Stationary phases for supercritical fluid chromatography

Open tubular columns. Here, the separation, in most cases, takes place according to the volatility of the analytes by means of a non-polar stationary phase. Stationary phases, with greater polarity are also used, but owing to the high /3-value (section 2.2.2), selectivity is not particularly powerful in open tubular columns. 

Packed columns. In this technique separation is by virtue of selectivity. A number of different types of modified silica particles developed for HPLC are applicable. In addition, there are a number of in situ modified packings that are of interest in connection with lipid analysis (Demirbiiker and Blomberg, 1991, 1992 Demirbiiker, Hagglund and Blomberg, 1990 Hag-glund, Demirbiiker and Blomberg, 1994) 

Initially, 5 pm particles were used in our columns, but an appreciable improvement in separation efficiency was achieved when these were replaced by 4 pm particles (Demirbiiker et al., 1990). 

In SFC, argentation columns separate TG according to degree of chemical unsaturation, chain length [e.g. POL (P = palmitate, O = oleate, L = linoleate) is separated from SOL (S = stearate) (Fig. 2.2)], position of double bonds in a fatty acid moiety [e.g. TG containing an a-linolenic moiety is separated from a TG where this moiety is exchanged for a y-linolenic moiety 

For the group separation of polar lipids, the packing material should be as inert as possible. Otherwise lipids such as phosphatidylethanolamine (PE) are eluted as tailing peaks. However, when there is some residual adsorptive activity from surface silanol groups on the packing these can be partially deactivated when the mobile phase contains an additive that has a deactivating ability, for example methanol. Class separation of moderately polar lipids, under subcritical conditions, was obtained on columns packed with diol-modified silica and a mobile phase modified with 19 mol% methanol (Fig. 2.5). Also, lipids with greater polarity, such as PE, could be 

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