Ninhydrin Test for Soil Testing

Purpose:
The purpose of the ninhydrin test is to determine the presence of amines or α-amino acids in a given sample. This is applicable to the science mechanism of the rover because the presence of these amines or α-amino acids can indicate the presence of extant life within a soil sample.

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.

Procedure:

1. First, a 2% solution of ninhydrin must be prepared by dissolving 2g of ninhydrin in 100ml of acetone. The rest of the acetone is left alone.

2. The 2% ninhydrin is mixed with the corresponding dirt experimental samples and the pure acetone is mixed with the control samples.

3. The test tubes are then left for a minimum of 5 minutes (5 to 15mins) and then observed.

4. Note any colour changes to the experimental samples and compare to each control. Refer to the next section for results.

Interpreting Results:
After the initial experiment is conducted, the samples must be observed for colour changes. If a yellow-orange colour develops, this indicates hydroxyproline and proline in the sample. Blue-Purple indicates ammonia, primary/secondary amines, and/or amino acids in the sample. A brown colour indicates asparagine. If no colour change is observed, this is a negative result, meaning none of the chemicals tested for are in the sample.