Supercritical fluid chromatography of nitrogen-containing compounds on packed columns

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Virginia Polytechnic Institute and State University

The separation of basic nitrogen-containing compounds has been investigated via supercritical CO₂ and 1% methanol modified CO₂. Packed columns with the following stationary phases were employed: silica, octadecyl (C₁₈), propylamino (NH₂), and polystyrene-divinyl benzene (PRP). Without modifier the range of basicities which could be eluted increased in the order of silica < PRP < C₁₈ = NH₂. Chromatographic peak shapes and selectivity were much better with propylamino column. Different aromatic amines and azaarenes were successfully separated on both analytical scale and microbore propylamino bonded phase packed columns with 100% supercritical CO₂. Separation is compared with both reversed phase and normal phase high performance liquid chromatography (HPLC). The retention mechanism study for these aromatic amines and azaarenes shows that the elution order not only depends on basicity and steric hindrance, but also on the solubility of the solute in CO₂.

New cross-linked cyanopropyl and phenyl bonded phases are studied as stationary phases for packed column SFC, as well as for separation of nitrogen-containing compounds. The cross-linked bonded phase impedes access to uncapped silanol sites, thereby giving rise to better peak shapes, and more rapid elution without the necessity of a polar modifier in the mobile phase. Experiments both at elevated temperature and in the presence of methanol modifier revealed that there is no short or long term deleterious effect on the column.

The separation of model mixtures of nitrated diphenylamine and nitrated anilines via SFC employing cyanopropyl packed and capillary columns is described. Peak identification and peak purity were performed by on-line Fourier transform infrared spectrometric detection. Supercritical CO₂ is employed with cyanopropyl packed columns for separation of non-polymeric components in double-base rocket propellants. Both supercritical CO₂ and CH₂Cl₂ were compared as a solvent for extraction of nonpolymeric components in "good" and "bad" double-base propellant.

Finally, twenty-four phenylthiohydantion amino acids (PTH-AA) have been rapidly and efficiently separated on a cyanopropyl packed column by gradient elution of supercritical CO₂ and tetramethyl ammonium hydroxide-modified methanol. Complete or partial resolution of 22 derivatives is observed with only valine co-eluting with norleucine and lysine co-eluting with asparagine. No modifier was required for elution of neutral PTH-AA's from the cross·linked stationary phase. The addition of base plays a major role in elution of acidic and basic PTH-AA's.