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The simplified equation proposed by Van Deemter in 1956, is well known for packed GC columns (Eq. (1.37)). The expression links the plate height H (HETP) to the average linear velocity of the mobile phase ū in the column (Figure 1.10):

(1.37)

This equation reveals that there exists an optimal flow rate for each column, corresponding to the minimum value of H, as shown by the curve of this equation. The loss in efficiency as the flow rate increases is obvious and represents what occurs when an attempt is made to rush the chromatographic separation by increasing the mobile phase flow rate. However, the loss in efficiency that occurs when the flow rate is too slow is less intuitive. To explain this phenomenon, the origins of the terms A, B, and C must be reviewed. Each of these parameters represents a domain of influence that can be perceived on the graph (Figure 1.10).

The three basic experimental coefficients A, B, and C are related to diverse physico‐chemical parameters of the column and to the experimental conditions. If H is expressed in cm, A will also be in cm, B in cm²/s and C in s (where velocity is measured in cm/s). The curve of the Van Deemter equation is a hyperbola that goes through a minimum (H_min) when:

(1.38)

Figure 1.10 Van Deemter curve in gas chromatography with the domains of parameters A, B, and C indicated. There exists an equation similar to Van Deemter’s that considers temperature: H = A + B/T + CT.