For an isocratic separation all peaks broaden as they move through the column, and the longer the retention time the broader the peak, therefore, the later eluting peaks will be broader in a system where the extra column volumes are minimized. If there are extra column volumes in your system, which will lead to distortion of your peaks, then a decrease in N for earlier eluting peaks will be seen (i.e. broader, less efficient peaks). Extra column effects typically only produce a small increase in retention time but a comprehensive increase in peak dispersion/band broadening (Figure 2). 



Figure 2: Effect of extra column volume. 

For example, the separation of six nitroaromatic compounds under identical analytical conditions but on two HPLC systems with differing extra system volumes exhibit peaks with markedly different efficiencies (Table 1). The greater effect of the extra column volumes can be clearly seen on the early eluting peaks. 



Table 1: Peak efficiencies for the separation of six nitroaromatic compounds on HPLC systems with different extra column volumes. The greater the value of N the more efficient (narrower) the peak. 

Band broadening/efficiency (N) in the column is given by Equation 1.


Where: t_{r} = retention time W_{b} = baseline peak width W_{1/2} = peak width at half height Which, can be expressed in volume units V (baseline band volume, Equation 2). Where, V_{R} is the retention (peak) volume (Equation 3). Therefore, substituting V_{R} (Equation 3) into Equation 2, gives Equation 4.
V_{m} is the column dead volume and is proportional to the internal volume of the column (i.e. as the column volume increases so will the dead volume). Bandwidths will be smaller for shorter, narrower columns (small V_{m}) packed with smaller particles (larger N per unit length). V is also smaller for bands that are less retained (smaller k values). When V is small extra column effects will have a greater impact, hence, greater band broadening will be seen for early eluting peaks as they have the smallest volume (i.e. narrowest peaks). This also means that when working with small volume columns or UHPLC equipment, extra volume effects must be minimized in order to maintain the efficiency gained from using smaller column volumes or UHPLC equipment. The use of optimized lengths and internal diameters of capillary tubing is important to retain the highest efficiency in LC systems. Tubing length is important but the internal diameter is of much greater importance, which can be demonstrated by the ArisTaylor equation (Equation 5). Where: F = flow through the tubing L = tubing length D_{M} = analyte diffusion coefficient i.d. = tubing internal diameter The overall dispersion of the analyte band is proportional to the length of the capillary tubing but is proportional to the capillary tubing internal diameter raised to the 4th power, thus, producing a profound effect upon analyte peak dispersion. 
