The separation of two peaks in terms of their average peak width at base (\(t_{\text{R2}}>t_{\text{R1}}\)): \[R_{\text{s}}=\frac{t_{\text{R2}}\,-\,t_{\text{R1}}}{\frac{w_{\text{b1}}\,+\,w_{\text{b2}}}{2}}=\frac{2\ (t_{\text{R2}}- t_{\text{R1}})}{w_{\text{b1}}\,+\,w_{\text{b2}}}\] In the case of two adjacent peaks it may be assumed that \(w_{\text{b1}}\approx w_{\text{b2}}\), and thus, the width of the second peak may be substituted for the average value: \[R_{\text{s}}\approx \frac{t_{\text{R2}}\,-\,t_{\text{R1}}}{w_{\text{b2}}}\]
Orange Book, 2nd ed., p. 108 [Terms] [Book]
PAC, 1993, 65, 819. (Nomenclature for chromatography (IUPAC Recommendations 1993)) on page 847 [Terms] [Paper]