An iPhone-based digital image colorimeter for detecting tetracycline in milk

•A portable iPhone-based digital image colorimeter was fabricated.•It was utilized for monitoring tetracycline in bovine milk.•A ColorConc application was developed to work on iPhone.•We use an image matching algorithm based on Euclidean distance theory.•The analysis is carried out after preconcentration on C18 SPE sorbent. An iPhone-based digital image colorimeter (DIC) was fabricated as a portable tool for monitoring tetracycline (TC) in bovine milk. An application named ColorConc was developed for the iPhone that utilizes an image matching algorithm to determine the TC concentration in a solution. The color values; red (R), green (G), blue (B), hue (H), saturation (S), brightness (V), and gray (Gr) were measured from each pictures of the TC standard solutions. TC solution extracted from milk samples using solid phase extraction (SPE) was captured and the concentration was predicted by comparing color values with those collected in a database. The amount of TC could be determined in the concentration range of 0.5–10μgmL−1. The proposed DIC-iPhone is able to provide a limit of detection (LOD) of 0.5μgmL−1 and limit of quantitation (LOQ) of 1.5μgmL−1. The enrichment factor was 70 and color of the extracted milk sample was a strong yellow solution after SPE. Therefore, the SPE-DIC-iPhone could be used for the assay of TC residues in milk at the concentration lower than LOD and LOQ of the proposed technique.