There are 7 types of TC but only 3 (tetracycline, chlortetracycline
and oxytetracycline) are used in agriculture, both in plants and animals.
The frequent use of TCs is high cost and continue to cause the
problems of drug residues in tissues, especially in milk. Milk is a
popular diary product consumed across all ages because it is a
source of 5 groups of nutrients including proteins, fats, carbohydrates,
minerals and vitamins. However, if the manufacturer does
not check the quality of milk before selling or exporting, antibiotic
residues might affect the health of consumers. TCs residues in milk
can be analyzed by various methods. Sample preparation techniques
especially solid phase extraction (SPE) is needed before
analysis due to the advantages over traditional liquid–liquid
extraction. Milk is normally processed under reversed phase or
ion exchange SPE condition (Sigma–Aldrich, 2013). C18 sorbent
was found to provide good recoveries (Furusawa, 2003) thus it
was selected to be used in this work for extraction of TC in milk
before analysis by the proposed iPhone-based digital image
colorimeter comparing with the UV–Visible spectrophotometer