4. My sample isn’t soluble in alkane-alcohols. What sample solvents can I use?
For analytical purposes, high sample concentrations are usually not necessary. A sample preparation of 1 mg/ml, or even less, in mobile phase is usually sufficient. If your sample is an acid salt of a base, then addition of 0.1% DEA to the sample solvent may help solubility by converting the material to the free base, which usually is more soluble. Conversely, if your sample is a salt of an acid, then addition of 0.1% trifluoroacetic acid may improve solubility.
For optimal method robustness, it is always preferred having the sample dissolved in the mobile phase used to resolved the target molecules. Nevertheless, when this is not possible, there are several options.
If you are using one of the immobilised phases, try to dissolve your sample in some other solvent in which it has good solubility. When the application is analytical, there should be no issues with sample solvent-generated effects on the results, providing the sample injection volume is small. This is especially true if the chosen solvent has a similar solvent strength to that of the mobile phase. For preparative applications, this can be more problematic since the sample can precipitate from solution, once the injection mixes with the mobile phase. In such cases, it is best to use the sample solvent as one of the mobile phase components to enhance the mobile phase solubility.
Where the column used is a traditional, coated column, you may try to dissolve the sample in 100% methanol, ethanol, isopropanol, or acetonitrile. These polar solvents can be used with nearly all of our most popular chiral stationary phases. Mixtures of acetonitrile and alkane must be strictly avoided as they are not miscible and will destroy those columns. In general, organic solvents such as toluene, chloroform, methylene chloride, MTBE, tetrahydrofuran, acetone, MEK, ethyl acetate, dimethylformamide, dimethyl sulfoxide, and pyridine must be avoided when dissolving samples to be injected into the traditional coated columns. Even small amounts of these solvents, not removed during sample clean-up, can dissolve the chiral stationary phase and lead to dramatically shortened column life.
If your sample will only dissolve in aqueous solvents, than it can be resolved in reverse-phase mode with IA, IB, IB N, IC, ID, IE, IF, IG, IH, and IJ columns and RP coated columns. Mobile phases and dilution solvents of methanol/H2O ethanol/H2O, isopropanol/H2O, or acetonitrile/H2O can be used in reverse-phase mode. Please consult the Instruction Sheet for your specific column for reverse-phase composition limitations associated with specific CSPs. Care should be taken to control the pH between 2 and 9 in order to avoid dissolving the silica support of the chiral stationary phase.
