A novel method of solid state ninhydrin reaction and its application in the quantification of oil-soluble amine polymers†

The ninhydrin reaction is a specific chromogenic reaction often used to detect amines. It has high chemical research and application value and is widely used in many fields. Gasoline detergents are typical oil-soluble amine polymers that can clean and protect automotive fuel systems, improving engine performance and reducing environmental pollution. In this paper, a quantitative method based on the solid state ninhydrin reaction was proposed to detect the concentration of detergents in gasoline. This method simplified the conditions of the traditional ninhydrin reaction and broke through its limitations. There was no need to add a buffer system to maintain the pH of the solution, which was essential in liquid ninhydrin reactions. The heating temperature could be as high as 120 °C because the solid state reaction was not limited by the boiling point of the aqueous solution, which was also vital for the chromogenic reaction of amine polymers. Fourier transform infrared spectrometry (FT-IR) showed that the formation of intramolecular hydrogen bonds between amino groups and polar groups would hinder their reaction with ninhydrin. The solid substrate and high temperature of the solid state reaction could effectively break hydrogen bonds. The reaction mechanisms of amine polymers were also studied by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) and were different from those of traditional ninhydrin reactions. This method was successfully applied to quantify three main types of gasoline detergents, and the minimum detectable concentration content was 25 mg kg⁻¹. The relative errors were within 15%. In addition, this work provided a new method for quantifying oil-soluble amine polymers in the fields of coatings, adhesives, surfactants, and curing agents.