Van Deemter’s curves have been plotted for each carrier gas (He, N2and CO2) for pressures from 10 to 60 bar and for temperatures between 25◦C and 120◦C to study the influence of carrier gases nature on Hmin. With such monolithic columns, the optimal velocity (6–8 cm s−1) was lower than the optimum velocity observed for open tubular column (15–20 cm s−1). For all tested columns, the minimum plate height always reach the lowest value when carbon dioxide was used, followed by nitrogen, and helium (typical Hmin values are 15 m, 30 m, 100 m respectively for CO2, N2and He).
In order to illustrate the ability of silica monolithic capillary columns to perform separations in HPGC, Fig. 8 shows the chromatogram obtained with mixture A (CO2Pi= 20 bar, Toven= 25◦C). All light hydrocarbons are well separated in a very short time, without any peak asymmetry. Furthermore, the system was able to maintain the chromatographic performances when high sample pressure was used: using mixture B which contains 15% methane and 15% ethane, no significant change on retention factor, efficiency and asymmetry have been observed when Psample increase from 1 to 8 bar: Fig. 1