Choosing an Optimal Co-Solvent for Method Development in Supercritical Fluid Chromatography (SFC) for the Whelk-O®1, RegisPack®, and RegisCell® Chiral Stationary Phases

The criteria for choosing an optimal co-solvent is twofold. The first is to obtain the best suitable separation and resolution. Secondly, when scaling to a preparative separation, the ability of the co-solvent to solubilize the sample is of highest priority.

With all of the covalently bonded chiral stationary phases (CSPs) available in the market, the choice of solvents has increased. A study of various solvents will be tested on bonded and coated phases. Twelve randomly chosen samples will be run with a variety of solvents on the Whelk-O®1, RegisPack®, and RegisCell® phases. 

Experimental Conditions

The instrumentation consisted of a Waters Thar analytical Method Station SFC System. The Whelk-O®1 is bonded with 1-(3,5-Dinitrobenzamido)-1,2,3,4,-tetrahydrophenanthrene. The RegisPack® polysaccharide-based CSP is coated with tris – (3, 5 – dimethylphenyl) carbamate of amylose. The RegisCell® polysaccharide-based CSP is coated with tris – (3, 5 – dimethylphenyl) carbamate of cellulose. All phases were based on 5 micron silica and packed into 25 cm × 4.6 mm ID columns.

Results

The solvents chosen for this study include methanol (CH₃OH), 2-propanol (IPA), ethanol, acetonitrile (CH₃CN), tetrahydrofuran (THF), and methylene chloride (CH₂CL₂).

Methyl tertiary butyl ether was taken out of the study as most of the compounds did not elute off the column. THF and CH₂CL₂ were only run on the Whelk-O®1 column as these two solvents would strip off the phase on the RegisPack® and RegisCell® CSPs.

Table I summarizes the number of hits for each possible solvent. There are a total of 36 hits possible for each of the solvents except THF and CH₂CL₂ where there are 12 possible for each solvent.

Table I: Solvent separation hits

With this particular set of solvents and samples, IPA appears to be the best co-solvent. The Whelk-O®1 had the greatest number of hits as shown in Table II.

Table II: CSP success rates (%)

Conclusions

The majority of the samples in this study and other studies show that a majority of compounds can be separated by the use of the three most common alcohols: IPA, CH₃OH, and ethanol. Solubility becomes a crucial factor in preparative chromatography. Combinations of solvents can be used to improve solubility when the chosen co-solvent has poor solubility of the compound in question.

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