QUECHERS Method (Part 1 of 2)
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 , 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.
In brief the procedure entails the following steps:
- Weigh 10 g of sample
- add 10 mL acetonitrile and internal standard
- agitate intensively
- add NaCl, MgSO4 and buffering salts for phase-separation and pH-adjustment
- agitate intensively and centrifuge ⇒ Raw extract
- take an aliquot of the upper organic phase and subject it to dispersive SPE cleanup (d-SPE) by mixing it with MgSO4 and a sorbent (e.g. PSA) to remove water and undesired co-extractives
- agitate shortly and centrifuge (optionally add Analyte Protecting Agents) ⇒ Final extract
- the final extract can be analyzed directly by GC- and / or LC-techniques
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 many different types of 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 labor, 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 small number of quick and easy steps
- Little need of laboratory glasses, bench space and equipment
- Low consumption of solvents
- No chlorinated solvents
- Broad range of pesticides can be determined
- Rugged method
- High recovery rates of analytes
- Safe to perform
The QuEChERS method is 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 interfering lipophilic ingredients of the matrix such as fats and waxes. Furthermore, when using acetonitrile, water residues can be better removed from the sample with magnesium sulfate. 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, large amounts of salt reduce the solubility of very polar pesticides. Thus the salt concentration must be adjusted to the matrix and the analyte to be determined.
By subsequent addition of a loose powdered 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 – also called dispersive SPE – and another centrifugation step, the cleaned pesticide extract is separated as supernatant and analyzed by an instrumental technique.
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 .
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 .
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 SPE phases as adsorbent additives clearly shows a higher clean-up efficiency.