Dimethyl carbonate as a green mobile phase modifier for reversed phase liquid chromatography of polar aromatic and polyaromatic hydrocarbons

Abstract

Linear alkyl carbonate solvents such as dimethyl carbonate (DMC) are considered to be environmentally safe with a low toxicology risk. The modest viscosity of 0.625 mPa·s and the UV cutoff wavelength of about 215 nm makes DMC a potential alternative aprotic HPLC modifier solvent to acetonitrile. The main issue is the limited solubility of DMC in water, about 14 %. A polarity parameter (P’) for DMC of 3.4 ± 0.1 was determined by ratioing Hildebrand solubility parameters for three alcohols and DMC to the corresponding alcohol P’ values. A test mixture of polar aromatic compounds (benzene sulfonate, aminobenzoic acid, nitrobenzoic acid, salicylic acid, and N,N-dimethylaminobenzoic acid) has a reasonable retention factor (k) range between 1 and 9 over a DMC modifier percentage of 10 to 2 % with no ethanol as the co-solvent using a C-18 column. For a similar retention factor range using methanol, the composition required variation from 60 to 10 %. Compatibility of DMC with the ion pairing agent tetrabutylammonium has been shown, increasing the retention of benzene sulfonate and lowering the retention of the aminobenzoic acids. Average solvent strength parameters calculated from quadratic fits of ln k versus organic mobile phase fraction for caffeine, sulfathiazole, and two nitroaniline isomers have shown DMC to be significantly higher (2.8) than ethanol (1.7) or acetonitrile 1.6). This water solubility limitation of DMC can be overcome by using ethanol as a co-solvent, still maintaining the green nature of the mobile phase, at a 5:3 DMC:ethanol ratio. DMC has proven to be an effective RPLC modifier solvent, with either UV or fluorescence detection, for hydrophobic polyaromatic hydrocarbons (PAHs) such as the seven ring coronene, reducing its retention by a factor of 5 as compared to acetonitrile.