You will be shown an abstract from a biomedical research paper. Given this abstract, your task is to extract all unique entities of the following types: ["Chemical"].

Please return the output as a JSON object of the format: {"Chemical": ["Arsenic", ...]}. The keys should be entity types and values should be lists of extracted entities belonging to the corresponding type. If you cannot find entities belonging to a specific type, the value should be [].

Only output the JSON object and do not include any additional text.

Abstract:

Determination of aflatoxin M1 in milk by high-performance liquid chromatography in Mashhad (north east of Iran). The aim of this study was to evaluate aflatoxin M1 (AFM1) contamination in milk samples in Mashhad in Iran. A total of 60 milk samples were collected from retail stores of five regions in June 2011. The occurrence and concentration range of AFM1 in the samples were investigated by high-performance liquid chromatography method. AFM1 was found in 100% of the examined milk samples by average concentration of 16.16 ng/L and the contamination level ranging between 2 and 64 ng/L. The concentration of AFM1 in all the samples was lower than the Iranian national standard and Food and Drug Administration limit (500 ng/L), and only in one (1.6%) of the samples, the concentration of AFM1 was greater than the maximum tolerance limit (50 ng/L) accepted by European Union and Codex Alimentarius Commission. Statistical evaluation showed no significant difference between the mean concentrations of AFM1 in milk samples taken from different regions (p > 0.05).
{"Chemical": ["aflatoxin M1", "AFM1"]}