Читать книгу Chemical Analysis - Francis Rouessac - Страница 67

The mobile phase is a gas (helium, hydrogen or nitrogen), either drawn from a commercially available gas cylinder or obtained, in the case of hydrogen or nitrogen, from an on‐site generator (water electrolysis for H₂ and air separation for N₂), dedicated to the installation. To eliminate all harmful traces of water vapour and oxygen from polar stationary phases and detectors, a double filter, for drying and reducing, is placed before the injector.

The nature of the carrier gas has no significant influence upon the values of the partition coefficients K of the solutes between the stationary and mobile phases, owing to an almost total absence of interaction between the gas and the solutes. Temperature is the only significant modification factor. By contrast, the viscosity and speed of the carrier gas, related to its flow rate or pressure at the head of the column, have an effect on the analytes’ dispersion in the stationary phase and on their diffusion in the mobile phase (see the Van Deemter equation, Chapter 1, Section 1.10). These two factors have a direct impact on column efficiency N (Figure 2.2).

Figure 2.2 Efficiency as a function of the nature and linear velocity of the carrier gas. The Van Deemter curves show the relationship between HETP and linear velocity of the carrier gas for a given compound. Comparison of the viscosities of these three gases. Note the increase in the viscosity of these gases with temperature.

The pressure at the head of the column (several tens to hundreds of kPa – a few tenths to a few bar) is stabilized by electronic pressure control (EPC), so that the flow rate remains constant at its optimal value. This device is valuable because if the analysis is performed with ascending temperature programming (temperature gradient), the viscosity of the stationary phase and, by consequence, the pressure drop in the column, increase with time. By controlling the pressure, we conserve a constant and optimal speed of the carrier gas. The result is a faster analysis with the same efficiency.

The comparison of the three Van Deemter curves shows that the minimum of each is obtained for various linear velocities of the carrier gas: low for nitrogen, higher for helium and even higher for hydrogen. This means that the latter gas reduces the analysis time. For hydrogen also, the growth of the curve after the peak is less quick than for the other two gases, which gives more latitude in choosing the carrier gas speed without impacting the column’s efficiency (Figure 2.2). The three curves plotting the viscosity of these gases versus temperature T again show that hydrogen has a lower viscosity than the other two carrier gases. Lower viscosity means a lower pressure drop and an increased column life.