6. Basic Chromatographic Terms
Retention time (tR)
The retention time tR shows the arrival time of the peak center at the detector. It is important to confirm the reproducibility of the retention time because the peak identification in HPLC is based on tR.
Retention factor (k)
The retention factor k is calculated from the retention time tR and t0. t0 is the elution time of an unretained compound in the HPLC conditions. Unlike tR, retentivity among various column lengths or flow rates can be compared.
Resolution Rs describes how well two adjacent peaks are separated. In general, Rs ≥ 1.5 is defined as baseline separation. Rs is given by the equation below with the retention factor k, the theoretical plate number N and the separation factor α:
Peak asymmetry (tailing factor)
Peak asymmetry (Asy) can be expressed as follows:
where W0.05 is the peak width at the 5% peak height and f is the distance between two vertical lines: one from the peak top and the other one from the 5% peak height of the leading edge (see figure on the right).