Avocado – a challenging, high-fat matrix with increased analytical demands
The widely used QuEChERS method was originally developed for high-water-content matrices such as apples or tomatoes. With avocados, however, significant amounts of lipids and other matrix components are co-extracted. These can influence the extraction yield, recovery rates and the quantification of the analytes. Furthermore, residual co-extractants can cause pronounced matrix effects in LC-MS/MS or GC-MS/MS analysis.
To overcome these challenges, adapted sample preparation and purification are required, as well as, as a rule, matrix-specific calibration. Lipophilic (active) substances, in particular, tend to distribute preferentially within the fatty components of the matrix. As as many lipids as possible must be removed for the analysis, there is a risk that some analytes may also be lost in the process. Residual lipids and other co-extractants can also influence the measurement signals and thus compromise the accuracy of the quantification.
Results of the multi-method approach
Problems with the analysis were particularly evident in the case of the highly lipophilic active substances bifenthrin and lufenuron. The pyrethroid bifenthrin was not detected by some laboratories, even though the (active) substance was included in the specified test range. Isolated false-negative results were also observed for lufenuron. However, the majority of errors for lufenuron were due to quantification errors, predominantly in the form of under-quantification (recovery < 70%).
Active substances such as diazinon and procymidone also proved challenging in this matrix, as evidenced by an increased number of quantification errors. There tended to be more quantification errors in the low spiked concentration ranges between 0.03 and 0.06 mg/kg. The moderately lipophilic active substances carbaryl and boscalid posed hardly any difficulties for the participating laboratories. The results highlight the particular importance of matrix validation and appropriate compensatory measures for matrix effects in high-fat matrices such as avocado.
The individual method for fosetyl/phosphonic acid
The current residue definition for the monitoring of phosphonic acid is ‘phosphonic acid and its salts, expressed as phosphonic acid, excluding fosetyl’. As plant protection products containing the active substance fosetyl-Al are used particularly in third countries, fosetyl-Al was specifically spiked as part of the laboratory performance assessment. To evaluate the analytical quality of the phosphonic acid determination, the measured values for both fosetyl and phosphonic acid were recorded and converted to fosetyl-Al. A quantitative evaluation could then be carried out on this basis.
This task also revealed a higher proportion of quantification errors. The analytical challenge lay less in the high-fat matrix than in the specific properties of the (active) substance. Due to the high polarity of fosetyl and, in particular, its degradation product phosphonic acid, there are increased requirements on extraction, recovery and quantification.
The details page is therefore closer in scope to the previous year’s version: the news section contains the overarching results and context, whilst the details page describes the technical background and specific anomalies relating to the individual (active) substances.
