Coupling SFC with ELSD: Study of the Parameters Ruling Response

The use of an evaporative light scattering detector (ELSD) with high performance liquid chromatography (HPLC) has been well documented. However, because the mobile phase is nebulized to produce droplets the use of supercritical fluids as the mobile phase with an ELSD is also favourable because the fluid depressurizes before the detector entrance, which allows the formation of aerosol. This article investigates the ELSD response variation with supercritical carbon dioxide–based mobile phases and compares the response (peak area) with HPLC.

The use of an evaporative light scattering detector (ELSD) with high performance liquid chromatography (HPLC) overcomes some of the drawbacks and limitations of using ultraviolet (UV) detection: ELSD can be used for the detection of compounds lacking a chromophore group, provides no baseline deviation during gradient elution and can be used with solvents with a high degree of UV absorption (1,2).

The principle of ELSD is based on nebulizing the mobile phase — a process where an aerosol is produced by the addition of a nebulizer gas (nitrogen) (3–8). This process takes place in a nebulizing chamber that can have various dimensions and volumes depending on the manufacturer. It can also be adapted to the type of chromatography used — HPLC, ultrahigh-pressure liquid chromatography (UHPLC) or micro-LC. The droplets, consisting of the analyte molecules and an unknown amount of solvent, are introduced into a drift evaporative tube and heated at varied temperatures. The droplet size is reduced by further solvent evaporation as the droplets pass through this tube. Finally, the droplets go through a light source and the deflected light is collected by a photomultiplier.

The response depends on the particle (droplet) size, which seems to increase with the increasing compound concentration and which is related to numerous physical properties of the solvents composing the mobile phase: viscosity, surface tension and volatility. This explains that the response (A) variation with the concentration (m) is generally nonlinear for noncapillary columns and follows the relation:

Consequently, using a gradient elution in reversed-phase (RP) HPLC, that is, increasing the organic solvent content in the mobile phase, leads to changes in the droplet size that favour the response of the compounds in relation to their retention time (4,9,10). A compensation gradient added after the column makes it possible to limit this drawback by keeping the mobile-phase composition at the detector entrance constant (10,11).

Other parameters induce great response changes, such as the atomization pressure (3,4), which is related to the flow rate of the nebulizer gas; the geometry of the nozzle (mainly the capillary diameter) (7,8); and the location of the mixing point of the mobile phase with this gas. The wavelength of the light source used also plays a role in the response intensity (6).

On the other hand, because of the nebulization process required, coupling supercritical fluid chromatography (SFC) with ELSD has been successfully performed previously (12–18). The specificity of the carbon dioxide–modifier (an organic solvent) mobile phase used in SFC is the pressure required at the column outlet to ensure the dense state of the fluid. This back-pressure is achieved with a back-pressure regulator or a restrictor, located after the column. However, after the back-pressure regulator or restrictor, carbon dioxide returns back to a gaseous state, thus favouring aerosol formation in the nebulizer chamber. However, this carbon dioxide depressurization cools the outlet capillary, which can induce dry-ice formation and possibly plug the capillary. Heating, provided either by the back-pressure regulator or by an additional transfer line, avoids this drawback.

Beyond these studies addressing this coupling of SFC with ELSD, however, few studies are available describing the varied effects of the numerous chromatographic parameters affecting SFC separations. The difficulty of interfacing SFC with a low-pressure detector, given the phase separation between the liquid solvent and gaseous carbon dioxide, has been addressed (19), but no clear comparison has been performed between SFC and HPLC to evaluate how the use of carbon dioxide instead of a liquid affects ELSD responses in terms of area or sensitivity. This comparison is the topic of this paper. In this study, we used an ELSD system with a slightly adapted nebulizer, mainly with carbon dioxide–ethanol mobile phases, with the goal of operating in "green" chromatographic conditions.