Ninhydrin is a chromogenic analytical probe for the quantification of proteins and amino acids.

Chromogenic analytical probe:

I. Amino Acid Analysis
1. Material Preparation
1) Ninhydrin solution: Prepared as 0.2%–1% ethanol or propanol solution.
2) Amino acid standard solution or sample to be tested.
3) Buffer: Commonly used citric acid buffer or other buffers with pH 5.5–6.5.
4) Heating equipment: such as water bath or constant temperature heating device.
2. Steps
1) Prepare the reaction system: Add a certain volume of sample solution and an appropriate amount of Ninhydrin solution (usually 1:1 of the sample volume) to the test tube.
Add buffer to adjust the reaction conditions (optional).
2) Heating color development: Heat the reaction system at 90–100°C for 10–20 minutes to generate blue-purple or yellow compounds (Ruhemann's purple).
3) Cooling and detection: Cool the reaction system to room temperature; use a spectrophotometer to measure the absorbance at 570 nm (blue-purple) or other appropriate wavelengths to calculate the amino acid concentration.
II. Protein detection
1.. Material preparation
1) Ninhydrin solution (0.5%–1% propanol solution).
2) Protein sample solution.
30 Buffer (such as phosphate buffer).
2. Steps
1) Reaction preparation: Add an equal volume of Ninhydrin solution to the sample solution and mix gently.
2) Heating color development: Heat the mixture to 90–100°C for 15 minutes. The color reaction depends on the type and concentration of amino acids.
3) Detection: After cooling, use a spectrophotometer to measure the absorbance at a specific wavelength (usually 570 nm) and use a standard curve for quantitative analysis.
Precautions
1) Operation safety: Ninhydrin is a potential allergen. Gloves and goggles should be worn during operation, and experiments should be performed in a well-ventilated area.
2) Storage in the dark: Ninhydrin is light-sensitive. The solution should be stored in the dark. It is recommended to store it at 4°C for short-term storage and -20°C for long-term storage.
3) Optimize reaction conditions: According to the characteristics of the sample, appropriately adjust the pH, temperature and reaction time to obtain the best results.