Des études récentes menées par l'USDA [1] et la NVWA en Europe [2] ont montré que les raisins secs, ainsi que les snacks fait de raisins séchés, sont en tête de liste des fruits et légumes qui contiennent des niveaux inacceptables de résidus de pesticides.
80 % des raisins secs importés aux Pays-Bas sont contaminés par en moyenne 11,3 pesticides différents, et presque toutes les marques de raisins secs commercialisées aux États-Unis contiennent au moins deux résidus de pesticides différents [3]. La préoccupation fondamentale en matière de santé est que les effets cumulatifs à long terme de la consommation d'une variété de pesticides sont inconnus. Il est clair que cela remet en cause l'hypothèse selon laquelle les raisins secs constituent une alternative saine et adaptée aux enfants aux snacks transformés. Pour répondre à ces préoccupations en matière de sécurité alimentaire, il est nécessaire d'effectuer des tests rapides et précis pour dépister les substances potentiellement dangereuses dans les échantillons alimentaires. Dans cette note d'application, le MISA (Metrohm Instant SERS Analyzer) de Metrohm Raman excelle dans la détection du pesticide acétamipride sur des raisins secs vendus dans le commerce. MISA est une alternative viable aux tests analytiques en laboratoire dans la quête d'empêcher les aliments contaminés d'atteindre et de nuire aux consommateurs.

Acetamiprid is a highly effective systemic neonicotinoid insecticide. Although toxicity to humans and other mammals is low, it is moderately to highly toxic for birds and aquatic life, posing a potential threat to wildlife and the food chain. This Application Note demonstrates the rapid and sensitive detection of acetamiprid extracted from raisins using the Metrohm Instant SERS (Surface-Enhanced Raman Scattering) Analyzer.

As direct point-and-shoot Raman spectroscopy is unsuitable for trace analyte detection, SERS was used in this experiment. Dilutions of 1 mg/mL acetamiprid in methanol were pipetted onto individual 1 g portions of raisins, yielding samples containing 100, 25, 5, and 2 μg/mL (ppm) and 500 ng/mL (ppb) acetamiprid. Each sample was dried and placed in a vial with 0.2 mL of dichloromethane (DCM). Each tube was vortexed for two minutes and rested for 30 minutes, and then the supernatant was transferred to a clean vial for evaporative drying. After 0.9 mL of silver colloid was added, each vial was vortexed for one minute. This was followed by the addition of 0.1 mL of 500 mmol/L NaCl and gentle agitation of the contents. Each vial was inserted into the vial holder attachment of MISA for measurement.

As shown in Figure 1, SERS spectra for DCM extracts of acetamiprid on raisins are identical to the reference spectrum for pure acetamiprid (in dark blue). The highly resolved signature peaks tend to correlate in intensity to analyte concentration.

Information content from Raman spectra is greatest at higher analyte concentrations. Some poorly resolved signature peaks in Figure 1 persist at 500 ng/mL (ppb), yet sensitivity at this level is essential because it corresponds to the maximum residue level accepted for acetamiprid in Europe.

At very low concentrations, the following two strategies may improve SERS detection:

1. Combine multiple extract aliquots into one test sample. In this case, three to four 0.2 mL DCM aliquots (from the same test batch of raisins) would be combined into one vial before evaporative drying.
2. Longer integration times on the instrument may improve sensitivity. The Auto Integration feature on MISA is adequate for higher concentrations; lower concentrations may require manual setting of integration times to four to eight seconds, for example.

Figure 2 overlays spectra for samples containing one aliquot of 5 μg/mL, 2 μg/mL, and 500 ng/mL acetamiprid with a sample that contains four aliquots of 500 ng/mL acetamiprid. This figure provides visual confirmation for improved signal through combined aliquots.