Chromatography is a collection of laboratory techniques for separating a mixture of substances. These techniques involve passing the mixture from a mobile phase to a stationary phase. Each substance in the mixture does this at a specific rate, causing it to become separated as it moves into the stationary phase. The retardation factor (RF) is the relative amount of time that a particular substance spends in the mobile phase.

### Step 1

Define the retardation factor mathematically. It is the ratio of the time that a substance spends in the stationary phase to the time that it spends in the mobile phase. Thus, Rf = Ds/Df ,where Rf is the retardation factor, Ds is the migration distance of the substance, and Df is the migration distance of the solvent front.

### Step 2

Dip a strip of filter paper into a solution that contains a mixture of substances to be analyzed and a solvent. As the solvent is absorbed through the filter, the various test substances will drop out of the solution. The period of time that a test substance remains in solution is its stationary phase. The period of time in which the solvent is moving is its mobile phase.

### Step 3

Examine the results of the chromatography performed in step 2. Measure the distance that the test substance traveled before dropping out of the solution (Ds) and the distance that the solvent traveled(Df). The retardation factor, Rf, is then given by Ds/Df.

### Step 4

Interpret the retardation factor. The retardation factor is only meaningful if the test substance moves some measurable distance, and it can never move farther than the solvent. Therefore, a meaningful retardation factor will always be between 0 and 1.

### Step 5

Find a known substance with a retardation factor similar to that of the test substance. Chemists typically combine this procedure with other chromatography techniques in order to identify an unknown substance.