GC010 Chromatographic terms and symbols - Part 3

 

In a typical chromatogram for a single solute A, we can see a small peak at the beginning, short time after the injection (which is performed at time zero). The solutes are characterized by their retention times or retention volumes (tr, or Vr). These quantities are depicted as the distance between the injection point and the peak maximum. The formula to calculate the retention volume Vr is:

Retention volume equals (“retention time” multiplied by “gas flow rate”)

Or we can express it in terms of the retention time as:

retention time equals (“Retention volume” divided by “gas flow rate”)

Or in formula Vr = tr x Fc  or  tr = Vr / Fc; where Fc is the gas flow rate

The small initial peak corresponds to a component that does not is sorb into the stationary phase, thus it is an un-retained component. The IUPAC defines Vm, the holdup volume as: “the volume of the mobile phase (MP) required to elute the un‐retained compound from the chromatographic column and reported at column temperature and ambient pressure.” In GC, air or methane are frequently used as the un-retained component. The holdup time tM is defined as “the time required for the MP to pass through the chromatographic column.”

Retention time (tR) is equal to (time spent moving in the mobile phase (tm) plus time spent sorbed not moving in the stationary phase (t’R)). In formula:

tR = tm + t’R

The total time required for the elution is composed then of two parts, the time that the solute takes to pass through the gas that fills the column (tm) and the time that the solute spends not moving in the stationary phase, which is determined by the distribution constant.