Low-Level Immunoassay Screen for 2,4-Dichlorophenoxyacetic Acid in Apples, Grapes, Potatoes, and Oranges: Circumventing Matrix Effects

Abstract

We have been able to circumvent the variability in immunoassay data due to matrix effects by development of a standard addition ratio method. This technique was used to develop a screen for ≥10 ppb of 2,4-dichlorophenoxyacetic acid (2,4-D) in apples, grapes, oranges, peaches, and potatoes using a commercially available (Ohmicron) immunoassay. To compensate for matrix effects observed at 0.5 g produce/mL, standard is added to an aliquot of each sample extract and run alongside it, without need of a standard curve. The apparent response of each sample varies according to its matrix, but the ratio of spiked to blank sample is consistent (±4.6%) and characteristic of the quantity spiked. For a positive sample (≥5 ppb in the assay), the ratio is ∼22% higher than for a negative sample. Comparing immunoassay with gas chromatography values for 226 samples gave 2−18.7% false positives and 2.9% negatives. The immunoassay is carried out by diluting an acetonitrile extract produced in support of other analyses with aqueous buffer, avoiding a separate extraction, derivatization, and cleanup necessary for GC. The immunoassay screen can reduce analysis time from 2 days to 5 h for an 18-sample set. Keywords: Immunoassay; phenoxyherbicides; 2,4-dichlorophenoxyacetic acid; 2,4-D; apples; grapes; oranges; peaches; potatoes