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containing strong additives) can in fact take place in our most common SFC systems? The question is most relevant since recent investigations showing the common co-solvent MeOH adsorbed strongly to common SFC stationary phases [14,15].
Figure 4. The Figure shows overloaded elution profiles of valerophenone at different co-solvent plateau levels in the mobile phase going from 0% to 7.3% MeOH in the eluent from left to right. Sample injections: 2, 4, 6, 8 and 10 µL injections of valerophenone100 g /L. Other conditions as in Figure 4.
Figure 5. The Figure shows the natural logarithms of the retention factors at different modifier fractions of the solute valerophenone and of the MeOH perturbation peak at a set BPR pressure of 140 bar and a temperature of 40°C. The red dotted line is the calculated retention factors of the MeOH perturbation and the dashed coloured lines the fit to experimental solute elution profile.
retention of a perturbation peak generally decreases faster than a solute peak with increased plateau concentration. The underlying more complicated reason for this is that the perturbation peak is more ‘a wave phenomena’, than a moving mass [15]. To summarise if both requirements described above are fulfilled, the overloaded solute
elution band turns from overloaded Langmuirian shape with a steep front and diffuse rear to being anti-Langmuiran shaped (diffuse front, steep rear) [15].
The question is if this strange phenomena that cannot take place in common LC separations (only in rare LC separations
The answer is yes as we can see in the following and which will be explained in more detail and for more various SFC systems at the SPICA 2016 meeting [16]. In Figure 4 we can see that this is in fact the case in SFC and that the peak deformation effect is due to strongly adsorbing MeOH co-solvent according to the general requirements described above. In Figure 4 we inject different injection volumes of a highly concentrated valerophenone (i.e. the solute) solution at different concentrations of MeOH in the mobile phase, going from neat carbon dioxide to a concentration plateau level of 7.3% MeOH (cf. Figure 4). The injection volumes were kept small between 2 and 10 µL in order to avoid the plug effect (cf. Figure 2) taking place at larger injection volumes, but the sample concentration was very large (100 g/L) in order to guarantee overloaded effects. When going from 0, to 6.2% and then further to 7.3% MeOH eluent fractions, the corresponding solute elution profiles are turning from having Langmuir shape at lower MeOH fractions to having a more rounded shape at the 6.2% MeOH fractions to having a clear anti- Langmuir shape at 7.3% MeOH (cf. Figure 4). This is in perfect agreement with the previous LC theory [15]. When we compare the logarithms of the retention factors of the 6.2% MeOH fraction peak with that of the perturbation peak of the additive (see Figure 5) we can see that the at 0% MeOH fraction the strongly adsorbing additive has a perturbation peak with a larger (extrapolated) retention factor than that of the solute, while at the 6.2% MeOH fraction, when the overloaded solute band shapes are rounder as predicted according to the LC investigations, the retention factors of the solute and perturbation peak are very close to each other [16]. Most interestingly, at slightly higher MeOH fraction 7.3%, a level at which the overloaded valerophenone band profile has turned from Langmuir to an anti-Langmuir shape (cf. Figure 4), the MeOH perturbation peak has passed and has a smaller retention factor compared to the solute peak of valerophenone (cf. Figure 5). This is in a clear agreement with the LC theory described above for the retention behaviour of a perturbation peak versus an ordinary solute peak. The fact that the co-solvent MeOH appears to adsorbs much
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