Soil Testing (Ninhydrin Test)

The organic molecules amino acids are the building blocks of life on Earth (https://www.sciencedaily.com/releases/2016/06/160628110040.htm)

Ninhydrin Test

The ninhydrin test is a chemical test that is used to check whether a given analyte contains amines or α-amino acids. In this test, ninhydrin (a chemical compound with the formula C9H6O4) is added to a test solution of the analyte. The development of a deep blue color indicates the presence of ammonia, primary/secondary amines, or amino acids in the analyte.

The amino group of the analyte forms a compound similar to diketohydrin. This compound has a deep blue color, often referred to as Ruhemann’s purple.

Ninhydrin Test Principle

The amino group belonging to a free amino acid undergoes a chemical reaction with ninhydrin, which behaves as an oxidizing agent. When exposed to ninhydrin, the amino acid undergoes oxidative deamination, resulting in the liberation of CO2, NH3, and an aldehyde along with hydrindantin (which is a reduced form of ninhydrin).

Now, the ammonia goes on to react with another ninhydrin molecule to form diketohydrin (which is also known as Ruhemann’s complex). This complex is responsible for the deep blue colour. When the analyte contains Imino-acids like proline, a yellow-coloured complex is formed. When asparagine is used, the colour of the resulting complex is brown.

Ninhydrin Test Procedure

1. First, a 2% solution of ninhydrin must be prepared by dissolving 0.2 grams of ninhydrin in 10ml of either ethanol or acetone.
2. This solution is now inserted into test tubes pre-filled with deionized water (Sterile), test tubes are loaded onto the rover.
3. Auger/scoop collects soils and soil is inserted into the test tubes. Test tube openings are covered. The mixture is mixed well to ensure uniformity.
4. The test tube must be kept in a warm water bath for approximately 5 minutes. [check if this is feasible – heat speeds up the test process]
5. The development of a deep blue/violet color indicates the presence of amino acids.

Ninhydrin Test Result Interpretation

1. For ammonia, primary/secondary amines, and amino acids, deep purple color is obtained – indication of extant life
2. For hydroxyproline and proline, a yellow color is obtained - indication of extant life
3. For asparagine, brown color is obtained – indication of extant/extinct life
4. If no color change is observed, the analyte does not contain amino acids, amines, or ammonia – no life

All three results should be pre-loaded as controls to be compared to the final result.