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:

A multi-residue method for the determination of 124 pesticides in rice by modified QuEChERS extraction and gas chromatography-tandem mass spectrometry. A modified QuEChERS method for simultaneous determination of 124 pesticide residues in rice was established and validated. The rice samples were initially extracted with acetonitrile (MeCN), and the targeted pesticides were purified following the dispersive solid phase extraction (d-SPE) cleanup method and detected by gas chromatography-tandem mass spectrometry (GC-MS/MS). The efficiency of citrate-buffered and unbuffered method in liquid extraction procedure was compared; the amount of primary secondary amine (PSA) was optimised. In the method validation, correlation coefficients (r(2)) were higher than 0.990 with the linear ranging from 10 to 200 μg kg(-1). At the fortification levels of 20-200 μg kg(-1), average recoveries ranged from 70% to 122.7% with the RSD <20%.
{"Chemical": ["MeCN", "PSA", "primary secondary amine", "citrate", "acetonitrile"]}