After this step, which is also called dispersive SPE, and another centrifugation step, the cleaned pesticide extract is separated as supernatant and transferred to a subsequent analysis.

The reliable applicability of this „standard“ QuEChERS method was shown in 2002 by an EU proficiency test for pesticides from oranges, in which 111 laboratories participated [2].

As some pesticides show a higher stability at low pH value, the standard method is enhanced by the addition and variation of buffer systems (sodium acetate pH ~4.5 or disodium hydrogen citrate pH ~5.5). The analysis of pesticide residues in low-fat products demonstrates this improvement [3].

The addition of graphitized carbon in a defined mixing ratio is successful for the preparation of complex food samples with a relative high amount of chlorophyll and carotinoids (e.g. carrots, bell peppers). In this case chlorophyll is removed by adsorption on the surface of carbon.
For samples with higher fat content (e.g. avocados) a combination of diamino and C18 ec phases as adsorbent additive shows clearly higher clean-up efficiency.

To meet the demand on a quick, easy, but effective and safe sample preparation also for complex samples, special CHROMABOND® QuEChERS mixes were developed.

The food sample is homogenised in a 50 ml centrifuge tube (REF 730223) and then efficiently and economically cleaned up with pre-weighed mixes of salts and adsorbents in a 15 ml centrifuge tube.

high-precision weighted mixtures
high reproducibility
individual combinations possible

economical, because less time- and personnel-consuming

For standard samples of food with low fat content, the sample, homogenised in acetonitrile, is mixed with the ready-to-use acetate extraction mix (II) and centrifuged. Then 1 ml supernatant is transferred to the centrifuge tube, which is pre-filled with Diamino clean-up mix (III). It is shaken and again centrifuged.
Instead of (III), the Diamino/Carbon Clean-up mix (IV) with a low amount of graphitized carbon is used for standard samples of food with low content of chlorophyll and/or carotinoids.

Further QuEChERS mixes, like combinations of citrate extraction mix (I) and e.g. Diamino/Carbon Clean-up mix (V) with higher amount of graphitized carbon or Diamino/C18 ec Clean-up mix (VI) are used for complex food samples depending on their properties.
The detailed composition of the mixes is described on the CHROMABOND® Diamino/QuEChERS website.

Since its introduction the QuEChERS method, especially with the high-precision weighted ready-to-use mixes, meets the demands on an economical and environmentally compatible sample analysis. Numerous publications and applications, not only for pesticide analysis of fruit and vegetables, show the importance of QuEChERS and comparable methods in the future [4]. Also further foodstuffs, like (instant) coffee or cacao will be brought into the focus of QuEChERS – thus, further interesting applications can be expected.

[1] M. Anastassiades, S.J. Lehotay, D. Stajhbaher, F.J. Schenk, Fast and Easy Multiresidue Method Employing Acetonitrile Extraction/Partioning and “Dispersive Solid-Phase Extraction” for the Determination of Pesticide Residues in Produce, Journal of AOAC Int. 86(2): 412-431

[2] CVUA Stuttgart, Pesticide Residue Laboratory, Participation in Proficiency Tests using the QuEChERS-Method, April 2006, 1-5

[3] QuEChERS – A Mini-Multiresidue Method for the Analysis of Pesticide Residues in Low-Fat Products, M. Anastassiades, CVUA Stuttgart, 1-12

[4] R.E. Majors, Sample Preparation Perspectives: QuEChERS – A New Technique for Multiresidue Analysis of Pestcides in Foods and Agricultural Samples, LCGC Asia Pacific, February 2008, 22-31

More information about CHROMABOND® Diamino/QuEChERS here!